Life history and spatial ecology of Skagerrak coastal cod (Gadus morhua)

Abstract

Practically every Atlantic cod (Gadus morhua) stock have suffered from depletion and overfishing, placing the fish on IUCN’s Red List as ‘Vulnerable’. The Skagerrak Coastal Cod stocks are in no better shape, reported to be experiencing harvesting pressure comparative to the Canadian cod stocks prior to the collapse in the early 1990s. Sub-populations of coastal cod have been found to exhibit different behaviour and life-history strategies. This may cause the various populations to respond differently to fishing pressure indicating a need for localized ecosystem-based management, reflecting biologically meaningful population units. To improve the management of coastal cod, more knowledge and a better understanding of the various populations’ fine-scaled behaviour and ecology is required. My study aspires to provide supplementary information to the knowledge base regarding the coastal cod’s movement and behavioural ecology. By using a network of 44 moored acoustic receivers and tagging 80 codfish with transmitters, the spatial and behavioural ecology of 39 individuals was investigated over a three month period (June-July-August) during the summer of 2012. Home range sizes and vertical distribution was investigated by using acoustic telemetry, kernel density estimation and linear mixed-effect models. I predicted that the life history of the cod (e.g. body size) will affect the behavioural traits observed; that smaller fish will have a more constrained behaviour and that faster growing fish will exhibit elevated activity pattern and a bold behavioural type. I found that home range size varies with respect to month, as did cumulative distance travelled, vertical range, mean depth and max depth. Min depth was not found to be affected by any of the included variables. Body size (at age 1 = L1) was only found to influence vertical range, decreasing in parallel with a decrease in L1. However, as L1 was based on age estimation from cod scales the results from these remains speculative. My results indicate that the spatial distribution of coastal cod in Sømskilen is controlled more by environmental factors and their subsequent influence on behaviour, than life-history traits per se. Further, these findings support the notion that the meta-populations of coastal cod acts as localized entities and should be managed accordingly.