Simms et al.: Distribution, growth, and mortality of larval Istiophorus platypterus in the northern Gulf of Mexico 
487 
2005 (n=524) 
Log e N, = 7.131 -0.288 (age) 
r 2 = 0.94 
o 0 
10 
15 
20 
2006 (n =71 2) 
Log e N, = 9.1 24 - 0.394(age) 
r 2 = 0.95 
10 
Age (days) 
15 
Figure 5 
Regression plots of log e (abundances 1) on age for ten-day cohorts 
of sailfish ( Istiophorus platypterus) larvae collected from the 
northern Gulf of Mexico in 2005 and 2006. Regression equations 
and plots are arranged by year. 
ond; Vukovich and Maul, 1985), which indicate 
that planktonic larvae may encounter multiple 
oceanographic features during their planktonic 
larval duration. Further, pelagic larvae from 
broad oceanic areas have been reported to ac- 
cumulate within frontal zones (Richards et al., 
1993; Bakun, 2006), making it difficult to de- 
termine where individuals spend the majority 
of their lives and therefore, in which feature(s) 
most of their early growth occurs. 
Estimated growth of sailfish in the Gulf 
(g=0.113 to 0.127) varied temporally and rates 
were comparable to or slightly slower than 
those reported for sailfish in the Straits of 
Florida (g=0.130 to 0.146; Luthy et al., 2005; 
Richardson, 2007; Richardson et al., 2009a) 
and blue marlin from the Bahamas (g= 0.098 
to 0.125; Serafy et al., 2003; Sponaugle et al., 
2005) and the Straits of Florida (g=0.089 to 
0.114; Sponaugle et al., 2005; Richardson, 
2007). Observed differences in growth among 
studies are minor and similarities are not un- 
expected because the timing of collections and 
environmental conditions between the regions 
were comparable. Sampling in the Straits 
of Florida was conducted between April and 
September in waters ranging from 26.1°C to 
30.6°C (Luthy et al., 2005; Richardson, 2007). 
This range of temperatures is similar to tem- 
peratures present in the Gulf during our May 
to September sampling period (26.4-30.4 °C). 
Moreover, observed ranges in salinity were 
nearly the same between the Straits of Flor- 
ida (34.0-36.7 ppt) and Gulf (35.2-36.5 ppt). 
Temporal variations in growth of marine lar- 
vae have been shown to be correlated with 
temperature (Rilling and Houde, 1999), and the most 
rapid growth of sailfish larvae was observed during 
July 2005 when the warmest mean temperature was 
reported. Nevertheless, the second fastest growth rate 
for sailfish was observed during May 2005 when the 
lowest mean temperature was reported. Thus, other 
factors known to affect growth, such as density of con- 
specifics or prey availability (Jenkins et al., 1991; Lang 
et al., 1994; Wexler et al., 2007), may be responsible 
for observed variations in growth of sailfish larvae. 
Although intra-annual differences in mortality were 
limited, losses were substantial throughout the early 
life interval examined (Z = 0.23 to 0.38). Daily instan- 
taneous mortality rates reported in our study are 
10-45% lower than mortality rates for sailfish and 
blue marlin larvae from the Straits of Florida and 
Exuma Sound, Bahamas (Richardson et al., 2009a). 
However, the losses reported in our study are compa- 
rable to those of other pelagic larvae, such as bluefin 
tuna ( Thunnus thynnus ) (Z=0.20; Rooker et al., 2007), 
yellowfin tuna ( Thunnus albacares) (Z= 0.33; Lang et 
al., 1994), and members of the suborder Scombroidei, 
which includes tunas, billfishes, and the barracuda 
Sphyraena barracuda (Z = 0.34; Houde and Zastrow, 
1993). Predation has been observed to be a major 
cause of mortality during the early life interval of pe- 
lagic species (Leggett and Deblois, 1994; Houde, 2002) 
and it may be responsible for high losses of sailfish 
larvae. Recent studies indicate that istiophorids are 
preyed upon by conspecifics and congeners (Llopiz and 
Cowen, 2008; Tidwell, 2008) and, if so, cannibalism or 
predation pressure by other istiophorids may represent 
an important source of mortality for sailfish during 
early life. 
Observed G:Z ratios were greater than 1.0 during 
all but the August 2006 survey, indicating conditions 
were likely favorable for production during the life 
stage examined. Houde and Zastrow (1993) reported a 
mean G:Z ratio of 0.89 for marine fish larvae (pooled 
across several taxa), which is lower than the range 
reported in our study for sailfish (0.91-1.66). However, 
indices reported for individual species or taxonomic 
groups ranged from 0.26 to 2.42 for larvae abundant 
in upwelling and shelf zones, indicating wide-ranging 
stage-specific production potential for pelagic fishes. 
Collection of larvae peaked in July and it is possible 
that increased G:Z coincided with increased temper- 
ature or prey availability, both of which have been 
