SCIENTIA MARINA 78(1)
March 2014, 000-000, Barcelona (Spain)
ISSN-L: 0214-8358
doi: http://dx.doi.org/10.3989/scimar.03898.06D
Feeding of European pilchard (Sardina pilchardus)
in the northwestern Mediterranean:
from late larvae to adults
David Costalago 1,2, Isabel Palomera 1
1 Institut
de Ciències del Mar (ICM-CSIC), Passeig Marítim de la Barceloneta 37-49, 08003 Barcelona, Spain.
E-mail: nauplius97@gmail.com
2 Present address: Department of Zoology, Nelson Mandela Metropolitan University, P.O. Box 77000, Port Elizabeth,
6031 South Africa
Summary: We assessed the relative importance of different prey types of the European pilchard (European sardine) from
the late larval to the adult stage. Two different methodologies for analysing stomach contents were used to describe the
trophic dynamics of sardine and the relationship of sardine feeding behaviour with the ontogenetic development of body
structures used for feeding, such as gill rakers and pyloric caeca. This information is essential to accurately depict the use
of the planktonic resources in the area by sardine and to discuss the extent to which the sardine population could be affected
by environmental changes in the Mediterranean Sea. We showed that cladocerans in summer and diatoms in winter were
numerically the most important prey types for both juveniles and adults. However, decapod larvae were the most important
prey during all seasons in terms of carbon content. Accordingly, differences in methodology should be considered in the
analysis of sardine diets. An analysis of the composition of the plankton showed that small copepods were strongly selected
by sardines at all ages and in both seasons. We also observed that the pyloric caeca began to grow when the sardines
were approximately 4-5 cm standard length (SL) and ended their development when the sardines reached approximately
8 cm SL, whereas the gill rakers appeared to be completely functional when the sardines reached 7 cm SL. Therefore,
filter feeding of small particles could be performed with total efficacy beginning at 7-8 cm SL. In view of the energetic
advantage of filter feeding in a well-adapted filter-feeding species such as sardine, the prospective limited availability of
small particles hypothesized by certain authors for the Mediterranean could have negative consequences for sardine. This
study demonstrates that sardine populations, given their extremely high dependence on the lower marine trophic levels, could
be strongly affected by alterations in the environment and in the planktonic community.
Keywords: Sardina pilchardus; northwestern Mediterranean; ontogeny; trophic ecology; small pelagic fish.
Alimentación de la sardina europea (Sardina pilchardus) en el Mediterráneo noroccidental: de post-larva a adulto
Resumen: Analizamos la importancia relativa de los diferentes tipos de presas de la sardina europea, desde post-larvas
hasta adultos. Se usaron dos métodos diferentes de análisis de los contenidos estomacales para describir la dinámica trófica
de la sardina y la relación de su comportamiento alimenticio con el desarrollo ontogénico de las estructuras corporales que
usa en el proceso alimenticio, como las brianquispinas y los ciegos pilóricos. Esta información es esencial para obtener
una imagen precisa de los recursos planctónicos de los que dispone la sardina en el área y para discutir de qué forma la
población de sardina podría verse afectada por cambios medioambientales en el mar Mediterráneo. Mostramos que los
cladóceros, en verano, y las diatomeas, en invierno, fueron los tipos de presas más importantes tanto para juveniles como para
adultos. Sin embargo, las larvas de decápodos fueron las presas más importantes durante todas las estaciones en términos de
contenido en carbono. En consecuencia, las diferencias metodológicas deberían ser tenidas en cuenta en el análisis de dietas.
Con el análisis de la composición del plancton se mostró que los copépodos pequeños eran fuertemente seleccionados por
sardinas de todas las edades y en ambas estaciones. También observamos que los ciegos pilóricos empezaron a crecer cuando
las sardinas alcanzaban aproximadamente 4-5 cm de longitud estándar (LE), y su desarrollo acababa cuando las sardinas
alcanzaban aproximadamente 8 cm LE, mientras que las branquispinas parecieron ser completamente funcionales cuando las
sardinas alcanzaron los 7 cm LE. Por tanto, la alimentación por filtración de pequeñas partículas podría ser llevada a cabo con
total eficacia cuando las sardinas tienen 7-8 cm LE. En vista de la ventaja energética de la alimentación filtradora en especies
bien adaptadas a la filtración como la sardina, la potencialmente limitada disponibilidad de pequeñas partículas que ciertos
autores han hipotetizado para el Mediterráneo podría tener consecuencias negativas para la sardina. Este estudio demuestra
que las poblaciones de sardina, dada su extremadamente alta dependencia de los niveles tróficos marinos inferiores, podrían
verse fuertemente afectadas por alteraciones en el medio y en la comunidad planctónica.
Palabras clave: Sardina pilchardus; Mediterráneo noroccidental; ontogenia; ecología trófica; peces pelágicos pequeños.
Citation/Como citar este artículo: Costalago D., Palomera I. 2014. Feeding of European pilchard (Sardina pilchardus)
in the northwestern Mediterranean: from late larvae to adults. Sci. Mar. 78(1): 000-000 doi: http://dx.doi.org/10.3989/
scimar.03898.06D
Editor: E. Massutí.
2 • D. Costalago and I. Palomera
Received: May 28, 2013. Accepted: December 13, 2013. Published: March 6, 2014.
Copyright: © 2014 CSIC. This is an open-access article distributed under the Creative Commons Attribution-Non
Commercial Lisence (by-nc) Spain 3.0.
INTRODUCTION
Currently, most marine fish ecologists consider that
the dietary habits of a fish species may depend upon
both the availability of prey (Frederiksen et al. 2006)
and the anatomy of the fish (Gerking 1994, Wainwright
et al. 1995). However, the mechanisms that fishes employ for feeding are diverse. In clupeids, two different feeding methods are generally assumed, namely,
particulate (selective) and filter (non-selective) feeding
(James 1986). The switch from one feeding mode to
the other depends primarily on the concentration of
food (Bulgakova 1996) and can also shift in response
to changes in the presence and abundance of particular
prey items (van der Lingen 1994) if the ontogenetic
development of the individuals so permits (Turingan
et al. 2005). Therefore, changes in the lowest trophic
level of the ecosystem can have drastic consequences
for fish recruitment (Cushing 1990, Beaugrand et al.
2003), particularly in small pelagic fish species, whose
prey is exclusively planktonic (Durbin 1979, Blaxter
and Hunter 1982, Checkley et al. 2009).
The European sardine or pilchard, Sardina pilchardus, is a rapidly growing and short-lived small
pelagic fish species. It is one of the most important
fish resources throughout its range in the northeastern
Atlantic, from the North Sea to the Senegalese coast,
including the Mediterranean and the Black Seas. Studies of its feeding dynamics have been conducted on
the Atlantic coast of Spain, where the diets of juvenile
and adult sardine have been found to consist entirely of
plankton (Bode et al. 2004, Garrido et al. 2007, 2008)
and where adults are able to perform both filter and particulate feeding. Due to this difference between stages,
the adults show a greater fraction of phytoplankton in
their stomachs than the juveniles do. In contrast, it has
been shown that sardine larvae in the same region generally feed on copepods (Conway et al. 1994, Munuera
and González-Quirós 2006).
Few previous studies have investigated the feeding behaviour of sardine in the Mediterranean Sea
(Nikolioudakis et al. 2011, 2012, Borme et al. 2013).
Information on this topic is particularly scarce for the
western Mediterranean. Two non-recent studies (Massuti and Oliver 1948, Lee 1961) considered sardine primarily zooplanktivorous, especially after reaching the
juvenile stage, and only two other more recent studies,
by Rasoanarivo et al. (1991) and Morote et al. (2010),
have considered sardine feeding behaviour in the Gulf
of Lions and in the Catalan Sea, respectively. Both of
these more recent studies focused strictly on larvae
smaller than 15 mm standard length (SL). Additionally, a study of all ontogenetic stages from larvae to
adults by Costalago et al. (2012) investigated the diet
of sardine in the Gulf of Lions using stable isotopes.
Despite the ecological and commercial importance
of sardine in the northwestern Mediterranean (Palom-
era et al. 2007), information on the trophic ecology of
this species in the area remains scarce. Therefore, the
main goal of this study was to fill that gap in current
knowledge and to contribute to an improved understanding of the functioning of the pelagic ecosystem
in the northwestern Mediterranean. Ecological models based on trophic web interactions and capable of
quantitatively describing the structure and function of
exploited marine ecosystems (e.g. Ecopath/Ecosim,
see Banaru et al. 2012, Coll et al. 2006) have described
sardine in the northwestern Mediterranean as a key
species in the trophic web but, with the exception of
data from Lee (1961) used in Banaru et al. (2012),
these models have used data on the diet of sardine from
regions other than the northwestern Mediterranean,
such as the eastern Mediterranean (Demirhindi 1961)
and the Atlantic coast of Spain (Bode et al. 2004, Garrido et al. 2008). Moreover, detailing the ontogenetic
fractions of key species included in the models (e.g.
sardine) from an ecological point of view is one of the
major tasks that remain to be carried out.
The area of study, the Gulf of Lions in the northwestern Mediterranean, is highly productive compared
with the generally oligotrophic Mediterranean Sea
(Bakun and Agostini 2001), owing primarily to water
discharges from the Rhône River and the dominant
northern winds that distribute nutrients and particulate
organic matter along the entire continental shelf (Estrada 1996, Salat 1996) and can even cause occasional
small upwelling events (Forget and André 2007). Additionally, this area is a very important nursery habitat
for both European anchovy (Engraulis encrasicolus)
and sardine (Giannoulaki et al. 2011, 2013). Accordingly, the Gulf of Lions has an important fishery for
small pelagic fish, particularly anchovy and sardine
(Lleonart and Maynou 2003, Palomera et al. 2007)
that is linked directly to the environmental features
described above (Lloret et al. 2004).
However, according to the General Fisheries Commission for the Mediterranean (GFCM; see GFCM
2012), the sardine in the Gulf of Lions has experienced
a decrease during the last decade in both tonnes of
catches and biomass, and its population has reached a
critical state. In addition, the mean size-at-age of these
fish has also decreased in recent years (Voulgaridou
and Stergiou 2003, D. Roos, personal communication
for data from 2005 on), with potential consequences
for the viability of the population. To maintain this
commercial fishing activity on this population without
jeopardizing the future of the stock, management must
be based on comprehensive studies of the ecology of
the exploited species and the environment. Accordingly, environmental and trophodynamic drivers must
receive thorough study because fish biomass trends,
and therefore all subsequent anthropogenic activities
(such as fisheries), depend strongly on these drivers
(Fu et al. 2012).
SCI. MAR., 78(1), March 2014, 000-000. ISSN-L 0214-8358 doi: http://dx.doi.org/10.3989/scimar.03898.06D
Feeding behaviour of European pilchard through ontogeny • 3
Fig. 1. – Study area (Gulf of Lions, NW Mediterranean), indicating fish and plankton sampling locations.
In this study, we assessed the relative importance
of different prey types of sardine using two methods
for analysing stomach contents. The first method is
based on the numerical abundance of each prey item
in the stomachs and the second estimates the carbon
content of the prey items and hence indicates the actual
importance that each prey type can have in the carbon
flux from one trophic level to the next. Although some
recent and very extensive works, such as the one by
van der Lingen et al. (2009), have used data obtained
mainly through the numerical analysis of preys in the
stomachs of small pelagic fish, we consider that a comparison of the two methods is still essential to determine whether one of them describes the diet of these
species more accurately.
Additionally, the current study represents the first
attempt to identify a pattern in the daily ration and
consumption rates. Owing, presumably, to the practical difficulties of conducting this type of observation
at sea, there are, to our knowledge, no previous field
studies of the daily feeding activity of sardine in the
western Mediterranean. Therefore, although our results
on this topic are based on only a single 24-h cycle, we
consider that they are important and should be reported.
The final aim of this study was to describe the
trophic dynamics of sardine from late larvae to adults
through the analysis of stomach contents and to show
how feeding behaviour is related to the ontogenetic
development of body structures, such as gill rakers
and pyloric caeca, used in feeding. This information
is essential to obtain an accurate picture of the ways
in which sardine utilizes the available planktonic re-
sources in the area and to discuss the ways in which
prospective changes in the plankton community could
affect sardine populations in the Mediterranean Sea.
MATERIALS AND METHODS
Study area and sample collection
This study was conducted in the Gulf of Lions (Fig.
1), one of the most productive areas of the northwestern Mediterranean (Salat 1996). In terms of fish biomass, it is also one of the most important areas in the
Mediterranean for small pelagic fish species (Barangé
et al. 2009).
Two cruises were performed during two different seasons (summer and winter) on board the N/O
L’Europe (Ifremer, France). The summer cruise
(PELMED07) was conducted in July-August 2007 and
the winter cruise (JUVALION09) was in January 2009.
Plankton samples were collected during each season (16 plankton sampling stations in summer and 13
in winter). To obtain the 200- to 3000-µm mesozooplankton fraction, a standard WP2 net with a mesh size
of 200 µm was used, with sieving through a 3000-µm
plankton mesh. Similarly, to obtain the 53- to 200
-µm microplankton fraction, a scaled-down WP2 net
with a mesh size of 53 µm was used, and the samples
were sieved through a 200-µm plankton mesh. All
plankton samples were split with a Motoda plankton
splitter (Motoda 1959). One-half of each sample was
preserved in buffered 4% formaldehyde-seawater solution for subsequent qualitative analysis of plankton
SCI. MAR., 78(1), March 2014, 000-000. ISSN-L 0214-8358 doi: http://dx.doi.org/10.3989/scimar.03898.06D
4 • D. Costalago and I. Palomera
Table 1. – Mean standard lengths (SL) and SL ranges of sardines caught during the summer 2007 and winter 2009 cruises. N, number of
individuals analysed.
N
Mean SL (cm)
SL range (cm)
Late larva
Winter
Summer
334
3.09
2.2-3.9
223
7.73
4.0-10.9
Juveniles
community composition, and the other half was frozen
(–21°C) on board for biomass measurements.
We collected sardine juveniles (SL range: 4.0-11.0
cm) and adults (SL range: 11-19.5 cm) on both cruises
and also collected late larvae (SL range: 2.2-4.0 cm)
in winter, as shown in Table 1. The size at which individuals were considered adults (11 cm) was based on
the observations by Tsagarakis et al. (2012) of the shift
in schooling behaviour (at a size of 10.7 cm), almost
coinciding with the minimum landing size for sardine
in the Mediterranean (11 cm, EC 1967/2006). To perform the daily ration analysis, consecutive hauls were
performed during a 24-h cycle on each cruise, every
three hours in PELMED07 and every four hours in JUVALION09. All specimens were caught with a pelagic
trawling net equipped with a small-mesh codend (mesh
length 5 mm, ISO 1107) and towed at an average speed
of 3.6 knots over a 30-40 min period. The samples were
immediately frozen (–21°C) after sorting on board by
age groups.
Anatomical analysis
The following morphological measurements were
performed: body wet weight (BW) in g and SL and total body length (TL) in mm. In all, 13 specimens of late
larvae between 2.2 and 4.0 cm SL, 56 juveniles between
4.0 and 11.0 cm SL and 67 adults between 11.0 and 19.5
cm SL were randomly selected from the two cruises to
count the number of pyloric caeca and to measure the
characteristics of the gill rakers according to the pro-
Winter
Summer
272
9.52
7.8-10.9
367
13.05
11-17.5
Adults
Winter
340
12.42
11-19.5
cedures in Tanaka et al. (2006). In the first branchial
arch (the lower or ceratohypobranchial branch) of the
left side of the body (Fig. 2), the number of gill rakers
was counted and the length of the ceratohypobranchial
arch and the width of the gill raker spacings (SGR) in
mm were measured. The SGR was averaged from five
gill raker spacings on the basis of these rakers. Data on
the number of pyloric caeca in specimens larger than
12 cm SL were not presented. The measurements were
made with ImageJ 1.4 software.
ANOVA was used to test the significance of the
slopes of the regressions calculated for the anatomical
variables.
Daily ration analysis
Up to 20 stomachs from each haul within the 24-h
cycle in each season were dissected and the stomach
contents carefully removed. Dissection was performed
under a stereomicroscope, and the entire contents of
each stomach were extracted individually in a Petri
dish. The contents of the intestine were discarded to
reduce bias caused by different rates of digestion and
gut passage times or codend feeding (Hyslop 1980),
and no regurgitation was detected. Particular care was
used to separate the stomach epithelium from the prey
items. The contents of each stomach were filtered on
pre-dried, pre-weighed Whatman GF/C filters (25 mm
Ø) and subsequently dried at 60°C for 48 h. The dry
weight (DW) of the stomach contents was measured to
the nearest 0.1 mg.
The stomach fullness index (SFI) for each individual was calculated by dividing the DW of the stomach
contents by the total fish wet weight (wet BW) according to the following equation:
SFI = (DW / wetBW) 1000.
Fig. 2. – Photo of the first left branchial arch of a sardine adult,
indicating how the different structures were measured: 1, length of
ceratohypobranchial arch (blue line); 2, length of a central gill raker
(green line); 3, space between two consecutive gill rakers, at their
bases (pink line).
Daily ration estimates were obtained with two different models in which the mean SFI value from each
tow was used:
a) the Elliott and Persson (1978) model, in which
consumption over a given time interval t is described
by the equation
Rt ( St − S0 e− Rt ) ,
Ct =
1 − e− Rt
where C is consumption over time t, St is the mean SFI
over time t, S0 is the mean SFI starting at time 0 and R
is the instantaneous gastric evacuation rate (Elliott and
Persson 1978). The total consumption over an entire
cycle (Ct) is equal to the sum of the partial consumptions calculated for the n time intervals between tows
over a complete cycle:
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Feeding behaviour of European pilchard through ontogeny • 5
n
Ct = ∑ Ctn .
1
b) the Eggers (1979) model, in which the consumption over a given time interval t is described by the following equation:
Ct − ( St − S0 ) = SRt ,
where Ct is the consumption over the feeding interval
considered, St is the mean SFI at the end of the interval,
S0 is the mean SFI at the beginning of the interval, S
is the mean SFI over the entire interval, R is the instantaneous evacuation rate and t is the duration of the
interval.
For both models, the gut evacuation rate (R) was
calculated as described by the Elliott (1972) equation:
St = S0 e–Rt
where St is the mean SFI at time t, S0 is the mean SFI at
the beginning of the time interval and R is the instantaneous evacuation rate.
A semi-logarithmic transformation of this equation
was used to calculate R for each consecutive pair of
samples showing a decrease in the value of the mean
SFI. The maximum R value calculated was selected to
represent the instantaneous gastric evacuation rate.
Diet composition analysis
In all, the stomachs of 334 late-larval, 145 juvenile
and 268 adult sardine were dissected and opened under
a stereomicroscope. As previously explained for the
analysis of the diet, the contents of the intestine were
discarded in this analysis, and no regurgitation was detected. Because no items were found in the esophagus,
regurgitation due to sampling stress was considered
absent. Only food items that could be identified were
recorded (e.g. van der Lingen 2002). For juveniles and
adults, pools of the contents of up to 20 stomachs for
each tow, if available, were diluted to a known volume
of filtered seawater as in van der Lingen (2002), and
stomachs of late larvae were analysed individually. All
the prey items were counted and identified up to the
lowest possible taxonomic level.
A SIMPER analysis provided the average dissimilarity between seasons in the prey composition of both
juveniles and adults. A one-way analysis of similarity
(ANOSIM) was used to test the significance of the
differences in the composition of the diet between
seasons and size classes. The PRIMER software package (Version 6.1.9) (Clarke and Warwick 2001) was
used to perform the SIMPER and ANOSIM analyses,
grouping prey species categories based on Bray-Curtis
mean similarities.
The contribution of each prey item to the diet was
calculated with the index of relative importance (IRI,
Pinkas et al. 1971) based on the following equation:
IRI = (%Wi + %Ni) %FOi,
where W = dry weight of prey type i in µg, N = number
of individuals of prey i in stomachs and FO = frequency of occurrence of prey i in stomachs. W was obtained
from estimates by Uye (1982), Saiz and Calbet (2007)
and Borme et al. (2009).
Prey selectivity was estimated with Ivlev’s diet selectivity index, E (Ivlev 1961) for each case analysed.
The value of the index was calculated with the following equation:
E = (ri – pi) / (ri + pi),
where ri is the proportion of prey item i in the stomach
and pi is the proportion of prey item i available from
the marine environment. The mesozooplankton and
microplankton fractions in the samples were considered together, as total plankton, for the calculation of
the Ivlev index.
The carbon content of each prey type, used for comparison with the numerical prey content, was estimated
using equations and tables from Espinoza and Bertrand
(2008) and Borme et al. (2009).
Plankton analysis
The qualitative analysis of plankton was performed in the laboratory. Individuals were identified
to the lowest possible taxonomic level under the
stereomicroscope (Wild M12, at 100× magnification). The mesozooplankton samples were analysed
in aliquots representing approximately 10% of the
sample and repeated until at least 400 copepods had
been counted in each sample; additional subsamples
were also taken for any other abundant organism (i.e.
cladocerans during summer). Microplankton samples
were subsampled differently: 1% to 2% of the original volume was analysed to estimate the presence of
nauplii, dinoflagellates, ciliates and diatoms; small
copepods (primarily copepodites) were analysed in
volumes sufficient to count at least 400 individuals.
Individuals of each identified taxon were counted and
abundances (ind m–3) were calculated. The genera
Clausocalanus, Ctenocalanus, Paracalanus and Parvocalanus were classified as the ‘Clauso-Paracalanidae’ group.
Differences between seasons in plankton biomass
and abundance were evaluated with a nonparametric
Kruskal-Wallis test at a significance level of p<0.05.
RESULTS
Development of trophic-related structures
The means of all the morphological measurements
are shown in Table 2.
Sardines of between 2.2 and 2.5 cm SL already had
approximately 10 short (<5 mm) gill rakers in the first
branchial arch but no pyloric caeca. The number of gill
rakers (GR) maintained a significantly positive trend
(p<0.05) with increasing size (Fig. 3A), but the slope
of the GR/SL relationship was gentler in specimens
larger than 5 cm SL.
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6 • D. Costalago and I. Palomera
Table 2. – Means±Standard deviations of anatomical parameters of
sardine.
Late larvae
Standard length (cm)
Pyloric caeca
Gill rakers
Arch length (mm)
Gill raker spacing (mm)
Juveniles
Adults
3.15±0.58
7.28±1.88
13.61±0.99
0
90.39±34.93 98.55±16.35
20.17±6.11 43.15±9.43 55.67±3.74
2.68±1.10
8.76±2.76
18.87±3.25
0.12±0.03
0.20±0.02
0.15±0.05
Pyloric caeca in sardines were first observed in
specimens of 4 cm SL. After the pyloric caeca appeared, their number continued to increase significantly
(p<0.05) until 12 cm SL, where their number reached
a plateau between values of 120 and 140. Although
the number of pyloric caeca after 12 cm SL remained
within this range, we did not present these data because
the counts of the exact number of caeca were extremely
uncertain owing to the accumulation of fat around the
caeca (Fig. 3B).
The mean length of the ceratohypobranchial arch
was always positively correlated with the SL (Fig. 3C),
ranging from approximately 1 mm in the smaller larvae
to approximately 20 mm in larvae of 17 cm SL. The
density of gill rakers (DGR; number of GR / length of
branchial arch in mm) showed a negative correlation
with SL (Fig. 3D), varying from approximately 7.5 GR
mm–1 branchial arch in the smallest larvae analysed to
less than 4 GR mm–1 branchial arch in sardines of 17
cm SL.
Daily ration and consumption rates
A daily feeding pattern was observed for sardine
juveniles in summer 2007 (Fig. 4A). The SFI values
increased throughout the day, reaching a peak before
sunset (sunrise time, 5:04 GMT; sunset time, 20:02
GMT). Subsequently, the SFI values decreased during the night. For sardine juveniles in summer 2007,
the feeding period began at 5:12 GMT and ended at
16:56 GMT. During the same season, adults showed
an increasing SFI from almost 0 at 8:00 until midnight
(23:02), although the peak occurred at 11:07 (Fig. 4B).
In winter (sunrise time, 8:17 GMT; sunset time,
17:41 GMT), the juveniles had a feeding period of approximately 12 hours, from 4:34 to 16:15, when the SFI
began to decrease (Fig. 4C). The adults did not show
a clear pattern of feeding periodicity and appeared to
feed continuously, although they had an especially low
SFI after sunset (20:12) (Fig. 4D).
The evacuation and consumption rates of adults
and juveniles in both seasons and calculated with both
consumption models are shown in Table 3.
Fig. 3. – Relationships between standard length of sardine and different anatomical structures related to feeding. A, number of gill rakers GR;
B, number of pyloric caeca PC; C, length of the ceratohypobranchial arch LCA (mm); D, density of gill rakers GRd (number of gill rakers /
length of ceratohypobranchial arch).
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Feeding behaviour of European pilchard through ontogeny • 7
Fig. 4. – Mean stomach fullness index±95% CI plotted over time, as obtained in a sampling carried out in consecutive 24 hours. A, juveniles
in summer; B, adults in summer; C, juveniles in winter; D, adults in winter. (Summer sunrise time, 5:04 GMT; summer sunset time, 20:02
GMT; winter sunrise time, 8:17 GMT; winter sunset time, 17:41 GMT).
Diet composition
Only 6 of the 334 stomachs of late larvae analysed
contained prey, and only 2 of those prey items could
be identified (Corycaeidae). For this reason, the diet
of sardine late larvae is not presented in this study. Instead, we used the results from Costalago et al. (2012),
based on isotopic signals, as indicative of the diet of
sardine larvae.
Expressed as a percentage of the total number of
prey, Cladocera dominated the stomachs of juveniles
in summer (84.30%) but occurred in only 60% of the
stomachs analysed (Table 4). In winter, diatoms were
the most abundant prey (72.26%) and appeared in all
the stomachs (Table 4). The stomachs of adult sardine
in summer contained primarily Cladocera (42.95%),
Oncaea spp. (13.34%) and other copepods (10.99%),
and all these prey types were found in all the stomachs (Table 3). In winter, diatoms and appendicularians were the most abundant prey types (66.61% and
12.06%, respectively) and the only prey types, together
with other copepods, that were found in all the stomachs (Table 4).
The IRI values (Table 4) confirmed the importance
of cladocerans in the diet of both size classes in summer
and the importance of diatoms in the diet of sardine in
winter. Additionally, other crustaceans (primarily decapod larvae) were present at low levels of numerical
abundance in the stomachs but had a high IRI owing to
their relatively high weight.
The SIMPER analysis of the numerical composition
of the diet of juveniles showed that the average squared
distance between seasons was 12.55%. The prey items
that contributed most heavily to this dissimilarity were
Cladocera (21.47%), decapod larvae (16.28%), Temora
spp. (11.38%), Clauso-Paracalanidae (10.19%) and M.
rosea (10.15%). In the SIMPER analysis based on the
carbon composition of the prey, the average squared
distance between seasons was 30.45%, and the prey
items that contributed most heavily to this result were
decapod larvae (40.49%), Cladocera (19.19%) and unidentified copepods (14.36%).
For adults, the SIMPER analysis based on the numerical composition of the prey showed an average
squared distance between seasons of 7.61%. The major
contributor to that dissimilarity was phytoplankton
Table 3. – Estimates of gastric evacuation Rmax (h−1) and consumption rates C (DW 1000 g–1 wet BW) for sardine adults and juveniles in
summer and winter. Consumption values are also expressed as percent total weight (%TW). DW, dry weight; CE, daily ration according to the
Eggers model; CE-P, daily ration according to the Elliott-Persson model.
Adults
Juveniles
Summer
Winter
Summer
Winter
Rmax (±SE)
DW (±SE)
0.259 (0.242)
0.508 (0.087)
0.099 (0.076)
0.082 (0.042)
1.58 (0.06)
4.53 (0.09)
3.94 (0.20)
1.48 (0.07)
CE
%TW
DW (±SE)
1.21
3.48
2.95
1.14
1.77 (0.64)
4.88 (0.68)
4.91 (1.55)
0.58 (0.31)
CE-P
%TW
1.36
3.75
3.78
0.45
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8 • D. Costalago and I. Palomera
Table 4. – Total stomach contents by weight (W), numerical abundance (N) of prey, frequency of occurrence (F) of prey in the stomachs and
index of relative importance (IRI).
Juveniles
Adults
Summer
Winter
Summer
Winter
W(%) N(%) F(%) IRI(%) W(%) N(%) F(%) IRI(%) W(%) N(%) F(%) IRI(%) W(%) N(%) F(%) IRI(%)
0.00 0.00
Acartia spp.
Appendicularia
0.00 0.00
0.00 0.00
Calanus spp.
0.00 0.00
Candacia spp.
0.00 0.00
Centropages spp.
Cladocerans
1.21 84.30
Clauso-Paracalanidae 0.32 1.02
Corycaeidae
0.50 4.09
Diatoms
0.00 0.00
0.00 0.00
Euterpina spp.
Foraminiferans
0.00 0.00
0.06 3.58
Microsetella rosea
Molluscs
0.00 0.00
0.00 0.00
Oithona spp.
0.00 0.00
Oncaea spp.
Other copepods
3.64 1.02
Other crustaceans
92.26 0.34
Polychaeta larvae
0.00 0.00
0.93 1.36
Temora spp.
Tintinnids
0.00 0.00
0
0
0
0
0
60
40
80
0
0
0
60
0
0
0
40
40
0
40
0
0.00 0.00 0.00
0.00 0.16 1.62
0.00 0.00 0.00
0.00 6.06 6.06
0.00 0.21 0.21
51.47 0.00 0.00
0.54 0.30 0.30
3.69 0.47 0.47
0.00 0.00 0.00
0.00 0.12 0.12
0.00 0.00 0.00
2.19 0.05 0.05
0.00 0.22 0.22
0.00 0.00 0.00
0.00 0.10 0.10
1.87 3.46 3.46
37.16 87.52 87.52
0.00 0.34 0.34
0.92 0.88 0.88
0.00 0.07 0.07
0
80
0
40
20
0
80
20
100
20
100
100
20
0
40
100
60
20
20
40
0.00 0.00 0.00
0.93 0.16 0.12
0.00 0.00 0.00
1.75 6.04 0.25
0.06 0.00 0.00
0.00 1.13 42.95
1.83 0.30 2.62
0.18 0.47 3.50
47.75 0.00 8.24
0.08 0.01 4.37
1.14 0.00 0.00
1.28 0.05 5.37
0.04 0.22 1.50
0.00 0.00 0.00
0.37 0.10 13.34
9.26 3.45 10.99
34.51 87.17 0.32
0.06 0.00 0.00
0.12 0.88 2.37
0.60 0.00 0.00
0
0.00 0.00 0.00
25
2.51 0.16 12.06
0
0.00 0.00 0.00
12.7 1.27 6.01 1.05
0
0.00 0.21 0.17
100 14.29 1.12 5.42
62.5 6.51 0.30 0.17
75
7.85 0.47 0.52
75
8.32 0.22 66.61
62.5 6.69 0.00 0.00
0
0.00 0.00 0.00
87.5 9.29 0.05 0.52
50
5.15 0.00 0.69
0
0.00 0.00 0.00
100 11.34 0.10 1.57
100 11.10 3.43 8.74
37.5 3.81 86.69 1.57
0
0.00 0.34 0.17
50
5.24 0.87 0.69
0
0.00 0.00 0.00
0
0.00
100 9.63
0
0.00
33.3 1.85
16.7 0.05
16.7 0.86
16.7 0.06
16.7 0.13
100 52.65
0
0.00
0
0.00
50
0.23
33.3 0.18
0
0.00
83.3 1.10
100 9.58
33.3 23.18
16.7 0.07
33.3 0.41
0
0.00
Fig. 5. – Ivlev’s dietary index. A, sardine juveniles summer; B, sardine juveniles winter; C, sardine adults summer; D, sardine adults winter.
(16.80%). In the SIMPER analysis of adult prey items
based on carbon content, the average squared distance
between seasons was 8.55%. The prey items contributing most heavily to this dissimilarity were decapod
larvae (47.14%) and unidentified copepods (12.93%).
According to the SIMPER analysis of the number
of prey in summer, the average squared distance between the juvenile and adult diets was 12.99%, owing
primarily to Cladocera (13.71%) and phytoplankton
(11.10%). In winter, the average squared distance
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Feeding behaviour of European pilchard through ontogeny • 9
Fig. 6. – Stomach contents for the juveniles and adults of sardine in number (N) and in carbon content (C) of prey.
between size classes was 3.38%, and the prey items
contributing most heavily to this dissimilarity were tintinnids (18.88%) and Clauso-Paracalanidae (11.13%).
In the SIMPER analysis of the carbon content of prey
in summer, the average squared distance between the
diets of juveniles and adults was 22.59% and was
primarily due to decapod larvae (40.34%), Cladocera
(15.30%) and unidentified copepods (14.71%); in winter, the average squared distance between size classes
was 4.73%, and the main prey items explaining this
dissimilarity were decapod larvae (62.25%) and Candacia spp. (13.52%).
An ANOSIM confirmed the significant differences
in the composition of the diet between seasons (significance level of sample statistic: 0.001% and 0.01%
for data based on the number of prey and the carbon
fraction of the prey, respectively) and the significant
differences between juveniles and adults (0.3% and
0.008% for data based on the number of prey and the
carbon fraction of the prey, respectively).
Ivlev’s index of prey selectivity (Fig. 5) showed that
the preferred prey items of sardine juveniles in summer were copepods, particularly Harpacticoidae, and
cladocerans. In winter, Candacia spp., Corycaeidae and
phytoplankton (radiolarians and diatoms) were the preferred prey items. Sardine adults in summer selected primarily Corycaeidae, Microsetella spp. and cladocerans.
In winter, Corycaeidae, Temora spp. and Centropages
spp., together with cladocerans and appendicularians,
were the most positively selected prey types.
The proportion of carbon content of the prey showed
the importance of the group ‘other crustaceans’ (composed primarily of decapod larvae) in both juvenile and
adult sardine during the two seasons (Fig. 6). The data
on the numerical percentage of prey types showed that
cladocerans were more important in summer, diatoms
in winter (Fig. 6).
Table 5. – Mean mesozooplankton and microplankton stock in terms
of abundance and biomass during the two periods (Min, minimum;
Max, maximum; SD, standard deviation).
Mesozooplankton
Total abundance
(ind m–3)
Biomass
(mg m–3)
Microplankton
Total abundance
(ind m–3)
Biomass
(mg m–3)
Summer
Winter
Min
Max
Mean
SD
Min
Max
Mean
SD
3767.1
11881.2
8024.2
2484.3
14.2
54.6
33.6
10.3
1211.7
34522.3
14559.7
12273.6
7.6
484.1
53.2
96.6
Min
Max
Mean
SD
Min
Max
Mean
SD
11186.6
452339.3
107734.7
138214.8
9.1
907.2
190.0
277.5
44731.1
487819.5
299637.6
194726.2
7.8
335.1
52.9
72.6
Plankton composition
Information on total microplankton and mesozooplankton abundance (ind m–3) and biomass (mg m–3)
during the surveys is summarized in Table 5. The
abundance and biomass of both mesozooplankton and
microplankton were higher in winter except for the
microplankton biomass, but no significant differences
were found between the summer and winter abundances of either microplankton or mesozooplankton.
Neither mesozooplankton nor microplankton biomass
differed significantly between the two seasons.
The plankton was dominated by phytoplankton
(primarily diatoms) and copepods during the two
seasons (52.8% and 30.1% in summer and 42.3%
and 49.1% in winter for phytoplankton and copepods,
respectively). The most abundant copepod species in
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10 • D. Costalago and I. Palomera
summer were Paracalanus parvus (243602 ind m–3),
Microsetella rosea (243622 ind m–3), Candacia spp.
(55326 ind m–3), Calocalanus spp. (55326 ind m–3)
and Centropages typicus (17061 ind m–3). In winter,
the most abundant copepod species were Ctenocalanus
spp. (12353.57 ind m–3), Paracalanus spp. (12206.59
ind m–3), Labidobcera spp. (11139.83 ind m–3) and
Oithona spp. (7717.51 ind m–3). Excluding copepods
and phytoplankton, the planktonic group with the highest relative abundance in summer was cladocerans
(11.7%). In winter, similar percentages (~8.5%) of
Mollusca, appendicularians and tintinnids were counted. Cnidaria, Doliolida, Echinodermata, Euphausiacea
and Polychaeta also occurred in small percentages in
both seasons (<5%).
DISCUSSION
To depict how sardine in the Mediterranean Sea
interact with the environment and to assess how prospective changes in the plankton community could
affect sardine populations, an accurate description of
the trophic dynamics of the species during all its life
stages, from larvae to adults, is essential.
In this study, the stomach contents of sardine specimens were analysed to obtain information about the
prey items consumed in summer and winter. Then,
based on the study of the development of certain ontogenetic features, we also sought to determine the
body size at which sardine have already developed a
completely functional filtering mechanism and can begin to shift efficiently to a diet richer in phytoplankton.
We observed that pyloric caeca do not appear until
metamorphosis begins (at approximately 4 cm SL). The
subsequent development of the pyloric caeca is very
rapid. No new pyloric caeca are formed after sardine
reach approximately 8 cm SL. Although the existing
pyloric caeca may continue growing in volume well
after 8 cm SL, we can hypothesize that the digestive
function of pyloric caeca (Buddington and Diamond
1986) is fully active as soon as sardine reach 8 cm SL.
Gill rakers appeared at an SL of 2.2 cm in our study.
Similarly, in the studies of Lee (1961) and Andreu
(1969), which thoroughly analysed the development of
gill rakers in sardines from the western Mediterranean,
a TL of 20 mm has been identified as the size at which
gill rakers begin to grow in sardines. Van der Lingen
et al. (2009), based on studies by Andreu (1969) and
Garrido et al. (2007), presented information that can be
compared with our results. Sardines from the Atlantic
coast of the Iberian Peninsula seem to have more gill
rakers than the sardines analysed for this study in the
Mediterranean. This is a very interesting finding that
may indicate that there are also important differences in
the diet of these two separate populations that led their
morphology to evolve in divergent ways and should be
further investigated. Van der Lingen et al. (2009) also
showed that sardines in the northeastern Atlantic, from
Vigo, NW Spain, had an apparently significant higher
number of gill rakers than anchovies of the same size
in that region. How these two similar species differ in
their feeding patterns in different environments, such
as the Atlantic versus the Mediterranean, is another
matter that needs to be researched in depth.
We also found that the increasing trend in the number of gill rakers becomes much less pronounced when
sardines reach 7 cm SL. This result means that filter
feeding on small particles (according to Garrido et al.
(2007), <750 µm is the prey size threshold for filter
feeding) can be totally effective beginning at that body
size. This result is also consistent with the results of
Nikolioudakis et al. (2012) on sardine in the Aegean
Sea and with the results of Scofield (1934), who studied
Sardinops sagax in California and stated that sardines
with a TL of 70-100 mm are able to filter diatoms.
We observed that the length of the ceratohypobranchial arch increases linearly with SL. Moreover,
because the number of gill rakers on this arch becomes
stable at a certain SL, the gap between gill rakers
would also increase with SL. This result might mean
that the ability to filter feed on the smallest particles
would be reduced, but we believe that the denticles
could help to compensate for the loss in filtering capacity resulting from the wider inter-raker gaps (King
and Macleod 1976).
The information obtained from a complete analysis of the daily ration can also be useful to validate
the ongoing development of bioenergetic models for
small pelagic fish species (Urtizberea et al. 2008).
Because at least two full 24-h cycles with sampling
every 3 h or less are needed for a confident assessment
of the diel variation in feeding intensities (Tudela and
Palomera 1995), we cannot guarantee that the pattern
described here represents the normal behaviour of
sardines in the northwestern Mediterranean. Nevertheless, the results of our analysis coincide with those
of previous studies that performed a more comprehensive analysis of the daily ration. For example, we
observed a general pattern of diurnal feeding activity
that extends until dusk, as observed also by Andreu
(1969) in the Atlantic and Nikolioudakis et al. (2011)
in the Aegean Sea. In addition, though the ElliottPersson model is claimed to be appropriate only if
the frequency of sampling is every three hours or less
(Elliott and Persson 1978), we found no differences
between the alternative models that we evaluated.
Moreover, we found no clear patterns of differences
in consumption rates between sizes and seasons, although higher rates are normally expected in summer
(Nikolioudakis et al. 2011). However, we found that
evacuation rates were always higher in adults than
in juveniles. This result may mean that larger individuals have higher metabolic rates, but this is also
expected because instantaneous evacuation rates are
affected by fish size (Elliott and Persson 1978).
Nevertheless, high evacuation rates in sardine
adults could make sense if larger sardines (7 cm SL
and higher) use filter feeding rather than particulate
feeding, as we demonstrated here, owing to the higher
energetic requirements imposed by continuous swimming activity relative to the energy requirements of
smaller individuals or similar species (e.g., anchovy),
whose typical locomotor pattern consists of a glide following a tail beat (Lasker 1970).
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Feeding behaviour of European pilchard through ontogeny • 11
Studies of the diet of European sardine on the
southern coast of England (Lebour 1921) and in
Turkish waters (Demirhindi 1961) have reported
contrasting results. Lebour (1921) stated that sardine
shift after metamorphosis to a diet with a higher proportion of phytoplankton, whereas Demirhindi (1961)
claimed that the diet consisted almost entirely of zooplankton at all ages. This discrepancy appears to have
been resolved by the present study and by other recent
studies. Based on observations of the high relative
importance of prey <750 µm in the diet, Bode et al.
(2004) and Garrido et al. (2007), in Atlantic waters,
and Nikolioudakis et al. (2012), in the eastern Mediterranean, have suggested that filter feeding is the
principal feeding behaviour of adult sardines in the
wild. The findings of the present study in the western
Mediterranean that sardines above 7 cm SL can efficiently feed on phytoplankton confirm the results of
the other recent studies cited above. In fact, sardines
above 4 cm SL in the western Mediterranean were
eating primarily diatoms and decapod larvae in winter, exhibiting their ability to perform both filter feeding and particulate feeding. However, sardine larvae
in this region, according to Morote et al. (2010) and
to Costalago et al. (2012), generally ate copepods and
did not appear to be able to feed on phytoplankton.
This absence of phytoplankton feeding is most likely
due to a lack of the specific body structures needed to
filter small particles.
Bulgakova (1996) explained that anchovy Engraulis encrasicolus could shift between filter feeding and
particulate feeding depending on the concentrations of
different prey items. Our results imply that such shifts
could also be the case in sardine. Other authors (Bode et
al. 2004 and Garrido et al. 2007) have stated that larger
prey, such as copepods and decapod larvae, can also be
an important component of sardine stomach contents,
particularly if the abundance of other prey items is relatively low. This observation suggests that particulate
feeding might also be used in the wild to compensate
for periods of low food availability (Margalef 1960).
Similarly, we observed that in winter, when the abundances of both micro- and mesozooplankton were
higher, both juvenile and adult sardines relied more, in
terms of numerical abundance, on diatoms than on any
other prey type. In summer, however, both juveniles
and adults fed heavily on cladocerans. The selection
of cladocerans rather than copepods in summer may
result from the greater ability of copepods to avoid
capture by fish (Strickler et al. 2005).
Although this study and others (Morote et al. 2010,
Costalago et al. 2012) have shown that sardine larvae
are obligate particulate feeders whose basic prey is
copepods, juvenile and adult sardine are opportunistic
feeders and show a more heterogeneous diet than similar species (e.g., anchovy) (Tudela and Palomera 1997,
Costalago et al. 2012). In addition, several authors
have found correspondences between the plankton
in the environment and in the stomachs (Varela et al.
1990, Bode et al. 2003), suggesting that sardine are essentially non-selective filter-feeders and that their diets
reflect the ambient plankton composition.
Comparisons of the numerical composition of prey
in the stomachs of sardine and anchovy suggest that
the juveniles of these two species show no interspecific
dietary overlap and that interspecific dietary overlap
is also most likely absent in the adults (Costalago et
al. 2014). However, the larvae of both species might
share the same alimentary resources if sea surface temperatures continue to increase (Costalago et al. 2011).
Moreover, other species that have not generally been
common in the Gulf of Lions but have increased in
recent years, such as Sardinella aurita (Sabatés et al.
2006) and Sprattus sprattus (GFCM 2012, D. Roos,
personal communication), are potentially intraguild
competitors with sardine (Palomera et al. 2007, Morote et al. 2008) and could place further pressure on its
population.
In addition, although cannibalism has not been
reported in S. pilchardus in the Mediterranean, it is
probable that intraspecific diet overlap occurs between
juveniles and adults, as confirmed by the similar feeding patterns observed here in all sardine individuals
larger than 7 cm SL. Because this factor can naturally
control population growth, it should be considered in
proposals for plans to manage the resource.
The analysis of stomach contents is a classical and
widely used technique for studying the ecology of
fish (Hynes 1950, Hyslop 1980, Wootton 1999). The
numerical composition of the prey items in fish stomachs provides information about the diet of the fish and
can be highly useful in comparing similar species of
predators (van der Lingen and Hutchings 1998, Costalago et al. 2014). However, numerical analysis of
the prey items in the stomachs of clupeid species can
overestimate the contribution made by phytoplankton
because of the small size and low carbon:volume ratio
of phytoplankton relative to those of zooplankton. For
this reason, the assessment of the stomach contents of
planktophagous fish is more informative if the methods used are based on the carbon fraction of ingested
prey, as is the case for several recent studies (van
der Lingen 2002, Garrido et al. 2008, Borme et al.
2009) and for the present study. We have compared
the results of both methods (carbon and numerical
contents) and shown that differences between the
two analytical techniques exist for adult sardine. In
the analyses based on carbon content, for example,
the largest prey types (decapod larvae and copepods)
contributed more to the seasonal differences in the
diet than any other prey type regardless of their numerical importance. However, the use of numerical
percentages indicated that phytoplanktonic prey items
were the most important. Similarly, we found that dietary differences between juveniles and adults in both
summer and winter based on carbon content were due
to decapod larvae and large copepods and were due to
cladocerans in summer. However, the analyses based
on the numerical composition of prey items in the
stomachs indicated that those differences were due
primarily to small copepods (Clauso-Paracalanidae)
and phytoplankton and were also due to cladocerans in
summer. We have observed that in juveniles, the prey
types that best described the diet (Cladocera, decapod
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12 • D. Costalago and I. Palomera
larvae and copepods) were almost the same according to both methods for measuring stomach contents.
We hypothesize that the reason for this finding is that
juveniles did not prey on phytoplankton during summer but generally fed on cladocerans, whereas adults
in summer showed diatoms in 75% of the stomachs,
representing 8.24% of the total number of prey items.
These numbers would increase the importance of diatoms in the adult diet relative to their role in the diet
of juveniles.
The composition of the diet based on the percentage
by number of prey types differed from the composition based on the percentage by weight. The numerical
percentage gave more importance to cladocerans and
diatoms in summer and winter, respectively, but the
weight percentage was much higher for decapod larvae
than for any other prey type in both seasons. However,
the IRI tended to give more importance to the numerically more dominant prey rather than to those with
higher weights. In addition, the results of the SIMPER
analysis showed greater mean distances for the diet
composition based on carbon content.
We compared our results, based on stomach contents, to those of Costalago et al. (2012), which were
based on stable isotope analysis and therefore provided a longer-term view of the diet prior to capture. The
isotope analysis indicated that appendicularians were
always the most important prey for both juveniles and
adults in both summer and winter, except that juveniles in summer consumed primarily cladocerans. In
contrast, the current study found that appendicularians were among the most important prey types only
in terms of the IRI in adults in winter. This difference
demonstrates that certain prey that are more easily digested, such as appendicularians, can often be
underestimated by analyses of fish stomach contents
(Capitanio et al. 2005).
The results based on the Ivlev selectivity index
showed that the prey types most positively selected by
juveniles and adults (and also by larvae, according to
Costalago et al. (2012)), in addition to cladocerans in
summer, were small copepods, such as Corycaeidae
and Harpacticoidae (primarily Microsetella spp.), in
summer and winter. This result is of particular interest
in view of the importance of this type of copepod in the
pelagic food web (Turner 2004). These copepods represent a link through small pelagic fishes that connects
bacterial plankton with ecologically and economically
important species that prey on small pelagic fishes (de
Laender et al. 2010).
Molinero et al. (2005) showed that high positive
anomalies in water temperature in the northwestern
Mediterranean can cause a decrease in the population
of copepods. An obvious effect of such a decrease on
the trophic dynamics of sardine in the region is that sardine would be forced to rely more heavily on primary
producers as food. In addition, Conversi et al. (2009)
claimed that species of small copepods would most
likely increase as a result of the anticipated warming
of the Mediterranean Sea. This increase would impose
additional limits on the expansion of phytoplankton
species. Given the energetic advantage of filtering for
a well-adapted filter-feeding species such as sardine
(van der Lingen et al. 2006), the limited availability of
small food particles could have negative consequences
for sardine populations. Moreover, the expected future
decrease in the cold period in the Mediterranean could
limit the spawning season of sardine (Coll et al. 2008a)
and could cause competition between sardine larvae
and anchovy larvae (Costalago et al. 2011).
Sardine populations support a large community
of species at higher trophic levels. Several of these
species are commercially important (Coll et al. 2006,
Preciado et al. 2008, Banaru et al. 2012). Conservation
of sardine and the suitable management of its fishery
in the Gulf of Lions are therefore required to guarantee both socio-economic and ecological stability in the
region. This study demonstrates that, in addition to the
effects of fisheries on sardine populations (Coll et al.
2008b), these populations can also be strongly affected
by changes in the planktonic community.
ACKNOWLEDGEMENTS
The authors gratefully acknowledge the collaboration of Ignacio Álvarez-Calleja and Marta Albo for their
highly valuable work on the plankton analysis and the
morphological parameter analysis, respectively, and
of B. Liorzou, J.L. Bigot, D. Ross L. Buttay, B. Moli
and all the crew of the N/O L’Europe for their help
during the cruises. This work was conducted under the
European project SARDONE (FP6 - 44294). D.C. was
funded from 2007 to 2010 with a PhD contract by the
SARDONE project. ECOTRANS project (CTM201126333) contributed to the final edition of the paper.
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