372 
Fishery Bulletin 95(2), 1 997 
versus 179 individuals/1,000 m 3 outside), the most 
abundant being E. tenera, E. mutica, and E. 
americana. Species richness of euphausiids was also 
greater inside the CCR than outside: five species 
(Euphausia pseudogibba, E. brevis, Nematoscelis atlan- 
tica, Thysanopoda monoacantha, and Nematobrachion 
flexipes) were found only in collections made inside the 
CCR. Details are available from the authors in a 3- 
page table. We speculate that the presense of the later 
three mesopelagic euphausiid species within the 
CCR, but not recorded outside the CCR, reflects the 
extension of their upper vertical distribution limits: 
where cold water domed shallower than 100 m, these 
mesopelagic species reached up into the zone where 
our nets collected samples at night. 
We have calculated a mean acoustic backscatter 
intensity (ABI) 0-200 m by time-averaging ADCP 
data from the 15-20 minute periods when net tows 
were made. The ADCP was calibrated, as explained 
by Zimmerman (1993), with mean ABI expressed as 
dB re(Mx47t) -1 . We also computed the integrated ABI 
(I ABI): the amount of backscatter that was greater 
than the grand mean of -74 dB for the upper 200 m, 
bin by bin, from the 8-12 m bin to the 96-100 m bin) to 
provide a summary number for comparison with wet 
displacement volume of zooplankton collected from 0 
to 100 m in the net tows. Figure 3 summarizes the mean 
ABI during the ensembles when net collections were 
being made. These acoustic data have been corrected 
for sound attenuation with depth, which was modeled 
from the T/Z relationship at each XBT station. Sub- 
surface regions of locally intensified return (locally 
higher backscatter) are presumed to be local concen- 
trations of biological scatterers. Although these regions 
of locally enhanced backscatter were concentrated into 
the vertical range of 60-100 m during the day, they 
reached closer to the surface and occurred over a greater 
vertical range of the water column at night. The ABI 
data, however, are not sufficient to distinguish whether 
at night euphausiids were more abundant and more 
species rich within the CCR than without. 
Discussion 
In the decade since lies and Sinclair (1982) recog- 
nized the existence of larval retention zones caused 
by oceanographic features, the relations between 
stocks of phytoplankton, zooplankton, larval nekton, 
and frontal zones have been an area of intense re- 
search. For example, it is now well known that local 
aggregations of phytoplankton can develop along and 
within week-long meanders and eddies in the Gulf 
Stream (Lee et al., 1991) and that elevated fish stocks 
often co-occur in these frontal disturbances (Atkinson 
and Targett, 1983). In the Gulf of Mexico, frontal 
zones at the periphery of meanders and eddies that 
are seaward of the continental margin are typically 
expressed as sharp gradients in temperature. These 
may have secondary expression as gradients in sa- 
linity, particularly in local convergences that can 
entrain low-salinity water and transport it off shelf 
as plumes or jets. For example, Biggs and Muller- 
Karger ( 1994) reported that some cyclone-anticyclone 
geometries in the Gulf of Mexico create flow 
confluence zones that can transport high-chlorophyll 
shelf water seaward several hundreds of kilometers. 
Sharp frontal zones may also be created during peri- 
ods of northern extensions of the Loop Current. 
Lamkin ( 1997) found a significant positive correlation 
between the abundance of larval nomeid fish and the 
location of the northern edge of the Loop Current by 
analyzing NOAA annual icthyoplankton survey data 
from 1983 to 1988. Lamkin’s data indicate that 
Cubiceps pauciradiatus, in particular, is a species 
whose adult spawning grounds and larval habitat 
are tied to sharp temperature gradients. Peak lar- 
val abundance was found close to the frontal inter- 
face, and peak abundance occurred just above the 
region of peak sea surface temperature (SST) gradi- 
ent. Lamkin went on to speculate that the extent of 
the frontal systems in the Gulf of Mexico would be 
expected to impact annual recruitment of a species 
that is tied to a frontal habitat. 
Table 2 
Comparison of net-collected with acoustic characterization of zooplankton stocks. See text for explanation of how Acoustic Back- 
scatter was calculated. CCR = cold-core ring; IABI = integrated acoustic backscatter intensity. 
Plankton Tow 
(0-100-0 m) 
Total wet 
displaced volume 
(mL/1,000 m 3 ± SD) 
Acoustic 
Backscatter (db) 
(IABI, 10-100 m ±SD) 
Euphausiids 
Pteropods 
Siphonophores 
(numbers per 1,000 
m 3 ±SD) 
1 (day: NW of CCR) 
36 
46.3 
112 
93 
212 
2-4 (night: inside CCR) 
90 ±12 
87.7 ±11.5 
574 ±138 
204 ±19 
580 ±78 
5-7 (day: SE of CCR) 
39 ±6 
36.2 ±31.2 
154 ±56 
104 ±48 
453 ±253 
8 (night: SE of CCR) 
67 
82.7 
806 
198 
840 
