Morphological defences do not protect an invasive catfish from a mammalian top predator, the Eurasian otter Lutra lutra

A key component of quantifying the threat of introduced exotic species and implementing their management is knowledge of whether their natural enemies counteract their invasion. Anti-predator defensive structures can help invaders by undermining biological control by native species. The invasive brown bullhead Ameiurus nebulosus has been highlighted as a potential driver of freshwater biodiversity loss in central and eastern Europe. The species is equipped with stout venomous spines in the pectoral fins, which may mitigate predatory pressure. We compared brown bullhead relative abundance in invaded water bodies in eastern Poland with its consumption, assessed through faecal analysis, by a top predator, the Eurasian otter Lutra lutra. The mean Jacobs’ index value for bullhead calculated from 14 study sites, tested against the neutral index value, indicated that otters showed a preference for this species. Also, otters displayed positive selectivity for littoral cyprinids and negative selectivity for open-water cyprinids. Bullhead was the otter’s main food source (on average 25.4% of fish prey by numbers and 23.6% by relative frequency of occurrence). Pectoral spines were among bullhead bones frequently recovered from otter faeces, indicating that otters had ingested them while eating the fish. Otters tended to take smaller bullheads than those caught by gillnetting or electrofishing at the study sites; smaller individuals may pose a lower risk of inflicting injury. Our results indicate that the brown bullhead’s defensive weapons provide no significant advantage over native fish species against otter predation, and that otters can contribute to the biotic resistance of the invaded community.