Shifts in diet of an apex predator following the colonisation of an invasive fish

Roach is an invasive cyprinid fish species that has been introduced to many Irish lakes, causing broad changes in fish community dynamics. This paper examines whether roach invasion is associated with temporal change in the diet of pike in colonised systems. The seasonal diet of pike in three Irish lakes was compared between a historical (pre-roach) data set collated on a monthly basis in the 1960s and 1970s, and recent samples collected monthly over 1year in 2016-2017. Statistical models indicated a significant increase between sampling periods in the probability of observing cyprinids in pike stomachs, and corresponding significant decreases in the probability of observing perch or brown trout. Small pike were significantly less likely than large pike to have salmonid prey in their stomach. There were seasonal effects on diet, with invertebrates and sticklebacks being consumed more in Winter-Spring compared to Autumn-Summer. In the recent period, prey selection indices indicated positive selection for roach and negative selection for perch; indices for trout tended towards neutrality. The dietary shift in pike following the establishment of roach may have alleviated predation pressure on native trout (and perch), with implications for food web structure in invaded lakes.