Piscivore-prey fish interactions - consequences of changing optical environment

Abstract: Predator-prey interactions are a primary structuring force in aquatic systems. A change in the predator-prey interactions may cause a change in the strength of trophic cascades and even resulting in ecosystem shifts. However, individual properties of predators and prey, as well as environmental conditions, may affect the relative strength of predator-prey interactions. In our studies we observed two different behavioural defences against predators. Both crucian carp and perch decreased their activity when exposed to pike chemical or visual cue. However, the behavioural response elicited by the chemical cue was context dependent, i.e. crucian carp only reacted to the cue if information from visual senses was limited (turbid or brown water). Another behavioural response to predation threat is schooling as shown in roach, a common prey fish, that group into schools to limit predation risk. The interaction between the predator and prey fish was influenced by optical conditions. Reaction distance of pike showed to be negatively affected by reduced visual conditions. The reaction distance and attack distance of pike and escape distance of roach was also context dependent, where brown water increased the reaction and attack distance in pike and reduced the escape distance in roach. This resulted in an overlap in attack and escape distance, which may have positive effects on the capture success of pike. Pikeperch was less affected by changes of the visual conditions in the water. However, pikeperch was indirectly affected by changing optical conditions through a change in prey behaviour. Pikeperch showed a strong preference for perch under good optical conditions, but shifted their food preference to roach in poor optical conditions. The underlying mechanism to this pattern was observed in behavioural studies. Roach escaped long before the pikeperch initiated an attack, whereas perch was inactive which allowed pikeperch to approach and successfully attack. In poor optical conditions roach shoals were splitting up and roach escape distance was strongly reduced, which increased the foraging success of pikeperch resulting in that both roach and perch were included in the diet. These changes in the predator-prey interactions may explain some of the changes in fish growth rate observed in lakes along a gradient of brown colouration. Here, I found that the growth rate of pike and roach were reduced in browner lakes. Changes in environmental drivers, such as eutrophication and brownification, affecting the optical climate should thus have consequences for the strength of predator-prey interactions. This in turn may affect lake ecosystems through higher-order interactions.