Steimie Population, growth, and production of Echinsrachnius parms 



183 



Figure 6 



Growth curves from the shift of the mean widths of prom- 

 inent Echuiarachnius parma cohorts, over time, at sta- 

 tion 17. Middle Atlantic Bight. 



Figure 7 



Growth curves from the shift of the mean widths and 

 ash-free dry-weight values of tlie prominent 1977-78 

 Echbmrachniiis parma cohort, over time, at station 15A, 

 Middle Atlantic Bight. 



Figure 8 



Growth curves from the shift of the mean widths of the prominent 

 1978 Echinarachmus parvia cohort, over time, at station 23, south- 

 western Georges Bank. 



estimated cohort age (Fig. 9), based on the modal pro- 

 gression analysis or estimated growth rates, for all sta- 

 tions combined produced a skewed normal curve. 



Discussion 



The population dynamics of E. parma, with variable 

 annual recruitment and cohort survival patterns (Figs. 

 2-5), are in general agreement with population dyna- 

 mics reported for other echinoderms (Ebert and Dex- 

 ter 1975, Lane and Lawrence 1980, Ebert 1983, Beu- 

 kema 1985). Recruitment is evident as peaks in the 0-2 

 mm size-frequency columns of Figures 2-5. Station 17 

 (Fig. 3) was the exception, however, to the almost 

 annual or frequent recruitment pattern; detectable 

 recruitment was evident there in only 2 years. This 

 recruitment pattern may be associated with anoxic con- 

 ditions that defaunated this area during the summer 

 of 1976 (Steimie and Radosh 1979). The definition of 

 the first definable cohort at this station in July 1977 

 included some animals larger (to 12 mm) than the 6-mm 

 estimated maximum annual growth rate, although the 

 size-frequency mode was about 6 mm for the initial July 

 1977 collection (Fig. 3). It must be assumed any 

 preanoxia recruitment in 1976 did not survive the 

 event, as few living E. parma were found in the area 



