Response of a Stream Fish Assemblage to a Severe Spate in Northern Spain

Abstract

I assessed the effects of a devastating spate upon the populations of brown trout Salmo trutta, Atlantic salmon S. salar, and European eel Anguilla anguilla of the Esva River basin in northern Spain. Numbers and lengths were determined for fish sampled with electrofishing techniques at nine sites along three streams before and after the spate. In addition, brown trout and Atlantic salmon that had been marked in two streams prior to the spate permitted a direct evaluation of the immediate effects. Because the spate occurred at the spawning time and destroyed reproductive habitats, I also determined its effects upon the recruitment of that year-class of brown trout. There was no evidence of negative effects of the spate upon the variables examined. The persistence of the site-specific populations after the spate was independent of site characteristics and the corresponding numbers and sizes of fish. The recruitment of brown trout was successful and similar to that of previous years. I hypothesize that mechanical responses related to microhabitat use permit brown trout and Atlantic salmon to withstand spates.