Zischke and Griffiths: Stock assessment of Acanthocybium solandn in the Pacific Ocean 
413 
10 -| 
8 - 
O ' v* y = -1.48x + 9.83 
' „ R 2 = 0.93 
0 0.5 1 1.5 2 
Age (years) 
Figure 3 
A length-converted catch curve for wahoo (Acanthocybium 
solandri) in the southwest Pacific Ocean (all fisheries com- 
bined) between 2008 and 2010. Numbers of fish in each age 
class were corrected with selectivity probability at age for all 
fisheries combined. Only values indicated by the solid black 
symbols were used in the regression analysis to estimate to- 
tal mortality. R 2 =coefficient of multiple determination. 
thetical implementation of shorter soak times in the 
pelagic longline fishery, thereby allowing more fish to 
remain alive at haul back, and introduced a 20% dis- 
card rate similar to that observed for the recreational 
fishery. All analyses were undertaken with R soft- 
ware, vers. 3.0.2 (R Core Team, 2013). 
Mortality estimates 
Estimates of M obtained from the use of mean 
biological parameters (Table 1) in the 2 empiri- 
cal equations were 0.66-0.74/year. Catch-curve 
analysis where the mean biological parameters 
for all fisheries were combined between 2008 and 
2010 produced an estimate of Z of 1.48/year (Fig. 
3). Subtracting M from Z, the annual F current for 
wahoo in the southwest Pacific Ocean was esti- 
mated as 0.74-0.82/year. 
Sensitivity and management simulations 
In the base scenario (SI), F current was lower than 
both limit reference points ( i.e. , F max and Fssb 25* 
and the target reference point Fq i, but it was 
higher than Fssb 40 ( Table 2). Introduction of a 
10-fold increase in M for young fish (S2) caused 
a decrease in overall Y/R and SSB/R (Fig. 4) but 
resulted in F curren t exceeding the reference points 
in a lower percentage of iterations than the per- 
centage seen in SI (Table 2). In all of the fishery 
management scenarios (F1-F4) examined, F cur _ 
rent was lower than all reference points (Table 2). 
Scenario F2 produced the most favorable results 
with respect to Y/R as F current exceeded the tar- 
get and limit reference points in 2% and 0% of 
iterations (Table 2). With respect to SSB/R, scenario 
F3 produced the most favourable results, as F current 
exceeded the target and limit reference points in 27% 
and 2% of iterations (Table 2). 
Discussion 
Results 
Size and age structure of exploited wahoo 
Length- and age-frequency distributions of wahoo were 
similar for the 3 fisheries examined in the southwest 
Pacific Ocean, but the recreational fishery tended to 
catch slightly smaller and younger fish (Fig. 2). The 
median FLs for the ETBF, New Caledonia, EC Rec 
were 1400 mm (mean: 1366), 1320 (1304) mm, and 
1209 (1222) mm. Corresponding median ages for the 
ETBF, New Caledonia, and EC Rec were 1.25 (mean: 
1.31) years, 1.08 (1.18) years, and 0.82 (0.95) years. 
These median and mean ages are underestimates, how- 
ever, because age could not be calculated for fish with 
an FL greater than 1499 mm (i.e., U. 
Age and size selectivity of fisheries 
The selectivity probability at age for wahoo was similar 
for the 3 fisheries. The age (and FL) at which 50% of 
wahoo were selected in the ETBF, EC Rec, and New 
Caledonia was 0.58 years (mean: 1041 mm), 0.60 years 
(1055 mm), and 0.63 years (1076 mm), respectively. 
Size selectivity of fisheries 
The 3 fisheries examined in this study had similar se- 
lectivity probabilities at age; however, there were dif- 
ferences in the size distribution of wahoo caught in 
these fisheries, particularly between the commercial 
and recreational fisheries off eastern Australia. Nu- 
merous characteristics of pelagic longline gear may in- 
fluence the probability of capture. These characteristics 
include the time of day (i.e., night or day) when the 
gear is soaking in relation to the peak feeding times 
of wahoo, the depth of hooks in relation to the depth 
distribution of the fish, and the size of the hook (and 
bait) relation to the maximum gape of the mouth. For 
example, in a trial where circle hooks were 57% wider 
than the traditional Japanese tuna hooks resulted in 
lower catch rates of smaller nontarget species, includ- 
ing wahoo (Curran and Bigelow, 2011). Circle hooks are 
widely used in the ETBF; and, therefore, these larger 
hooks may select for larger fish. 
Recreational fishing gear used to catch wahoo (ei- 
ther intentionally or incidentally) off eastern Australia 
varies considerably among individual fishermen and in- 
cludes heavy tackle, large hooks, baits, and lures used 
