AVERAGE MONTHLY BOTTOM 

 TEMPERATURES 



The distribution of recaptured tagged lobsters by 

 month and grouped by 6-minute squares against average 

 bottom water temperatures (°C) from Colton and Stod- 

 dard (1973) are presented in Figures 34-45. Bottom 

 isotherms are plotted from data collected during the 

 period 1940-66. Only recaptures whose month and loca- 

 tion of recapture are known (N = 584) are plotted. 



Relating lobster distribution to average bottom water 

 temperatures, it is apparent that the offshore lobster 

 population generally maintains itself within a 

 temperature regime of 8°-14°C. The two apparent excep- 

 tions to this generalization, evident in Figures 36 

 (March) and 37 (April), are predictable. Bottom 

 isotherms represent average temperature conditions for a 

 26-yr period, and the temperature conditions for a given 

 month vary considerably from year to year and within a 

 given month (Colton and Stoddard 1973; and 

 Chamberlin 2 ). 



During the first quarterly period, January through 

 March, offshore lobsters are distributed along the outer 

 continental shelf and upper slope (Figs. 34-36). Bottom 

 water temperatures during this period ranged from 8° to 

 12°C (Colton and Stoddard 1973; Chamberlin 2 ). In con- 

 trast, the inshore, shallow-water lobster populations are 

 in a state of reduced activity in coastal waters of 0°-4°C 

 (Cooper et al. 1975). 



During the second quarterly period, April through 

 June, the onset of shoalward migration has begun, oc- 

 curring first (April and May) in the western half of the 

 shelf (Figs. 37, 38) and next (June) in the eastern half of 

 the shelf (Fig. 39). Bottom water temperatures in the 

 latter half of May along southern Long Island, Block 

 Island Sound, and Buzzards Bay are 8°C and warmer 

 (Colton and Stoddard 1973). An intensive fishery for 

 lobsters occurs along southern Long Island from late May 

 through mid-July, directed primarily toward the onshore 

 migrants emanating from Hudson to Veatch Canyon 

 (Cooper and Uzmann, unpubl. studies). Lobster 

 migrations into the southern Long Island coastal waters 

 are evident from Figures 6, 15, and 16. 



Figures 40-43 (July-October) demonstrate that the 

 offshore lobster population is widely distributed over the 

 southern New England continental shelf, including the 

 shoal waters of Georges Bank and the coastal waters of 

 Long Island, Rhode Island, southern Massachusetts, and 

 Cape Cod. Bottom water temperatures in areas of ap- 

 parent lobster abundance during July-September are 8°- 

 14°C. 



The return migration to the outer shelf-upper slope 

 waters probably begins in August (Fig. 41) and continues 

 through September, October, and November (Figs. 42- 



"Chamberlin, J. L. Bottom temperatures on the continental shelf and 

 slope south of New England during 1974. In J. Goulet (editor), Environ- 

 ment of the United States living marine resources — 1974, p. 18-1 to 18-7, 

 figs. 18.1-18.6 (NMFS unpubl. manuscr.) 



44). Migration to deep water first occurs in the western 

 half of the shelf and then in the eastern half. 



During the first month (October) of the last quarter 

 (October-December) there are still some lobsters dis- 

 tributed over the shoals of Georges Bank and immediate- 

 ly south of Nantucket Island (Fig. 43) with bottom water 

 temperatures of 10°-14°C. By December the offshore 

 lobster population is again distributed along the outer 

 continental shelf and upper slope waters (Fig. 45) where 

 bottom temperatures are 8°-12 c C. 



CONCLUSIONS 



The distribution of tag returns from a 4-yr tagging and 

 recapture study has demonstrated that at least 20% of 

 the offshore lobster population moves into shoal water in 

 the spring and summer and returns to the outer shelf and 

 upper slope by early winter. This migratory behavior 

 appears to be motivated by temperature, as the seasonal 

 distribution of tagged lobsters according to depth is well 

 correlated with bottom temperature. The extensive 

 seasonal migrations undertaken by offshore lobsters con- 

 trast sharply with the localized movements of coastal 

 stocks. This apparent difference may be partially ex- 

 plained by the very high exploitation rate inshore such 

 that most lobsters of recruit size are quickly harvested 

 within the bounds of locally intensive fisheries. 



Whether the offshore stocks are genetically distinct 

 from the coastal stocks has not been established, but it is 

 evident that the shelf edge and upper slope is a perma- 

 nent habitat from which small- and large-scale excur- 

 sions are made with seasonal regularity. We believe that 

 the continental slope habitat lacks sufficiently high 

 temperatures during the summer to promote extrusion of 

 eggs, molting, and subsequent mating, and that the 

 deficiency is compensated by seasonal shoalward migra- 

 tion to warmer water. In situ observations of offshore 

 lobsters from the research submersible Nekton Gamma 1 

 at Corsair, Lydonia, Oceanographer, Hydrographer, and 

 Veatch canyons during June-July of 1973 and 1974 sub- 

 stantiate this belief. Evidence of lobster molting (shed 

 exoskeleton) was observed only at depths shoaler than 

 100-110 fathoms (183-201 m), whereas lobsters were dis- 

 tributed to depths of at least 170 fathoms (311 m). 



The magnitude of variation in depth at recapture by 

 month suggests that the migration toward shoal water is 

 not a total population response, nor is it likely a well- 

 coordinated one. We hypothesize that some lobsters 

 migrate early, some late, and some not at all. Superim- 

 posed upon these variations in migratory behavior is an 

 apparent tendency of some lobsters to move laterally east 

 or west along the outer shelf and upper slope. Hence, the 

 concept of discrete canyon populations is unlikely. 



SUMMARY 



1. This report has presented the results of an offshore 

 lobster tag and recapture study to define the seasonal 



Research submersible operations provided by NOAA's Manned Under- 

 sea Science and Technology Office. 



62 



