Seasonal shifts in feeding patterns: Individual and population realized specialization in a high Arctic fish.

Marine Cusa,Øystein Varpe,Jørgen Berge

doi:10.1002/ece3.5615

Marine Cusa, Øystein Varpe + Show 1 more

Open Access

https://doi.org/10.1002/ece3.5615

Copy DOI

Abstract

Species with a broad and flexible diet may be at an advantage in a rapidly changing environment such as in today's Arctic ecosystems. Polar cod (Boreogadus saida), an abundant and ecologically important circumpolar Arctic fish, is often described as a zooplankton generalist feeder, which suggests that it may cope successfully with changes in prey composition. This description is justified based on the relatively broad diet of polar cod across sites and seasons. In this case study, we used polar cod dietary data from fall and winter and from two distinct environments, dominated either by Arctic or Atlantic water masses in Svalbard. Our results point to the importance of time and space when drawing conclusions on dietary plasticity and degree of specialization. Polar cod diet differed significantly between fall and the winter and between Arctic and Atlantic domains. Polar cod from Arctic domains displayed a strong realized population specialization on Themisto libellula in fall, and the larger dietary niche width observed in the winter was the product of realized individual specialization, with increased feeding on fish prey. Overall, we did not observe a generalized feeding behavior. If dietary niche width is to inform conservation management, we argue it must be recognized that populations from a single species may adopt seasonally contrasting degrees of dietary specialization and that these populations may differ in their vulnerability to climate‐induced changes in prey community composition.

Highlights

Generalist feeders display large dietary niche width comprising of high prey taxa diversity, whereas specialists display narrow dietary niche width composed of few prey taxa (Gerking, 1994)
The PERMANOVA indi‐ cated that polar cod diet differed significantly between fall and win‐ ter (F1,6 = 7.3167; p = .002), and between Arctic and Atlantic domains (F1,6 = 2.69; p = .042); 40% of the total variance was explained by the Season factor (r2 = .401), whereas 15% of the total variance was ex‐ plained by the Domain factor (r2 = .147)
Our study indicates that adult polar cod populations from Svalbard act as tertiary consumers with a diet primarily composed of sec‐ ondary consumer amphipod in fall and fish prey in the winter

Summary

| INTRODUCTION

Generalist feeders display large dietary niche width comprising of high prey taxa diversity, whereas specialists display narrow dietary niche width composed of few prey taxa (Gerking, 1994). Polar cod have been de‐ scribed as opportunist feeders (Ajiad & Gjøsæter, 1990; Majewski et al, 2016; Nakano et al, 2016) and more rarely as specialist feed‐ ers (Cui, Grebmeier, & Cooper, 2012) This variability partly stems from evaluating diet at varying spatial and temporal scales and from inconsistencies regarding the choice of a fundamental or realized approach to describing degrees of specialization. We studied polar cod stomach content to evaluate and contrast the (a) seasonal and geographical variations in dietary niche width, (b) variation in ingested prey taxa between fall and winter from Atlantic and Arctic domains, and (c) degree of realized indi‐ vidual and population specialization in fall and winter from Arctic domain fjords and an Atlantic domain fjord using a metric of spe‐ cialization developed by Amundsen et al (1996). We aim to reveal the importance of acknowledging organizational levels (individual, population, and species) when drawing conclusions on dietary specialization

| MATERIALS AND METHODS

| DISCUSSION

| Limitations

| Implications, outlook, and conclusion

Findings

CONFLICT OF INTEREST