Surface Currents as Determined by Drift Card 
Releases Over the Continental Shelf Off 
Central and Southern California 
JAMES L. SQUIRE, Jr.' 
ABSTRACT 
During March 1964 through February 1966, 8,320 plastic drift cards were released at selected 
points from an aircraft to measure surface current drift over two areas: from the coast to about 48 
n.mi. off central California between Point Arena and Point Sur; and from the coast to about 90 n.mi. 
off southern California between Point Arguello and Punta Salsipuedes, Baja California, Mexico. The 
recovery rate was 3.5% in the central area and 5.7% in the southern area. An average 79.4% of the 
recoveries were found within 2 wk following the date of release. Results lend support to studies con- 
cluded by earlier investigators. The distribution of the directions from which drift cards were re- 
turned increased the evidence for the presence of an eddy off the coast between San Francisco and 
Monterey Bay during May through July, and of the large gyre and associated southern California 
countercurrent south of Point Conception during April through August and to a lesser extent in Oc- 
tober and December. 
INTRODUCTION 
Dispersal and survival of the planktonic eggs and 
larvae of marine fishes in the ocean euphotic zone is 
affected by surface currents. The purpose of this study 
was to develop additional information on the drift 
patterns to which surface planktonic forms are subjected 
over the continental shelf of the west coast of the United 
States. The general direction of surface drift was deter- 
mined and is compared with wind velocity during the 
survey period to expose any general relationship between 
wind direction and inshore surface currents. Johnson and 
Squire (1970) published the results found along the 
northwestern coast of the United States. This paper 
covers the central area from Point Arena to Point Sur, 
Calif., and the southern area from Point Arguello, Calif., 
to Punta Salsipuedes, Baja California, Mexico. 
Field work for the study was carried out concurrently 
with an airborne infrared sea surface temperature survey 
of the continental shelf conducted with the cooperation 
of the U.S. Coast Guard (Squire 1971). 
Increased interest in coastal surface currents as they 
may pertain to the drift of surface pollutants, such as 
petroleum, has prompted the documentation of this drift 
study. 
METHODS 
Talbot (1964) showed that specially prepared drift 
bottles could be successfully dropped from an aircraft; 
however, drift cards were chosen for this study due to the 
‘Southwest Fisheries Center La Jolla Laboratory, National Marine 
Fisheries Service, NOAA, La Jolla, CA 92038. 
ease of handling and the limited space available in the 
aircraft. The individual drift cards are identical to those 
described by Johnson and Squire (1970) and were a 4 X 
5%-in, numbered, postage paid, self-addressed, fluores- 
cent red postcard. A plain fluorescent red card was used 
as backing, and both were sealed in a clear plastic 
envelope with a steel washer as ballast. This study was 
conducted from March 1964 through February 1966, dur- 
ing which period 16 drift card releases were made off 
southern California and central California. From March 
1964 through February 1965, airborne releases of drift 
cards were made once each 3 mo. Beginning in March 
1965 and continuing until the end of the study, the fre- 
quency of drift card releases increased to one per month. 
Ten cards were released at each drop station. 
The U.S. Coast Guard aircraft were flown at an 
average altitude of 500 ft over a prescribed pattern cover- 
ing the central area to about 48 n.mi. offshore and the 
southern area to about 90 n.mi. offshore. Twenty-five 
release stations were located on the flight transects over 
the central area, and 27 release stations over the southern 
area. Dead-reckoning techniques, aided by the aircraft’s 
tacan, loran, and radar navigation equipment were used 
to locate release points. 
For visual comparison of wind direction, average wind 
speed, and the direction of surface drift, a wind rose sym- 
bol developed by Tabata (1961) was used on each drift 
chart (Figs. 1, 2). The symbols give the percent frequen- 
cies for the prevailing winds and calms (wind velocity 1 
kn) based on eight points of the compass and the average 
wind speed in knots for a 2-wk period—1 wk before re- 
lease and 1 wk after (drift cards were released on the 
last day of the first week of the 2-wk period). Wind data 
were obtained for geographical locations as near the 
