Intrabasin variations in age and growth of bullhead: the effects of temperature

The variability of growth variables and age structure in bullhead Cottus gobio was studied in a single river catchment by comparing populations inhabiting river reaches with different thermal regimes. Age and growth variables were determined by examining 540 sagittal otoliths from specimens ranging between 19 and 146 mm total length. Growth of young bullhead (0þ and 1þ years) was higher in the warmest sites, especially in relation to water temperature during the growing season (spring and summer). Growth of older bullhead (2þ to 5þ years) was negatively related to mean water temperature of the site. The growth performance of bullhead over its whole life cycle was negatively correlated with water temperature, which could be partly explained by a higher energy allocation to reproduction in the warmest sites. Life span was longer in the colder sites (7 years) than in the warmer ones (4 or 5 years), and bullhead matured earlier in the warmer sites (age 1þ or 2þ years) compared to the colder ones (age 2þ or 3þ years). These results, combined with previous data on reproductive traits, suggest the possible effects that global warming could have on bullhead population dynamics.