Spatial and temporal variation in assemblage structure of fish larvae in mediterranean–type streams: contrasts between native and non-native species

Larval fish ecology remains poorly understood in freshwater ecosystems. This study analysed the larval ecology of native and non-native fishes in a mediterranean-type watershed in Southern Iberian Peninsula. Assemblage structure of fish larvae was quantified at four distinct rivers sites, every 2 weeks between March and October 2004, and analysed against 16 variables reflecting river flow, temperature and habitat context. There was considerable spatial variation in taxonomic richness and abundance of larval assemblages, with either native or non-native fishes dominating in different sites. There was also a clear temporal separation between native and non-native fishes, with native cyprinids generally peaking earlier in the year than non-native fishes. Temporal fluctuations in larval assemblages across sites were mostly associated with variations in water temperature and transparency, but flow was an important factor shaping local assemblage structure. Larvae of native fishes appeared to found most suitable conditions in naturally flowing sites early in spring, when flow is high and water temperature stills low. These results suggest that preservation of natural flow peaks and adequate thermal contexts may be crucial for conservation of native fish fauna in mediterranean-type streams.