Simms et at: Distribution, growth, and mortality of larval Istiophorus platypterus in the northern Gulf of Mexico 
485 
including the Bahamas (Serafy et al., 2003) and 
the Dominican Republic (Prince et al., 2005), 
were less than 10 istiophorid larvae per hour, 
which is markedly lower than rates reported in 
our study for the Gulf of Mexico (24.4 sailfish 
larvae per hour). Sailfish frequency of occur- 
rence from the Gulf (45.0%) also corresponds 
closely to the 41.1% occurrence reported in the 
Straits of Florida and Bahamas by Luthy (2004), 
which included all Atlantic billfishes. Although 
CPUE was standardized for tow length, sampling 
gear, and number of tows, variations in towing 
methods as well as in the timing of sampling 
may have affected differences in catch rates of 
larvae. However, the relatively high catch rate of 
sailfish larvae reported in our study supports the 
premise that this region is an important spawn- 
ing and nursery ground of sailfish — a contention 
supported by high bycatch rates of adult sailfish 
during summer months (NMFS 1 ). 
The highest density of larvae was reported 
within mesoscale frontal features and highest 
catch rates were observed from June through 
August. Lowest densities of sailfish larvae were 
observed in cold core features during all surveys; 
however, as with other pelagic species (Richards 
et al., 1993, Hoffmeyer et al., 2007), catches were 
higher within fronts and anticyclones associated 
with the western margin of the Loop Current. 
In fact, highest densities were observed within 
frontal features during three of five surveys and 
over the course of all surveys combined. Higher 
catches of marine fish larvae have been reported 
at frontal features created by riverine discharge 
or converging oceanic currents in both temper- 
ate and tropical oceans (Hoffmeyer et al., 2007; 
Richardson et al., 2009b). The accumulation of 
larvae near or within fronts may simply be due 
to hydrodynamic convergence which has been 
shown to aggregate marine larvae in the Gulf 
and other regions (Govoni and Grimes, 1992; Bakun, 
2006). Alternatively, elevated primary and secondary 
production within frontal features often increases the 
availability of planktonic prey (Govoni et al., 1989; 
Grimes and Finucane, 1991) and therefore may effect 
higher survival for larvae within these oceanographic 
features (Grimes and Finucane, 1991; Biggs, 1992). 
In a recent study in the Straits of Florida, larval 
sailfish density was observed to peak at eddy frontal 
zones where there was a corresponding increase in 
density of common sailfish prey items (Llopiz and 
Cowen, 2008; Richardson et al., 2009b). This finding 
supports the premise that larvae are more abundant 
at fronts because of the increased availability of prey. 
As with spatial trends in sailfish density, temporal 
patterns in Gulf collections were comparable to those 
in the Straits of Florida; sailfish larvae were more 
abundant during spring and summer months (Post et 
al., 1997; Luthy, 2004). Elevated catches of sailfish in 
the summer correspond to peak spawning activity of 
sailfish in the North Atlantic (de Sylva and Breder, 
1997; Richardson, 2007). 
Spatial variation in growth of sailfish was limited 
despite elevated densities of sailfish larvae within 
fronts. The general lack of growth variation in sailfish 
among oceanographic features is somewhat unexpected 
given that cyclonic and frontal features often display 
increased primary productivity relative to anticyclones 
(Grimes and Finucane, 1991; Biggs, 1992) and that 
growth variation during early life is influenced by pri- 
mary productivity and prey availability (de Vries et 
al., 1990; Wexler et al., 2007). The lack of variation 
in growth among oceanographic features may be at- 
tributed to the fact that larvae in the northern Gulf 
likely spend time in multiple oceanographic features 
during early life. Oceanographic currents in this re- 
gion have been observed to have speeds up to 0.8 me- 
ters per second (69.1 kilometers per day) (Govoni and 
Grimes, 1992), and higher velocity currents have been 
recorded within the Loop Current (1.0 meters per sec- 
