480 
Fishery Bulletin 108(4) 
Density and catch rate 
The total number of sailfish caught at each sampling 
station was divided by the surface area sampled to 
determine the density of larvae in number of individuals 
per 1000 m 2 . Mean density did not vary between the two 
mesh sizes (500 pm and 1200 pm) in any survey (paired 
t-test, all P>0.05), indicating no difference in capture 
success between net sizes. However, mean standard 
length was smaller in the 500-pm net gear (5.2 mm vs. 
5.6 mm; F a 3116) =21.3, PcO.Ol), indicating that a larger 
fraction of small larvae were retained by the finer mesh. 
Frequency of occurrence was calculated for each survey 
as the number of collection stations which produced at 
least one sailfish larva by using the equation 
Frequency of occurrence = 
( number of stations with 
>1 larva / total number of (1) 
stations during survey) x 100. 
In order to compare catch rates with those from other 
ichthyoplankton studies, catch per unit of effort (CPUE) 
of sailfish larvae was estimated with the equation 
CPUE (larvae per hour) = 
number of larvae collected / 
([ number of towsxtow length (min) x (2) 
2 ( number of nets towed)] / 60). 
Oceanographic features 
Remotely sensed sea surface height (SSH) data were 
used to delineate oceanographic features into one of 
four categories (Leben et ah, 2002). Briefly, stations 
with a SSH greater than 17 cm were considered to be 
within the core of the Loop Current or an associated 
eddy system and classified as “anticyclones” (warm core 
eddies) (Leben et ah, 2002). Further, the core of adjacent 
“cyclones” (cold core eddies) were identified by a SSH of 
less than -10 cm. Frontal features associated with the 
Loop Current and anticyclones have been reported to 
extend up to 60 km from the 17-cm contour. Therefore, 
stations between 17 cm and -10 cm SSH and within 60 
km of the 17 cm SSH contour were classified as being 
in a “front” (Tidwell, 2008). Remaining stations were 
classified as “open ocean.” 
Analysis of otolith microstructure 
Sagittal otoliths were extracted, cleaned of tissue in 
immersion oil, and preserved in mounting media (Flo- 
texx, Fisher Scientific #14-390-4, Pittsburgh, PA) for 
a subset of sailfish larvae. Mounted otoliths were pho- 
tographed under high magnification (400x) with an 
Olympus BX41 light microscope, and daily growth 
increments were enumerated with the use of Image- 
Pro Plus software (vers. 4.5, Media Cybernetics Inc., 
Bethesda, MD) by counting the first visible incre- 
ment beyond the hatch check as day one (Luthy et al. 
2005). Inner increments of large otoliths are occasion- 
ally difficult to enumerate; therefore a regression of 
growth increment radius on age was used to predict 
the number of increments at various distances from 
the core (Rooker et al., 1999). The number of incre- 
ments within unclear regions was estimated with this 
regression and added to the increment count for the 
enumerated section to produce final age estimates. 
Less than 25% of age estimates were corrected with 
this method, and if the unclear region comprised more 
than 33% of the final age estimate, the otolith was not 
used for age and growth assessments. Two independent 
readings of daily increments were conducted by a single 
reader for each otolith. When two readings were within 
10% variance of each other, one reading was randomly 
selected for analysis. If readings differed by >10%, a 
third reading was performed. If the third reading was 
within 10% variance of one of the former readings, 
one of the two similar readings was randomly selected 
for analysis. If each reading differed from the others 
by >10%, the otolith was not used for further analysis 
(n = 94, 7.1%). 
Growth, mortality and hatch dates 
Growth rates were determined by using otolith-derived 
age estimates (n = 1236) and standard length data. 
Because ages varied among surveys, growth rates were 
based on a limited age range (<17 days or the oldest 
larva in May 2005 collections) to minimize any effect 
of variable age ranges on growth analyses (Rilling and 
Houde, 1999). Daily instantaneous growth coefficients 
(g) were calculated from an exponential model: 
L t = L 0 es f , (3) 
where L t = length (mm SL) at time t; 
L 0 = estimated length at hatching; 
g = instantaneous growth coefficient (d); and 
t - otolith-derived age (days after hatching). 
Ages of sailfish without an otolith-derived age were 
predicted by using age-length relationships (n=1118). A 
small number of sailfish larvae were damaged (n=72), 
and therefore length and age estimates could not be 
determined for these larvae. 
Daily mortality was estimated for each survey from 
regressions of the decline in log,, {abundances 1) on age. 
In order to minimize the influence of gear avoidance by 
larger larvae (Houde, 1987), mortality estimates were 
determined over a short time interval (10 days), and 
thus for a limited size range of larvae (<10 mm). Mor- 
tality was calculated for several time intervals rang- 
ing from 5 to 10 days and differences were negligible, 
indicating that the 10-day duration was appropriate 
for the target life stage. The age of peak abundance for 
each cohort was used as the initial point for catch-curve 
analysis and ranged from 9 to 11 days after hatching. 
Daily instantaneous mortality rates (Z) were calculated 
from the exponential model 
