Effects of environmental factors on survival, growth, and production of American shad larvae

Episodic increases in temperature of 5°C above 20° C, over 48 h or declines in pH of 1·0 unit from pH 7·0 reduced survival of yolk-sac and feeding-stage larvae of American shad Alosa sapidissima . Over 16 days all measures of survival, growth, and production were more favourable at each higher temperature in the 15–25° C range. More favourable responses were also obtained at the higher prey level (500 v . 50 Artemia nauplii l^-1) and at the higher pH (7·5 v . 6·5). Combinations of high temperature and high prey levels, at pH 7·5, led to highest larval production. Little growth or production occurred at 15° C, regardless of pH or prey level. The effect of pH was strong with respect to survival, but weak with respect to growth. In attempts to restore American shad populations by larval stocking, release times and sites can be critical to optimize survival and eventual returns. Releases of larvae potentially will be most effective when made at temperatures >20° C, pH>7·0, and prey levels >50 1^-1. These conditions are most likely to occur in Maryland tributaries of Chesapeake Bay between mid-May and early June.