Buoyancy aptitude of the European wels catfish in relation to vertical migration and hunting behaviour

The European wels catfish (Silurus glanis) is a large, physostomous apex predator of freshwater habitats. Its current range expansion both within and beyond its native distribution, is strongly influenced by global warming. This study investigates patterns of diving behaviour of a wels catfish population that has recently increased significantly in a large, deep, cold-water peri-alpine lake. Pressure and temperature loggers were implanted into 42 free-ranging wild caught wels catfish in Lake Constance. High-resolution data from 13 recaptured individuals was subsequently analysed. Two distinct vertical migration modes previously undescribed for this species were identified: fast vertical movements, which lasted for less than 5 minutes, and slower vertical movements in which the vertical movement enacted over a period exceeding 15 min. Swimming depth was shown to be strongly affected by season and water temperature. Fish occupied deeper waters in winter, and shallower depths during the warmer summer months. During summer, vertical movements occurred predominantly around dusk and night with few vertical movements during daytime, indicating nocturnal foraging behaviour. During winter however, vertical movements were more equally divided over the day. Fish making relatively fast vertical movements returned to their original depth, whereas slow vertical movements appeared to be migration between depth layers. The amplitude per period of vertical movement of both fast and slow vertical migrations was mostly restricted to a pressure change of less than 30%. This limitation is likely to be related to the physostomous nature of the species, wherein buoyancy is controlled by a swimbladder with a limited capacity for volume change under pressure. These results provide new insights into the seasonal timing and diving aptitudes of a highly significant aquatic predator. Vertical movements were mostly made in darkness in summer, and during low light conditions during winter, in agreement with wels catfish having a nocturnal lifestyle in which they forage by detecting and following the wakes of their prey. The narrow amplitude of pressure changes during the vertical movements underscores the importance of tight control over buoyancy during navigation, which may be an important factor allowing it to colonize large deep lakes.