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Fishery Bulletin 95(2), 1997 
expression is limited, and generally these rings can 
be recognized better by direct sampling from ships 
or aircraft than from satellites (Hamilton, 1992). 
Materials and methods 
Collections in this study were gathered from the 
NOAA Ship Oregon II. Annual surveys were con- 
ducted that covered most of the U.S. Exclusive Eco- 
nomic Zone (Richards, 1984). These surveys followed 
a grid pattern with stations at every 30 minutes of 
latitude and longitude. Each station consisted of con- 
ductivity-temperature-depth (CTD) casts to 200 
meters or consisted of an expendable bathythermo- 
graph (XBT) drop. Biological samples were collected 
with 60-cm paired bongo nets of 0.333-mm mesh 
towed to 200 m or to within 5 m of the bottom at 
stations <200 m. The nets were towed at a speed of 
approximately 1.5 kn with a wire angle of 45° and 
retrieved at a rate of 20 m per minute. A neuston- 
net tow of 10-min duration (vessel speed approxi- 
mately 2.5 kn) was also conducted at each station. 
In 1983 and 1984 only one survey of the northern 
Gulf of Mexico was completed (Table 1). In the fol- 
lowing years, two surveys were completed (1986, 
1987, and 1988), each about two weeks apart, al- 
though with fewer stations and reduced geographic 
coverage. There was no survey in 1985. In addition 
to the normal survey, six transects across the Loop 
Current were made in 1987. The transect locations 
were selected on the basis of real-time satellite im- 
agery and frontal analysis and the frontal positions 
were radioed to the vessel (Richards et al., 1989). 
Transects 1-6 consisted of stations 2 km apart, and 
transects 7-8 were 3.6 km apart. 
In 1988, a line of stations was sampled along 86°W 
running from 29.5°N to 27.6°N. All stations except 
Table 1 
NOAA ship Oregon II cruise dates covering the period 
1983-88. 
Cruise 
Leg 
Date 
Year 
No. of 
bongo stations 
QT-134 
25 April-16 May 
1983 
99 
OT-143 
23 April-7 May 
1984 
98 
OT-159 
1 
22 April-6 May 
1986 
36 
2 
9 May-22 May 
1986 
37 
OT-166 
1 
15 April -2 May 
1987 
35 
2 
7 May-20 May 
1987 
35 
OT-173 
1 
15 April -2 May 
1988 
34 
2 
12 May-26 May 
1988 
35 
the first two were 8.3 km apart. Biological samples 
were taken until the 22°C isotherm rose to 100 m. 
The XBT drops were continued in order to provide a 
more detailed definition of the water mass. A 1-m 
Tucker trawl, with three nets and two opening and 
closing bongo nets, was deployed in addition to the stan- 
dard gear. Samples were fixed in buffered formalin and 
transferred to 70% ethanol within 48 hours. 
Bulk zooplankton biomass was estimated from wet 
displacement volume (dv) (Ahlstrom and Thrailkill, 
1960). Samples from the annual surveys were pro- 
cessed at the Plankton Sorting and Identification 
Center, Szczucin, Poland. The samples from the 
transects in 1987 and 1988 were processed at the 
Southeast Fisheries Science Center, Miami, Florida. 
Fish were identified from the descriptions of 
Ahlstrom et al. (1976). Catches of larvae were stan- 
dardized to number under 10 m 2 . Bulk plankton 
standing stocks were standardized to mL of wet dis- 
placement volume per 1,000 m 3 . 
To test the relationship between larval C. pauci- 
radiatus and the close proximity (<5 km) of a frontal 
feature, a chi-square (^ 2 ) test for one-dimensional 
count data was performed. Because the sampling grid 
was held constant, the total number of stations <5 
km from a zone of surface-temperature gradient var- 
ied in relation to the spatial location and size of the 
frontal features present in that year. To account for 
the fact that in some years most of the stations were 
within 5 km of a frontal zone and in other years few 
were, the analysis was performed to test whether the 
percentage of stations with C. pauciradiatus <5 km 
from a frontal feature was greater than the percent- 
age of stations with C. pauciradiatus >5 km from a 
frontal feature. This procedure has the added ben- 
efit of accounting for random distribution; i.e. if 90% 
of the stations are within 5 km of a feature, then 
expectations are such that 90% of the stations with 
C. pauciradiatus would be within 5 km. Pearson cor- 
relation coefficients (r) were also obtained from the 
relationship between larval C. pauciradiatus and 
plankton volume and between total larvae (number 
under 10 m 2 ) and plankton volume from the 
transects. 
The frontal edge of the Loop Current and anticy- 
clonic rings was defined as 22°C at 100 m depth fol- 
lowing Leipper, ( 1970) and Maul and Herman (1985). 
Results 
Physical oceanography 
The circulation patterns preceding and during the 
1983-88 April-May ichthyoplankton cruises varied 
