398 
PACIFIC SCIENCE, Vol. XXI, July 1967 
ond year of life the crabs assume a strictly 
benthic existence and become segregated from 
younger animals by assuming a deeper environ- 
ment. 
The number of crabs per square meter of 
bottom may be roughly estimated by regarding 
the 10-ft otter trawl as a quantitative sampling 
device. At one station the trawl was towed for 
25 minutes at two knots; it should have swept 
an area of 50,800 ft. 2 The weight of the catch 
of crabs was estimated at 400-500 pounds. 
Since 100 crabs of the size caught in the trawl 
weighed 0.96 pounds, it is estimated that there 
were 0. 8-1.0 crabs per ft 2 of bottom, or 9-11 
crabs per m 2 . Estimated densities for other sta- 
tions were similar. 
PELAGIC DISTRIBUTION 
The center of the pelagic distribution (re- 
gion of greatest abundance of crabs), as de- 
lineated by data from the numerous plankton 
tows of the California Cooperative Oceanic 
Fisheries Investigations and miscellaneous 
cruises from Scripps Institution of Oceanogra- 
phy, is on the continental shelf of the western 
shore of southern Baja California. Presumably 
the crabs are distributed from this population 
center by the influences of the oceanic current 
systems. The surface circulation along the 
western coast of Baja California (Fig. 3) is 
complex, but in general there are two currents, 
acting in opposite directions (Reid, I960). 
The more obvious of these is the California 
Current, which sweeps in a southerly direction 
along the California coast and swings west- 
ward in the latitude of southern Baja Cali- 
fornia. Its hydrography and fauna change grad- 
ually, and eventually it becomes or joins the 
North Pacific Equatorial Current. The effect 
of this southwesterly swing of the California 
Current on the distribution of P. planipes can 
Fig. 3. A schematic presentation of the currents along the coast of California and Baja California. 
