Lamkin: The Loop Current and abundance of larval Cubiceps pauciradiatus 
251 
Current in the eastern Gulf and the large warm-core 
anticyclonic and smaller cold-core cyclonic eddies in 
the western Gulf. These features form frontal zones 
across a wide area of the Gulf of Mexico and may be 
areas in which adult C. pauciradiatus are abundant. 
If C. pauciradiatus are abundant around the edges 
of gyres and upwelling areas, the Gulf of Mexico could 
be expected to support an extensive population. 
This relationship between larval fish and frontal 
zones has been an area of intense research since lies 
and Sinclair (1982) first proposed the existence of 
larval retention zones caused by oceanographic fea- 
tures. Thermal fronts are defined as a boundary be- 
tween two water masses that usually have a sharp 
temperature gradient over short (<10 km) distances 
(Brandt and Wadley, 1981; Owen, 1989). The biologi- 
cal implications of these features have been recog- 
nized by several authors (Brandt and Wadley, 1981; 
Le Feure, 1986; Richardson et al., 1986, 1989). Ther- 
mal fronts are often associated with abrupt changes 
in salinity, color, turbidity, primary productivity, and 
phytoplankton species composition and abundance. 
Fronts may also be considered ecotones and may pose 
a zoogeographic barrier to both adult and larval fish 
(Brandt and Wadley, 1981; Richards et al., 1993). 
Changes in the distribution and abundance of phy- 
toplankton species across frontal zones have been 
reported by Seliger et al. (1981), Holligan et al. 
(1984), Richardson et al. (1985), and Richardson et 
al. (1986). These authors have reported increased 
abundance across these features, but the duration 
and long-term effect of increased phytoplankton 
abundance on trophic levels have yet to be deter- 
mined. In a series of papers examining larval her- 
ring patches in the Buchan area of Scotland, 
Richardson et al. (1986) found phytoplankton bio- 
mass was highest at a transition zone created by 
warming waters and tidal mixing. Increased zoo- 
plankton abundance across fronts has also been re- 
ported (Tranter et al., 1983; Kiprboe and Johansen, 
1986; Richards et al., 1989). Trantor et al. (1983) and 
Kiprboe and Johansen ( 1986) both reported increased 
zooplankton biomass concurrent with increased phy- 
toplankton abundance. In a series of transects across 
the Loop Current, Richards et al. (1989) found in- 
creased zooplankton volumes in thermally mixed 
water close to the outer perimeter of highest surface- 
current velocity. This occurrence coincided with in- 
creases in surface chlorophyll measurements. 
The purpose of this paper is to describe the large- 
scale (Gulf-wide) distribution and abundance of lar- 
val C. pa uciradiatus and their interaction with meso- 
scale oceanographic features in the Gulf of Mexico. I 
will show that C. pauciradiatus are retained on the 
cool side of thermal fronts in areas of high produc- 
tivity. I hypothesize that the temporal persistence of 
northern excursions of the Loop Current directly af- 
fects the abundance and probably the survival of this 
species. Because this study focuses on larval, rather 
than adult, C. pauciradiatus , the results of this study 
will help define the role that these oceanographic 
features play in larval distribution and may help to 
determine the size of future year classes. 
Physical oceanography of the Gulf of 
Mexico 
The Gulf of Mexico is a semi-enclosed body of water, 
the circulation of which is dominated by the Loop 
Current. Water enters through the Yucatan Chan- 
nel and exits through the Straits of Florida. The Loop 
Current is very dynamic and unstable, pushing as 
far as 29 degrees north latitude into the Gulf of 
Mexico and at other times flowing almost directly 
out through the Straits (Vukovich et al., 1979). These 
characteristics have caused considerable confusion 
over the years, and only recently have we begun to 
understand the dynamics of this system (Leipper, 
1970; Behringer et al., 1977; Maul, 1977; Vukovich 
et al., 1979; Vukovich, 1988; Maul and Vukovich, 
1993). 
Among the more significant features of the Loop 
Current are the large (200-300 km at formation) 
anticyclonic rings generated when the northward 
intrusion separates from the rest of the Current. 
These rings are pinched off from the Loop Current 
and move into the western Gulf shelf where they 
eventually spin down and break up (Merrell and 
Vazquez, 1983; Lewis and Kirwan, 1987; Lewis, 
1992). The exact mechanism of ring genesis is un- 
clear, but it seems to involve the formation of a nar- 
row intrusion of cold water between the ring and the 
remainder of the Loop Current (Cochrane, 1972; 
Vukovich and Maul, 1985; Vukovich, 1986). Hurlburt 
and Thompson (1980, 1982) used numeric models 
that showed that inherent instabilities exist within 
the flow field and eventually result in ring separa- 
tion. Ring separation occurs every 6-17 months (on 
average every 11 months [Maul and Vukovich, 1993]). 
As these warm-core anticyclonic rings move west- 
ward, adjacent mesoscale (20-80 km) cyclonic circu- 
lations may develop (Elliot, 1979; Merrell and 
Morrison, 1981; Merrell and Vazques, 1983; Lewis 
and Kirwan, 1985). These cyclonic rings may be im- 
portant biologically; Biggs (1992) found elevated ni- 
trate concentrations just below the mixed layer. Cy- 
clones such as these exist for 6 months or more, dur- 
ing which time they may move tens to hundreds of 
km (Hamilton, 1992). However, their cold surface 
