Fishery closures increase pike aggression and trigger a trophic cascade in coastal Baltic Sea bays

Sammanfattning

Fisheries often intentionally target large predatory fish, because of their high commercial value or trophy size. Fisheries also unintentionally target fish with a high activity and aggression. This is because static fishing gears like gillnets, traps and baited-hooks have a higher likelihood of catching fish which swim around more or strike baits more often. This selective impact on large, active individuals shifts target populations toward smaller, more timid fish. A parallel and growing body of literature suggests that prey organisms live in fear of their predators, and that this fear can alter prey behaviour enough to cause trophic cascades. In this study, we investigated whether fishery induced timidity causes trophic cascades, and if fishery closures can help reverse this. Our hypothesis was that fisheries decrease predatory activity enough to decrease fear in prey, therefore causing higher prey feeding rates and subsequently a trophic cascade. Our study system was coastal Baltic Sea bays in the Stockholm and Öregrund (Sweden) archipelago, an area where fisheries mainly consist of recreational catch-and-release (C&R) fishers targeting large predator fish like pike with lures. A C&R event, although often non-lethal, causes enough stress to alter the behaviour (e.g. habitat choice, swimming distance, aggression) of caught pike. Our surveys included abundance estimations of pike (using rod-and-reel fishing), mesopredators (using gillnets), mesograzers (using fauna traps) along with vegetation surveys. This was coupled with surveys of pike behaviour (attack/landing rate estimations during the rod-and-reel fishing), mesopredator feeding behaviour (using baited-remote-underwater-video cameras) and mesopredator predation rate (using gammarid tethering). To assess what effect different levels of fishery absence have on pike and trophic interactions, the surveys were conducted in year round fishery closures (MPAs), seasonal fishery closures (pMPAs), and areas open to fishing. We found that pike aggression was higher inside fishery closures and that this variable was negatively correlated with predation pressure on gammarids; a clear sign of a trophic cascade. Additionally, we found a negative correlation between pike and mesopredator abundance, and higher predation pressure on gammarids with increasing levels of fishery protection. Importantly, when the seasonal protection of pMPAs (2.5 months/year) was in effect, predation pressure on gammarids was higher inside of pMPAs compared to areas open to fishing, but after seasonal protection had ended, predation pressure was similar. Our results are the first to suggest that fishery induced timidity can trigger a trophic cascade and that fishery closures can help reverse this effect. Our results also suggest that mesopredators in shallow coastal areas of the Baltic Sea live in a landscape of fear, shaped by exposure to threats from pike and human (fishing related) activities, and that this fear is strong enough to cause a trophic cascade. These findings have major implications for our continued understanding and management of shallow coastal ecosystems.