Jill A. Olin

Rescaling the trophic structure of marine food webs.

Measures of trophic position (TP) are critical for understanding food web interactions and human-mediated ecosystem disturbance. Nitrogen stable isotopes (δ15N) provide a powerful tool to estimate TP but are limited by a pragmatic assumption that isotope discrimination is constant (change in δ15N between predator and prey, Δ15N = 3.4‰), resulting in an additive framework that omits known Δ15N variation. Through meta-analysis, we determine narrowing discrimination from an empirical linear relationship between experimental Δ15N and δ15N values of prey consumed. The resulting scaled Δ15N framework estimated reliable TPs of zooplanktivores to tertiary piscivores congruent with known feeding relationships that radically alters the conventional structure of marine food webs. Apex predator TP estimates were markedly higher than currently assumed by whole-ecosystem models, indicating perceived food webs have been truncated and species-interactions over simplified. The scaled Δ15N framework will greatly improve the accuracy of trophic estimates widely used in ecosystem-based management.

Stable isotopes and elasmobranchs: tissue types, methods, applications and assumptions.

Stable-isotope analysis (SIA) can act as a powerful ecological tracer with which to examine diet, trophic position and movement, as well as more complex questions pertaining to community dynamics and feeding strategies or behaviour among aquatic organisms. With major advances in the understanding of the methodological approaches and assumptions of SIA through dedicated experimental work in the broader literature coupled with the inherent difficulty of studying typically large, highly mobile marine predators, SIA is increasingly being used to investigate the ecology of elasmobranchs (sharks, skates and rays). Here, the current state of SIA in elasmobranchs is reviewed, focusing on available tissues for analysis, methodological issues relating to the effects of lipid extraction and urea, the experimental dynamics of isotopic incorporation, diet-tissue discrimination factors, estimating trophic position, diet and mixing models and individual specialization and niche-width analyses. These areas are discussed in terms of assumptions made when applying SIA to the study of elasmobranch ecology and the requirement that investigators standardize analytical approaches. Recommendations are made for future SIA experimental work that would improve understanding of stable-isotope dynamics and advance their application in the study of sharks, skates and rays.

Maternal meddling in neonatal sharks: implications for interpreting stable isotopes in young animals.

Stable isotopes of neonatal vertebrates reflect those of their mothers diet and foraging location. Evaluating feeding strategies and habitat use of neonates is consequently complicated by the maternal isotopic signal and its subsequent elimination with growth. Thus, methods that measure the loss of the maternal signal, i.e. when the isotopic signal of a neonate reflects its own diet, are needed. Values of δ(13)C and δ(15)N were measured in liver and muscle tissues of <1 year old bull (Carcharhinus leucas) and Atlantic sharpnose (Rhizoprionodon terraenovae) sharks and related to age using, total length, date sampled and umbilical scar stage (USS). We observed a decline in δ(13)C and δ(15)N values with age that was different among species, similar among isotopes, and greater in liver than in muscle; highlighting that retention of the maternal signal is dependent on species-specific life history and tissue characteristics. USS was most effective for assessing the loss of the maternal isotopic signal in the faster growing Atlantic sharpnose shark, but was less effective for the slower growing bull shark. Total length and date sampled were overall less effective and may be more informative for slower growing species when coupled with USS, as variable size at birth and misclassification of animals >1 year old, which remain in nursery habitats, increase the variability of the isotopic values. Consideration of the maternal signal and measuring its loss are thus necessary when analyzing the stable isotopes of young animals, as there is potential to misinterpret feeding strategies, over-estimate trophic position and incorrectly assign carbon source.

Variable δ15N Diet-Tissue Discrimination Factors among Sharks: Implications for Trophic Position, Diet and Food Web Models

The application of stable isotopes to characterize the complexities of a species foraging behavior and trophic relationships is dependent on assumptions of δ15N diet-tissue discrimination factors (∆15N). As ∆15N values have been experimentally shown to vary amongst consumers, tissues and diet composition, resolving appropriate species-specific ∆15N values can be complex. Given the logistical and ethical challenges of controlled feeding experiments for determining ∆15N values for large and/or endangered species, our objective was to conduct an assessment of a range of reported ∆15N values that can hypothetically serve as surrogates for describing the predator-prey relationships of four shark species that feed on prey from different trophic levels (i.e., different mean δ15N dietary values). Overall, the most suitable species-specific ∆15N values decreased with increasing dietary-δ15N values based on stable isotope Bayesian ellipse overlap estimates of shark and the principal prey functional groups contributing to the diet determined from stomach content analyses. Thus, a single ∆15N value was not supported for this speciose group of marine predatory fishes. For example, the ∆15N value of 3.7‰ provided the highest percent overlap between prey and predator isotope ellipses for the bonnethead shark (mean diet δ15N = 9‰) whereas a ∆15N value < 2.3‰ provided the highest percent overlap between prey and predator isotope ellipses for the white shark (mean diet δ15N = 15‰). These data corroborate the previously reported inverse ∆15N-dietary δ15N relationship when both isotope ellipses of principal prey functional groups and the broader identified diet of each species were considered supporting the adoption of different ∆15N values that reflect the predators’ δ15N-dietary value. These findings are critical for refining the application of stable isotope modeling approaches as inferences regarding a species’ ecological role in their community will be influenced with consequences for conservation and management actions.

Age‐related polychlorinated biphenyl dynamics in immature bull sharks (Carcharhinus leucas)

Polychlorinated biphenyls (PCBs) were quantified in liver tissues of bull sharks (Carcharhinus leucas) ranging in age from <4 wk to >3 yr. Summed values of PCBs (ΣPCBs) ranged from 310 ng/g to 22 070 ng/g (lipid wt) across age classes with ΣPCB concentrations for the youngest sharks in the present study (<4 wk; 5230 ± 2170 ng/g lipid wt) determined to not significantly differ from those quantified in >3-yr-old sharks, highlighting the extent of exposure of this young life stage to this class of persistent organic pollutants (POPs). Age normalization of PCB congener concentrations to those measured for the youngest sharks demonstrated a significant hydrophobicity (log octanol/water partition coefficient [KOW ]) effect that was indicative of maternal offloading of highly hydrophobic (log KOW ≥6.5) congeners to the youngest individuals. A distinct shift in the PCB congener profiles was also observed as these young sharks grew in size. This shift was consistent with a transition from the maternally offloaded signal to the initiation of exogenous feeding and the contributions of mechanisms including growth dilution and whole-body elimination. These results add to the growing pool of literature documenting substantially high concentrations of POPs in juvenile sharks that are most likely attributable to maternal offloading. Collectively, such results underscore the potential vulnerability of young sharks to POP exposure and pose additional concerns for shark-conservation efforts.

Isotopic Ratios Reveal Mixed Seasonal Variation Among Fishes from Two Subtropical Estuarine Systems

Characterizing dietary resources and species interactions in estuaries is challenging, particularly when considering the dynamic nature of these ecosystems, the ranges in body sizes of species, and the potential for trophic roles to vary with ontogeny. We examined the influence of season and location on relationships between body size and δ15N, δ13C, and δ34S values across a range of fishes from two subtropical estuaries. The results suggest that isotopic values of estuarine fishes are independent of body size. However, seasonal variation propagated throughout the assemblage as the majority of fishes integrated different δ15N, δ13C, or δ34S values. The absence of δ15N–, δ13C–, and δ34S–body size relationships suggests that either (1) dietary preference of these fishes do not shift within the range of body sizes sampled, (2) these fishes shift to an alternate diet that is not isotopically distinct, or (3) that spatial and temporal variation in isotopic signatures of prey negate any size-based relationships. Seasonal variability in the isotopic values of these fishes suggests either movement to an alternative habitat or a shift in organic matter source associated with the transition of dry to wet seasons. Moreover, variance distributions of the best-fit models indicate that seasonal dietary preferences of conspecifics do not vary over moderate spatial scales. Seasonal variability among fishes in these estuaries suggests plasticity in feeding strategies that may afford greater adaptive flexibility to these species in response to changes in food availability resulting from variable environmental conditions.

Preservation Effects on Stable Isotope Values of Archived Elasmobranch Fin Tissue: Comparisons between Frozen and Ethanol-Stored Samples

AbstractElasmobranch fin tissue has been sampled and archived for decades to support genetics research. However, these collections have the potential to provide additional information on the trophic ecology of and habitat use by elasmobranch species. The use of fin tissue is especially attractive considering the threatened status of many elasmobranchs and the call for limiting mortalities. Yet, the use of fin samples for stable isotope analysis requires either that (1) storage methods do not alter tissue isotope values or (2) any alterations in isotope composition that occur during storage are predictable. In this study, paired fin tissues sampled from Smalltooth Sawfish Pristis pectinata and cownose rays Rhinoptera spp. were stored frozen and in ethanol and were subsequently analyzed for carbon (δ13C) and nitrogen (δ15N) isotope ratios. Fin δ13C and δ15N values were highly correlated between treatments for both taxa (r2 ≥ 0.80). For Smalltooth Sawfish, ethanol storage significantly increased fin δ13C val...

Loss of seasonal variability in nekton community structure in a tidal river: evidence for homogenization in a flow-altered system

Modifications to riverine systems that alter freshwater inflow to downstream estuarine habitats have resulted in altered patterns of nekton distribution and abundance. To examine how nekton assemblages respond to variable hydrologic patterns, we used trawl and seine survey data to compare the seasonal trends (dry vs. wet season) expected of a natural system to those of a river with regulated flow discharges that often magnify high flow events. Nekton assemblages differed between seasons in a representative natural system, similar to other estuaries of the region. For example, assemblage differences were characterized by significantly higher abundance and richness in trawl surveys, and significantly higher richness in seine surveys in the wet relative to the dry season. These seasonal trends were dampened in the altered system. Species important in defining seasonal dissimilarities in both systems were characterized as estuarine resident species, including Anchoa mitchilli, Menidia spp., Cynoscion arenarius, and Trinectes maculatus, yet were observed largely to have opposing seasonal trends in abundance between the two rivers. Our comparison provides evidence that flow modifications result in a loss of natural seasonal variability in estuarine nekton assemblages, but additional investigations of flow-altered systems are needed to confirm these findings.