Earlier Spring Warming of Coastal Streams and Implications for Alewife Migration Timing

Seasonal temperature increases have been shown to correlate with changes in the migration timing of fish. We looked at existing temperature and fish count data for anadromous alewives Alosa pseudoharengus in several southern New England streams. Run initiation, defined as occurring when the first 5% of the total annual run migrated, was associated with 9 degrees C. A weighted-mean migration temperature, considered an overall indicator of total run timing, revealed that 13 degrees C was a consistent predictor of run timing for one historic and three recent alewife streams over several years. Because historical and recent daily water temperatures with concurrent fish counts were unavailable for any one site, we used the occurrence of 9 degrees C and 13 degrees C stream temperatures to determine the magnitude of any detectable shift in the migration timing of alewives between the 1970s and recent years. Stream temperatures in the spring warmed to 13 degrees C about 12 d earlier in recent years than they did in the 1970s. This implied that alewife runs occurred about 12 d earlier on average. Run initiations occurred about 13 d earlier. Management of the water supply to fish passage facilities may need to be adjusted to accommodate these shifts in migration timing.