Seasonal Trends in Abundance and Size of Juvenile American Shad, Hickory Shad, and Blueback Herring in the St. Johns River, Florida, and Comparison with Historical Data

Abstract

AbstractAmerican shad Alosa sapidissima, hickory shad A. mediocris, and blueback herring A. aestivalis spawn in Florida's St. Johns River during winter and spring, and juveniles migrate out of the estuary during the following fall. We assessed the temporal trends of juvenile relative abundance and size as each species moved downstream past Palatka, Florida, and compared these values to historical data from the system. Juveniles were collected from April 2004 to March 2005 primarily with surface trawling at night and with supplemental daytime electrofishing. Relatively few juveniles were collected by trawl (23 American shad, 7 hickory shad, and 36 blueback herring) or by electrofishing (55 American shad, 4 hickory shad, and 179 blueback herring). Trawl catch rates were highest in autumn, and electrofishing catch rates were highest in autumn and early winter. Both sampling methods caught juveniles at similar sizes. The surface trawl catch rates of juvenile American shad in 2004 and 2005 were not significantly different from comparable historical catch rates for 1969 and 1970, suggesting similar overall juvenile recruitment levels. Trawl collections were sufficient to document the seasonal downstream migration of shad juveniles. However, given the low catch rates, we found electrofishing to be a useful alternative method for describing seasonal abundance and habitat use by juvenile shad.