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

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.

Mixed origin of juvenile Atlantic cod (Gadus morhua) along the Swedish west coast

“…Second, the coastal ecotype may be better adapted to the environmental conditions associated with inshore habitats and therefore dominate these areas (Knutsen et al ., 2018; Barth et al ., 2019; Oomen, 2019). Third, the pattern might be attributed to local spawning populations inhabiting coastal areas that are closely related to Kattegat and Öresund cod (Svedäng et al ., 2019).…”

“…Despite claims that most cod along the Swedish west coast have an offshore origin, the relative proportions of offshore and coastal ecotype cod have not been assessed explicitly. Biophysical models of larval drift suggest that spawning populations in the Kattegat and Öresund supply the majority of recruits to the Swedish west coast (Jonsson et al, 2016;Barth et al, 2017), while recent observations of early egg stages inside Swedish fjords suggest that local fjord spawning may still also occur (Svedäng et al, 2019). In addition to the lack of knowledge regarding the origin of recruits, recent declines in the offshore stocks in the North Sea (ICES, 2021a;2021c) have raised questions about whether the Skagerrak-Kattegat coastal zone has lost some of its function as a nursery habitat for juvenile cod, regardless of ecotype.…”

“…Genetic and tagging studies have identified two sympatric cod ecotypes residing in the North Sea-Skagerrak-Kattegat region (Knutsen et al ., 2011; André et al ., 2016; Barth et al ., 2017; Svedäng et al ., 2019). One of these ecotypes is genetically similar to North Sea cod and dominates in the offshore and outer coastal regions (Knutsen et al ., 2018).…”

Sea surface temperature dictates movement and habitat connectivity of Atlantic cod in a coastal fjord system

“…This local protection promotes survival in the Fjord region, which potentially acts as a source to nearby unprotected populations. This is strengthened by recent evidence that spawning events are highly likely to be occurring in the Gullmar Fjord and other nearby coastal areas in this region (Svedäng et al, ). However, there is debate on whether local, genetically separated subpopulations exist in this region or whether they stem from a mix of western Baltic and North Sea Atlantic cod (Cardinale, Mariani, & Hjelm, ; Svedäng et al, ).…”

Depletion of coastal predatory fish sub-stocks coincided with the largest sea urchin grazing event observed in the NE Atlantic

“…Similar findings are presently underway for a suite of species, with coastal and fjordic affinities and widely different life histories. This shift in our appreciation of implicit vulnerabilities challenges traditional stock assessment models and classical management—assuming discrete populations within large administrational areas—calls for sub-stock-specific management recommendations (see e.g., Reiss et al 2009 ; Lindegren et al 2013 ; Dahle et al 2018 ; Svedäng et al 2018 ).…”