Influence of Spring Floods on Year‐Class Strength of Fall‐ and Spring‐Spawning Salmonids in Catskill Mountain Streams

AbstractClimate change in northeastern North America has already led to earlier snowmelt and increases in the magnitude and frequency of large storms; associated changes are expected to intensify over the next several decades. Alterations in flooding regimes associated with climate change have the potential to negatively affect fish species with earlier fry emergence and alter the fish communities in tributary streams where fall‐ and spring‐spawning salmonids coexist or where there may be invasion potential by a given species. This 6‐year study (2002‐2007) assessed the effects of the timing and magnitude of spring flood events on year‐class strength in sympatric populations of fall‐spawning brook troutSalvelinus fontinalis, fall‐spawning brown troutSalmo trutta, and spring‐spawning rainbow troutOncorhynchus mykiss. The relative abundances of young‐of‐year (age‐0) and age‐1 fall‐ and spring‐spawning salmonids were documented for 4 and 6 years in streams from two watersheds in the Catskill Mountains, New York. Fall‐spawning age‐0 fish dominated in all years except 2005. In spring 2005 a large flood occurred at nearly the same time of year as the 2004 snowmelt, but discharge was nearly an order of magnitude greater. The dominant salmonid for that year‐class shifted to rainbow trout, but fall‐spawned age‐0 trout were again dominant in the following year. These results indicate that the timing and magnitude of spring high flows can increase the relative abundance of spring‐spawning age‐0 salmonids, decrease the abundance of fall‐spawning age‐0 salmonids, or both. The overall dominance of fall‐spawned fish appears to be resilient as long as displacing floods occur a few years apart. If the trend toward larger, more frequent, and earlier spring floods continues, differential survival of age‐0 fish of the three salmonid species will probably cause shifts in the dominant trout species in many Catskill Mountain streams.