STEVENSON and PIERCE: PANDALUS MONTAGUI AND DICHELOPANDALUS LEPTOCERUS 



females prior to the subsequent breeding season, 

 assumed external female characteristics during the 

 winter and spring. The appearance of a distinct size- 

 group of 9-11 mm CL females in late January (Fig. 

 3F) which was not present 2 mo earlier (Fig. 3B), 

 suggests that transition was well underway by late 

 January. The two size-groups of non-ovigerous 

 females in January and March (Fig. 31) were as- 

 sumed to belong to the same age-group, the smaller 

 females being those which did not function as males 

 in their first year (they may have started life as 

 females) and the larger females being those which 

 were still males in November-December (Fig. 3A). 

 Transitional shrimp made up a larger percentage of 

 samples collected in moderate and deep waters and 

 in areas 3 and 4 (Tables 7, 8). 



Transition of the younger age-group which was 

 first captured in November-December (Fig. 3A) was 

 incomplete since a great many shrimp remained as 

 males for another entire year before undergoing 



Table 7.— Percent total number of male, transitional, and 

 female Pandalus montagui collected at all depthis and times 

 of year in four areas in Penobscot Bay during 1980-81. 

 (Females are categorized by reproductive stage.) 



Table 8. — Percent total number of male, transitional, and 

 female Pandalus montagui collected in all areas and times of 

 year by deptfi range in Penobscot Bay during 1980-81. 

 (Females are categorized by reproductive stage.) 



transition during their third spring (at age 2). Given 

 the fact that a few male and transitional shrimp > 15 

 mm CL were sampled at various times of year, the 

 possibility that a few individuals do not change sex 

 until their fourth year (age 3) could not be ruled out. 

 Even though the transition of younger (age 1) 

 shrimp in the spring was incomplete, a sizable 

 number of non-ovigerous Stage I shrimp which com- 

 pleted transition in their first year were collected in 

 March (Fig. 3J). These new females were consider- 

 ably larger (by about 2 mm CL) than their male 

 counterparts, suggesting that it was the larger, 

 faster growing, individuals which underwent transi- 

 tion at age 1. Allen (1963) also reported that the 

 largest age-group males changed sex first. 



Older shrimp which changed sex in their second 

 year had not yet appeared as females in March (Fig. 

 3J), suggesting that sex transition in older shrimp 

 was delayed; it may also have been less rapid, par- 

 ticularly since growth was considerably reduced 

 after the first year. Earlier studies of P. montagui in- 

 dicated that sex transition in two locations in the 

 North Sea persisted for most or all of the year. 

 Mistakidis (1957) noted that sex transition occurred 

 from May to December in the Thames estuary 

 whereas Allen (1963) collected transitional in- 

 dividuals throughout the year in Northumberland, 

 but primarily in June. The timing of minimal sex 

 transition in Penobscot Bay and Northumberland 

 was identical (November-December); Allen (1963) 

 reported that this was when males were sexually ac- 

 tive. Couture and Trudel (1969b) reported that most 

 sex transition occurred in October at Grand-Riviere; 

 a few transitionals were collected in July and August 

 but none in May and June. Sex transition in the 

 North Sea, as reported by Mistakidis (1957) and 

 Allen (1963), was accelerated in comparison with 

 Penobscot Bay; some individuals changed sex in 

 their first year, but most did so in their second year. 

 Ages at sex transition at Grand-Rivifere, on the other 

 hand, were the same as in Penobscot Bay, i.e., some 

 in their second year, most in their third year, and 

 some in their fourth year. 



Seasonal Changes in Size (Age) Composition 



Two size-groups of male D. leptocerus were ob- 

 served in the Penobscot Bay during the winter (Fig. 

 2 A, E, G) and only one in the summer (Fig. 2K); 

 similarly, female length-frequency distributions in 

 November-December were bimodal (Fig. 2B), but a 

 single size-group was dominant in the summer (Fig. 

 2L). Presumably, most of the older (age 2) males 

 either die or migrate out of the upper Bay in the 



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