Isotopic niche overlap between co-occurring capelin (Mallotus villosus) and polar cod (Boreogadus saida) and the effect of lipid extraction on stable isotope ratios

Abstract The ability of otoliths to withstand digestion in a seal stomach appears to differ between fish taxa with the otoliths of gadoids being particularly resistant to gastric erosion. Few studies have examined the effect of otolith size on the rate of otolith dissolution. We exposed 50 otoliths (total sagittal length: 1.4–7.0 mm) from capelin (Mallotus villosus, n = 20), herring (Clupea harengus, n = 10), and polar cod (Boreogadus saida, n = 20) as well as nine whole capelin (total body length = 100–120 mm) to digestive solutions assumed to mimic the gastric environment of Arctic seals in vitro (pH: ∼2, temperature: ∼37°C, pepsin concentrations: 0–750-μg ml−1 solution). Pepsin did not affect the dissolution rates of sagittae but resulted in a complete digestion of whole capelin within 10 h. Irrespective of species examined, sagittae fitted the same pattern of dissolution rate with the smaller sagittae being dissolved faster than did the larger ones. We suggest that scaling controls sagittal dissolution rates (DR, μm min−1), and that the sagittae from the three forage fish combined dissolve according to sagittal length (SL, mm) as shown by the equation: DRCommon = 27.348e−0.120 SL (n = 46, r2 = 0.688, p < 0.001). Given this relationship, the numerical importance of prey taxa with the smaller otoliths – i.e. also the small-sized gadoids (polar cod) – is likely to be underestimated in feeding studies with Arctic seals.

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Dietary characteristics of co-occurring polar cod (Boreogadus saida) and capelin (Mallotus villosus) in the Canadian Arctic, Darnley Bay

Polar Biology ◽

10.1007/s00300-015-1834-5 ◽

2015 ◽

Vol 39 (6) ◽

pp. 1099-1108 ◽

Cited By ~ 19

Author(s):

D. G. McNicholl ◽

W. Walkusz ◽

G. K. Davoren ◽

A. R. Majewski ◽

J. D. Reist

Keyword(s):

Canadian Arctic ◽

Polar Cod ◽

Boreogadus Saida ◽

Mallotus Villosus ◽

Capelin Mallotus Villosus ◽

Polar Cod Boreogadus Saida

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Stable isotope analysis reveals that humpback whales (Megaptera novaeangliae) primarily consume capelin (Mallotus villosus) in coastal Newfoundland, Canada

Canadian Journal of Zoology ◽

10.1139/cjz-2020-0257 ◽

2021 ◽

Author(s):

Kelsey Johnson ◽

Gail Davoren

Keyword(s):

Stable Isotope ◽

Stable Isotope Analysis ◽

Isotope Analysis ◽

Niche Overlap ◽

Prey Type ◽

Megaptera Novaeangliae ◽

Humpback Whales ◽

Mallotus Villosus ◽

Δ13c And Δ15n ◽

Capelin Mallotus Villosus

On the Newfoundland, Canada foraging ground, humpback whales (Megaptera novaeangliae (Borowski, 1781)) are found associated with a dominant forage fish species, capelin (Mallotus villosus (Müller, 1776), that experienced a population collapse in the early 1990s and has not recovered. Our primary goal was to reconstruct dietary proportions of humpback whales on their summer foraging grounds off the northeastern Newfoundland coast during July-August, 2016 and 2017 using a Bayesian stable isotope mixing model (MixSiar). Modelled dietary proportions were similar in both years, with capelin comprising ~90% of the diet. However, both δ13C and δ15N in humpback whale skin differed significantly between years, resulting in minimal dietary niche overlap (9%). Lipid-extracted and nonlipid-extracted skin samples were used to develop a lipid normalization equation: ∆13C = – 3.184 + 1.011(C:N). Overall, findings suggest that capelin is the primary prey type of humpback whales in coastal Newfoundland, despite the continued collapsed state of the capelin population. Findings also reiterate that dietary reconstruction from stable isotope analysis of cetacean skin can be misinterpreted without concurrently sampled isotopic ratios of potential prey types.

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