Historic changes in length distributions of three Baltic cod (Gadus morhua) stocks: Evidence of growth retardation

Abstract

Understanding how combinations of fishing effort and selectivity affect productivity is central to fisheries research. We investigate the roles of fishing regulation in comparison with ecosystem status for Baltic Sea cod stock productivity, growth performance, and population stability. This case study is interesting because three cod populations with different exploitation patterns and stock status are located in three adjacent but partially, ecologically different areas. In assessing stock status, growth, and productivity, we use survey information and rather basic stock parameters without relying on age readings. Because there is an urgent interest of better understanding of the current development of the Eastern Baltic cod stock, we argue that our approach represents partly a novel way of interpreting monitoring information together with catch data in a simplified yet more informative way. Our study reports how the Eastern and Western Baltic cod have gone toward more truncated size structures between 1991 and 2016, in particular for the Eastern Baltic cod, whereas the Öresund cod show no trend. We suggest that selective fishing may disrupt fish population dynamic stability and that lower natural productivity might amplify the effects of selective fishing. In support of earlier findings on a density‐dependent growth of Eastern Baltic cod, management is advised to acknowledge that sustainable exploitation levels for Eastern Baltic cod are much more limited than perceived in regular assessments. Of more general importance, our results emphasize the need to embrace a more realistic view on what ecosystems can produce regarding tractable fish biomass to facilitate a more ecosystem‐based fisheries management.

Relative impacts of fishing and eutrophication on coastal fish assessed by comparing a no-take area with an environmental gradient

“…Fishing is globally known to cause, for example, changes in species composition, truncated size structures, and habitat alterations (Jennings and Kaiser 1998 ), as also seen in the Baltic Sea. In offshore areas of the Baltic Sea, extensive fishing has been connected to dramatic declines in target species, especially of larger fish, and changes in food web structure (Casini et al 2009 ; Svedäng and Hornborg 2017 ). The level of commercial fishing in coastal areas is considerably lower in comparison, but recreational fishing may on the other hand be substantial.…”

Compensatory Feeding in Eastern Baltic Cod (Gadus morhua): Recent Shifts in Otolith Growth and Nitrogen Content Suggest Unprecedented Metabolic Changes

“…On the other hand, estimates based on accurately aged material comprising ages 1-3 as well as length-frequency analysis, support the view of moderately reduced individual growth in EBC over the last two decades (Hüssy et al, 2018). Also, the continued decline in the length class diversity and the mean length in the survey catches between 1995 and 2015 indicates lower individual growth (Svedäng and Hornborg, 2017) or increased natural mortality for fish above 30 cm (c.f. ICES, 2019a).…”

“…However, the enhanced recruitment was not, transformed into higher biomass, as landings continued to wane through the 2000s and 2010s. Body condition and hepatosomatic index (HSI) started to decline in the mid-1990s (Eero et al, 2012), while the size distribution of EBC became increasingly truncated (Svedäng and Hornborg, 2017) and the fish mature at progressively smaller sizes (ICES, 2019a). Due to the deteriorated status of EBC, a fishing ban was eventually introduced in 2019.…”

Biodiversity and ecosystem services in Nordic coastal ecosystems

“…55). As opposed to the development of the eastern and western Baltic cod stocks, the size-distribution of the Sound cod is becoming neither wider nor more truncated (Svedäng & Hornborg, 2017). The Sound differs in exploitation patterns from the Kattegat and the western and eastern Baltic, particulary in terms of the selection of gear.…”