LARVAL DYNAMICS OF THE DUNGENESS CRAB, CANCER 

 MAGISTER, OFF THE CENTRAL OREGON COAST, 1970-71 



R. Gregory Lough^ 



ABSTRACT 



The larval dynamics of the economically important Dungeness crab, Cancer magister, were investi- 

 gated from plankton samples collected bimonthly during 1970 and 1971 along a trackline near 

 Newport, Oreg. Larvae appeared at maximum densities (8,000/1,000 m^) within 15 miles of the coast 

 in late January 1970 and remained in the plankton until late May for an approximate larval period of 

 130 days. The bulk of the larval population was retained in the nearshore area by the strong along- 

 shore and onshore components of the surface currents and to some extent by the behavior of larvae in 

 determining their position in the water column. During the 1971 season, larvae appeared initially 

 at about the same time and densities, but a mass mortality may have occurred in the early zoeal 

 stages coinciding with the unusually severe weather in February and March. A significant difference 

 between the 1970 and 1971 larval populations was suggested by analysis of covariance using sea 

 surface temperature and salinity as environmental variables. However, the effect of the low tempera- 

 ture and salinity values that occurred during the winter of 1971 were not clearly indicated by 

 multiple regression analyses of laboratory experimental data to be the prime factors directly affect- 

 ing larval survival. Neither did a gut-fullness study of planktonic larvae substantially explain the 

 1971 larval mortality. Therein various hypotheses are explored in view of the present knowledge of 

 processes affecting larval survival and recommendations are suggested for further research. 



It is well known that many species of economi- 

 cally important marine resources fluctuate 

 greatly in number and location. These fluctua- 

 tions may be explained in part by changes occur- 

 ring in the larval populations. That the larval 

 stage is the most critical period for the majority of 

 marine animals was originally emphasized by 

 Hjort (1914, 1926) for fish larvae and by Thorson 

 (1946) for marine invertebrate larvae. Survival 

 through this period is usually considered the 

 major factor in determining the strength of the 

 year class. The causes or extent of larval mortal- 

 ity, however, are still relatively unknown. 



Bimonthly plankton samples were collected 

 from 1969 through 1971 along a transect off the 

 central Oregon continental shelf to document the 

 species of crab larvae present, their seasonality 

 and abundance, and their onshore-offshore dis- 

 tribution in relation to seasonal changes in 

 oceanographic conditions (Lough 1975b). A major 

 effort was made to assess the larval population of 

 the Dungeness crab. Cancer magister Dana, as it 

 supports one of the most important fisheries in 

 the Pacific Northwest. 



Cancer magister occurs along the Pacific coast 



'School of Oceanography and Marine Science Center, Oregon 

 State University, Corvallis, OR 97331; present address: North- 

 east Fisheries Center, National Marine Fisheries Service, 

 NOAA, Woods Hole, MA 02543. 



fi'om Unalaska to lower California and ranges 

 from mean low water to 50 fathoms (91 m) 

 (Schmitt 1921). Although it prefers sandy or 

 sandy-mud bottoms of the nearshore area, speci- 

 mens have been found on all bottom types within 

 estuaries and on the continental slope. Adult fe- 

 males generally reach maturity by their second 

 or third year and may produce three or four 

 broods during a life-span (MacKay 1942; Cleaver 

 1949; Butler 1960). Egg-carrying females are 

 found in Oregon waters from October to March 

 with essentially one brood produced per year 

 (Waldron 1958). Field observations (Waldron 

 1958) and laboratory rearings by Poole (1966) 

 and Reed (1969) indicate that larvae hatch off 

 northern California and Oregon from January 

 through March and are present in the nearshore 

 waters through July for a total estimated larval 

 life of 128 to 158 days. Cancer magister passes 

 through five zoeal stages and one megalops dur- 

 ing its larval development before settling out 

 of the water and metamorphosing to the benthic 

 juvenile. 



HYDROGRAPHIC FEATURES 

 OF STUDY AREA 



The surface waters along the U.S. west coast 

 are dominated by the California Current; a slow, 



Manuscript accepted September 1975. 

 FISHERY BULLETIN; VOL. 74, NO. 2, 1976. 



353 



