Genetic assignment predicts depth of benthic settlement for 0-group Atlantic cod

Abstract

Atlantic cod is a keystone species that remains among the most economically important demersal fish in the North Atlantic. Throughout its distribution range, Atlantic cod is composed of populations with varying environmental preferences and migratory propensities. This life-history variation is likely to have contributed to the niche width and large population sizes of Atlantic cod, and its relative resilience to environmental change and exploitation. The Icelandic cod stock is currently managed as a single unit, but early research indicates population variation by depth and temperature and distinct offshore and inshore spawning components. Pelagic 0-group juveniles from different spawning grounds coexist in nursery areas around Iceland, but their genetic composition or habitat partitioning had not been examined post benthic settlement. In the current study we examine the genetic composition of Atlantic cod juvenile aggregations at nearshore nursery grounds in NW-Iceland and report distinct segregation by the depth of offshore and inshore juvenile cod. The physiological mechanism of this segregation is not known, but the pattern demonstrates the need to consider population structure at nursery grounds in the application of marine spatial planning and other area-based conservation tools.

Fig 1. Map of sample sites.

The map shows the four areas and the sample sites within each area (areas and site code matching Table 1). Beach seining sites are indicated by a black dot and trawl sites by a black triangle. The coastline presented in the map is based on the National Land Survey of Iceland (Landmælingar Íslands) IS 50V database, made available to the authors by CC BY 4.0, downloaded 12/2018 (www.lmi.is).

Fig 2. Standard length of juvenile Atlantic cod populations and genotypes across fishing gear and sample sites.

The violin plots show the distribution of juvenile size in the areas sampled and by different fishing gear. The SNP assignment to inshore and offshore populations (a) and Pan I genotypes (b & c) are depicted separately and note that although the two sample years are depicted for the Pan I genotypes (b & c) the figure does not reflect variation in sampling time post benthic settlement.

Fig 3. Results from the DAPC analysis.

The figure shows how the juveniles separated to the two identified clusters based on the SNP data and how the identified groups association to fishing gear.

Fig 4. Assignment to inshore / offshore populations and Pan I genotypes.

The barplots specifically depict the relative assignment of juveniles from different sample sites and times to inshore and offshore populations (a), Pan I genotypes in the 2017 sample groups (b) and Pan I genotypes in the 2019 sample groups (c). The month of sampling is indicated after the site code.