Simms et al.: Distribution, growth, and mortality of larval Istiophorus plcitypterus in the northern Gulf of Mexico 
479 
and increased growth have been observed for larvae 
within frontal features created by riverine discharge 
and hydrodynamic convergence (Lang et ah, 1994; Hoff- 
meyer et ah, 2007). It is likely that oceanographic fea- 
tures also influence the early life ecology and population 
dynamics of sailfish in this region, and therefore an 
improved understanding of the effects of these features 
on distribution, growth, and survival will aid in deter- 
mining the value of the Gulf as spawning and nursery 
habitat for sailfish. 
The objectives of this research were threefold: 1) to 
characterize spatial and temporal patterns of abun- 
dance of sailfish larvae in the northern Gulf of Mexico; 
2) to relate spatial variation in distribution and growth 
to oceanographic features to determine the causal fac- 
tors responsible for recruitment variability; and 3) to 
estimate demographic parameters within and across 
years to determine whether these traits varied tempo- 
rally. Estimates of growth (G) and mortality (Z) were 
combined ( G:Z ) to determine indices of stage-specific 
production potential and to assess the functional role 
of the Gulf as spawning and nursery habitat of this 
species. 
Materials and methods 
Field collections 
Five ichthyoplankton surveys were conducted in shelf 
and slope waters of the northern Gulf during spring 
and summer of 2005 (May, July, and September) and 
summer of 2006 (June and August). Surveys were con- 
ducted in an area bounded by 27° to 28°N latitude and 
88 to 94°W longitude. This sampling area was selected 
because bycatch rates of adult sailfish by U.S. longliners 
were high in this region during the presumed spawning 
period of Atlantic sailfish (NMFS 1 ). Istiophorid larvae 
were collected with paired neuston nets (2-m widthxl-m 
height frame) with two mesh sizes (500 pm and 1200 pm) 
to account for potential differences in capture success 
between mesh sizes. Nets were towed through the top 
meter of the water column at approximately 2.0 knots for 
10 minutes. Paired tows were taken at 60 to 70 sampling 
stations spaced approximately 15 kilometers (km) apart 
during each survey. Sampling was conducted at 15-km 
intervals to allow coverage of a large area encompassing 
multiple oceanographic features. The September 2005 
survey was shortened (39 stations sampled) because of 
inclement weather. 
At each station, sea surface temperature (°C), salin- 
ity (ppt), and dissolved oxygen (mg/L) were recorded 
by using a Sonde 6920 Environmental Monitoring Sys- 
tem (Yellow Springs Instruments Inc., Yellow Springs, 
OH). A probe malfunctioned during sampling in August 
2006, preventing dissolved oxygen measurements. Sea 
surface height (cm) for each station was determined 
from archived satellite altimetry data provided by the 
Colorado Center for Astrodynamics Research (CCAR) 
Real-Time Altimetry Project (argo.colorado.edu; R. Leb- 
en, personal commun. 2 ). Flowmeters (General Oceanics, 
model 2030R, Miami, FL) placed within each net were 
used to determine the surface area sampled by each 
net. A formula provided by the manufacturer was used 
to determine distance towed during each collection 
which was multiplied by net width to determine surface 
area sampled during each collection (m 2 ): surface area 
sampled (m 2 ) = distance sampled (m) x 2 m (net width). 
Fish larvae and associated biota were preserved on- 
board in 95% ethanol and istiophorids were sorted from 
each sample in the laboratory with the use of a Leica 
MZ stereomicroscope and stored in 70% ethanol. Istio- 
phorid larvae were photographed and standard length 
(SL) was measured to the nearest 0.1 mm. Istiophorids 
do not have a full complement of fin rays until reaching 
20 mm SL (Richards, 1974), and 99.8% of specimens col- 
lected in the Gulf were less than 20 mm. Even though 
a small number of our largest specimens were over 20 
mm and may be considered early juveniles in- 3), for the 
purposes of this study, all sailfish collected are referred 
to as larvae. 
Genetic identification 
A percentage of istiophorid larvae (22.2%) were iden- 
tified to the species level by following the protocol of 
Bangma (2006). The protocol was subsequently modi- 
fied and the remaining istiophorid larvae (77.8%) were 
identified according to the protocol of J. Magnussen and 
M. Shivji (personal commun. 3 ). Briefly, a single eyeball 
was removed from each larva and DNA was extracted by 
using a QIAGEN DNeasy blood and tissue kit (QIAGEN 
#69506, Valencia, CA). A multiplex polymerase chain 
reaction (PCR) was performed by using an Eppendorf 
mastercycler gradient, QIAGEN Hot Star Taq DNA 
Polymerase (QIAGEN #203203), and PCR grade dNTP 
mix (QIAGEN #201901). Four primer pairs were used 
in each PCR reaction: a universal billfish primer set, 
and species-specific primers for sailfish, white marlin, 
( Kajikia albida), and blue marlin ( Makaira nigricans). 
PCR reactions were examined by means of gel electro- 
phoresis with 1% agarose gels containing ethidium bro- 
mide and species identification was based on gel banding 
patterns. This multiplex assay was employed to identify 
sailfish larvae as follows. For samples consisting of less 
than 10 individuals, all specimens were assayed. For 
samples with 10-50 istiophorid larvae, 40—60% of larvae 
were processed. For samples with >50 istiophorid larvae, 
25% of larvae were processed. If all larvae in a particu- 
lar subsample were identified as conspecific, remaining 
larvae from the sample were assigned to that species. 
If more than one istiophorid species was detected in a 
subsample, all remaining larvae from the sample were 
identified genetically. 
2 Leben, Robert. 2008. Colorado Center for Astrodynamics 
Research, Univ. Colordao, Boulder, CO. 
3 Shivji, Mahmood. 2007. Guy Harvey Research Institute, 
Nova Southeastern Univ., Ft. Lauderdale, FL. 
