Fish are not safe from great cormorants in turbid water

Great cormorant populations have shown near-exponential increase in Western Europe over recent decades, generating wildlife management conflicts across the region. It is essential to detail the foraging strategies of this piscivorous predator to understand the functional mechanisms determining its actual impact upon freshwater fisheries. Great cormorants have been shown to be highly efficient predators, yet their underwater vision is limited in turbid water. We therefore tested the hypothesis that the predatory performance of great cormorants will be lower in turbid than in clear water. We used parallel data sets on water turbidity, great cormorant diet, great cormorant population size, fish biomass and fish-cormorant co-occurrence collected over 3 decades, with a focus on 1997 and 1998, in 2 Macedonian lake systems with contrasting features. In one lake system, water turbidity was lower, fish abundance was also lower, and the local great cormorant population did not increase markedly in size. Local populations of Alburnus belvica, which is the preferred prey of great cormorants in this region, increased over the study period. In the second lake system, water turbidity was higher, fish abundance was also higher, and the local great cormorant population has increased exponentially since the 1980s. Along the highly turbid affluent of this lake, great cormorants foraged en masse at the time of highest fish abundance, and local fish biomass decreased over the study period. Overall, our data strongly suggest that great cormorants remain highly efficient fish predators even in the most turbid water. We propose that they switch from visual to tactile cues to maintain high foraging efficiency when diving in water above a certain turbidity threshold.