66 



Fishery Bulletin 102(1) 



anchovy. Modal lengths of young-of-the-year (YOY) bay 

 anchovy cohorts were determined from length-frequency 

 distributions in MWT catches and a modal analysis (Bhat- 

 tacharya, 1967; King, 1995). Based on the modal analysis 

 of summer and fall survey data, the maximum TL of YOY 

 bay anchovy and, therefore, the minimum TL of spawners, 

 were estimated (Table 2). 



Length-dependent gear selectivity for bay anchovy was 

 adjusted by comparing catches of the MWT and a 2-m 2 

 Tucker trawl with catches from 707-iim meshes at the 

 same stations during a September 1998 baywide survey. 

 The length-specific MWT:Tucker-trawl catch ratios (N^^j/ 

 iVj^catch per unit of effort MWT 4- catch per volume of 

 water Tucker trawl) for anchovies 21-70 mm TL indicated 

 that both gears fished with a consistent selectivity for bay 

 anchovy of 30-48 mm TL, and with a slight decrease in N TT 

 for 48-70 mm TL. However, the values ofN MWT IN TT were 

 lower by factors of 1-7 for 21-30 mm TL fish, indicating 

 that small anchovies were collected less efficiently by the 

 MWT. We concluded that length classes of anchovies >30 

 mm TL were equally vulnerable to the MWT and those >48 

 mm TL were less vulnerable to the Tucker trawl. Accord- 

 ingly, we adjusted MWT catches of ^30 mm TL anchovy 

 by multiplying them by a weighting factor estimated from 

 the regression of values of iV MH , T /./V. r7 . for 21-30 mm TL 

 bay anchovy. 



( Weighting factor) = -0.59 TL + 19.08, (r 2 =0.96) 



where TL = total length. 



The weighting factor equals 1.0 for anchovy >30 mm TL 

 because MWT selectivity is constant for anchovy >30 mm 

 TL. To estimate water sampled in a 20-min MWT tow, 



and 



where D« 



d n = n mwt/ v mwt = ( 1/s ' x Nt/Vtt 



MWT — ^ x ^-^ MWT TT x TT  



bay 



N, 



MWT 



N 



77' 



the concentration of 31-48 mm TL 

 anchovy at a station (i.e. number/m 3 ); 

 the number of 31-48 mm TL bay anchovy 

 collected per 20-min MWT tow at a station; 

 V MWT = the effective water volume sampled 

 by a 20-min MWT tow (m :! ); 

 the number of 3 1-48 mm TL bay anchovy col- 

 lected by the 2-m- Tucker trawl at the same 

 station; 



vulnerability to the Tucker trawl (s=l if all 

 bay anchovies in water volume, V^, are col- 

 lected); and V TT is the volume filtered by the 

 Tucker trawl (m 3 ) estimated from a flowme- 

 ter in its mouth. 



The mean of Af WHT /./V 7T for 30-48 mm TL bay anchovy 

 during the September 1998 survey indicated that V' WUT = 

 4961 m\ if 30-48 mm TL bay anchovy did not significantly 

 avoid the mouth of the 2-m 2 Tucker trawl (i.e. s=l). Assum- 

 ing s=l (i.e. V MVVT =4961 m 3 ), we estimated "relative" bay- 



wide abundance and biomass of YOY and spawners for the 

 18 surveys from 1995 to 2000. 



To coarsely estimate a typical value of s. "absolute" bay- 

 wide spawner biomasses in June— August were estimated 

 for 1995-2000 according to an egg production method 

 (Parker, 1985; Rilling and Houde, 1999a). Bay anchovy 

 eggs had been collected in a 1-m 2 Tucker trawl during the 

 same surveys and provided estimates of egg abundance. 

 The coverage of stations and sampling design for the 

 Tucker trawl was comparable to that of the MWT, but the 

 Tucker trawl was deployed during both day and night. We 

 presumed that all eggs collected between 00:00 and 20:00 

 h had been spawned near a midnight peak 1 00:00 h) (Za- 

 strow et al., 1991) and decreased in abundance at a mean 

 instantaneous mortality (reported for bay anchovy eggs 

 in Chesapeake Bay as M = 0.066/h; Dorsey et al., 1996). 

 Based on the estimated number of eggs spawned at 00:00 

 h for each station, the regional mean weight of individual 

 spawners (defined by the minimum TL in Table 2) in MWT 

 catches, and the reported fecundity-weight relationship for 

 females (Zastrow et al., 1991), we were able to coarsely 

 estimate "absolute" baywide spawner biomass. We as- 

 sumed that the spawning fraction of adult females per day- 

 was essentially 1.0 (i.e. all adult females participated in 

 spawning, Zastrow et al., 1991) and the fecundity-weight 

 relationship was constant over years. 



Comparison of the baywide estimates of spawner bio- 

 mass in June-August based on the egg production method 

 ("absolute" biomass) with estimates based on the MWT 

 catch-per-unit-of-effort ("relative" biomass) indicated that. 

 on average, for 1995 to 2000, s is equal to 0.20. Therefore, 

 the mean effective water volume fished by a 20-min MWT 

 tow was 4961x0.20 = 989 m 3 . 



