Seascape ecology of Atlantic cod (Gadus morhua) in coastal Skagerrak: population structure, connectivity, and role in fish assemblage

Abstract

Atlantic cod populations along the Norwegian Skagerrak coast consist of two known ecotypes. During the last decade, there has been a dramatic decline of larger cod and other piscivorous fish species in eastern Skagerrak. In my PhD-project, I set out to study the outer Oslo fjord seascape with a main focus on the Atlantic cod (Gadus morhua) populations residing in the system, with the aim to generate fundamental knowledge on ecology, and evaluate potential for restoration of local populations. We conducted studies on genetic structuring of the two known ecotypes of cod present in this area, and combined genetic and oceanographic data to reveal potential spawning sites. Moreover, the absence of top predators gave the opportunity to investigate how the ecosystem compared to a contrasting seascape harboring a system of MPAs. First, we studied the potential for natural selection to act on a specifically designed single nucleotide polymorphism (SNP) panel used to differentiate between the two ecotypes of Atlantic cod, testing if these high-graded SNPs were unreliable for discriminating populations. We found no evidence for selection and concluded that high-graded marker panels under putative natural selection indeed represent a valid tool for identifying population structure in this species. Second, we investigated the fish assemblage in outer Oslo fjord to quantify and characterize the species present and compared the observed patterns to a protected seascape in southern Norway. We found evidence of a mesopredatory release in outer Oslo fjord, likely resulting from fishing down of the larger top predator species. Average length of cod sampled in the partially protected seascape was significantly larger than for cod caught in outer Oslo fjord. Mesopredatory fish species was in general more variable in the outer Oslo fjord seascape, and more stable in the Tvedestrand MPA. Third, we used genetic data from 0-group and 1-group cod coupled with a biophysical model of ocean drift to infer likely sources of cod recruits to the outer Oslo fjord seascape. Three potential spawning areas were revealed inside the sampling areas, and three potential extant sources were pointed out in the North Sea and Kattegat. We found that both ecotypes of Atlantic cod were present in outer Oslo fjord as both 0-group and as older fish. Fourth, we followed the 2016 cohort of Atlantic cod from 2017 to 2019 with the aim of distinguishing whether there was a higher survival of local ‘fjord’ cod compared to ‘North Sea’ assigned cod. We found that the ‘fjord’ ecotype had a higher proportion of individuals surviving their first year. However, both ecotypes disappeared from the system at around 3 years of age. This thesis demonstrates that: (1) The use of high-graded SNPs under putative selection can serve as a valuable tool in population identification. (2) Absence of larger top-predator species such as Atlantic cod act to increase the abundance of lower level carnivores, which can lead to a shift in ecosystem dynamics. (3) Both ecotypes of Atlantic cod are contributing to viable recruits in the area studied. (4) Both ecotypes experience high mortality, and few survive beyond age 3. Thus, present harvesting modes and -levels seem incompatible with rebuilding of age structure and biomass of local cod populations.