Dorsey et al.: Cohort abundances and mortality of Anchor mitchilli 



259 



Collections 



Collections were made from the research vessel RV 

 Henlopen during the periods 2-9 July and 18-24 July 

 1991, the peak of the spawning season for bay an- 

 chovy. The lower Chesapeake Bay (sites 1-4) was 

 surveyed on the first cruise and the mid-Bay (sites 

 5-7) was surveyed on the second cruise (Fig. 1). 



Anchovy eggs and larvae were collected in a 40- 

 cm diameter plankton net with 280-^m meshes. A 

 flowmeter in the net mouth was used to determine 

 the volume of water sampled. Vertical lifts of the net 

 from near bottom (never deeper than 25 m) to the 

 surface were made at 1 m/sec. The mean volume 

 sampled in 312 net lifts was 3.00/m 3 (SD=1.38/m 3 ). 

 Upon collection, samples were immediately preserved 

 in 5% formalin-seawater. At each experimental site, 

 sampling began between 0000 and 0400 h local time, 

 shortly after peak spawning had occurred on each 

 day. Duplicate samples were collected at 2-h inter- 



2.0 40 



Kilometers 



6 



Figure 2 



Vessel tracks from two surveys of bay anchovy egg and 

 larval abundance in Chesapeake Bay. Samples were col- 

 lected every 2 hours on the vessel track. (A) Track for 

 site 6, 19 July 1991. (B) Track for site 3, 5 July 1991. 

 The locations of sites are illustrated in Figure 1. 

  = start; • = end. 



vals until approximately 2100 h. By sampling at 2-h 

 intervals we documented declines in abundances of 

 eggs and yolk-sac larvae from which mortality rates 

 of daily cohorts were estimated. In this way, the level 

 of mortality and its variability were determined for 

 12 daily cohorts. 



Abundances of gelatinous predators were esti- 

 mated from collections in a 60-cm, 280-jUm mesh, 

 opening and closing net. Collections were made in 

 partitioned oblique tows: 1 ) from near-bottom to the 

 pycnocline; and 2) from the pycnocline to the sur- 

 face. They were made -12 times daily adjacent to 

 the drifting mesocosm array and concurrent with 

 collections of anchovy eggs and larvae. Total 

 biovolumes of C. quinquecirrha andM. leidyi >10 mm 

 diameter in each sample were measured immediately 

 upon collection. 



Salinity, temperature, dissolved oxygen, and chlo- 

 rophyll a fluorescence were determined from surface- 

 to-bottom casts of a conductivity-temperature-depth 

 (CTD) probe. Mean values during each 24-h experi- 

 ment were based upon three to five CTD casts. Posi- 

 tion data for plankton samples and CTD casts were 

 logged by the vessel's navigation system to delineate 

 drift tracks of the mesocosm deployments and the 

 locations of sampling stations. 



In the laboratory, fish eggs and larvae (yolk-sac 

 and older) were removed from all samples. Eggs were 

 identified and counted. Yolk-sac larvae were sorted, 

 identified, and separated on the basis of >6 samples 

 from each day's collections. 



Settled volumes of zooplankton from the 40-cm net 

 collections, determined from a minimum of three 

 samples each day, were recorded and expressed as 

 mL under 1 m 2 of bay surface. The zooplankton in 

 the 280-jUm mesh net potentially represented prey 

 of adult bay anchovy, rather than prey of first-feed- 

 ing larvae. 



Egg and yolk-sac larval abundance 



Densities (number/m 3 ) and abundances (number 

 under 1.0/m 2 ) of anchovy eggs and yolk-sac larvae in 

 each sample were estimated. Duplicate samples, col- 

 lected between 0000 and 0600 h, were examined to 

 determine the sample pair on each day which had 

 the maximum abundance of eggs or yolk-sac larvae. 

 Those samples provided an estimate of the mean "ini- 

 tial abundance" for each experiment. The decline in 

 abundance over the next 18 to 20 h was a measure of 

 mortality. 



Some yolk-sac larvae are extruded through 280- 

 /jm meshes. However, abundances of yolk-sac larvae, 

 which are of uniform length (approximately 2.0 mm), 

 were not adjusted for extrusion in this analysis be- 



