264 



Fishery Bulletin 94(2), 1996 



20,000 



10,000 



5,000 



2,000 

 1,000 



3 July 



Log, abund = 9 632 - 109 h 

 r- =0 54 



£ 5,000 r 

 CD 



c 2 OOOl 



5 1 000* 



5 500 



en 200 



S 100 



10,000 



I July 



Log, abund = 7 390 - 060 h 



5,000, 



2 000 

 1,000 

 5.000 

 I 

 1,000 

 500 







50 



Log e abund =8 771 - 080 h 

 r^ = 067 



20 July 



Log, abund =8382-0 185 h I 

 r- = 70 I 



24 July 



10 15 



Time of day (h) 



20 



Figure 4 



Regressions of log p abundances on elapsed time in hours (t) for 

 five experimental surveys of bay anchovy eggs in Chesapeake Bay 

 during July 1991. The five regressions are examples which dem- 

 onstrate the method to obtain mortality rates of eggs and yolk-sac 

 larvae for the 12 surveys summarized in Table 2. 



mean daily egg mortality was 86%, with a range of 

 26% to 97%. 



Egg mortality rates of bay anchovy obviously are 

 high. But, they are lower than predicted at their tem- 

 perature of development (27"C), based upon Pepin's 

 ( 1991 1 temperature-dependent mortality model, Z = 

 0.030 e° lHT , in which Z is the daily instantaneous 

 mortality rate for marine fish eggs and Tis tempera- 

 ture (°C). At 27°C the model predicts a mortality of Z 

 = 3.91 (=0.16/h), a rate more than twice our mean 

 estimate of Z = 1.58/d (=0.066/h). Pepin's ( 1991) model 

 for marine fish eggs predicts mortality rates of Z = 

 2.81 to 4.63/d for temperatures in the range 25.23°C 

 to 27.99°C which were observed during this study. 

 These predicted mortality rates are higher than all 

 except one of our estimated rates and are also higher 

 than rates estimated for bay anchovy eggs by Leak 



and Houde (1987) in Biscayne Bay, Florida. 

 Castro and Cowen ( 1991 ) determined mortal- 

 ity of bay anchovy eggs and yolk-sac larvae at 

 lower average temperatures in Great South 

 Bay. At their two-year mean of 23.4°C, they 

 estimated a mean mortality rate of 2.36/d, a 

 value rather close to Pepin's ( 199 1 ) model-pre- 

 dicted 2.02/d. Pepin ( 1991 ) had suggested that 

 species with short development times (i.e. high 

 temperatures ) may have lower than predicted 

 daily mortality rates because of their relatively 

 short exposure to predators. 



Mortality rates for bay anchovy eggs are 

 higher and perhaps more variable than rates 

 reported for eggs of other anchovy species. 

 Only the Peru anchoveta, Engraulis ringens, 

 has been reported to experience similar daily 

 egg mortality rates, a consequence of canni- 

 balism by adult anchoveta and of high preda- 

 tion rates by other pelagic fish (Smith et al., 

 1989). The reported stage-specific egg mortali- 

 ties of other anchovies, while high, probably 

 do not exceed 50% to 60% ( Dorsey and Houde 2 ) 

 and appear to be considerably lower than rates 

 estimated for bay anchovy. 



Instantaneous mortality rates for bay an- 

 chovy eggs also are higher than rates reported 

 for most teleost fishes (Houde, 1989b; Houde 

 and Zastrow, 1993). However, if stage-specific 

 mortalities are compared, rather than daily 

 rates, the bay anchovy resembles many fishes. 

 For example, Harding et al. ( 1978) estimated 

 that the mean mortality rate for plaice, 

 Pleuronectes platessa, eggs was Z = 0.075/d in 

 the Southern Bight of the North Sea at mean 

 temperature 5.65"C. Egg-stage duration of pla- 

 ice is about 22 days (Coombs et al., 1990). Thus, 

 the stage-specific egg mortality is Z nm = 1.65, equiva- 

 lent to 81%, and similar to that of bay anchovy 

 (Z =1.32) despite the more than 20-fold difference in 



cunt r 



hourly rates (0.003 for plaice, 0.066 for bay anchovy). 

 Yolk-sac larvae are essentially embryos free of the 

 chorion, and their expected mortality may be more 

 comparable to eggs than to feeding-stage larvae. Our 

 estimated mortality rates of bay anchovy yolk-sac 

 larvae ranged from Z = 0.41 to 4.24/d (33.5 to 98.5%/ 

 d). The mean rate was Z = 1.27/d (72.0%/d), a value 

 nearly equal to the egg-stage mortality rate. Because 

 the yolk-sac larval stage of bay anchovy is approxi- 

 mately one day in duration, the daily mortality rate is 



2 Dorsey, S. E., and E. D. Houde. 1992. Daily variability in 

 mortality of bay anchovy, Anchoa mitchilli, eggs in Chesapeake 

 Bay. Int. Counc. Explor. Sea, Pelagic Fish Committee, Coun- 

 cil Meeting 1992/H:17. 11 p. 



