Steimie Population, growth, and production of Echinsrschnius parma 



185 



Figure 9 



Relationship between i;ir(.iduction (P) to biomass (B) ratios 

 (mean and ranges) and estimated age for all definable 

 Echhiarachnius parma cohorts from all stations, Middle 

 Atlantic Bight and Georges Bank. 



The period of peak recruitment varied slightly be- 

 tween the Middle Atlantic Bight stations and that of 

 Georges Bank. Recruitment was heaviest or most con- 

 sistent in the winter to early spring (December-April) 

 at Stations 11 and 15A (Figs. 2,4) which is consistent 

 with most reports of fall E. pnrmn spawning (Fewkes 

 1886. Cocanour and Allen 1967, Ruddell 1977). Recruit- 

 ment of the two cohorts found at Station 17 (Fig. 3) 

 iwurred prior to when sampling began, but the 1978 

 ^■ot\in was probably recruited in the winter of 1977-78. 

 (.''r: (.ux^rges Bank (Fig. 5), however, recruitment ap- 

 peared in all collections except those of September, and 

 most did not persist. Collie's (1987) observation of 

 yellowtail iloiinder selectively feeding on < 12 mm size 

 E. pcDDia on Georges Bank is relevant to the poor sur- 

 vival of most new cohorts there. 



The life span for persistent cohorts in the Middle 

 Atlantic Bight, estimated from recruitment in 1976 to 

 the end of the study, was at ieas,t 7-8 years, e.g., see 

 Figure 3. The presence of a few larger individuals, to 

 54 mm, suggests a maximum life span of perhaps 15-1- 

 years, given the probable very slow growth rates of the 

 larger/older segment of the population. However, max- 

 imum widths were generally below 40 mm which sug- 

 gests that most of the population did not live much 

 more than 8 years. At the Georges Bank station, 

 recruitment prior to the 1978 cohort was not evident, 

 thus reasonable estimates of maximum size, life span, 

 or other population variables were not possible because 

 collections there covered less than 5 years. 



The estimated life span of E. parma in the study area 

 is most likely less than the estimated 21 years reported 

 by Brykov (1975) for a Pacific population of this dis- 

 junctly trans-subboreal species, based on assumed an- 

 nual growth-ring counts to estimate age. However, 



individuals of E. parma have been measured that 

 generally exceed 80 mm (maximum reported is 92 mm) 

 in the northern Gulf of Maine (Lohavanijaya 1964, 

 Cocanour 1969), and if one uses a mean growth rate 

 of about 3 mm per year (range 1.5-4 mm/year) as deter- 

 mined from tagged, middle-sized (33-55 mm) indivi- 

 duals measured after 2 years in the Gulf of Maine 

 (Cocanour 1969), then 20 -i- year life spans could occur 

 in boreal waters. If annual ring counts to size estimates 

 of Cocanour (1969) alone were used, it appears a lesser 

 age would be reasonable because 60-70 mm animals 

 had only 6-7 growth rings. The reduced growth rates 

 of a cohort as it reaches senescence may not produce 

 distinguishable growth rings and juvenile rings may 

 also be weak or obscure, so ring counts could under- 

 estimate age. Also, Cocanour (1969) has suggested that 

 mature E. parma may alternate years of growth and 

 gametogenesis, at least in the Gulf of Maine; there is 

 no evidence of this alternate-year growth pattern in 

 the growth curves (Figs. 6-8). The maximum width 

 measured in the Middle Atlantic Bight was 54 mm. This 

 apparent difference in maximum size, and possibly life 

 spans, between populations in the temperate Middle 

 Atlantic Bight and the boreal Gulf of Maine suggests 

 a latitude or temperature-related size cline for this 

 species, as predicted for echinoids in general by Ebert 

 (1975). Other factors, e.g., food supply, genetic vari- 

 ability, differences in methodologies, or habitat or 

 predation pressure, could also be involved. 



Although the collection periods caused some prob- 

 lems in accurately defining the specific time of peak 

 recruitment, the quarterly-to-semiannual sampling 

 over at least the initial 5-year period of the study 

 was probably sufficient to estimate growth and produc- 

 tion. Parsons et al. (1977), for example, suggest the 



