Rutherford and Houde. The influence of temperature on growth of Morone saxatilis 



319 



River occurred in April in each year when tempera- 

 tures rose to >12°C. A secondary spawning peak was 

 evident in mid-May of 1989 (Fig. 2). Peak spawning 

 in the Upper Bay occurred in mid to late May when 

 water temperatures rose to >14°C (Fig. 3). Striped 

 bass egg abundances were highest in 1989 and low- 

 est in 1988 (Rutherford et al. 4 ). Egg abundances were 

 nearly an order of magnitude higher in the Upper Bay 

 in 1989 (77.9 eggs-m" 2 ) than in 1988 (8.2 eggs-m" 2 ) but 

 did not differ significantly (P>0.05) among years in 

 the Potomac (25.1 to 73.5 eggs-m" 2 ). 



Episodic mortalities of striped bass eggs and lar- 

 vae occurred in the Potomac River in each year after 

 spring storms, which caused flood conditions and 

 sharp declines in river temperatures to or below the 

 12°C lethal limit. In 1987, a storm on 16-17 April 

 and a subsequent temperature drop caused complete 



150 



100 



50 



C 



o 



0) 



o 

 c 

 co 



■o 

 c 



3 



n 



(0 



O) 



en 

 w 



32 

 28 

 24 

 20 

 16 



1 10 20 30 10 

 April May 



20 



30 9 

 June 



(D 



3 

 ■o 



(D 



0) 

 C 

 (D 



1 10 20 30 10 

 April May 



30 9 

 June 



Figure 3 



Areawide egg abundances ( millions) of striped bass, Morone 

 saxatilis, estimated on each survey date (bars) during the 

 two years of sampling effort in the Upper Bay, 1988-89. 

 Water temperatures recorded at Conowingo Dam during 

 the spawning season also are given (open diamonds). The 

 12°C critical low temperature, at which 100% egg and lar- 

 val mortality may occur, is indicated by the dotted line. 

 Note that the Y-axis scales change between years. 



mortality of all striped bass eggs and larvae, effec- 

 tively eliminating >50% of the season's egg produc- 

 tion (Rutherford et al. 4 ). Although no larvae survived 

 that event, relatively minor spawning (Fig. 2) that 

 occurred later, combined with favorable conditions 

 for larval growth and survival, resulted in higher 

 mean larval abundances than those in 1988 or 1989 

 (Rutherford et al. 4 ). After 20 April 1987, Potomac 

 River temperatures increased steadily and were both 

 warmer and less variable than in 1988 or 1989. Tem- 

 perature profiles in the Upper Bay were similar in 

 both 1988 and 1989 (Fig. 3). In the Upper Bay, the 

 initial order of magnitude difference between years 

 in egg abundance was maintained through the lar- 

 val stage (Rutherford et al. 4 ). 



Mean densities of zooplankton that were poten- 

 tial prey for larval striped bass cohorts during the 

 first 5-20 days posthatch were highest in the 

 Potomac River in 1987 and in the Upper Bay in 1989 

 (Fig. 4). Densities of copepod nauplii, Eurytemora 

 affinis, and rotifers (including Brachionus,Asplanchna, 

 Synchaeta, Polyarthra), in particular, which are ini- 

 tial prey of striped bass larvae (Takacs, 1992; Beaven 

 and Mihursky 6 ), were higher in the Potomac River 

 in mid to late May 1987 than in 1988 or 1989. 



Growth 



Body length and otolith-radius relationships Lar- 

 val length was strongly correlated with otolith size 

 in each year (Table 1). The best regression relation- 

 ships were linear for most cohorts with adequate 

 sample sizes (n>4) and ranges in body length. Expo- 

 nential models provided better fits for some Potomac 

 River cohorts, and age was a significant covariate of 

 otolith size for two cohorts in 1989 (Table 1). 



With few exceptions, cohorts hatched early in the 

 season, experienced mean temperatures <17°C dur- 

 ing the first 20 days posthatch, and had relatively 

 larger otoliths per unit body length than did cohorts 

 hatched when temperatures were warmer. Differ- 

 ences in body length-otolith radius relationships 

 among Potomac River striped bass cohorts were sig- 

 nificant in 1989 (ANCOVA; P<0.001), but not in 1987 

 (ANCOVA; P>0.10) or 1988 (ANCOVA; P>0.20). 



Growth rates Growth rates of striped bass larval 

 cohorts, estimated by both aggregate and back-cal- 

 culated methods, indicated that growth varied sea- 



6 Beaven, M. S., and J. A. Mihursky. 1979. Food and feeding hab- 

 its of larval striped bass: an analysis of larval striped bass stom- 

 achs from 1976 Potomac estuary collections. Ref. No. 

 [UMCEES]CBL 79-045, Chesapeake Biological Laboratory. 

 Available: University of Maryland Center for Environmental 

 and Estuarine Studies, Chesapeake Biological Laboratory, 1 Wil- 

 liams St., Solomons, MD 20688. 



