Climate change or mismanagement?

Abstract

Climate change and deoxygenation are affecting fish stocks on a global scale, but disentangling the impacts of these stressors from the effects of overfishing is a challenge. This study was conducted to distinguish between climate change and mismanagement as possible causes for the drastic decline in spawning stock size and reproductive success in cod (Gadus morhua) and herring (Clupea harengus) in the Western Baltic Sea, when compared with the good or satisfactory status and reproductive success of the other commercial species in the area. Available data on water temperature, wind speed, and plankton bloom during the spawning season did not reveal conclusive correlations between years with good and bad reproductive success of cod or herring. Notably, the other commercial species in the area have very similar life history traits suggesting similar resilience against stress caused by climate change or fishing. The study concludes that severe, sustained overfishing plus inappropriate size selectivity of the main fishing gears have caused the decline in spawning stock biomass of cod and herring to levels that are known to have a high probability of impaired reproductive success. It is pointed out that allowed catches were regulated by management and adhered to by the fishers, meaning that unregulated fishing did not occur. Thus, mismanagement (quotas that were too high and gears that selected too small sizes) and not climate change appears to be the primary cause of the bad status of cod and herring in the Western Baltic Sea.

Ecosystem-based fisheries management increases catch and carbon sequestration through recovery of exploited stocks: The western Baltic Sea case study

“…Catch and biomass predictions for the years 1994-2019 obtained with fishing mortality as a driver match values of stock size and catch remarkably well. This finding directly supports the hypothesis that the depletion of cod and herring in the WBS and the retraction of sprat towards the northern central Baltic were not primarily caused by changes in carrying capacity but were driven by overfishing (ICES, 2020f;Möllmann et al, 2021;Froese et al, 2022).…”

“…Exploration of future scenarios considered fish extraction as the main driver altering fish stock biomass, an assumption corroborated by previous studies on the WBS (Möllmann et al, 2021;Froese et al, 2022). However, when food web resilience is eroded by excessive fishing the importance of other stress factors cannot be neglected (Möllmann et al, 2021).…”

“…The Baltic Sea ecosystem is affected by multiple stress factors including nutrient pollution, deoxygenation, ocean warming and acidification (Reusch et al, 2018); these stressors act concurrently and their effect may be exacerbated in the presence of heavily depleted stocks (Möllmann et al, 2021;Polte et al, 2021). Although excessive fish extraction represents the primary and most pervasive cause behind the collapse of iconic stocks like western Baltic cod and western Baltic spring-spawning herring (Froese et al, 2022), this study also developed a wider view of future scenarios that account for changes in phytoplankton biomass and ocean warming. Nutrient enrichment enhances phytoplankton productivity leading to increased density.…”

“…These changes had a strong negative impact on catches of herring (-85%) and cod (-56%). Moreover, cod experienced repeated recruitment failures resulting in a population now dominated by a single year class (ICES, 2019a;ICES, 2019b;Froese et al, 2022). Herring and sprat are planktivorous fish that occupy strategic positions in the WBS food web (Casini et al, 2004;Andreasen et al, 2017).…”

Population dynamics of Baltic herring since the Viking Age revealed by ancient DNA and genomics

“…The Atlantic spring spawners are divided into two groups, with the transition zone between the Baltic and Atlantic separating on PC2. This is likely due to the presence of Norwegian fjord herring in the transition-zone cluster, which show signatures of adaptation similar to the Baltic herring due to the brackish fjord conditions ( 87 ). Winter- and autumn-spawning specimens from the Atlantic cluster together.…”