Local cod (Gadus morhua) revealed by egg surveys and population genetic analysis after longstanding depletion on the Swedish Skagerrak coast

Svedäng, Henrik; Barth, Julia M I; Svenson, Anders; Jonsson, Patrik; Jentoft, Sissel; Knutsen, Halvor; André, Carl

doi:10.1093/icesjms/fsy166

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Journal article

Local cod (Gadus morhua) revealed by egg surveys and population genetic analysis after longstanding depletion on the Swedish Skagerrak coast

Svedäng, Henrik Institute of Marine Research, Department of Aquatic, Resources, Swedish University of Agricultural Sciences (SLU), Box 4, SE-45330 Lysekil, Sweden
Barth, Julia M I Zoological Institute, University of Basel, Vesalgasse 1, 4051 Basel, Switzerland
Svenson, Anders Institute of Marine Research, Department of Aquatic, Resources, Swedish University of Agricultural Sciences (SLU), Box 4, SE-45330 Lysekil, Sweden
Jonsson, Patrik Institute of Marine Research, Department of Aquatic, Resources, Swedish University of Agricultural Sciences (SLU), Box 4, SE-45330 Lysekil, Sweden
Jentoft, Sissel Centre for Ecological and Evolutionary Synthesis, Department of Biosciences, University of Oslo, N-0316 Oslo, Norway
Knutsen, Halvor Centre for Coastal Research, University of Agder, N-4604 Kristiansand, Norway
André, Carl Department of Marine Sciences-Tjärnö, University of Gothenburg, SE-452 96 Strömstad, Sweden

2018-11-30

Published in:

ICES Journal of Marine Science. - Oxford University Press (OUP). - 2018, vol. 76, no. 2, p. 418-429

Ecology, Evolution, Behavior and Systematics

English Abstract
Dramatic and persistent reductions in Atlantic cod (Gadus morhua) are common in many coastal areas. While offshore cod stocks still were abundant and productive, the Swedish west coast showed signs of diminishing adult cod abundance at the beginning of the 1980s, where the local cod component was considered to be extirpated. To survey the present cod spawning activity and stock composition, we initiated egg trawling over two consecutive years (203 hauls in total) in combination with population genetic analyses (425 individually genotyped eggs). Here, we provide evidence of cod spawning at the Swedish Skagerrak coast, suggesting recolonization or that local cod has recovered from a nearly depleted state. Early stage eggs were found inside fjords too far to have been transported by oceanic drift from offshore spawning areas. The cod eggs were genetically similar in early to late life-stages and cluster mainly with the local adult cod, indicating that eggs and adults belong to the same genetic unit. The cod eggs were genetically differentiated from adult North Sea cod, and, to a lesser degree, also from the Kattegat and Öresund cod, i.e. indicating a possible recovery of local coastal stock. The patterns of the genetic structure in the inshore areas are, however, difficult to fully disentangle, as Atlantic cod in the North Sea-Skagerrak area seem to be a mixture of co-existing forms: local cod completing their entire life cycle in fjords and sheltered areas, and oceanic populations showing homing behaviours. The egg abundances are considerably lower compared with what is found in similar studies along the Norwegian Skagerrak coast. Nevertheless, the discovery of locally spawning cod along the Swedish west coast—although at low biomasses—is an encouraging finding that highlights the needs for endurance in protective measures and of detailed surveys to secure intraspecific biodiversity and ecosystem services.

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