Estimating Northern Pike population dynamics and capture probability by recreational angling using spatial capture–recapture models in a Baltic Sea spawning area

ABSTRACT Objective Coastal populations of Northern Pike Esox lucius in the Baltic Sea are declining in both abundance and individual size. Management measures, including seasonal spawning closures, have been implemented for selected vulnerable populations. To ensure effective management, it is crucial to estimate spawning population densities and relate them to the number of individuals that are caught by recreational angling, as this is a primary factor that influences Northern Pike populations. Methods This study presents findings from a 6-year spatial capture–recapture study using standardized recreational angling in a Baltic Sea archipelago bay. Results The estimated population size of Northern Pike measuring ≥42 cm was 2,449 individuals (95% CI 1,264 to 4,889), or 65 individuals per hectare, whereas the population of Northern Pike measuring ≥60 cm was estimated at 1,156 individuals (95% CI 647 to 2,079), or 31 individuals per hectare. On average, two anglers that fished for 4 h captured 1.1% (95% CI 0.6% to 2.1%) of the total population, equivalent to 27 individuals. Although the maximum captures for a single day amounted to 2.9% (95% CI 1.5% to 5.6%). These results suggest that if recreational angling is frequent and continuous, a large percentage of the population will be captured, which could likely influence the population density and size structure in the long term. Over the 6-year period, previously captured Northern Pike had a higher recapture probability than those that were not caught previously, indicating a behavioral effect such that some individuals are more susceptible to capture, making a portion of the population more vulnerable. Additionally, there was weak evidence suggesting that smaller Northern Pike (≥42 cm and 60 cm) had a lower capture probability than larger individuals (≥60 cm). However, a bell-shaped length distribution of the captured Northern Pike suggests that smaller individuals may have been undersampled. Grey seals Halichoerus grypus, a piscivorous top predator, were observed in the bay during the fourth year of the study. The presence of seals significantly affected the capture probability of Northern Pike, reducing angling success more than influencing the estimated population density. The presence of seals likely caused a change in the behavior of Northern Pike that resulted in lower catchability rather than reduced population size. Conclusion This study highlights that stock indices based on catch per unit effort in standardized angling can be biased when capture probability varies, such as in the presence of grey seals. However, this bias can be corrected by using capture–recapture models. The insights that are gained from this study can help improve the ongoing monitoring and management of recreational angling and the effects of grey seals, benefiting Northern Pike populations in the Baltic Sea.