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The location of reproduction is a key life history trait, as it influences the bio-physical conditions that offspring experience and, thus, fitness. Capelin Mallotus villosus is a small (< 200 mm), short-lived forage fish that spawns in two habitats in coastal Newfoundland: warm beaches and cool, deep water (15–40 m). From 2009 to 2014, we investigated temperature-dependent spawning habitat use by quantifying hourly temperature at spawning sites during July within each habitat along with population-level spawning site use. Capelin did not spawn at sites with temperatures <2 °C or consistently >12 °C, supporting the estimated suitable spawning temperature range (2–12 °C). Spawning typically occurred exclusively at beaches early in July when deep-water habitat was too cold (i.e. <2 °C), and then switched to deep-water habitat later in July when beaches became too warm (i.e. >12 °C). Spawning overlapped for 1–3 d in both habitats when temperatures were within suitable ranges (2011, 2013), but capelin also spawned exclusively in one habitat when temperatures remained suitable in the other. The latter suggests that other factors influence spawning habitat selection, such as conspecific egg densities and other environmental conditions. Overall, the flexible use of spawning habitats, along with wide tolerance ranges of capelin eggs to environmental conditions, are likely key adaptations to maximize fitness and stabilize population dynamics of this important forage fish species in an unpredictable environment.

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Lipid extraction is an important component of many ecological and ecotoxicological measurements. For instance, percent lipid is often used as a measure of body condition, under the assumption that those individuals with higher lipid reserves are healthier. Likewise, lipids are depleted in C compared with protein, and it is consequently a routine to remove lipids prior to measuring carbon isotopes in ecological studies so that variation in lipid content does not obscure variation in diet. We provide detailed methods for two different protocols for lipid extraction: Soxhlet apparatus and manual distillation. We also provide methods for polar and nonpolar solvents. Neutral (nonpolar) solvents remove some lipids but few non-lipid compounds, whereas polar solvents remove most lipids but also many non-lipid compounds. We discuss each of the methods and provide guidelines for best practices. We recommend that, for stable isotope analysis, researchers test for a relationship between the change in carbon stable isotope ratio and the amount of lipid extracted to see if the degree of extraction has an impact on isotope ratios. Stable isotope analysis is widely used by ecologists, and we provide a detailed methodology that minimizes known biases.

This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

The presence of lipids and urea in elasmobranch tissues can affect carbon (δ13C) and nitrogen (δ15N) stable isotope values, introducing bias in food web interpretations. Information on how lipids and urea affect δ13C/δ15N is only available for < 5% of ~ 1150 described elasmobranch species and results are highly variable among existing studies. Here, we investigate the effects of lipid and urea extraction on δ13C, δ15N, and C:N ratios in muscle and blood plasma of two batoids (Pastinachus ater and Himantura australis) and examine the influence of lipid/urea extraction on isotopic niche size and overlap. Lipid extraction (LE) did not affect δ13C, δ15N, or C:N ratios, suggesting low lipid content. Urea extraction (UE), however, increased δ15N and C:N in muscle. The ~ 1‰ δ15N increase represents a shift of ~ 0.5 trophic levels relative to bulk samples, highlighting the importance of UE to accurately assess trophic positions. Although there was no effect of any treatment on niche size, the probability of P. ater occurring within the niche of H. australis increased following UE. Overall, results suggest that urea should be removed from muscle prior to analysis, but LE is not required. Given the interspecific variability in the effects of lipid/urea on elasmobranch δ13C/δ15N, more studies are needed to assess the effects of lipid and urea on a broader range of species to produce a generalized understanding. Where no species‐specific data are available, we recommend pilot samples are analyzed to determine if LE is needed prior to preparation of the overall sample set.

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