Recent decline in cod stocks in the North Sea–Skagerrak–Kattegat shifts the sources of larval supply

Abstract

Cod stocks in the North Sea, including the Kattegat and the Skagerrak, have declined dramatically since the 1970s. Occasionally there is a high recruitment of juveniles in Kattegat/Skagerrak, without leading to the rebuilding of adult cod stocks despite reduced fishing mortality. In a biophysical model of egg and larval drift, we examined the potential importance of extant and historical spawning grounds for recruitment of cod in the Kattegat/Skagerrak seas using data of spawning stock biomass from the 1970s and from today's reduced stocks. The results suggest that Kattegat in the 1970s relied on largely locally retained (83%) larvae with little annual variation in recruitment. Kattegat also provided a substantial proportion of larvae recruiting in Swedish Skagerrak (72%). This is in contrast to present conditions where the Kattegat spawning stock has been reduced by 94%, and Kattegat only provides 34% of locally retained larvae and 30% to Swedish Skagerrak. Instead, the protected area in the Öresund and the Belt Sea are expected today to provide most larvae recruiting in Kattegat. Also, the inflow of larvae from the North Sea to Skagerrak and Kattegat can be significant although highly variable between years, with a positive correlation to the North‐Atlantic Oscillation index (NAO). The rebuilding of healthy spawning areas in the Kattegat may be key for restoring local cod stocks in both Kattegat and along the Skagerrak coast. This poses a management challenge if cod with local ‘Kattegat’ adaptations, e.g., in terms of egg density and migration patterns, are lost or reduced to non‐resilient densities.

Genome architecture enables local adaptation of Atlantic cod despite high connectivity

“…Similarly, local recruitment was also predicted along the western Skagerrak coast, although these values may be underestimates as the model does not resolve the complex geomorphology with high retention within fjords. No local recruitment was assumed for the eastern Skagerrak coast where spawning stocks are negligible (see Jonsson et al., ).…”

“…Contrary to these well defined populations, the eastern Skagerrak fjords appeared to be composed of a mix between North Sea‐like and western Baltic‐like individuals, indicating that these fjords are part of the distributional area of the two major evolutionary units detected in this study. These fjords may experience larval recruitment through a strong influx of central North Sea water into the Skagerrak, as well as less‐saline Kattegat water entering along the coast (André et al., ; Danielssen et al., ; Jonsson et al., ; Knutsen et al., ; Stenseth et al., ). In agreement with these predominant ocean currents, a large fraction of individuals from the eastern Skagerrak fjords appeared to be of North Sea origin (Figure ), while our biophysical model suggested greater larval connectivity with the Kattegat and western Baltic (Figure ).…”

“…Physical transport and connectivity of Atlantic cod eggs and larvae was quantified using a biophysical model to explore geneflow potential and connectivity by predicting the most important sources of larvae settling along the Skagerrak coast and the Kattegat. A full description of the biophysical model is given in Jonsson, Corell, André, Svedäng, and Moksnes (). Briefly, the dispersal of eggs and larvae was modelled with a Lagrangian particle‐tracking routine in off‐line mode driven by flow fields from an ocean circulation model (BaltiX; Hordoir, Dieterich, Basu, Dietze, & Meier, ).…”

“…The velocity, temperature and salinity were updated for all grid boxes in the model domain every three hours, and the trajectory calculations were done with a 15‐min time step. To simulate dispersion of cod larvae, we used an individual‐based drift model with a wide range of combinations of spawning time, egg and larval drift depth, as well as pelagic larval duration time (for a detailed description see Jonsson et al., ). Briefly, eggs were simulated to drift at depths between 5 and 15 m and hatched after 20 days.…”

“…The measure of connectivity of the biophysical model only predicts the probability per egg to be transported from i to j . To obtain a relative estimate of the abundance of eggs reaching a settlement area, we also scaled the inferred connectivity with recent estimates of the spawning stock biomass (SSB, for calculations see Jonsson et al., ).…”