Evaluation of methods to assess relative abundance, size/age structure and gear-specific traits in northern pike (Esox lucius) sampled in brackish lagoons

Accurate information on fish populations, including abundance, age and size structure, growth rate, and condition, is crucial for fisheries science and management. In order to acquire such information, some type of sampling protocol and gear has to be applied in a random pattern in a given aquatic ecosystem. All gears have a certain type of bias, which is related to factors such as catchability, seasonality, fish behaviour and the habitats sampled. Those biases need to be considered when deriving population-level statements from a sample taken from this population. Northern pike (Esox lucius) are a widespread species in the northern hemisphere and is an important species in many fisheries. In the Baltic lagoons around the island of Rügen, Germany, pike stocks have been drastically declining in the recent years and no standardized monitoring approach is in place. The present thesis compared five different sampling gears and methods – e-DNA, angling, gillnetting, ROUV’s and longlining – with each other over different seasons and along a gradient of areas with varying fishing pressure and protection. The goal was to derive relative abundance estimates of pike and to compare growth rates, age/size structures and condition revealed by each gear. From all the five gears applied, only three gears – e-DNA, angling and gillnetting – were providing sufficient data.
It was found that angling was the most consistent gear across all the seasons, while e-DNA and gillnetting were mainly applicable in spring, when pike are spawning. However, pike captured with angling showed a very high growth rates in comparison to gillnet pike. That indicates that angling is strongly selective towards highly active fish, while less active and more shy pike are underrepresented due to timidity. This was found to be a large bias in angling based assessments of abundance of pike. The effect of timidity might be as strong as the effect of abundance, when comparing protected and unprotected sites witch each other. In order to rule out timidity, a second gear must be applied, that is more resistant to timidity. Gillnets have the potential in doing so, but higher efforts are required than in this thesis. Pike, being a low abundance and low activity species remains to be a challenging species to sample. Given that all tested gears exhibit some form of bias, it is crucial for fisheries management to implement a multi-gear sampling strategy to mitigate these biases and improve the accuracy of population assessments of pike. Angling, despite its biases, should be included due to its consistency across different seasons.