Life-history traits of non-native black bullhead Ameiurus melas with comments on its invasive potential

Life-history traits of non-native black bullhead Ameiurus melas from an oxbow of the River Váh (near the city of Komárno), located in the middle Danube reaches (Slovakia) were investigated during 2006 and 2007, including samples from the pre-spawning (March-May), spawning (May-June), post-spawning (late June-August) and resting (September) periods. In total, 600 specimens were examined. The sex ratio oscillated close to 1 : 1. Females were found to mature at 98.6-120 mm SL (118-145.1 mm TL, compared to 171-254 mm TL in the native North American populations). Spawning took place in May and June, although the spawning period in 2007 was more protracted. Based on the analyses of annual gonadal cycle and oocyte size distribution, the black bullhead is considered a multiple spawner with asynchronous oocyte development and indeterminate fecundity. Absolute fecundity ranged from 1238 to 4755 (mean 2610), and relative fecundity from 36.2 to 154.3 (mean 56.1) oocytes g⁻¹. The high flexibility in the life history of the black bullhead as demonstrated by its non-native populations, as well as its extreme tolerance and capability to live in systems with poor water quality, suggests that this species has a high potential to invade new areas and establish viable populations.