L Litherland

Australian east coast spotted mackerel, Scomberomorus munroi, is a pelagic fish species harvested by commercial, charter and recreational fishers. It forms a single genetic stock in Queensland and New South Wales. The stock is shared with NSW although over 80 per cent of the harvest is from Queensland waters. Spotted mackerel exhibit schooling behaviours that make them susceptible to overfishing. The Queensland commercial line and net fishery operates under a quota system for spotted mackerel (140 tonnes total allowable commercial catch). Recent commercial harvest (2013–2018) was around 64 tonnes per year. Catch shares (in 2013–14) in Queensland were around 55 per cent commercial and 45 per cent recreational. The formal stock assessment was requested to update the status of the stock and inform fishery management agencies and stakeholders on estimates of sustainable harvest that will build and maintain the fishery in the long term. The 2018 stock assessment combined data in an annua...

There has been no previous stock assessment for school mackerel. This stock assessment used an age structured model with a yearly time step and length based selectivity. Only the south-east stock was considered for assessment as harvests for the north-east stock were small. Data inputs included total harvest, standardised catch rates and age and length structures. The south-east coast stock model uses data from the 1988-89 to 2017-18 fishing years (comprising commercial harvest (1988-89 to 2017-18); recreational harvest (1994-95 to 2013-14); length structures (2014-15 to 2017-18) and age-length information (1991-92 to 1994-95)). Model analyses suggested that biomass declined from the mid-2000s to around 65% of unfished biomass in 2017-18. Maximum sustainable yield (MSY) was estimated at 104–119 t per year, and the yield consistent with a biomass ratio of 60% was estimated at 84–95 t. The time series of harvests modelled may be underrepresented, causing an overestimate of the biomass...

The topographic analysis of retinal ganglion and photoreceptor cell distributions yields valuable information for assessing the visual capabilities and behavioral ecology of vertebrates. This study examines whole-mounted retinas of four elasmobranch species, the ornate wobbegong,Orectolobus ornatus; the whitetip reef shark,Triaenodon obesus; the epaulette shark,Hemiscyllium ocellatum; and the east Australia shovelnose ray,Aptychotrema rostrata, for regional specializations mediating zones of improved visual ability. These species represent a range of lifestyles: benthic, mid-water, diurnal, and nocturnal. Both photoreceptors (visualized using differential interference contrast optics) and ganglion cells (stained with cresyl violet) in the retina are extensively sampled, and their spatial distribution is found to be nonuniform, exhibitingareaeor “visual streaks.” In general, the topographic distributions of both cell populations are in register and match well with respect to the loca...

The topographic analysis of retinal ganglion and photoreceptor cell distributions yields valuable information for assessing the visual capabilities and behavioral ecology of vertebrates. This study examines whole-mounted retinas of four elasmobranch species, the ornate wobbegong, Orectolobus ornatus; the whitetip reef shark, Triaenodon obesus; the epaulette shark, Hemiscyllium ocellatum; and the east Australia shovelnose ray, Aptychotrema rostrata, for regional specializations mediating zones of improved visual ability. These species represent a range of lifestyles: benthic, mid-water, diurnal, and nocturnal. Both photoreceptors (visualized using differential interference contrast optics) and ganglion cells (stained with cresyl violet) in the retina are extensively sampled, and their spatial distribution is found to be nonuniform, exhibiting areae or In general, the topographic distributions of both cell populations are in register and match well with respect to the location of regi...

The visual abilities of sharks show substantial interspecific variability. In addition, sharks may change their habitat and feeding strategy throughout life. As the eyes of sharks continue to grow throughout the animal’s lifetime, ontogenetic variability in visual ability may also occur. The topographic analysis of the photoreceptor and ganglion cell distributions can identify visual specializations and assess changes in visual abilities that may occur concurrently with eye growth. This study examines an ontogenetic series of whole-mounted retinas in two elasmobranch species, the sandbar shark,Carcharhinus plumbeus, and the shortspine spurdog,Squalus mitsukurii, to identify regional specializations mediating zones for improved spatial resolution. The study examines retinal morphology and presents data on summation ratios between photoreceptor and ganglion cell layers, anatomically determined peak spatial resolving power, and the angular extent of the visual field. Peak densities of photoreceptors and ganglion cells occur in similar retinal locations. The topographic distribution of neurons in the ganglion cell layer does not differ substantially with eye growth. However, predicted peak spatial resolution increases with eye growth from 4.3 to 8.9 cycles/deg inC. plumbeusand from 5.7 to 7.2 cycles/deg inS. mitsukurii. The topographic distribution of different-sized ganglion cells is also mapped inC. plumbeus, and a population of large ganglion cells (soma area 120–350μm2) form a narrow horizontal streak across the retinal meridian, while the spatial distribution of ordinary-sized ganglion cells (soma area 30–120μm2) forms anareain the central retina. Species-specific retinal specializations highlight differences in visually mediated behaviors and foraging strategies betweenC. plumbeusandS. mitsukurii.

Understanding the processes driving species distributions is a key question in ecology, yet obtaining pertinent data for many species can be challenging. Tag-recapture data from voluntary recreational fisheries programmes is an undervalued citizen science data resource and can
provide valuable observation records for data-poor species. Here, we highlight the utility of such data by compiling four tag-recapture data-
sets (n = 20 878 tags) to describe the movements and distribution of a piscivorous predator, tailor Pomatomus saltatrix, with respect to their
environment. We compared species distribution models that explored a trade-off between temporal duration and spatial extent. Both models
revealed sea surface temperature (SST) was an important factor in describing the distribution and density of tailor. However, the model with
broader spatial extent indicated a non-linear response to SST, peaking at 21.5 C, while the model with temporal longevity showed a linear
SST response. This disjunct showcases the importance of building models across an adequate geographic extent to ensure covariate responses are not truncated. This is the first study to comprehensively describe the distribution and migrations of adult tailor across their eastern Australian distribution. The results are discussed in light of ocean warming along eastern Australia, and the subsequent implications for using tag-recapture data in fisheries management.

Four species of large mackerels (Scomberomorus spp.) co-occur in the waters off northern Australia and are important to fisheries in the region. State fisheries agencies monitor these species for fisheries assessment; however, data inaccuracies may exist due to difficulties with identification of these closely related species, particularly when specimens are incomplete from fish processing. This study examined the efficacy of using otolith morphometrics to differentiate and predict among the four mackerel species off northeastern Australia. Seven otolith measurements and five shape indices were recorded from 555 mackerel specimens. Multivariate modelling including linear discriminant analysis (LDA) and support vector machines, successfully differentiated among the four species based on otolith morphometrics. Cross validation determined a predictive accuracy of at least 96% for both models. An optimum predictive model for the four mackerel species was an LDA model that included fork length, feret length, feret width, perimeter, area, roundness, form factor and rectangularity as explanatory variables. This analysis may improve the accuracy of fisheries monitoring, the estimates based on this monitoring (i.e. mortality rate) and the overall management of mackerel species in Australia.

Several aspects of vision in juvenile and adult Green Turtles (Chelonia mydas) are examined, with special reference to retinal anatomy such as oil droplet topography, transmission electron microscopy of photoreceptors, spectral transmission measurements of the ocular media (cornea, lens, and vitreous humor), and measurements of focal length and optical sensitivity. A detailed study of the distribution of the different color classes of oil droplets shows that all oil droplets are found in high concentrations (>1000 mm−2) in the central/temporal parts of the retina. Red oil droplets were the largest, followed by yellow and clear. Oil droplet size varied in different parts of the retina. On average, red oil droplets were found in fewer numbers compared to yellow and clear oil droplets. Two types of clear oil droplets were identified: those that fluoresced under UV illumination and those that did not. We found that the majority (78.5%) of colorless oil droplets fluoresced when viewed under UV light. Spectral transmission measurements of the ocular media show that wavelengths to approximately 325 nm are transmitted. This may suggest ultraviolet (UV) vision in Green Turtles. The optical sensitivity of the Green Turtle eye was relatively low, suggesting an adaptation to high light intensities commonly experienced by this species.

Neuroethology and ecomorphology seek to understand ecology and behaviour from the perspective of specialised adaptations of sensory systems, such as vision. Sharks display a large variety of visual specialisations reflecting the diversity of different ecological niches they occupy. Many shark species are long-lived and wide ranging and often select different habitats for reproduction, growth, and feeding. Habitat complexity, ambient lighting conditions and feeding strategies can therefore change throughout a shark’s lifetime or between populations. Few comprehensive investigations of visual function exist for sharks as studies typically focus on a narrow aspect of visual function or a particular life history stage. Consequently, there is limited data on within-species plasticity of visual function in response to acclimation to different visual environments or ontogenetic development. The aim of this thesis is to undertake a functional analysis of the shark visual system. An integrated approach is employed to investigate optical, anatomical and physiological specialisations, linking such specialisations to known habitat and/or behavioural traits, with particular emphasis on ontogenetic, inter-population and inter-specific variability. Fundamental capabilities of the visual system are examined, including optical quality, eye morphology, spectral range, irradiance sensitivity, spatial and temporal resolution, contrast discrimination, and temporal and spatial summation. The main study species is the sandbar shark (Carcharhinis plumbeus; Carcharhinidae), a cosmopolitan species of ecological and economic importance. C. plumbeus occupies a wide range of natural habitats from highly turbid coastal estuaries, to relatively clear waters off the outer continental shelves and near pristine clear waters over the slopes of oceanic islands. This provides an opportunity to explore the relationship between habitat variability and the adaptation of visual specialisations and subsequent behaviour. For inter-specific comparison, the visual systems of two other species of shark with contrasting ecological niches are also assessed: the shortspine spurdog (Squalus mitsukurii; Squalidae) and the tiger shark (Galeocerdo cuvier; Carcharhinidae). The study finds marked differences in visual specialisations of the three species studied. The eyes of S. mitsukurii are adapted to enhance retinal illumination within a dim light environment with a large eye, immobile pupil, reflective tapetum and a relatively high optical sensitivity (2.72 μm2 steradians). Visual features include a short wavelength lenticular filter, a high spatial resolving power (7.2 cycles/degree) and a large binocular overlap in the dorsal visual field, suggesting adaptations may facilitate the visualisation of bioluminescent prey. In contrast, the eyes of C. plumbeus are optimised for vision under variable light conditions with a mobile pupil and an occlusible tapetum. The sandbar shark shows an optical sensitivity of 1.11 μm2 steradians. Visual resolution is highest in the lateral visual field, reaching a peak spatial resolution of 8.9 cycles/degree. An ERG derived spectral response curve for this species indicates maximal response to blue light between 460-490 nm. Interestingly, the tiger shark is maximally sensitive to a brighter range of light intensities compared to sandbar sharks, implying that tiger sharks occupy a more photopic light environment. However, sandbar sharks have a visual system with higher temporal resolution, as evaluated by the ERG response, (54 Hz) than tiger sharks (38 Hz). These results may reflect a difference in the importance of motion perception between C. plumbeus and G. cuvier. Phenotypic variability in visual function is shown between different populations of C. plumbeus occupying habitats with different ambient light conditions. This study provides new evidence of plasticity of visual function in response to acclimation to different visual environments within the same species. Sandbar sharks show an adaptive plasticity in visual sensitivity and temporal resolution, which appears to enable both temporal and population-specific adaptations to local light environments. In addition, the eyes of C. plumbeus and S. mitsukurii continue to grow even in adulthood. Visual performance, with respect to spatial resolving power and optical sensitivity, improve with eye growth. For example, peak spatial resolution increases with eye growth from 4.3 to 8.9 cycles/degree in C. plumbeus and from 5.7 to 7.2 cycles/degree in S. mitsukurii. These studies suggest that the light environment strongly influences visual function in this ancient class of vertebrates. Anthropogenically induced changes in water clarity may, therefore, impact on visually-mediated behaviours such as prey detection, agonistic signals or vertical migration. Anatomical and physiological parameters obtained from these studies provide a platform from which to model visual behaviours such as 1). Prey detection capabilities, 2). The impacts of water clarity on the limits of visually-mediated behaviour, and 3). The visual strategies that would allow sharks to maximise visual function, such as spatial and temporal summation under low light conditions. In conclusion, neuroethological studies can be a useful means to enrich information obtained from life-history and tagging studies and, together, can inform us of the functional role of sharks in marine ecosystems.

During 2018, Queensland net fishery operators participated in a survey that documented the relative importance of behavioural and economic factors that influence whether sharks are retained or not. 121 commercial net fishers were interviewed, spanning the diversity of net fishery operations in Queensland waters (Gulf of Carpentaria and east coast). Results suggest that discarding sharks (live or dead) is common practice in Queensland’s net fisheries with 76% of fishers responding that they don’t keep a lot or all of the sharks that they catch. A key finding of the survey was that a combination of regulatory and market forces dictate whether fishers keep sharks. When sharks are retained, 80% of fishers indicated the market is the reason for keeping sharks with 28% of fishers saying sharks are important to their business. Overall, the results of this survey suggest that the policy changes enacted in January 2018 which made the reporting of shark catch more difficult, may have encouraged net fishers to discard more sharks. New tools currently being developed by Fisheries Queensland (such as a commercial fishing app, Species Identification tools, and vessel tracking) should simplify future reporting requirements. A follow-up survey, conducted after the introduction of these new tools, would be informative.