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Fishery Bulletin 88(1), 1990 



sampling period for growth and production estimates 

 should be no greater than 10% of the generation time 

 (from egg to sexual maturity) of a species. Ruddell 

 (1977) reports E. panna reaches sexual maturity at a 

 width of about 27 mm in the Middle Atlantic Bight. This 

 width suggests an age of approximately 5 years from 

 the growth curve at Stations 17 and 15 A (Figs. 7 and 

 8); thus, semiannual sampling appears to meet this sug- 

 gested minimum. 



The slightly sigmoidal growth curves, for most prom- 

 inent cohorts (Figs. 6-8), differ from the more parabolic 

 growth curves reported for other sand dollar species 

 (Ebert and Dexter 1975, Lane and Lawrence 1980) and 

 sea urchins (Ebert 1982). However, Sime and Cranmer 

 (1985) and Nichols et al. (1985) report sigmoidal growth 

 curves in North Sea echinoid species. Some of the ir- 

 regularities in growth curves in the present study may 

 be due to a sigmoidal. intra-annual, seasonal growth 

 cycle that would have highest rates in the early sum- 

 mer and lowest in the early winter (Cocanour 1969). 

 These curves, based on defined cohort means, may be 

 somewhat imprecise because of the small sample sizes; 

 however, the modes are basically congruent with the 

 means and support the general shapes of the curves. 

 The apparent growth curves for Stations 17 (1976/77 

 cohort. Fig. 6) and 15A (1977/78 cohort. Fig. 7) are 

 almost congruent although the Station 17 (1977/78) 

 cohort has a lower slope, similar to that of the Station 

 15A (1981/82) cohort or that of the Station 23 (1977/78) 

 cohort curve. The cause of difference in these two sets 

 of slopes is unknown, at present. 



The mean annual growth rates (Table 1), estimated 

 by modal progression analysis, ranged from 4.0 to 6.5 

 mm per year and were greater than the average 1.5- 

 4.0 mm per year rates reported by Cocanour (1969), 

 based on tagged individuals, for an intertidal popula- 

 tion along the northern Maine coast. However, the 

 ratios of number of apparent growth rings to diameter 

 were generally higher, ranging from about 7.5 to 15 

 mm per ring. The rates estimated in the present study 

 are less, however, than the approximate 7 mm per year 

 from Woods Hole, Massachusetts (Durham 1955) or the 

 7.5 mm per year off northern New Jersey (Graef 1977), 

 both determined by growth ring counts for 45-48 mm 

 specimens. The diameter-to-ring ratio ranged from 4.3 

 to 11.3 mm per ring for eight smaller specimens in 

 Graef's study and appeared to peak at about 34 mm 

 with three rings evident. The size range examined by 

 these two studies is near the predicted size asymptote, 

 and senescence may be involved with zero or negative 

 growth which would underestimate the age and over- 

 estimate overall growth rates. Negative growth 

 (shrinkage), evident as slope declines on the right side 

 of curves found for some older cohorts, agrees with 

 similar findings by Cocanour (1969) for this species and 



for other sand dollars (Lane and Lawrence 1980). 

 Senescence is thought by these authors to be the cause 

 of this negative growth in older populations, although 

 Gordon (1929) and Cocanour (19(i9) rei)ort some shrink- 

 age in all age classes during winter months. There is 

 better agreement between the growth-ring count 

 estimates and the size-frequency estimates of growth 

 rate if it can be assumed that the relatively slow 

 juvenile growth, suggested by the sigmoid growth 

 curves and reported by Gordon (1929) and Highsmith 

 and Emlet (1986) for a Pacific population, may leave 

 an obscure ring for the first or perhaps even second 

 year of survival, thus lowering the size-age estimates 

 based on ring counts. On Georges Bank, the most per- 

 sistent or dominant cohort was the 1978 recruitment. 

 The growth patterns (Fig. 8) suggest a mean growth 

 rate of about 4 mm per year for the first 4 years of 

 this cohort, although there was some irregularity in 

 July 1979. This rate is slightly lower than most rates 

 for similiar cohorts at other stations. 



The overall, estimated annual cohort and population 

 production varied from station-to-station and from 

 year-to-year (Table 2), reflecting the variable dynamics 

 of recruitment, growth, and mortality of each popula- 

 tion. It is interesting to note that population produc- 

 tion peaked in the November 1979-October 1980 pro- 

 duction year at all three Middle Atlantic Bight stations 

 and later, during the May 1981 -April 1982 production 

 year for Georges Bank. The E. parma population at 

 Station 17 was the most productive for the period ex- 

 amined, and its total production peaked in 1979-80 at 

 8080 mg AFDW/m^ per year This production level (46 

 Kcal/m- per year), when converted to energy equiva- 

 lence (24 KJ or 5.7 Kcal/g AFDW; Steimie and Ter- 

 ranova 1985), is greater than the total benthic com- 

 munity |)roduction reported in many North Atlantic 

 continental shelf areas (table 5, Steimie 1985); even the 

 mean value for this station, 29 Kcal/m- per year is 

 comparable to some total community values. The high 

 production at Station 17 is probably related to the 1976 

 anoxia episode, mentioned previously. 



The P:B ratios (Table 2) varied from negative values 

 for older cohorts to a maximum of about 8.1 for some 

 juvenile cohorts at Station ISA. A comparison of mean 

 cohort P:B ratios against approximate cohort age (Fig. 

 9) for all stations comliined shows a skewed normal 

 curve, with mean ratios per age group ranging from 

 a maximum of about 2.4 for 1-2 year-old cohorts to less 

 than 0.5 for cohorts older than 5 years. The declining 

 part of this curve appears to be common in other 

 marine organisms, e.g., Warwick (1980). The low P:B 

 ratios for 0-1 year-old cohorts could suggest a relative- 

 ly slow growth of young-of-the-year juveniles (Figs. 

 6-8). The apparently low initial ratios could also be in- 

 fluenced by accurately establishing time of recruitment. 



