FISHERY BULLETIN: VOL. 83, NO. 3 



length-frequency data collected during this study, it 

 is unlikely that very many P. montagui survive in the 

 Bay beyond age 4. No D. leptocenis females older 

 than age 2 were caught in the Bay although some 

 males remained until their third fall (age, 2 yr and 9 

 mo). Conclusions concerning longevity were com- 

 plicated by the fact that larger individuals of both 

 species migrated down the Bay into deeper water as 

 the winter progressed and were not captured and by 

 the difficulty of inferring age from length data for 

 the larger size-groups, particularly for P. montagui. 



Both species were similar with respect to growth 

 and migratory behavior. Growth decreased with in- 

 creasing age and was seasonal, i.e., rapid in the 

 spring and summer of the first year and the spring of 

 the second year and negligible in the fall and winter. 

 Males of both species reached 7-9 mm CL at age 1 

 and 11-13 mmCL at age 2. The data suggested that 

 early (age 1) transitional P. montagui also grow con- 

 siderably more rapidly than the remainder of their 

 age group which undergo sex transition at age 2. The 

 slowest observed growth rate was for P. montagui 

 which apparently begin life as females and therefore 

 never undergo sex transition. For D. leptocerus, 

 females grow more rapidly than males and differen- 

 tial growth is a "fixed" sexual attribute. For P. nwn- 

 tagui, the fact that the faster growing individuals 

 change sex a year earlier and therefore function as 

 females for an additional year (assuming that fast- 

 and slow-growing shrimp have identical lifespans) 

 means that more rapid growth and early sex transi- 

 tion increase the reproductive potential of the 

 population, as long as enough males remain in the 

 population to mate with the females. Female maturi- 

 ty is clearly a function of size, not age. 



The life cycle of P. montagui in Penobscot Bay was 

 quite different in several respects from the life cycles 

 of populations which have been studied in the North 

 Sea and at Grand-Riviere, Quebec. Pandahis mon- 

 tagui populations in two locations in the North Sea 

 (Mistakidis 1957; Allen 1963) appeared to grow more 

 rapidly in their first year of life than in Penobscot 

 Bay and were composed of a considerably greater 

 proportion of early maturing females, many of which 

 never functioned as males. Growth over the entire 

 lifespan was considerably more rapid in Penobscot 

 Bay and the North Sea than at Grand-Riviere 

 (Couture and Trudel 1 969b), but the relative scarcity 

 of females which do not function first as males and 

 the delay of most sex transition until the third year 

 were common to the Canadian and United States 

 populations. In addition to a difference in the timing 

 of maximum sex transition from age 2 (Penobscot 

 Bay and Grand-Riviere) to age 1 (North Sea), the 



seasonal intensity of sex transition was not the same 

 in three of the populations. Most transitionals were 

 observed in March in Penobscot Bay, in June in 

 Northumberland (Allen 1963), and in October at 

 Grand-Riviere (Couture and Trudel 1969b). Eggs 

 were carried by females in both North Sea locations 

 and in Penobscot Bay during the winter (November- 

 March); at Grand-Riviere most ovigerous females 

 were observed in October. 



In Penobscot Bay and the North Sea, sex transi- 

 tion tended to follow the end of the breeding season, 

 whereas in Grand-Riviere, maximum sex transition 

 coincided with the time when most females were 

 carrying eggs (unless sex transition was more com- 

 mon later in the fall when no samples were 

 collected), suggesting that there was a 12-mo inter- 

 val between the appearance of external female 

 characteristics and spawning at Grand-Riviere, and a 

 6-9 mo interval in Penobscot Bay and Northumber- 

 land. More rapid growth rates in the latter two loca- 

 tions would explain the shorter time intervals 

 between sex transition and spawning. The reproduc- 

 tive cycle in Grand-Riviere was seemingly con- 

 tinuous, beginning in July and ending in June 

 (Couture and Trudel 1969b). 



Although an offshore migration of larger male P. 

 montagui was observed in Penobscot Bay, Grand- 

 Riviere, and Northumberland, this migration oc- 

 curred in the winter following the end of the spawn- 

 ing season in the Bay and in spring and summer, 

 prior to spawning, in the other two locations. Similar 

 movements of larger females have been noted in 

 both North Sea populations in the fall. Unlike the 

 other migrations, the one observed in Penobscot Bay 

 was not a spawning migration and may instead have 

 been a response of older shrimp to declining winter 

 temperatures in the shallower waters of the upper 

 Bay. The departure of significant numbers of older 

 shrimp from the Bay could certainly affect any con- 

 clusions concerning the size or age structure of 

 either population and their estimated maximum 

 lifespans. 



ACKNOWLEDGMENTS 



The authors wish to acknowledge Frank Spencer, 

 Director of the Fisheries Technology Division, and 

 Penn E stab rook. Director of the Bureau of Marine 

 Development, both of the Maine Department of 

 Marine Resources (DMR), whose support made this 

 project possible. Thanks are also due to Mike Brown, 

 formerly a DMR employee and leader of this project; 

 to Mike Dunton, the captain of the RV Explorer and 

 to the late Paul DeRocher, captain of the RV 



232 



