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Stock Status and Conservation Advice
(From ISC16 Plenary Report)

Blue marlin

Stock Status

Estimates of total Blue marlin (BUM) stock biomass show a long term decline. Population biomass (age-1 and older) averaged roughly 130,965 t in 1971-1975, the first 5 years of the assessment time frame, and has declined by approximately 40% to 78,082 t in 2014 (Figure 1). Female spawning biomass was estimated to be 24,809 t in 2014, or about 25% above SSBMSY (Table 1 and Table 2). Fishing mortality on the stock (average F, ages 2 and older) averaged roughly F = 0.28 during 2012-2014, or about 12% below FMSY. The estimated spawning potential ratio of the stock (SPR, the predicted spawning output at the current F as a fraction of unfished spawning output) is currently SPR2012-2014 = 21%. Annual recruitment averaged about 897,000 recruits during 2008-2014, and no long-term trend in recruitment was apparent. Overall, the time series of spawning stock biomass and recruitment estimates indicate a long-term decline in spawning stock biomass and suggest a fluctuating pattern without trend for recruitment (Figure 1). The Kobe plot depicts the stock status relative to MSY-based reference points for the base case model (Figure 2) and shows that spawning stock biomass decreased to roughly the MSY level in the mid-2000s, and has increased slightly in recent years (Table 1 and Table 2).

Based on the results of this 2016 stock assessment update, the Pacific blue marlin stock is not currently overfished and is not experiencing overfishing. Because Pacific blue marlin is mainly caught as bycatch, direct control of the annual catch amount through the setting of a total allowable catch may be difficult.

Table 1. Reported catch (t) used in the stock assessment along with annual estimates of population biomass (age-1 and older, t), female spawning biomass (t), relative female spawning biomass (SSB/SSBMSY), recruitment (thousands of age-0 fish), fishing mortality (average F, ages-2 and older), relative fishing mortality (F/FMSY), and spawning potential ratio of Pacific BUM.
Year 2008 2009 2010 2011 2012 2013 2014 Mean1 Min1 Max1
Reported Catch 17,828 18,282 20,086 18,165 19,407 20,727 20,356 18,232 9,160 25,589
Population Biomass 71,768 69,720 72,696 72,995 76,697 78,761 78,082 101,149 69,720 135,623
Spawning Biomass 22,706 23,065 22,392 23,182 23,432 24,771 24,809 41,717 20,972 71,807
Relative Spawning Biomass 1.14 1.16 1.13 1.17 1.18 1.25 1.25 2.10 1.06 3.62
Recruitment (age 0) 687 1031 702 1061 763 909 839 897 589 1181
Fishing Mortality 0.27 0.29 0.30 0.26 0.27 0.28 0.28 0.22 0.09 0.38
Relative Fishing Mortality 0.82 0.88 0.92 0.82 0.83 0.87 0.87 0.67 0.26 1.17
Spawning Potential Ratio 22% 21% 20% 22% 22% 21% 21% 31% 15% 57%
1During 1971-2014
Table 2. Estimates of biological reference points along with estimates of fishing mortality (F), female spawning stock biomass (SSB), recent average yield (C), and spawning potential ratio (SPR) of BUM, derived from the base case model assessment model, where “MSY Eand  E0% Eindicate reference points based on maximum sustainable yield and a spawning potential ratio of 20%, respectively.
Reference Point Estimate
FMSY (age 2+) 0.32
F20% (age 2+) 0.30
F2012-2014 (age 2+) 0.28
SSBMSY 19,853 mt
SSB20% 22,727 mt
SSB2014 24,809 mt
MSY 19,901 mt
C2012-2014 20,163 mt
SPRMSY 0.18
SPR2012-2014 0.21
Note: SSB values represent female spawning biomass only.
ISC16_BUM_Figure1
Figure 1. Time series of estimates of
  1. population biomass (age 1+)
  2. female spawning biomass
  3. recruitment (age-0 fish)
  4. instantaneous fishing mortality (average for age 2+, year-1)
for BUM derived from the 2016 stock assessment update. The solid circles represents the maximum likelihood estimates by year for each quantity and the shadowed area represents the uncertainty of the estimates (± 1 standard deviation), except for the total biomass time series. The solid horizontal lines indicate the MSY-based reference points for spawning biomass and fishing mortality.
ISC16_BUM_Figure2
Figure 2. Kobe plot of the time series of estimates of relative fishing mortality (average of age 2+) and relative spawning stock biomass of BUM during 1971-2014. The dashed lines denote the 95% confidence intervals for the estimates in the year 2014.

Conservation Advice

Since the stock is nearly full exploited, the ISC recommends that fishing mortality remain at or below current levels (2012-2014).