326 



Fishery Bulletin 93(2). 1995 



Potomac River 1987 



10 20 30 10 20 30 9 

 April May 



Potomac River 1988 



10 20 30 10 20 30 9 

 April May 



Date 



Potomac River 1989 



Upper Bay 1989 



10 20 30 10 20 30 

 April May 



Date 



4(1 

 30 

 20 



III 

 



D< 



3 *3 



3 m 



Figure 10 



River temperatures at Wilson Bridge (Potomac River) or Conowingo Dam ( Susquahanna River, 

 Upper Bay), percent egg production, and percent estimated abundances by hatch dates of 

 striped bass, Morone saxatilis, larvae at 8.0 mm TL in 3-day cohorts from the Potomac River, 

 1987-89, and Upper Bay, 1989. Larval abundances at 8.0 mm were estimated from 60-cm net 

 samples in the Potomac River, 1987-88, and from 60-cm net and 2-m 2 Tucker trawl catches in 

 the Potomac and Upper Bay, 1989. The dashed line indicates the 12°C critical low tempera- 

 ture at which 100% mortality of egg and yolk-sac larvae may occur. 



Discussion 



Temperature, larval survival, and growth 



Estuarine nurseries of anadromous fishes are dy- 

 namic environments, subject to fluctuating and some- 

 times unpredictable conditions for survival and 

 growth. Striped bass spawn in the freshwater, tidal 

 regions of mid- Atlantic estuaries during the spring 

 months, a transitional season when average tempera- 

 tures are increasing rapidly, but erratically, from 10 

 to 25°C. Frequent storms and flood conditions can 

 cause temperatures to drop quickly to lethal or near- 

 lethal levels. The unusually wide range of tempera- 

 tures to which striped bass eggs and larvae may be 

 exposed suggest that temperature may be a factor con- 

 trolling growth rate, stage duration, and survival rate. 



Our results confirmed that temperature affects 

 larval growth rates and stage durations. Except for 

 observed episodic mortalities associated with tem- 

 perature drops to or below the 12°C lethal limit, more 

 subtle, direct effects of temperature on mortality 

 rates could not be demonstrated. Despite episodic 

 mortalities which impacted early-spawned cohorts, 

 favorable temperatures during the latter half of 

 spawning seasons resulted in significant potential 

 recruitment from small cohorts of late-spawned eggs. 

 The relatively good potential recruitments in the 

 Potomac River, 1987, and Upper Bay, 1989, were 

 derived from modest, late-season spawning that pro- 

 duced fast-growing larvae in environments where 

 water temperatures were steadily increasing. 



The application of otolith-ageing to identify 3-day 

 cohorts and to derive cohort-specific growth and 



