STEVENSON and PIERCE: PANDALUS MONTAGUI AND DICHELOPANDALUS LEPTOCERUS 



considerably higher, i.e., 3°C west of Islesboro in 

 May 19706 and 11°-12°C at various stations in the 

 lower Bay in August 1976''. In Penobscot Bay, P. 

 montagui were smaller after their first year's growth 

 than in the two North Sea locations (Mistakidis 1957; 

 Allen 1963) but attained approximately the same size 

 by the end of the second year. Males in the Thames 

 estuary measured 10 mm average CL by November 

 of their first year, and in Northumberland (at 40-60 

 m depth) they averaged 9.5 mm CL by October; tran- 

 sitionals reached 12.5 mm CL by November of their 

 second year in both locations as compared with 

 11-13.5 mm CL males of the same age in Penobscot 

 Bay (Fig. 3A), while females in Northumberland 

 reached 14.8 mm CL by November of year 2 as com- 

 pared with 13-15 mm CL at the same age in 

 Penobscot Bay (Fig. 3C). One-year-old females in 

 Northumberland averaged 10.8 mm CL in October. 

 Allen's (1963) explanation for this difference be- 

 tween male and female lengths-at-age was that 

 shrimp which mature as females in their first year do 

 so 3-4 wk after males of the same age-group; since 

 growth virtually ceases in the fall and winter, the dif- 

 ference in length attained by the females in the first 

 year is maintained into the third year of life. In 

 Penobscot Bay, on the other hand, very few females 

 mature in their first year; however, if males mature 

 in their first year (this was not determined) and stop 

 growing in the fall before the females, Allen's ex- 

 planation might apply. It seems more likely that sex 

 transition is a function of size, not age, and that the 

 faster growing age-group shrimp complete sex 

 transition in their first year. Another possible ex- 

 planation for the difference in size of females which 

 change sex in their second and third years is that 

 there may be two distinct periods of larval produc- 

 tion and/or survival. Length-frequency data collected 

 at two different periods during the winter (Fig. 3C, 

 G) did suggest that older females may have spawned 

 earlier than younger females. A 5-yr study of P. 

 borealis in the Sheepscot River of Maine* failed, 

 however, to reveal any consistent bimodality in lar- 

 val production during February-April even though at 

 least two age-groups of ovigerous females are com- 



«Muirhead, C. R., and J. H. Wartha. 1971. Temperature- 

 salinity observations, Penobscot Bay, Maine, 1970. Open Data 

 Rep. NOS DR-13, U.S. Dep. Commer., NOAA, Natl. Ocean Surv., 

 Off. Mar. Surv. Maps, Oceanogr. Div., Descr. Oceanogr. Sect., 

 Rockville, MD. 



'Central Maine Power Co., unpublished data, courtesy Richard 

 Birge, Environmental Studies Department, CMP, August, ME. 



*Stickney, A. P. Environmental physiology of northern shrimp, 

 Pandalus borealis. Maine Dep. Mar. Resour., West Boothbay Har- 

 bor, ME, Annu. Rep. 1981-82, 15 p. 



monly observed in commercial catch samples^. We 

 contend, therefore, that the most plausible explana- 

 tion for differences in the sizes of shrimp which 

 become females in their second and third years is a 

 difference in growth rates, especially since a similar 

 difference in size was observed between male and 

 female D. leptocerus in which sex remains fixed 

 throughout life. 



Pandalus montagui which either began life as 

 females or became females early in their first year 

 were smaller than their male counterparts by 

 November-December of their first year (Fig. 3A, B). 

 It therefore seems probable that the smaller Stage I 

 (6.5-8.5 mm CL) females captured in late January 

 (Fig. 3F) did not grow as rapidly as the larger 

 females in the same age-group which apparently 

 completed sex transition in late December and early 

 January or as shrimp which remained as males for 

 the entire year (Fig. 3D). These differences in 

 length-at-age between age-group males and both 

 groups of females were also evident in late March 

 (Fig. 3H, J). The accelerated growth rate of age- 

 group individuals which changed sex during their 

 first year contrasts with the reported faster growth 

 of P. montagui in the North Sea and at Grand- 

 Rivifere which began life as females (Allen 1963; 

 Couture and Trudel 1969b). 



Winter Migration 



During November-December 1980, younger male 

 and non-ovigerous female D. leptocerus were 

 predominant in shallow water in area 1 (Figs. 4A, 

 5A) while the older age group predominated in 

 deeper water (Figs. 4B, C, 5B, C). By late January, 

 the older females were no longer being caught, and 

 the older males had disappeared completely from 

 depths < 50 m (Fig. 4D, E), but accounted for about 

 50% of the males collected in deep water in area 3 

 (Fig. 4F). There were older males in areas 2 and 3 

 (moderate and deep water) in March (Fig. 4G, H). 

 These results suggested that older male shrimp 

 migrated down the Bay into deeper water as the 

 winter progressed and as bottom water tempera- 

 tures dropped from about 8°C in early December to 

 1°C in late February and early March in the upper 

 Bay.i° Their disappearance from the catches. 



9Diodati, P., S. H. Clark D. Mclnnes, R. Tichko, and D. 

 Sampson. 1983. Gulf of Maine northern shrimp stock status 

 -1983. Northern Shrimp Technical Committee, November 1983, 9 



P- 

 i°Birge, R. P. 1982. Surface and bottom water temperatures, 



upper Penobscot Bay, Maine, March 1975 - December 1981. Cen- 

 tral Maine Power Co., Environmental Studies Department, Report 

 SI-82-3, 45 p. 



229 



