CalCOFI Reports, Vol. 11, 1967 - California Cooperative Oceanic ...

150 CSLIFORNIA COOPERATIVE OCEAXIC FISHERIES IXVESTIGATIOSSin the temperature regime, there are reasons to doubta causative relationship even though this was impliedby Radovich (1961) ; the area off Baja California towhich the species was previously restricted was OCcupiedthroughout the period of northern range extensionby apparently non-migratory crabs, and it is inany case doubtful if temperatures higher than thoseshown by Table 3 to be tolerated by pelagic crabswould have been encountered in the normal southernrange of the species. Further, there is other evidenceto show that organisms with little or no mobility werealso involved at this time in the general northernmovement of southern forms (Berner and Reid, 1961;Radovich, 1961).Some mechanism beyond simply an amelioratingtemperature regime is therefore required to explainthese migrations, and this is to be found in thechanges in the advective transport of surface waterswhich occurred in the California Current at thistime (eg., Sette and Isaacs, 1961). Although the200 m flow is to the northward along Baja Californiaand southern California throughout the year (Reidet al., 1958), the transport of adult pelagic crabs ispresumably mainly in the mixed layer which flowspredominately towards the south; there are fourfeatures of the California Current system (Reid etal., 1958, 1961) which prima facie seem to be relevantto the distribution of pelagic crabs: the DavidsonCurrent, a northward coastal flow in winter whichis effective only to the north of Point Conception;the great eddy south of Point Conception which leadsto onshore flow throughout most of the year nearEnsenada at around 32"N. and consequently to flownorth and south from this point along the coast, reachingsouth to the region of Bahia Sebastian Vizcaino ;the countercurrent which develops in winter along thecoast of Baja California from Cab0 San Lucas northto Punta San Eugenio where it meets the southerlyflow from Ensenada ; finally, the overall southwarddrift of the main current offshore of the above features.In particular, the normally year-round coastal flowto the south between Ensenada and Punta SanEugenio as a consequence of the Point Conceptiongyre is probably of great significance in preventingthe northward transport of southern species by thecoastal countercurrents.Examination of the charts of geostrophic flow forthis area in the mimeographed CalCOFI data reportsindicates that throughout 1955 and 1956 the situationwas as described above; however, in the second halfof 3957 a different pattern appeared; in July, for thefirst time, the whole coast from Bahia Sebastian Vizcainonorth to San Diego was occupied by very disturbedflair- containing a series of eddies which appearto have been capable of some transport towards thenorth, a situation already invoked by Johnson (1960)to explain the northward movement of phyllosomalarvae at this time; again, in October and December1957 and in ?January 1958 the eddy extended unusuallyfar south and close to the coast so that northwardcoastal flow effectively bridged the gap in northwardflow from Punta San Eugenio. These data indicatethat the correspondence between the start of rangeextension of pelagic crabs and the appearance of conditionsof flow which could transport them northwardsfrom Punta San Eugenio is very good; the flow tothe north in the permanent eddy and in the wintercountercurrent north of Point Conception are sufficientexplanation of transport further to the north once thelatitude of San Diego is reached.During early 1958 there is little evidence of coastalflow to the north except that connected with thepermanent eddy, but in October, in the month inwhich it has been suggested above that northwardmovement of crabs began again, possibilities of suchtransport recurred ; the permanent gyre extendedvery far to the south, at least to 29" 30' N. and wasvery close inshore at its southern end so that it producedcoastal northern transport again from BahiaSebastian Vizcaino ; such transport was then continuousfrom this latitude to beyond Point Conception,where Davidson Current conditions were in effect.This situation coincided with the first reports of massstrandings at San Pedro.Once again, in 1959 the same pattern was repeated :from June to September the permanent eddy extendedfurther south than usual and the northward turn ofthe onshore flow occurred close to the coast, thusplacing the beginning of northward flow farthersouth than usual; additionally, as in July 1957, theappearance of active eddies as far down as BahiaSebastian Vizcaino gave further possibilities of northerntransport. These eddies were contemporaneouswith an active countercurrent south of Punta SanEugenio in August, and with Davidson Current conditionsin the north from July until January 1960,in which month the final strandings of Pleuroncodesoccurred in Monterey.During April 1960, for the first time since the startof this series of observations in 1955, the eddy was SOreduced as to be absent from the charts of geostrophicflow which thus showed an uninterrupted southwarddrift along the whole coast, including the bight tothe south of Point Conception, from the latitude ofMonterey south to Baja California del Sur ; this patternwas repeated in July of the same year and suegestsa mechanism which could flush the area northof Bahia Sebastian Vizcaino once more clear ofPleuroncodes-a flushing which certainly occurredduring this period.During the rest of 1961 and throughout 1962 theconditions returned to normal and it has been shownalready that in these years Pleiwoncodes was scarcelyrecorded north of the gap in the coastal countercurrent.The two oceanic records in April 1958 referred toabove may be explained, perhaps, by the same mechanismsas that suggested by Berner and Reid (1961)for occurrence of Doliolunz. denticulatunz in the samearea at the same time-by the southwesterly flow ofa tongue of inshore water from an upwelling on theCalifornia coast to the north of Point Conception; itis likely that examination of the zooplankton fromthese stations would show the presence of a numberof southern organisms.