Great cormorant predation on Cisalpine pike: a conservation conflict

In the last decades, the distribution and abundance of great cormorants have extraordinarily increased throughout Europe. Many studies reported that great cormorants may impact fish populations not only by consuming large number of individuals but also by wounding them. Most studies regarded fish farms and cultured species, but there is less information about wild fish populations. In this study, we examined the incidence of wounds caused by great cormorants on an endemic and threatened species, the Cisalpine pike (Esox cisalpinus Bianco and Delmastro 2011). The object of our research was to quantify this impact and indirectly to estimate if cormorant predation may be one of the causes of the rapid decline of this Esocidae. In the years 2009–2013, 139 pikes were collected in some gravel pits in Northwestern Italy. More than a half of the specimens (57 %) reported wounds attributable to great cormorant attacks. Most wounds were localized in the dorsal and lateral surfaces of pikes. We detected a significant difference in the occurrence of wounds between fish sizes, with 73.5 % of adults showing some kind of injury. In a context of general freshwater habitat alteration, quarry lakes represent important sites for Cisalpine pike conservation. Unfortunately, pike breeding season overlaps with the presence of large colonies of overwintering cormorants, increasing the probability of interactions in a period of extreme importance for this Esocidae. Our data evidenced that the increase of cormorants represents an important menace for Cisalpine pike conservation. Finally, we suggest some management options to minimize the problem.