JAMIESON and PHILLIPS: OCCURRENCE OF CANCER CRAB MEGALOPAE 



130 days (range: 89-143) by Lough (1976), 45 and 

 108 days at 17.8° and 10.0°C, respectively, by Reed 

 (1969), and 128-158 days under natural conditions 

 by Poole (1966). In laboratory studies, Poole (1966) 

 found the total time required was 111 days at 

 10.5°C. Since water temperatures typically range 

 from about 8° to 13°C off the British Columbia- 

 California coast during January-February (Thom- 

 son 1981), the total larval period in the study area 

 is assumed to be about 110 days. 



Off Vancouver Island, megalopae can be collected 

 in abundance in open coast waters from mid- April 

 to August. Ovigerous Dungeness crab, collected at 

 Tofino and held in ambient temperature seawater 

 at Nanaimo, hatched their eggs from January to 

 March, with most hatching occurring in February 

 (G. Jamieson, unpub. data). This suggests that mega- 

 lopae collected between April and June could be 

 from local populations whereas most megalopae 

 collected in July and August may have largely 

 originated elsewhere. 



Lough (1975) stated that the hatching period of 

 C. oregonensis off Oregon extends from January to 

 July, with two broods indicated, one primarily hatch- 

 ing in February and the other in May-June. He 

 estimated larval duration of a brood at 155 days 

 (range: 123-203 days) from field sampling, but there 

 was possible intermixing of the larvae from differ- 

 ent broods, as well as population variability in the 

 timing of hatching. We have no data on the hatching 

 period of this species in British Columbia. 



Given the currents off the west coast of Vancouver 

 Island, then, Cancer larvae present there in the 

 spring could theoretically have originated anywhere 

 between northern California to perhaps southern 

 Alaska. It seems very unlikely that the larvae are 

 entirely the progeny of adult crabs on the west coast 

 of Vancouver Island. 



Onshore Movement 



Geographical location of larval settlement is very 

 dependent on currents. Adult Cancer of both species 

 are largely found in nearshore, shallow-water 

 habitats (Hart 1982), and such environments are 

 apparently both the origin and preferred destina- 

 tion of larval crabs (Butler 1956). Along the open 

 coast, Dungeness crab larvae are known to settle 

 in both estuarine and nearshore areas (Wild and 

 Tasto 1983; Stevens and Armstrong 1984; Arm- 

 strong and Gunderson 1985). In waters largely sur- 

 rounded by land, such as Puget Sound, Georgia and 

 Queen Charlotte Straits, and, to a lesser extent, 

 Hecate Strait, Dixon Entrance, and southeastern 



Alaska, larval crabs may perhaps remain nearshore 

 throughout their entire developmental period. How- 

 ever, studies suggest that while larvae may be 

 hatched nearshore along the outer coast, they subse- 

 quently move offshore and then inshore (Lough 

 1976; Reilly 1983). The extent to which this may oc- 

 cur can profoundly affect the degree of dispersal of 

 a local region's progeny. 



Evidence for an offshore-onshore movement of C. 

 magister during the larval development period is 

 ambiguous. In the field, three studies of the offshore 

 spatial pattern of larval distribution have been 

 undertaken: off central and northern California 

 (Reilly 1983), off Newport, Oregon (Lough 1976), 

 and off Tofino, British Columbia (this study). The 

 California study extended to about 185 km from 

 shore from San Francisco north to Cape Mendocino, 

 with some eariier CALCOFI data (1949-75) extend- 

 ing to about 275 km offshore included. Sampling 

 consisted of discrete-depth and oblique plankton 

 tows and was mostly during the day. The sampling 

 gear (0.5 m diameter opening) most often used was 

 preceded by the towing cable and bridle (P. Reilly^), 

 and gear avoidence by same megalopae may have 

 occurred. Because of the gear and protocol used, it 

 is difficult to interpret Reilly's (1983) results in a 

 quantitative sense. However, offshore movement of 

 larvae during zoeal stages II-V was indicated, as 

 was the later presence of megalopae in nearshore 

 waters following a period when stage V zoeae were 

 generally absent from within 40 km of shore, but 

 a mechanism to explain the onshore transport of 

 megalopae was not established. As part of the over- 

 all study (Wild and Tasto 1983), Hatfield (1983) 

 determined the intermolt stage of many larvae col- 

 lected by Reilly (1983). Earlier stage megalopae 

 were in general collected further offshore, earlier 

 in the year, and at a lower latitude. 



The Oregon study (Lough 1976) was on one track- 

 line extending 110 km offshore off Newport, with 

 mostly daytime, oblique bongo samples collected at 

 specified stations. Although early stage zoeae of 

 both species were abundant nearshore, late stage 

 zoeae were not, and were largely collected at the 

 offshore stations. Lough (1975) noted that C. ore- 

 gonensis megalopae were found further offshore and 

 later during the summer upwelling season than were 

 C. magister megalopae. In 1970, large numbers of 

 C. magister megalopae, which were not intermolt 

 staged, were found inshore. In 1971, virtually no C. 

 magister larvae older than stage III were collected 



^P. Reilly, California Department of Fish and Game, Menio Park, 

 CA 94025, pers. commun. February 1988. 



539 



