Influence of female age, body size and environmental conditions on annual egg production of the bullhead

A population of bullhead Cottus gobio inhabiting a mountainous river network, was examined for the influence of maternal condition (total length, L-T, and age class in years) and environmental factors (water temperature and fish density) on fecundity, egg size and net annual reproductive effort (E-R, defined as the ratio of total egg production mass to body mass) within several stocks. Regardless of location within the drainage network, bullhead reproduction was synchronous, with only a single batch and fecundity ranging from 69 to 896 eggs per female. As with L-T fecundity increased with age although the difference between age classes 5 and 6 years was smaller than between other age classes. Egg diameter also increased with age from age classes 2 to 4 years, but then tended to decrease for age classes 5-6 years. Multiple stepwise regression was used to examine the effect of female L-T, water temperature and fish densities on bullhead egg production for each of the four age classes (2, 3, 4 and 5-6 years) separately. When statistically significant, water temperature always had a positive effect and fish densities a negative effect, on fecundity and egg size. The L-T always had it positive effect on fecundity for all the age classes. In contrast, L-T had a positive effect on egg size and E-R only for female reaching maturity (2 years), whereas intermediate aged females (3-4 years) were insensitive and the oldest fish showed a negative response. (c) 2005 The Fisheries Society of the British Isles