Small‐scale biocomplexity in coastal Atlantic cod supporting a Darwinian perspective on fisheries management

Abstract

Harvesting of marine resources raises concerns about how to identify and preserve biocomplexity, including the diversity of life histories found within and among wild populations of a species. In order to fully accomplish this, there is a need to elucidate the underlying causes of phenotypic variation, and how this variation responds to environmental changes. In general, both evolutionary (genetic) and nonevolutionary (plastic) responses may occur. Plastic responses to environmental change are expected to shift the phenotype along a reaction norm, while an evolutionary response is expected to shift the reaction norm itself. Here, we assess the maturation patterns of coastal Atlantic cod (Gadus morhua) in Skagerrak, where studies using neutral markers have revealed genetically differentiated populations of this harvested fish within tens of kilometres of coastline. Our results suggest that physiological state prior to the spawning season, as well as juvenile growth, both influence the probability of completing sexual maturation at a given age. Furthermore, our results point towards a spatial structuring of this plasticity (i.e. the maturation reaction norms) comparable with population connectivity inferred from neutral markers. We argue that such fine-scale biocomplexity calls for a Darwinian approach to fisheries management.

Habitat effects on population connectivity in a coastal seascape

“…Genetic studies of coastal cod sampled in small fjords have documented population structure at a scale down to approximately 30 km (Knutsen et al 2003, Jorde et al 2007, where the pattern most likely arises from individuals sampled in sheltered locations far inside the fjords (Knutsen et al 2011). Differences in coastal cod life-history traits have been observed at a similar spatial scale, indicative of local adaptations (Olsen et al 2008). In contrast, genetic structure is less evident in more open continental shelf habitats (i.e.…”

“…1), the larvae metamorphose into juveniles and settle in nearshore nursery areas during May and June. Growth is about 10 to 15 cm yr −1 and maturity sets in at an age of 2 to 4 yr and a body length of 30 to 50 cm (Dannevig 1954, Olsen et al 2008. Spawning has been documented in some fjords, particularly in sheltered areas away from the Norwegian Coastal Current ), but comprehensive know ledge of spawning locations is lacking.…”

Theory, consequences and evidence of eroding population spatial structure in harvested marine fishes: a review

“…These consequences highlight the relevance of maintaining the 'biocomplexity' of exploited populations. That is, preserving different sub-populations or demes ensures high life history diversity and yields more temporal stability because of the independent but complementary dynamics (Hilborn et al 2003, Olsen et al 2008 (Fig. 1).…”

Diel vertical migration patterns in juvenile cod from the Skagerrak coast

“…The Skagerrak cod reach maturity at the age of 2 to 4 yr (Olsen et al 2004). Due to their relatively small body size, we expected the fish in our study were immature (see Olsen et al 2008), so they would not display any spawning behaviour.The area of Sømskilen, where all fish were released, is a semisheltered basin with several small islands and rocks (Fig. 1).…”