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Title: Dietary variability in two common Alaskan skates (Bathyraja interrupta and Raja rhina)
Description: Determining trophic relationships within and among species can provide insight into the structure and function of an ecosystem, and can inform the development of multi-species monitoring and management plans. The goal of this study was to address the need for dietary and trophic information of two common and abundant skates in Prince William Sound (PWS), the Bering skate, Bathyraja interrupta, and longnose skate, Raja rhina. Based on identification and analysis of stomach content data, both species were found to be generalist predators with diets dominated by crustaceans and supplemented with teleosts. The primary source of dietary variability for each species was total length, with spatial variables (i.e., latitude, longitude, and depth) also explaining a large portion of total dietary variability in the Bering skate. However, only a small proportion of the total intraspecific dietary variation was accounted for among the analyzed variables, suggesting substantial individual-based differences in the feeding habits of each species. Trophic level estimates indicated that the Bering skate and longnose skates <100 cm total length (TL) were secondary consumers, whereas longnose skates ≥100 cm TL were more piscivorous, tertiary consumers. Significant interspecific dietary differences were also evident, suggesting trophic separation, likely as a function of skate size. Given their abundance and generalistic feeding behavior, PWS skates can provide a means of monitoring demersal community health; information about their foraging ecology will be valuable in gaining a better understanding of trophodynamics within the PWS food web.
Author: Kemper, Bizzarro, Ebert

Title: Diet Composition and Trophic Ecology of Northeast Pacific Ocean Sharks
Description: Although there is a general perception of sharks as large pelagic, apex predators, most sharks are smaller, meso- and upper-trophic level predators that are associated with the seafloor. Among 73 shark species documented in the eastern North Pacific (ENP), less than half reach maximum lengths > 200 cm, and 78% occur in demersal or benthic regions of the continental shelf or slope. Most small (≤ 200 cm) species (e.g., houndsharks) and demersal, nearshore juveniles of larger species (e.g., requiem sharks) consume small teleosts and decapod crustaceans, whereas large species in pelagic coastal and oceanic environments feed on large teleosts and squids. Several large, pelagic apex predator species occur in the ENP, but the largest species (i.e., Basking Shark, Whale Shark) consume zooplankton or small nekton. Size-based dietary variability is substantial for many species, and segregation of juvenile and adult foraging habitats also is common (e.g., Horn Shark, Shortfin Mako). Temporal dietary differences are most pronounced for temperate, nearshore species with wide size ranges, and least pronounced for smaller species in extreme latitudes and deep-water regions. Sympatric sharks often occupy various trophic positions, with resource overlap differing by space and time and some sharks serving as prey to other species. Most coastal species remain in the same general region over time and feed opportunistically on variable prey inputs (e.g., season migrations, spawning, or recruitment events), whereas pelagic, oceanic species actively seek hot spots of prey abundance that are spatiotemporally variable. The influence of sharks on ecosystem structure and regulation has been downplayed compared to that of large teleosts species with higher per capita consumption rates (e.g., tunas, billfishes). However, sharks also exert indirect influences on prey populations by causing behavioural changes that may result in restricted ranges and reduced fitness. Except for food web modelling efforts in Alaskan waters, the trophic impacts of sharks are poorly incorporated into current ecosystem approaches to fisheries management in the NEP.
Author: Bizzarro, Carlisle, Smith, Cortés

Title: Burrowing behavior, habitat, and functional morphology of the Pacific sand lance (Ammodytes personatus).
Description: The Pacific sand lance (Ammodytes personatus) is a small, elongate forage fish that spends much of its life buried in the seafloor. We determined that the Pacific sand lance can burrow in a wide variety of sediments from silt to gravel, but it prefers coarse sand (0.50-1.00 mm grain size). In the absence of coarse sand, the Pacific sand lance chooses larger grain sizes over smaller ones. These preferences are independent of light or the compaction of sediment, and therefore indicate that visual cues and ease of entry are not primary means of choosing burial substrate. Instead, we speculate that the Pacific sand lance is morphologically adapted for rapid mobility in coarse sand and that coarse sand has enough interstitial spaces to enable respiration during protracted immersion. As an obligate burrower in specific sediments, the Pacific sand lance is a good candidate for habitat-based management. Substrate maps of 3 fishing grounds in southeast Alaska where the Pacific sand lance is abundant and where habitat-based management is practiced were used to create potential habitat maps. Different geologic histories have resulted in variable amounts of preferred (sand-gravel), suitable (sand mixed with silt, cobble-boulder, or rock outcrop), and unsuitable (mud, pebble-boulder) habitat for this species among regions. [ABSTRACT FROM AUTHOR], Article
Author: Bizzarro, Peterson, Blaine, Balaban, Greene, Summers

Title: Interactive effects of urea and lipid content confound stable isotope analysis in elasmobranch fishes
Description: Stable isotope analysis (SIA) is becoming a commonly used tool to study the ecology of elasmobranchs. However, the retention of urea by elasmobranchs for osmoregulatory purposes may bias the analysis and interpretation of SIA data. We examined the effects of removing urea and lipid on the stable isotope composition of fourteen species of sharks, skates, and rays from the eastern North Pacific Ocean. While effects were variable across taxa, removal of urea generally increased δ15 N and C:N. Urea removal had less influence on δ13C, whereas extracting urea and lipid generally increased δ15 N and C:N while also increasing δ13 C. Because C:N values of non-extracted tissues are often used to infer lipid content and adjust δ13 C, shifts in C:N following urea extraction will change the inferred lipid content and bias any mathematical adjustment of δ13 C. These results highlight the importance of urea and lipid extraction and demonstrate the confounding effects of these compounds, making it impossible to use C:N of non-urea- extracted samples as a diagnostic tool to estimate and correct for lipid content in elasmobranch tissues.
Author: Carlisle, Litvin, Madigan, Lyons, Bigman, Ibarra, Bizzarro