Combination of genetics and spatial modelling highlights the sensitivity of cod (Gadus morhua) population diversity in the North Sea to distributions of fishing

Abstract

Conserving genetic diversity in animal populations is important for sustaining their ability to respond to environmental change. However, the “between-population” component of genetic diversity (biocomplexity) is threatened in many exploited populations, particularly marine fish, where harvest management regions may be larger than the spatial extent of genetically distinct subpopulations. Using single-nucleotide polymorphism data, we delineated the geographic limits of three population units of Atlantic cod (Gadus morhua) in northwest European waters. Two of the populations cohabit the North Sea, and trawl survey data showed differing trends in their abundances. We developed a spatial model of these units to simulate population dynamics under spatial patterns of harvesting. Competition between units during the pelagic juvenile stages in the model led to suppression of the more localized northern North Sea (Viking) unit by the more widespread (Dogger) unit, and its premature extinction under some spatial patterns of fishing. Fishery catch limits for cod are set at the scale of the whole North Sea without regard to such subpopulation dynamics. Our model offers a method to quantify adjustments to regional fishing mortality rates to strike a balance between maximizing sustainable yield and conserving vulnerable populations.

Fishing impact and environmental status in European seas: a diagnosis from stock assessments and ecosystem indicators

“…); unexpected ecosystem effects due to changes in species assemblages; and the loss in the genetic biodiversity for some severely overexploited species (Heath et al . ).…”

Population‐level consequences of seismic surveys on fishes: An interdisciplinary challenge

“…Furthermore, there are many stock monitoring studies and predictive models for fish stock that take biotic, abiotic and anthropogenic influences into account (e.g. Cardinale & Svedäng, ; Heath et al., ).…”

Assessing the role of ontogenetic movement in maintaining population structure in fish using otolith microchemistry

“…(b) Chart showing adult sample locations in 2009 and 2010 (circles) overlaid on the density distribution of age 2 and 3 cod (gray shading). Locations of cod assigned to the Viking and Dogger population units (Heath et al., ; Poulsen et al., ) are also shown (squares), highlighting the segregation around depths >100 m in the northern North Sea.…”

“…Atlantic cod, Gadus morhua , is well suited to studying the nature of population structuring because there is already considerable information on their movements (Neuenfeldt et al., ) and genetic differentiation (Bradbury et al., ), including evidence for natal fidelity (Barth et al., ; Bonanomi et al., ). In the North Sea, studies of microsatellite DNA have supported a degree of reproductive isolation between the deeper northeastern region between 100 and 200 m and shallower depths, although the neutrality of some key markers has been questioned (Hutchinson, Carvalho, & Rogers, ; Nielsen et al., ), and single‐nucleotide polymorphic markers under selection considerably improve the significance of these differences (Heath et al., ; Poulsen, Hemmer‐Hansen, Loeschcke, Carvalho, & Nielsen, ). At a finer spatial scale, analyses of otolith chemistry indicate that juvenile cod settling off the Scottish coast mainly recruit to their local spawning area (Wright, Neat, et al., ).…”