FISHERY BULLETIN: VOL. 74, NO. 2 



spend considerable time resting on the bottom of 

 the rearing vessel. 



A species such as C. magister, which has a lar- 

 val life of approximately 130 days, could conceiv- 

 ably be transported northward about 600 miles 

 along the North Pacific coast as Wyatt et al. 

 (1972) reported that the winter surface currents 

 based on drift bottle studies have a mean speed of 

 0.2 knots, or a drift of 150 miles per month. The 

 Ekman transport of surface waters due to wind 

 stress decreases exponentially with depth due to 

 frictional resistance, so that when the current has 

 fallen to about one-twenty third that of the sur- 

 face, this subsurface flow is negligible or reverse 

 to that of the surface currents (Sverdrup et al. 

 1942). Recent studies indicate wind driven water 

 motion extends to a depth of about 10 m (Bourke 

 et al. 1971). If the larval population resides about 

 5m below the surface where the wind induced cur- 

 rent is about one-quarter that of the surface, then 

 the larvae would only be transported 150 miles in 

 a linear distance. Larvae located in the water col- 

 umn below 5m depth, particularly the later zoeal 

 stages, would experience relatively little trans- 

 port in any direction. Holton and Elliot (1973) 

 reported the greatest abundance and density of 

 zooplankton containing crab larvae occurred at 

 about 15m depth at nearshore stations off New- 

 port during the daylight hours. Hypothetically, 

 larvae released in January-February could be 

 transported north along the coast in the surface 

 currents and, after the transition period of cur- 

 rents in March, travel south a comparable dis- 

 tance in April and May. Or, taking into considera- 

 tion the fact that the older stages may reside 

 deeper into the water column, they could conceiv- 

 ably travel north in the surface currents as early 

 zoea and travel south again as late larvae in a 

 weak underlying countercurrent, but this seems 

 unlikely. Huyer et al. (1975) reported the north- 

 ward currents along the central Oregon coast es- 

 sentially are constant with depth during the win- 

 ter and southward at all depths in the spring but 

 stronger at the surface. Larvae occurring within 

 3-5 miles of the coast probably are caught within a 

 system of eddies and countercurrents characteris- 

 tic of this zone, retarding large-scale dispersal in 

 any direction. The mechanistic concepts of re- 

 cruitment seem too contrived and unnecessary if 

 stochastic processes are the general rule for 

 species producing large numbers of expendable 

 young. Most investigators would agree that the 

 great majority of the pelagic larvae of marine in- 



vertebrates are lost to the population and that 

 only a very small percentage of annual recruits 

 are normally required to maintain a stable popu- 

 lation for longer-lived adults. Cancer magister 

 lives 4 or 5 yr so that a population unexploited by 

 man would only require recruitment every other 

 year or so. The fact that the adult populations are 

 not retreating northward supports the view that 

 at least some of the larvae are retained in the 

 same general area as their point of origin. 



The low densities of late stage larvae collected 

 in the offshore area indicated that the small vol- 

 ume of water filtered on the inshore stations 

 could account for their disappearance or reduced 

 numbers. 



Knowledge of their vertical location within the 

 water column at different stages of development 

 is important in understanding their spatial dis- 

 tribution and local abundance. However, a sepa- 

 rate study of the larvae within the upper 150 m 

 was not undertaken. Most crab larvae are photo- 

 positive to light in their early stages and migrate 

 to the surface layers, whereas the late stages re- 

 spond photonegatively and are found in the 

 deeper layers near the bottom as they prepare to 

 molt to juveniles (Thorson 1964). The larvae of C 

 magister appear to follow this same general pat- 

 tern except that the early megalopal stage shows 

 anomalous behavior as they have been observed 

 to "swarm" near the sea surface along the coast 

 (Cleaver 1949; Gaumer 1971; pers. obs.). Personal 

 laboratory observations, as well as those by 

 MacKay (1942) and others, substantiate the fact 

 that the early zoea and megalopa of C. magister 

 are generally photopositive in contrast to the late 

 zoeal stages which are neutral or photonegative. 



A scheme is proposed which would explain 

 their distribution and abundance within 10 miles 

 of the coast taking into account the differential 

 behavioral response to light of the various larval 

 stages. Newly hatched zoeal larvae are strongly 

 photopositive and swim to the surface where cur- 

 rent transport during the winter is generally on- 

 shore. They become progressively heavier and 

 less photopositive with development until in the 

 late zoeal stages they are neutral or responding 

 negatively to light. As a consequence, the late 

 zoeal stages reside in the deeper layers of water, 

 possibly within a few meters of the bottom. They 

 are now maximally dispersed in the nearshore 

 area. Upon molting to the megalopic stage they 

 are temporarily strongly photopositive to light 

 and coupled with their increased powers of 



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