Disparate movement behavior and feeding ecology in sympatric ecotypes of Atlantic cod.

Kim Aarestrup,Even Moland,Anders Koed,Esben Moland Olsen,Halvor Knutsen,Kristi Källo,Peter Grønkjær,Martin Lykke Kristensen

doi:10.1002/ece3.7939

Kim Aarestrup, Even Moland

Open Access

https://doi.org/10.1002/ece3.7939

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Abstract

Coexistence of ecotypes, genetically divergent population units, is a widespread phenomenon, potentially affecting ecosystem functioning and local food web stability. In coastal Skagerrak, Atlantic cod (Gadus morhua) occur as two such coexisting ecotypes. We applied a combination of acoustic telemetry, genotyping, and stable isotope analysis to 72 individuals to investigate movement ecology and food niche of putative local “Fjord” and putative oceanic “North Sea” ecotypes—thus named based on previous molecular studies. Genotyping and individual origin assignment suggested 41 individuals were Fjord and 31 were North Sea ecotypes. Both ecotypes were found throughout the fjord. Seven percent of Fjord ecotype individuals left the study system during the study while 42% of North Sea individuals left, potentially homing to natal spawning grounds. Home range sizes were similar for the two ecotypes but highly variable among individuals. Fjord ecotype cod had significantly higher δ13C and δ15N stable isotope values than North Sea ecotype cod, suggesting they exploited different food niches. The results suggest coexisting ecotypes may possess innate differences in feeding and movement ecologies and may thus fill different functional roles in marine ecosystems. This highlights the importance of conserving interconnected populations to ensure stable ecosystem functioning and food web structures.

Highlights

The evolutionary divergence of ecotypes is common in both terrestrial and aquatic ecosystems and represents an important component of intraspecific diversity
The North Sea ecotype were more likely to leave the fjord system compared to local Fjord cod and had significantly lower values of stable δ13C and δ15N isotopes
Given the potential importance of cod as a top predator, differences in fjord residence and trophic ecology may have an important effect on the overall structure and functioning of temperate coastal ecosystems

Summary

Introduction

The evolutionary divergence of ecotypes is common in both terrestrial and aquatic ecosystems and represents an important component of intraspecific diversity. A large body of empirical and theoretical studies have examined the evolution of ecotypes, for instance in the context of ecological speciation (Hendry, 2017). Ecotype variation may have wide-ranging consequences for ecosystems. Anadromous salmon ecotypes support freshwater- and terrestrial ecosystems by transporting large amounts of nutrients from oceanic ecosystems as part of their feeding- and spawning migration (Carlson et al, 2011). Understanding potential eco-evolutionary effects of ecotype variation is highly relevant for conservation and management. A variety of morphs and life history forms have been recognised (Sherwood & Grabowski, 2010; Karlsen et al, 2013).

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