Length at maturity as a potential indicator of fishing pressure effects on coastal pikeperch (Sander lucioperca) stocks in the northern Baltic Sea

In the Baltic Sea, the pikeperch is one of the commercially important coastal fish species. Most of the local pikeperch stocks in this area can be classified as data-limited. There is an increasing need to monitor the state of all commercial fish stocks in European waters due to the EU Marine Strategy Framework Directive (MSFD). Proper stock assessment is not required in all cases, and alternative and less data demanding approaches and indicators can also be applied. In this study, we combined data on three of the best-studied coastal pikeperch stocks in the northern Baltic Sea to evaluate the performance of length at maturity indicators. A connection was found between intensive selective fishing and the length at maturity (TL50) of female pikeperch. Power analysis indicated that a sample size of 200-400 females, or around 35-70 samples per year, produces reasonable and almost maximal precision when determining the six-year mean TL50 for a stock. Our analysis also demonstrated that in cases where a monitored parameter is annually variable, an increase in the annual sample size does not continuously yield higher precision in the long-term mean, but it is crucial that samples are collected annually. We conclude that TL50 is a promising and cost-efficient indicator of the effects of fisheries on the maturation of coastal pikeperch stocks. This indicator could be further tested after a few years when more data become available.