182 



Fishery Bulletin 



1990 



100 

 50 



i 



5 80 



S 80 

 or 



u- 40 



g 80 



^y^ 



r^^. 



1- 

 80 

 40 



\L 



20 30 40 



SIZE(mm) 



APR 1978 '°° 

 N=141 50- 



JUL 1979 



SEP 1978 



N=29 40 



i 



APR 1979 80 

 N-97 40 



80 

 "n = 47 40 



SEP 1979 80 

 N=20 40 



DEC 1979 80 

 N=2211 



40 



DEC 1980 

 N=121 



AUG 1981 

 N-226 



ft- 



FEB 1982 

 N=929 



AUG 1982 

 N^416 



J^ -, 



--,JT-„ 



JUL 1980 



N = 70 10- 



rx 



iH 



JUL 1983 

 N:59 



Aug 1984 

 N=36 



JUL 1985 

 N=33 



20 30 40 



SIZE(mm) 



JUL 



Figure 4 



Ech inii rac/i » iiis pn rinii size-frcinKTioy 

 histograms piT ci)llectioii al station 

 15A, Middle Atlantic Bight. 



groups. The time series of population size-frequencies 

 available in this study could estimate poorly defined 

 cohorts and modal peaks by visual examination of the 

 overall modal patterns in the time series. 



Results 



The size-frequency distributions, grouped into 2-mm 

 size classes for all collections at each station, show a 

 high degree of variability in population structure (Figs. 

 2-5). The distributions for Station 1 1 , off Delaware Bay 

 (Fig. 2), show a population with a variety of size classes 

 evident in all collections; cohort definition is often 

 weak, largely because of small sample sizes for most 

 collections. Size-frequency distributions at Station 17. 

 off Central New Jersey (Fig. 3), in contrast to Station 

 11, show a very limited population, restricted entirely 

 to two annual recruitments in 1976 and 1978. Both of 

 the two cohorts at Station 17 appear to persist until 

 at least 1985 (the last collection of the study), although 

 they were inseparable after 1981. There is no evidence 

 of any additional recruits, even though the population 

 reached a size, >27 mm, when spawning should occur 

 (Ruddell 1977). The situation at Station 15A, off Fire 

 Island, is similar to that of Station 11, with several sizes 

 present in the population subsequent to initial collec- 

 tions (Fig. 4). There appear to be two recent strong 

 cohorts, that of 1978 and 1982. Some non-persistent 

 recruitment was evident in other years, however. The 

 E. parma population structure at station 23, on north- 

 east Georges Bank, was similar to that of Station 17, 

 with no evidence of a population prior to 1978 and with 

 the 1978 and 1979 cohorts the strongest, although 

 non-persistent recruitment was noted for other years 

 sampled. 



APR 1978 90p 

 N = 117 



80 



5 



FEB 1982 



SEP 1979 80-5 

 N=19 1,0 



10 20 



SIZECmm) 



Figure 5 



Er!u»in-i:chiii}is [iiirmii sizA'-frequency histograms per collection at 

 station 23. southwestern (ieorges Bank. 



The estimated growth curves (based on change in 

 cohort's modal size progressions over time) of persis- 

 tent and prominent cohorts at Stations 15A, 17 and 23, 

 were sigmoidal (Figs. 6-8). The highly variable size- 

 frequency distributions at Station 1 1 (Fig. 2) did not 

 produce reliable modal trends to use for growth esti- 

 mates. The mean cohort growth rates, approximated 

 from modal progression analysis for definable cohorts 

 ranged from 4.0 to 6.5 mm per year (Table 1). 



Table 2 summarizes the estimated annual production 

 (P), mean biomass (B), and P:B ratio for each cohort, 

 and the total annual population production and P:B 

 ratio for each station, showing much variability. A com- 

 parison of individual-cohort annual P:B ratios with 



