COS 1-7
On death and drying: The effects of summer drought on fish mortality in an intermittent stream
Mediterranean-climate regions are characterized by seasonal drought due to their predictable patterns of dry summers and wet winters. Consequently, streams in these regions undergo cycles of ecosystem contraction and expansion that coincide with seasonal cycles of precipitation. Intermittent streams, which cease flowing for a portion of the year, represent an extreme form of contraction. Our goal is to understand the response of juvenile steelhead to summer drought in a small intermittent stream in the Point Reyes National Seashore, California. This involved detailed study of stream physical dynamics and a mark-recapture study of juvenile steelhead rearing at the site. We studied a continuous series of riffle-pool habitats during the summer drought season across four recent years (2009-2012). In order to estimate how habitat availability (i.e., water quantity) decreases across the summer low flow season, we used a total station to obtain precise estimates of pool and riffle volumes across the summer for each of our study years. We also implanted fish with uniquely-coded PIT-tags at the beginning of each drought season and used a portable PIT-tag antenna to resight tagged fish each week across the summer, allowing us to estimate weekly scale survival using Program MARK.
Results/Conclusions
We were fortuitous to have two dry years (2009 and 2012) and two wet years (2010 and 2011) during the course of our study. In all summers, we observed reductions in both pool and riffle volumes across the drought season, with more marked reductions in volume and earlier onset of drying during the relatively dry years. Our mark-recapture study suggests that drought intensity has a strong influence on juvenile steelhead survival. We found that oversummer survival was lowest in the driest years. Nevertheless, our results suggest that resistance to drought is high: survival remains high and constant until the very end of the summer, when conditions begin to rapidly deteriorate. Our findings suggest that seasonal drought can influence greatly the survival of juvenile steelhead rearing in intermittent streams and that interannual variation in patterns of survival are related to interannual variation in precipitation. With stream runoff projected to decrease in the region, our findings are highly relevant to managers who seek to balance human water needs and those of stream-dwelling fishes.