The presence of grey seals affects the chance of catching pike
Using recreational fishing and mark–recapture methods, researchers estimated how many pike live in a Baltic archipelago bay—2,449 individuals. When grey seals appeared, catches fell sharply, not because pike disappeared, but because their catchability declined.
Populations of Northern pike have declined in both number and individual size along large parts of the Baltic Sea coast. To protect pike in the archipelago, fishing bans have been implemented in key spawning bays during the spawning season. In one of these protected areas in the southern Stockholm archipelago, SLU (Swedish University of Agricultural Sciences) and the Stockholm County Administrative Board conducted a six-year study using standardized recreational fishing methods—specifically, rod-and-line sampling known as the ReFisk method. The study has been published in the scientific journal Transactions of the American Fisheries Society.
A unique opportunity to study seal impact
In the study, captured pike were marked with individual ID numbers and released. When marked fish were recaptured, researchers used a capture-recapture model to calculate the total number of pike in the bay.
During the fourth year, grey seals were observed in the bay, and several fish carcasses were found. The following year, a net was installed to prevent seals from entering the bay while still allowing fish to pass through.
"This provided a unique opportunity to study the effect grey seals can have, not only on the fish but also on the fishing itself," says Henrik C Andersson, County Fisheries Consultant at the Stockholm County Administrative Board.
When the seals were present in the bay, catches decreased markedly, from an average of 27 pike on an occasion down to zero.
Seal presence alters pike behavior
"If you don't catch a single pike while fishing, it can be due to two things: either there are no pike, or the pike cannot be caught. Capture-recapture calculations can separate the two factors that influence recreational pike catches, the number of fish and the capture probability. If either is zero, the catch will also be zero," says Konrad Karlsson, researcher at the Department of Aquatic Resources at SLU.
"Our results suggest that the pike do not disappear entirely when seals arrive, but they change their behavior. Seals certainly eat pike and many other fish species, but the decreased catches are also due to the pike becoming more shy and harder to catch," Konrad Karlsson continues.
New methods for monitoring pike stocks
In the studied bay, pike were plentiful. According to the calculations, there were 2,449 individuals over 42 centimeters. Most stayed near the vegetation along the shoreline, where density reached 27 individuals per 900 square meters, before decreasing sharply further from the shore.
However, truly large fish were missing; the largest pike caught was 97 centimeters. This reflects the situation for pike across large parts of the Baltic Sea, with declining numbers of large individuals.
"The pike is one of Sweden's most popular sportfishing species but has poor status in much of the Baltic. We need to develop methods that make it possible to monitor stocks reliably, both along the coast and in our lakes. This study is an important piece of the puzzle in that work," says Göran Sundblad, researcher at the Department of Aquatic Resources at SLU.
The study “Estimating Northern Pike population dynamics and capture probability by recreational angling using spatial capture–recapture models in a Baltic Sea spawning area” has been published in Transactions of the American Fisheries Society:
Contakt:
Konrad Karlsson, researcher, Department of Aquatic Resources, SLU, konrad.karlsson@slu.se, 010-478 42 54
Göran Sundblad, researcher, Department of Aquatic Resources, SLU, goran.sundblad@slu.se, 010-478 42 92
Henrik C Andersson, Stockholm County Administrative Board, Henrik.C.Andersson@lansstyrelsen.se, 010-223 12 13