Niche partitioning and individual specialisation in resources and space use of sympatric fur seals at their range margin.

Ecological theory predicts niche partitioning between high-level predators living in sympatry as a mechanism to minimise the selective pressure of competition. Accordingly, male Australian fur seals Arctocephalus pusillus doriferus and New Zealand fur seals A. forsteri that live in sympatry should exhibit partitioning in their broad niches (in habitat and trophic dimensions) in order to coexist. However, at the northern end of their distributions in Australia, both are recolonising their historic range after a long absence due to over-exploitation, and their small population sizes suggest competition should be weak and may allow overlap in niche space. We found some niche overlap, yet clear partitioning in diet trophic level (δ15N values from vibrissae), spatial niche space (horizontal and vertical telemetry data) and circadian activity patterns (timing of dives) between males of each species, suggesting competition may remain an active driver of niche partitioning amongst individuals even in small, peripheral populations. Consistent with individual specialisation theory, broad niches of populations were associated with high levels of individual specialisation for both species, despite putative low competition. Specialists in isotopic space were not necessarily specialists in spatial niche space, further emphasising their diverse individual strategies for niche partitioning. Males of each species displayed distinct foraging modes, with Australian fur seals primarily benthic and New Zealand fur seals primarily epipelagic, though unexpectedly high individual specialisation for New Zealand fur seals might suggest marginal populations provide exceptions to the pattern generally observed amongst other fur seals.

Why aquatic scientists should use sulfur stable isotope ratios (ẟ34S) more often.

Over the last few decades, measurements of light stable isotope ratios have been increasingly used to answer questions across physiology, biology, ecology, and archaeology. The vast majority analyse carbon (δ13C) and nitrogen (δ15N) stable isotopes as the 'default' isotopes, omitting sulfur (δ34S) due to time, cost, or perceived lack of benefits and instrumentation capabilities. Using just carbon and nitrogen isotopic ratios can produce results that are inconclusive, uncertain, or in the worst cases, even misleading, especially for scientists that are new to the use and interpretation of stable isotope data. Using sulfur isotope values more regularly has the potential to mitigate these issues, especially given recent advancements that have lowered measurement barriers. Here we provide a review documenting case studies with real-world data, re-analysing different biological topics (i.e. niche, physiology, diet, movement and bioarchaeology) with and without sulfur isotopes to highlight the various strengths of this stable isotope for various applications. We also include a preliminary meta-analysis of the trophic discrimination factor (TDF) for sulfur isotopes, which suggest small (mean -0.4 ± 1.7 ‰ SD) but taxa-dependent mean trophic discrimination. Each case study demonstrates how the exclusion of sulfur comes at the detriment of the results, often leading to very different outputs, or missing valuable discoveries entirely. Given that studies relying on carbon and nitrogen stable isotopes currently underpin most of our understanding of various ecological processes, this has concerning implications. Collectively, these examples strongly suggest that researchers planning to use carbon and nitrogen stable isotopes for their research should incorporate sulfur where possible, and that the new 'default' isotope systems for aquatic science should now be carbon, nitrogen, and sulfur.

Can denticle morphology help identify southeastern Australian elasmobranchs?

Elasmobranchs are covered in scale-like structures called dermal denticles, comprising dentine and enameloid. These structures vary across the body of an individual and between species, and are frequently shed and preserved in marine sediments. With a good understanding of denticle morphology, current and historical elasmobranch diversity and abundance might be assessed from sediment samples. Here, replicate samples of denticles from the bodies of several known (deceased) shark species were collected and characterized for morphology before being assigned morphotypes. These data were used to expand the established literature describing denticles and to investigate intra- and interspecific variability, with the aim of increasing the viability of using sediment samples to assess elasmobranch diversity and abundance. Denticle morphology was influenced more by life-history traits than by species, where demersal species were largely characterized by generalized function and defense denticles, whereas pelagic and benthopelagic species were characterized by drag-reduction denticles. Almost all species possessed abrasion strength or defense denticles on the snout, precluding their utility for separating species. In a separate manipulative experiment, samples of denticles were collected from sediments in two aquaria with known elasmobranchs to determine their utility for reliably separating species. Visual examination of denticles, morphometric measurements, scaled photographs, and reference collections allowed for some precise identification, but not always to the species level. Ongoing work to develop denticle reference collections could help to identify past and present families and, in some cases, species.

Diving into the diet of provisioned smooth stingrays using stable isotope analysis.

Recreational fishing waste, produced from processing catches at shore-based fish cleaning facilities and discarded into adjacent waters, is foraged by various aquatic species. However, the potential alterations to the diet of consumers of these resources are poorly studied. Smooth stingrays (Bathytoshia brevicaudata) are a large demersal mesopredatory ray species and common scavenger of recreational fishing discards around southern Australia. Due to their attraction to fish cleaning sites, they are also common targets of unregulated 'stingray feeding' tourism where they are fed commercially produced baits (e.g., pilchards). This study provides a preliminary assessment of the diet of smooth stingrays provisioned recreational fishing discards and baits at two sites in southern New South Wales, Australia (Discard Site: recreational fishing discards only; Provisioning Site: recreational fishing discards and commercial baits) using stable isotope analysis of carbon (δ13 C) and nitrogen (δ15 N), and Bayesian stable isotope mixing models. Our results indicate that at both sites invertebrates, considered a main part of the natural diet of smooth stingrays, made a limited contribution to the diets of provisioned stingrays, while a benthic teleost fish that is a common recreational catch was the dominant contributor. As the assessed teleost is potentially a natural prey item for smooth stingrays, it remains unclear whether the contribution came from recreational fishing discards or natural foraging. However, due to smooth stingrays' typically opportunistic foraging strategy, we expected a greater mixture of resources from low to high trophic level prey than was observed. These results suggest that smooth stingrays have either lower reliance on invertebrates as a result of utilizing provisioned resources or higher reliance on teleost fishes than previously thought. Commercial bait products fed to stingrays at the Provisioning Site were not a major contributor to the diets of smooth stingrays, suggesting this activity has a low impact on their nutrition.

Integrating isotopic and nutritional niches reveals multiple dimensions of individual diet specialisation in a marine apex predator.

Dietary specialisations are important determinants of ecological structure, particularly in species with high per-capita trophic influence like marine apex predators. These species are, however, among the most challenging in which to establish spatiotemporally integrated diets. We introduce a novel integration of stable isotopes with a multidimensional nutritional niche framework that addresses the challenges of establishing spatiotemporally integrated nutritional niches in wild populations, and apply the framework to explore individual diet specialisation in a marine apex predator, the white shark Carcharodon carcharias. Sequential tooth files were sampled from juvenile white sharks to establish individual isotopic (δ-space; δ13 C, δ15 N, δ34 S) niche specialisation. Bayesian mixing models were then used to reveal individual-level prey (p-space) specialisation, and further combined with nutritional geometry models to quantify the nutritional (N-space) dimensions of individual specialisation, and their relationships to prey use. Isotopic and mixing model analyses indicated juvenile white sharks as individual specialists within a broader, generalist, population niche. Individual sharks differed in their consumption of several important mesopredator species, which suggested among-individual variance in trophic roles in either pelagic or benthic food webs. However, variation in nutrient intakes was small and not consistently correlated with differences in prey use, suggesting white sharks as nutritional specialists and that individuals could use functionally and nutritionally different prey as complementary means to achieve a common nutritional goal. We identify how degrees of individual specialisation can differ between niche spaces (δ-, p- or N-space), the physiological and ecological implications of this, and argue that integrating nutrition can provide stronger, mechanistic links between diet specialisation and its intrinsic (fitness/performance) and extrinsic (ecological) outcomes. Our time-integrated framework is adaptable for examining the nutritional consequences and drivers of food use variation at the individual, population or species level.

Lipid extraction has tissue-dependent effects on isotopic values (δ34 S, δ13 C, and δ15 N) from different marine predators.

RATIONALE: The use of sulfur isotopes to study trophic ecology in marine ecosystems has increased in the past decade. Unlike other commonly used isotopes (e.g., carbon), sulfur can better discriminate benthic and pelagic productivity. However, how lipid extraction affects sulfur isotopic values has not been assessed, despite its frequent use to remove lipid effects on δ13 C values. METHODS: We used white muscle and liver samples from two species of sharks and skin samples from two species of pinnipeds (sea lion and fur seal) to assess the effects of lipid extraction on stable isotope values for δ34 S, δ13 C, and δ15 N. Isotopic values were determined using a continuous flow-isotope ratio mass spectrometer coupled to an elemental analyzer. RESULTS: Lipid extraction significantly decreased δ34 S values in shark tissues, more so for liver than muscle (-4.6 ± 0.9‰ vs -0.8 ± 0.3‰, average change), with nearly no change in their standard deviations. Lipid extraction did not affect δ34 S values from pinniped skin samples (0.2 ± 0.8‰, average change). After lipid extraction, consistent increases in δ13 C values (0.2‰-7.3‰) were detected as expected, especially in tissue with high lipid content (C:N >4). After lipid extraction, significant increases in δ15 N values (0.5‰-1.4‰) were found in shark muscle and liver tissues. For pinniped skin samples, δ15 N values were not significantly lower after lipid extraction (-0.4‰ to -0.1‰). CONCLUSIONS: Lipid extraction did not have a strong impact on δ34 S values of shark muscle and pinniped skin (≤1‰). However, our results suggest it is essential to consider the effects of lipid extraction when interpreting results from δ34 S values of shark liver tissue, as they significantly depleted values relative to bulk tissue (~5‰). This may reflect selective removal of sulfolipids and glutathione present in higher concentrations in the liver than in muscle and skin and requires further investigation.

Patterns of mother-embryo isotope fractionation in batoids vary within and between species.

Patterns of mother-embryo fractionation of 13 C and 15 N were assessed for their predictability across three species of batoids caught as by-catch in south-eastern Australia. Stable isotope analysis of 24 mothers and their litters revealed that isotope ratios of embryos were significantly different from their corresponding mothers and that the scale and direction of the difference varied within and across species. The range of variation across species was 3.5‰ for δ13 C and 4‰ for δ15 N, equivalent to a difference in trophic level. In one species (Urolophus paucimaculatus) litters could be significantly enriched or depleted in 13 C and 15 N relative to their mothers' isotope signatures. These results suggest that patterns of mother-embryo isotope fractionation vary within and between species and that these patterns may not be explained only by developmental mode. Contrasting patterns of fractionation between and within species make it difficult to adjust mother-embryo fractionation with broad-scale correction factors.

Trophic niche of Australian cownose rays (Rhinoptera neglecta) and whitespotted eagle rays (Aetobatus ocellatus) along the east coast of Australia.

Australian cownose rays (Rhinoptera neglecta) and whitespotted eagle rays (Aetobatus ocellatus) are large myliobatiform rays that co-occur off temperate eastern Australia. Here, we performed stable-isotope analyses (δ13 C, δ15 N and δ34 S) on fin clips of both species to gain insights into their trophic interactions and isotopic niches, and assess the effect of preservation (ethanol-stored versus frozen) on isotopic values of fin-clip tissue of R. neglecta. Linear mixed models identified species as the main factor contributing to variation among δ15 N and δ34 S values, and disc width for δ13 C. Bayesian ecological niche modelling indicated a 57.4% to 74.5% overlap of trophic niches, with the niche of R. neglecta being smaller and more constrained. Because values of δ13 C were similar between species, variation in isotopic niches were due to differences in δ15 N and δ34 S values. Linear mixed models failed to detect differences in isotopic values of ethanol-stored and frozen fin tissue of R. neglecta. This study provides the first examination of the trophic ecology of R. neglecta and the comparison of isotopic niche with A. ocellatus, which will facilitate future research into the trophic interactions of these species and aid better resource management.

Diet-tissue discrimination and turnover of δ13 C and δ15 N in muscle tissue of a penaeid prawn.

RATIONALE: Stable isotopes are used to study trophic and movement ecology in aquatic systems, as they provide spatially distinct, time-integrated signatures of diet. Stable isotope ecology has been used to quantify species-habitat relationships in many important fisheries species (e.g., penaeid prawns), with approaches that typically assume constant values for diet-tissue discrimination and diet-tissue steady state, but these can be highly variable. Here we provide the first report of these processes in Metapenaeus macleayi (eastern school prawn). METHODS: Here we explicitly measure and model carbon (δ13 C) and nitrogen (δ15 N) diet-tissue discrimination and turnover in eastern school prawn muscle tissue as a function of experimental time following a change in diet to an isotopically distinct food source. RESULTS: Diet-tissue discrimination factors were 5 and 0.6‰ for δ13 C and δ15 N, respectively. Prawn muscle tissue reached an approximate steady state after approximately 50 and 30 days for δ13 C and δ15 N. Half-lives indicated faster turnover of δ15 N (~8 days) than δ13 C (~14 days). CONCLUSIONS: Our diet-tissue discrimination factors deviate from 'typical' values with larger values for carbon than nitrogen isotopes, but are generally similar to those measured in other crustaceans. Similarly, our estimates of isotopic turnover align with those in other penaeid species. These findings confirm muscle tissue as a reliable indicator of long-term diet and movement patterns in eastern school prawn.

Pelagic and benthic ecosystems drive differences in population and individual specializations in marine predators.

Individual specialization, which describes whether populations are comprised of dietary generalists or specialists, has profound ecological and evolutionary implications. However, few studies have quantified individual specialization within and between sympatric species that are functionally similar but have different foraging modes. We assessed the relationship between individual specialization, isotopic niche metrics and foraging behaviour of two marine predators with contrasting foraging modes: pelagic foraging female South American fur seals (Arctocephalus australis) and benthic foraging female southern sea lions (Otaria byronia). Stable isotope analysis of carbon and nitrogen was conducted along the length of adult female vibrissae to determine isotopic niche metrics and the degree of individual specialization. Vibrissae integrated time ranged between 1.1 and 5.5 years, depending on vibrissae length. We found limited overlap in dietary niche-space. Broader population niche sizes were associated with higher degrees of individual specialization, while narrower population niches with lower degrees of individual specialization. The degree of individual specialization was influenced by pelagic and benthic foraging modes. Specifically, South American fur seals, foraging in dynamic pelagic environments with abundant but similar prey, comprised specialist populations composed of generalist individuals. In contrast, benthic southern sea lions foraging in habitats with diverse but less abundant prey had more generalist populations composed of highly specialized individuals. We hypothesize that differences in specialization within and between populations were related to prey availability and habitat differences. Our study supports growing body of literature highlighting that individual specialization is a critical factor in shaping the ecological niche of higher marine predators.

Anatomy of the mechanosensory lateral line canal system and electrosensory ampullae of Lorenzini in two species of sawshark (fam. Pristiophoridae).

It has long been assumed that the elongated rostra (the saws) of sawsharks (family: Pristiophoridae) and sawfish (family: Pristidae) serve a similar function. Recent behavioural and anatomical studies have shed light on the dual function of the pristid rostrum in mechanosensory and electrosensory prey detection and prey manipulation. Here, the authors examine the distributions of the mechanosensory lateral line canals and electrosensory ampullae of Lorenzini in the southern sawshark, Pristiophorus nudipinnis and the longnose sawshark, Pristiophorus cirratus. In both species, the receptive fields of the mechano- and electrosensory systems extend the full length of the rostrum indicating that the sawshark rostrum serves a sensory function. Interestingly, despite recent findings suggesting they feed at different trophic levels, minimal interspecific variation between the two species was recorded. Nonetheless, compared to pristids, the pristiophorid rostrum possesses a reduced mechanosensory sampling field but higher electrosensory resolution, which suggests that pristiophorids may not use their rostrums to disable large prey like pristids do.

Overcoming multi-year impacts of maternal isotope signatures using multi-tracers and fast turnover tissues in juvenile sharks.

Stable isotopes are often used to determine the ecological role of different age classes of animals, but particularly for young animals this approach may be compromised. During gestation and or incubation body tissues of the young are derived directly from the mother. In neonates or post hatching, there is a period of transformation as the young grow and forage independently, but during this period different organs will continue to reflect the maternal isotopic signature as a function of their turnover rate. How long this maternal hangover persists remains poorly understood. We applied a multi-tracer approach (δ15N, δ13C and δ34S) to stable isotope signatures in juvenile bull sharks (Carcharhinus leucas) up to 6.5 years post parturition. We found that maternal provisioning was detectable for up to 3.5 years after birth in muscle but only detectable in young-of-the-year for liver. Inclusion of sulphur revealed when maternal signatures disappeared from low-turnover tissue, while also identifying the spatial and trophic ecology patterns from fast-turnover tissue. These results reveal the importance of sampling fast turnover tissues to study the trophic ecology of juvenile elasmobranchs, and how the use of only δ15N and δ13C isotopes is likely to make maternal patterns more difficult to detect.

Seasonal and developmental diet shifts in sympatric and allopatric intertidal gobies determined by stomach content and stable isotope analysis.

Resource partitioning facilitates the coexistence of sympatric species through spatial, temporal and/or trophic strategies. Fishes living in the intertidal zone demonstrate highly adaptive plastic behaviour, including resource partitioning, through spatial and temporal shifts in diet and microhabitat. Although intertidal fish assemblages are influenced by inter- and intraspecific competition, few studies have compared the extent of resource partitioning between sympatric species in the context of trophic niche plasticity. Here we used complementary approaches, stomach content and stable isotope (δ13 C and δ15 N) analyses, to evaluate seasonal and developmental shifts in trophic niche position in two sympatric (Favonigobius lentiginosus and Bathygobius krefftii) and one allopatric (Bathygobius cocosensis) species of intertidal goby. The results indicate that resource partitioning in the two sympatric species varied with season, with almost no trophic niche overlap in summer to about ~30% overlap in winter. Also, evidence of dietary changes was found in B. cocosensis, which is likely associated with a shift in microhabitat and intraspecific competition. The findings highlight the temporal range of behavioural plasticity in trophic niche position of intertidal gobies, which likely has high adaptive value in the dynamic intertidal zone.

Short- and long-term diets of the threatened longhorned pygmy devil ray, Mobula eregoodoo determined using stable isotopes.

Most mobulids are listed as near threatened to endangered. Nonetheless, effective conservation measures are hindered by knowledge gaps in their ecology and behaviour. In particular, few studies have assessed diets and trophic ecologies that could inform methods to avoid fishing mortality. Here, a shortfall in data for the longhorned pygmy devil ray, Mobula eregoodoo was addressed by describing temporal variability in dietary preferences using stable isotope analysis. During summer and autumn in 2017, five bather-protection gillnets were deployed off eastern Australia (29° S, 153.5° E). From the catches of these gillnets, 35 adult M. eregoodoo had liver, muscle and stomach contents sampled to determine δ13 C and δ15 N profiles. Analyses revealed that surface zooplankton and zooplanktivorous teleosts were important dietary components across short- and long-term temporal scales. Large quantities of undigested sandy sprat, Hyperlophus vittatus, in the stomachs of some specimens unequivocally confirm feeding on teleosts. A narrow isotopic niche and minimal isotopic overlap with reef manta rays, Mobula alfredi from the same geographic region in eastern Australia implies M. eregoodoo has unique and highly specialised resource use relative to other mobulids in the area. The species is clearly vulnerable to capture during inshore migrations, presumably where they feed on shallow-water shoaling teleosts. Female M. eregoodoo likely have a low annual reproductive output, so population recoveries from fishing-induced declines are likely to be slow. Measures to reduce the by catch of M. eregoodoo in local bather-protection gillnets, and artisanal fisheries more broadly, should be given priority.

Resource use of great hammerhead sharks (Sphyrna mokarran) off eastern Australia.

Great hammerhead sharks Sphyrna mokarran are the largest member of Sphyrnidae, yet the roles of these large sharks in the food webs of coastal ecosystems are still poorly understood. Here we obtained samples of muscle, liver and vertebrae from large S. mokarran (234-383 cm total length; LT ) caught as by-catch off eastern Australia and used stable-isotope analyses of δ15 N, δ13 C and δ34 S to infer their resource use and any associated ontogenetic patterns. The results indicated large S. mokarran are apex predators primarily relying on other sharks and rays for their diet, with a preference for benthic resources such as Australian cownose rays Rhinoperon neglecta during the austral summer. Teleosts, cephalopods and crustaceans were not significant components of S. mokarran diets, though some conspecifics appeared to rely on more diverse resources over the austral summer. Ontogenetic shifts in resource use were detected but trajectories of the increases in trophic level varied among individuals. Most S. mokarran had non-linear trajectories in ontogenetic resource-use shifts implying size was not the main explanatory factor. Stable isotope values of δ13 C and δ34 S in muscle suggest S. mokarran span coastal, pelagic and benthic food webs in eastern Australia.

Mother-embryo isotope fractionation in the pygmy devilray Mobula kuhlii cf. eregoodootenkee.

We determined stable-isotope ratios for replicate muscle tissues in 13 gravid Mobula kuhlii cf. eregoodootenkee (110.4-120.4 cm disc width; WD ) and their embryos (7.0-42.3 cm WD ) and also yolks and histrotroph, to assess the potential implications for juvenile nutrition and habitat use. Irrespective of their development in the uterus, embryos had similar δ13 C values in their muscle tissue as the mothers and both had greater values than in the histotroph. During gestation, δ13 C values increased across all sample types. However, while embryo muscle tissue and the histotroph were associated with increasing 15 N levels during embryonic development, this was depleted in the mothers' muscle tissue and yolk. Although speculative, the observed variation in stable-isotope ratios might imply a dietary shift among gravid females during their early gestation. Irrespective of the underlying mechanisms, the results indicate neonates will have relatively greater δ15 N values than post-partum females, which would probably confound juvenile foraging-ecology estimates.

Localized zinc distribution in shark vertebrae suggests differential deposition during ontogeny and across vertebral structures.

The development of shark vertebrae and the possible drivers of inter- and intra-specific differences in vertebral structure are poorly understood. Shark vertebrae are used to examine life-history traits related to trophic ecology, movement patterns, and the management of fisheries; a better understanding of their development would be beneficial to many fields of research that rely on these calcified structures. This study used Scanning X-ray Fluorescence Microscopy to observe zinc distribution within vertebrae of ten shark species from five different orders. Zinc was mostly localised within the intermedialis and was generally detected at levels an order of magnitude lower in the corpus calcareum. In most species, zinc concentrations were higher pre-birth mark, indicating a high rate of pre-natal zinc deposition. These results suggest there are inter-specific differences in elemental deposition within vertebrae. Since the deposition of zinc is physiologically-driven, these differences suggest that the processes of growth and deposition are potentially different in the intermedialis and corpus calcareum, and that caution should be taken when extrapolating information such as annual growth bands from one structure to the other. Together these results suggest that the high inter-specific variation in vertebral zinc deposition and associated physiologies may explain the varying effectiveness of ageing methodologies applied to elasmobranch vertebrae.

Stress profile influences learning approach in a marine fish.

The spatial learning skills of high and low stress juvenile mulloway (Argyrosomus japonicus) were tested in a dichotomous choice apparatus. Groups of fish were formed based on background blood cortisol levels and required to learn the location of a food reward hidden in one of two compartments. Low stress fish characterised by low background levels of the stress hormone cortisol had higher activity levels and entered both rewarded and unrewarded rooms frequently. Within the first week of exposure, however, their preference for the rewarded room increased, indicative of learning. Fish that had high background levels of cortisol, in contrast, showed low levels of activity but when they chose between the two rooms they chose the rewarded room most often but showed less improvement over time. After 12 days in the apparatus, both low and high stress fish had similar ratios of rewarded vs unrewarded room entrances. Our results suggest that proactive coping styles may increase exposure to novel contexts and thus favour faster learning but at the cost of reduced initial accuracy.

Ecological impacts and management implications of reef walking on a tropical reef flat community.

Continued growth of tourism has led to concerns about direct and indirect impacts on the ecology of coral reefs and ultimate sustainability of these environments under such pressure. This research assessed impacts of reef walking by tourists on a relatively pristine reef flat community associated with an 'ecoresort' on the Great Barrier Reef, Australia. Heavily walked areas had lower abundances of live hard coral but greater amounts of dead coral and sediment. Abundances of macroalgae were not affected between sites. Coral-associated butterflyfish were less abundant and less diverse in more trampled sites. A manipulative experiment showed handling holothurians on reef walks had lasting negative impacts. This is the first study to show potential impacts of such handling on holothurians. Ecological impacts of reef walking are weighed against sociocultural benefits of a first hand experience in nature.