Fine-scale population differences in Atlantic cod reproductive success: A potential mechanism for ecological speciation in a marine fish.

Nancy E Roney,Esben M Olsen,Halvor Knutsen,Rebekah A Oomen,Jeffrey A Hutchings

doi:10.1002/ece3.4615

Nancy E Roney, Esben M Olsen + Show 3 more

Open Access

https://doi.org/10.1002/ece3.4615

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Abstract

Successful resource‐management and conservation outcomes ideally depend on matching the spatial scales of population demography, local adaptation, and threat mitigation. For marine fish with high dispersal capabilities, this remains a fundamental challenge. Based on daily parentage assignments of more than 4,000 offspring, we document fine‐scaled temporal differences in individual reproductive success for two spatially adjacent (<10 km) populations of a broadcast‐spawning marine fish. Distinguished by differences in genetics and life history, Atlantic cod (Gadus morhua) from inner‐ and outer‐fjord populations were allowed to compete for mating and reproductive opportunities. After accounting for phenotypic variability in several traits, reproductive success of outer‐fjord cod was significantly lower than that of inner‐fjord cod. This finding, given that genomically different cod ecotypes inhabit inner‐ and outer‐fjord waters, raises the intriguing hypothesis that the populations might be diverging because of ecological speciation. Individual reproductive success, skewed within both sexes (more so among males), was positively affected by body size, which also influenced the timing of reproduction, larger individuals spawning later among females but earlier among males. Our work suggests that spatial mismatches between management and biological units exist in marine fishes and that studies of reproductive interactions between putative populations or ecotypes can provide an informative basis on which determination of the scale of local adaptation can be ascertained.

Highlights

The persistence of a species depends on the number, phenotypic differentiation, and genetic variability of its constituent populations (Schindler et al, 2010)
The present study examined correlates of reproductive success in a broadcast‐spawning marine fish at an exceptionally fine spatial and temporal scale
Based on daily estimates of parentage for al‐ most 4,500 offspring, several broad‐scale patterns emerged. Despite their small (

Summary

| INTRODUCTION

The persistence of a species depends on the number, phenotypic differentiation, and genetic variability of its constituent populations (Schindler et al, 2010). There is compelling evidence that cod inhabiting the inner waters of some Skagerrak fjords are phenotypically and genetically distinct from cod inhabiting outer waters of the same fjord (Knutsen et al, 2018; Øresland & André, 2008). In this respect, the most extensive data are available for Risør Fjord (58.7°N, 9.2°E). Notwith‐ standing the phenotypic and genetic differences between the two populations described above, it is not known whether individu‐ als would have equal reproductive success in a mixed‐population spawning situation. If one population experiences higher average reproductive success when cohabiting the same breeding environment, ceteris paribus it might indicate that genetic and life‐history differences between inner and outer populations reflect processes contributing to re‐ duced probability of interbreeding, and increased probability of re‐ productive isolation, between populations (Rundle & Nosil, 2005; Schluter, 2000)

| MATERIALS AND METHODS

| DISCUSSION

Findings

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