What to Expect After a Commercial Fishery Ban? Long Term Fish Size and Biomass Trends in Two Productive Water Bodies With Contrasting Management Regimes

ABSTRACT Commercial fishing is often assumed to have greater impacts than recreational fishing, and many fisheries assessments overlook the latter. Yet, numerous coastal and freshwater fish populations are subject to both fishing types and environmental pressures, all influencing biomass, body size and community structure and making management outcomes difficult to predict. In this study, we reconstruct 70 years of fish size and 30 years of biomass trends and explore environmental and harvesting impacts in two productive water bodies with contrasting management regimes: Kaunas Reservoir and the Curonian Lagoon (Lithuania). Commercial fishing has been banned in Kaunas Reservoir since 2013 but continues in the Curonian Lagoon, whereas recreational fishing occurs in both. Fish biomasses in the lagoon generally declined, while in the Reservoir biomasses of commercial target species increased up to fourfold since the fishery ban. Yet, recovery of popular angling species, including main predators, remains slow. Across species, body sizes were stable in the Curonian Lagoon and increased significantly in Kaunas Reservoir. Such positive size trends were partly explained by positive temperature impacts, although fishing had a larger effect on body sizes than temperature. Our findings indicate that rapid population recovery of temperate fish populations could be expected within a decade if all types of fishing mortality are removed, and that effective no‐take reserves could see a several‐fold increase in fish biomasses. Notably, due to often positive effects of warming on average fish sizes, body size trends may suggest a more optimistic population status than biomass data, and both should be studied simultaneously.