Rapid response of brook trout to removal of its intraguild prey, yellow perch

Abstract

Brook trout (BT) and yellow perch (YP) interact as intraguild predator and prey: while adult BT consume YP, juvenile BT and YP compete for food resources, i.e. zooplankton (mainly a pelagic resource) and zoobenthos (mainly littoral). YP appear to dominate the interaction, reducing BT abundance in lakes with YP present. We removed 90 % of YP from a 35 ha oligotrophic lake to test whether the removal of the dominant competitor would lead to an increase in growth and recruitment of juvenile BT. This manipulation shifted the YP:BT biomass ratio from 11:1 to 1.5:1. The food web response was evaluated using the pre-removal condition as the reference. The abundance of cladoceran zooplankton increased, especially large-bodied Dapnia catawba, and the body size of Daphnia dubia increased significantly, responses indicative of reduced zooplanktivory. Some benthic taxa (chironomid and Ephemeroptera larvae) also increased following YP removal. BT responded rapidly: mean length at age-1 increased from 11.6 cm to 12.8 cm (~35 % weight increase), the condition of 15–25 cm trout improved, and stable isotopes indicated increased consumption of littoral-linked prey by 20 to 30 cm BT. YP, despite their reduced abundance, remained the primary prey of large BT (> 25 cm). The suite of changes observed in response to YP removal support the view that although YP are the primary prey of large BT, they exert a strong competitive effect on juvenile BT. Further study would be required in order to assess whether piscivorous BT can establish persistent top–down control of YP due to enhanced juvenile BT recruitment.