484 
Fishery Bulletin 108(4) 
slopes, F { 2 282 ) = ®-^’ F=0.67 and ANCOVA, slopes, F (2 
241) =2.5, P=0.09, respectively). 
Interannual differences in daily instantaneous growth 
rates of sailfish were detected between 2005 {g= 0.123) 
and 2006 (g=0.114) (ANCOVA, slopes, F (1 i205) = 21.4, 
P<0.01) (Fig. 3). Further, significant intra-annual varia- 
tion in growth was observed in 2005 (ANCOVA, slopes, 
F [2 507) = 5.1, P=0.01); larvae collected in September 
(g- 0.113) displayed slower growth than larvae collected 
in July (g=0.127) (Fig. 4). In contrast, growth was sta- 
tistically similar between surveys in 2006 (ANCOVA, 
slopes, F (1 692)=0-7, P=0.39). 
Temporal variation in mortality 
Estimates of daily instantaneous mortality (Z) did not 
differ significantly between 2005 (Z=0.288) and 2006 
(Z=0.394) (ANCOVA, slopes, P (1 46) =1.0, P=0.33) (Fig. 
5, Table 5). Mortality rates ranged from 0.228 to 0.345 
among survey periods in 2005, but rates were statisti- 
cally similar (ANCOVA, slopes, P (2 24) =2.1, P=0.15) 
(Fig. 6). Further, mortality rates were not significantly 
different between survey periods in 2006, ranging from 
0.344 in June to 0.381 in August (ANCOVA, slopes, F (1 
16) =0.5, P=0.48). 
Production potential (G.Z) 
Instantaneous weight-specific growth coefficients ( G ) 
were 0.371 in 2005 and 0.347 in 2006, and G was indexed 
to daily instantaneous mortality (Z) to assess stage- 
specific production potential, G.Z (Table 5). Annual 
estimates of G.Z were 1.29 in 2005 and 0.88 in 2006. 
Production potential was highest in May 2005 (1.30) and 
July 2005 (1.66). In contrast, G.Z ratios of 
the September 2005 (1.01), June 2006 (1.02), 
and August 2006 (0.91) surveys were lower. 
400 
300 
200 
ioo- 
2005 
10 
120 
100 
- 80 
- 60 
- 40 
25 
400 
300 
200 
ioo- 
2006 
120 
- 80 
60 
- 40 
- 20 
0 5 10 15 20 25 
Standard length (mm)/age (days) 
Figure 2 
Standard length and age-frequency distributions of sailfish ( Istiopho - 
rus platypterus) larvae collected from the northern Gulf of Mexico 
in 2005 and 2006. Standard length distributions are shown in dark 
bars (n = 1037 and n = 1317 in 2005 and 2006, respectively). Age 
distributions are shown in light bars (n = 524 and n=712 in 2005 
and 2006, respectively). Note: 72 larvae indicated in Table 2 were 
damaged and no length measurements were taken for these larvae. 
Hatch-date distribution 
Age-based estimates of hatch date indicated 
that sailfish in our collections were spawned 
from May to September (Fig. 7). Hatching of 
sailfish peaked in mid-July, and the majority 
of larvae hatched in July: 56.1% according to 
back-calculated estimates, 73.4% according 
to mortality adjusted estimates. The percent- 
age of total catch from July for mortality- 
adjusted hatch dates was 52.1% in 2005, 
and 77.1% in 2006. Because the majority of 
sailfish were <20 days of age and sampling 
was conducted bimonthly, hatch-date dis- 
tributions comprised multiple cohorts from 
separate spawning events throughout the 
spawning season. 
Discussion 
The relative abundance of sailfish larvae 
collected in the present study was compa- 
rable to or higher than values reported from 
other putative spawning grounds of sailfish. 
Llopiz and Cowen (2008), collected 7.3 sail- 
fish larvae per hour in neuston tows over a 
two-year period in waters of the Straits of 
Florida; their catch rate was lower than the 
catch rate of 27.4 sailfish larvae per hour 
reported by Post et al. (1997) in the same 
region. Further, Richardson et al. (2009b), 
during a single week, reported a catch 
rate of 298 sailfish larvae per hour in the 
Straits of Florida; this catch rate indicates 
that very high, but ephemeral, abundances 
occur in this region. Catch rates of istio- 
phorid billfishes (sailfish, white marlin, 
and blue marlin) reported in other areas, 
