Rilling and Houde: Variability in growth and mortality of Anchoa mitchil/i 



565 



plii/L. In the present study, nauplii densi- 

 ties often exceeded that level in July, but 

 levels were lower and potentially limiting 

 during June, especially in the mid bay 

 (mean = 9.7 nauplii/L) and lower bay 

 (mean=17.7 nauplii/L). 



During each month, fastest gi'owth rates 

 were estimated for larvae from the upper 

 bay where copepod nauplii were most 

 abundant (means=84.8 nauplii/L in June 

 and 157.5 nauplii/L in July). Lowest 

 growth was estimated during June in the 

 mid and lower bay, where copepod nauplii 

 densities and mean temperatures were 

 lowest. Although temperature alone may 

 exercise an important control over lar\'al 

 growth, (Houde, 1989a: Pepin, 1991), a 

 combination of factors, including prey 

 availability, effects of body size, or growth- 

 rate dependent mortality, may operate to 

 control growth rates and survival poten- 

 tial (Bailey and Houde, 1989; Heath, 1992; 

 Leggett and DeBlois, 1994). In the case of 

 bay anchovy, all of these factors may oper- 

 ate, but temperature and prey level appar- 

 ently predominate. There also was an effect 

 of body size; growth rate of larvae >13 mm 

 declined. In a synthesis analysis, Houde 

 (1997b) reported that average weight-spe- 

 cific growth coefficients of bay anchovy de- 

 clined progressively from 0.573 ( 77.3T/d ) in 

 newly hatched lawae to 0.065 l6.7'J/d) in 

 near-metamorphosis individuals. 



Our baywide instantaneous daily mor- 

 tality rates decreased from 0.41 in June 

 to 0.23 in July 1993. This decline was co- 

 incident with increasing growth rate, sug- 

 gesting that cohorts of rapidly growing 

 larvae in July might have been less vul- 

 nerable to size-selective or growth-rate 

 dependent predation. From length-specific 

 and age-specific analyses of mortality, it 

 was clear that mortality rates were great- 

 est for the smallest and youngest lai"vae 

 (Table 3). Houde (1997b), analyzed the accumulated 

 data on bay anchovy larvae from Chesapeake Bay 

 and demonstrated that mortality rate (A/) declined 

 predictably with respect to body weight raised to the 

 -0.318 power In the present study, mortality from 

 the egg to 3-day-old larval stage was 2 to 7 times 

 higher than mortality from the 3- to 10-day-old lar- 

 val stage. Interestingly, mean baywide mortality 

 rates for the egg to 3-day-old stage (yolksac and first- 

 feeding larvae) were similar in June and July (82'7f/d 

 in June, 797c/d in July), but mortality rates for the 





o 

 O 



Log Jellyfish biovolume (mL/m') 



25 



25,5 



26 



26.5 



27 



Temperature (°C) 



Figure 8 



Relationship between I A) mean regional bay anchovy larval abundance 

 and gelatinous predator biovolumes and ( B i mean regional bay anchovy 

 larval growth rates and temperature at 3-m depth in Chesapeake Bay. 

 June and Julv 1993. 



10 to 18-day-old larval stage were considerably lower 

 in July (34'/f/d in June, I27c/d in July), implying that 

 conditions had become more favorable for feeding- 

 stage larvae in July. Baywide cumulative mortality 

 rates for egg to 18-day-old larvae indicated that 

 <0.1'7f of a daily cohort survived to 18 days after 

 hatching in June and that -1.6% survived to 18 days 

 in July 



Predation is a major cause of mortality in the early 

 life of marine fishes (Leggett, 1986; Bailey and 

 Houde, 1989; Leggett and DeBlois, 1994) and may 



