STAGE IV PRODUCTION AND 

 SUBSEQUENT STOCK 



In an extensive series of observations, Scarratt (1964, 1973) 

 examined the relationship between stage IV production in 

 Northumberland Strait and subsequent stock size. Stock size 

 was lagged by 6 yr to account for the delay between spawning 

 and recruitment (Wilder 1953). Stock estimates were based 

 on tagging studies conducted off Miminigash, P.E.I. Wide 

 variability in growth rates may result in a single cohort re- 

 cruiting to the fishery over a 2-3 yr period (Wilder 1953); 

 accordingly, Scarratt (1973) related 3-yr running averages of 

 stage IV larval production and stock size. Scarratt concluded 

 that sampling variability prevented accurate prediction of 

 stock size based on larval production estimates. 



Scarratt (1964, 1973) restricted consideration to a linear 

 relationship between larval production and subsequent stock 

 size. However, density (stock) dependent effects may result 

 in a nonlinear functional relationship between larval produc- 

 tion and subsequent stock. To further examine this possibility, 

 a modification of the Ricker stock-recruitment model (Saila 

 and Lorda in press) was used to evaluate the relationship be- 

 tween 3-yr running averages of stage IV production (P) and 

 stock size (5) lagged by 6 yr. The generalized model of 

 Saila and Lorda (in press): 



S = aPPe 6P 



was employed in this analysis. The derived curve provided a 

 reasonable representation (r = 0.87; df = 12) of the stage IV 

 production-recruitment observations (Fig. 1). This model may 

 assume either a nearly asymptotic or convex form and there- 

 fore retains great flexibility in evaluating siock-rccruitmcni 

 relationships. 



Although the many sources of variability in estimating stage 

 IV density and stock size must be recognized, this analysis 

 does provide an indication of a relationship between stage IV 

 production and stock size which merits further investigation. 



Stock recruitment relationships have proven difficult to con- 

 clusively demonstrate for marine species. Variable survival 



8 3- 



- 



• 





• 





_ 









• 



.— — "— • 







^** 



• 



• • 





• 

 • 

 • 



7 











1 



R 



1 



2431.37 P'- 5 ™'.- 

 r = Q.87 



1 



.3566 P 



1 



1 



10 15 



STAGE IV PRODUCTION 



Figure I. — Relationship between stage l\ production tno 3.430 m : > and sub- 

 sequent stock si/e ( ltn 6 yr later. Three-year moving average emplo>cd for both 

 variables. 



rates for egg and larval stages caused by variability in critical 

 environmental factors tend to obscure any underlying parental 

 stock effect. During the lengthy incubation period typical of 

 many crustacean species, however, the protection afforded to 

 the eggs by brooding behavior of the female reduces mortality 

 and variability in survival rates. Perkins (1971) estimated an 

 average egg loss rate of 36% during incubation for the 

 American lobster; egg mortality would undoubtedly be con- 

 siderably higher if the eggs remained unprotected. 



Although adequate time series of stock and recruitment 

 estimates are not widely available for crustacean stocks 

 (Hancock 1973), recent studies provide evidence for stock- 

 recruitment relationships in two species. Boddeke (1981) 

 demonstrated asymptotic relationships between egg produc- 

 tion and subsequent harvestable stock of the European brown 

 shrimp, Crangon crangon, in four areas off the Netherlands 

 and Belgium. Morgan et al. (1982) reported an asymptotic re- 

 lationship between puerulus settlement and recruitment for 

 spiny lobster, Panulius cygnus, off Western Australia. 



The asymptotic form noted in each of the above investi- 

 gations indicates that recruitment in crustacean stocks may 

 be relatively stable over a wide range of parental stock sizes. 

 Recruitment curves of this type further imply that the primary 

 population regulatory mechanism is limitation of available 

 resources (e.g., food or habitat). Boddeke (1981) suggested 

 that spatial limitations on the nursery grounds may limit popu- 

 lation size. Morgan et al. (I982) cited limitations on food 

 and shelter sites as possible regulatory factors. Although 

 there is no convincing evidence of food resource limitation on 

 American lobster populations, shelter availability is an im- 

 portant feature of lobster habitat (Cobb 1971) and shelter 

 may be a limiting resource, particularly for juvenile lobsters, 

 w hich are more vulnerable to predation. 



CONCLUSIONS 



American lobster larvae have been collected most consis- 

 tently at the surface during daylight. Abundance apparently 

 declines with increasing depth. Although laboratory observa- 

 tions have indicated clear photonegative responses during 

 portions of several larval stages, few larvae have been obtained 

 in subsurface collections. The limited number of samples 

 collected at night do not permit definitive conclusions but do 

 suggest some dispersal from surface waters. 



Transport of larvae in surface currents has been widely as- 

 sumed (Templeman 1937, 1939; Templeman and Tibbo 1945; 

 Scarratt 1964, 1968, 1969, 1973; Rogers et al. 1967; Lund 

 and Stewart 1970; Harding et al. 1979) and higher larval densi- 

 ties along windward coasts tend to support this inference. 

 Vertical migration has been implicated in position-keeping in 

 response to w ind-induced turbulence and surface drift (Squires 

 1970; Caddy 1979), however direct evidence of a behavioral 

 mechanism of this type has not been observed. 



Prevailing southwesterly winds along much of the north- 

 eastern coast of the United States may result in transport of 

 larvae from offshore locations to coastal sites. Rogers et al. 

 (1968) cited a coastward transport of up to 6.4 km/d off 

 southern New England, implying a possible dispersal range of 

 approximately 100-150 km during a 20-25 d developmental 

 period. Larval transport from offshore areas (which have only 

 recently been exploited) may provide some degree of larval 

 recruitment to coastal populations. Fishing mortality rates in 



