The most recent assessment for blue marlin was conducted in 2018 through a process that included a data preparatory meeting in March 2018 (Anon., 2018) and an assessment meeting in June 2018 (Anon., 2018). The last year of fishery data used in the assessment was 2016.

Atlantic blue marlin inhabits the upper parts of the open ocean. Blue marlin spend the majority of their time in the mixed surface layer (58% of daylight and 84% of nighttime hours), however, they regularly make short-duration dives to depths of around 300 m, with some vertical excursions down to 800 m. They do not confine themselves to a narrow range of temperatures, but most tend to be found in waters warmer than 17°C. The distribution of time at depth is significantly different between day and night. At night, the fish spent most of their time at or very close to the surface. During daylight hours, they are typically below the surface, often at 40 to 100+ m. These patterns, however, can be highly variable between individuals and vary depending on the temperature and dissolved oxygen of the surface mixed layer.

The central and northern Caribbean Sea and northern Bahamas have historically been known as the primary spawning area for blue marlin in the western North Atlantic. Recent reports show that blue marlin spawning can also occur north of the Bahamas in an offshore area near Bermuda at about 32º-34º N. Ovaries of female blue marlin caught by artisanal vessel in Côte d’Ivoire show evidence of pre-spawning and post-spawning, but not of spawning. In this area females are more abundant than males (4:1 female/male ratio). Coastal areas off West Africa have strong seasonal upwelling, and may be feeding areas for blue marlin. More information on blue marlin biology and ecology is published in the ICCAT Manual.

Blue marlin is widely distributed in subtropical and tropical waters of the Atlantic Ocean, with occasionally appearances in temperate waters.

Based on the biological information available for assessment purposes, it is currently assumed one single stock in the Atlantic (Figure 1).

Figure 1: Geographic distribution of blue marlin total catches by decade (last decade only covers 7 years).

Description of fisheries

The decadal geographic distribution of the catches is given in BUM-Figure 1. The Committee used Task I catches as the basis for the estimation of total removals (BUM-Figure 2). Total removals for the period 1990-2016 were obtained during the 2018 Blue Marlin Data Preparatory Meeting by modifying Task I values with the addition of blue marlin that the Committee estimated from catches reported as billfish unclassified. Additionally, the reporting gaps were filled with estimated values for some fleets.

During the 2018 blue marlin assessment it was noted that catches from 2013, 2014 and 2016 had been above the recommended TAC. Over the last 20 years, Antillean artisanal fleets have increased the use of Moored Fish Aggregating Devices (MFADs) to capture pelagic fish. Catches of blue marlin caught around MFADs are known to be significant and increasing in some areas, however reports to ICCAT on these catches are incomplete. Although historical catches from some Antillean artisanal fleets have been recently included in Task I, there still are an unknown number of artisanal fleets that may have unreported catches of blue marlin caught around MFADs. It is important that the amount of these catches be documented. Recent reports from purse seine fleets in West Africa suggest that blue marlin is more commonly caught with tuna schools associated with FADs than with free tuna schools. Preliminary Task I catches of blue marlin (BUM Table 1) in 2016 and 2017 were 2019 and 1987, respectively. These catches are likely underestimated because few CPCs have reported discards.

A series of indices of abundance for blue marlin were presented and discussed during the 2018 Blue Marlin Data Preparatory meeting. Ten CPUE series were used in the assessment. The standard errors from the CPUE standardized series as weights was applied in all assessment models. All estimated standardized CPUE index for blue marlin showed a sharp decline during the period 1960-1975, and thereafter have fluctuated around lower levels. (BUM-Figure 3).

Figure 2: Atlantic blue marlin (Makaira nigricans) Task I cumulative catches (landings + dead discards) (t) by gear type between 1950 and 2017 and TAC.

Figure 3: Plot of the indices of abundance used in the blue marlin stock assessment, 2018.

A full stock assessment was conducted for blue marlin in 2018, applying to the available data through 2016, using both surplus production and age-structured models. Both models estimated similar annual trends of biomass and fishing mortality (BUM-Figures 4 and 5). The results of the 2018 assessment indicated that the estimated B/BMSY and F/FMSY were such that the current stock status is overfished and undergoing overfishing. Since the mid-2000s, the biomass has ceased to decline and fishing mortality has shown a declining trend since its peak in 2003.

The 2018 results are similar to those of the 2011 assessment. The estimated MSY was determined to be 3,001 t with 10% and 90% confident limits of 2,399 to 3,537 t. The current status of the blue marlin stock is presented in BUM-Figure 6. The probability of being in the red quadrant of the Kobe plot was estimated to be 54%. The probability of the being in the yellow quadrants of the Kobe plot was estimated to be 42% and that of being in the green quadrant only 4%. However, the Committee recognizes the high uncertainty with regard to data and the productivity of the stock.

Figure 4: Trends in harvest rate relative to FMSY and biomass relative to BMSY for the Bayesian surplus production model (JABBA) fits to Atlantic blue marlin.

Figure 5: Trend in SSB/SSBMSY (top) and F/FMSY for the stock synthesis model, including approximate 95% confidence intervals.

Figure 6: Combined Kobe plots for the final base cases of Bayesian Surplus Production model (JABBA, blue) and stock synthesis model (SS3, pink) models for the Atlantic blue marlin.

Effects of current regulations

A 2006 recommendation (Rec. 06-09) established that the annual amount harvested by pelagic longline and purse seine vessels and retained for landing must be no more than 33% for white marlin and 50% for blue marlin of the 1996 or 1999 landing levels, whichever is greater. Furthermore, in 2012, the Commission established a TAC for 2013, 2014, and 2015 of 2,000 t (Rec. 12-04), placed additional catch and commerce restrictions in recreational fisheries for blue marlin and white marlin, and requested methods for estimating live and dead discards of blue marlin and white marlin/spearfish. In 2015, the Commission further strengthened the plan to rebuild blue marlin stock by extending for 2016, 2017, and 2018 the annual limit of 2,000 t for blue marlin (Rec. 15-05). However, the catches from 2013, 2014 and 2016 were above the recommended TAC. Furthermore, current assessment results indicate that catches need to be reduced below 2,000 in order to recover to Commission objectives.

The Committee is concerned with the significant increase in the contribution from non-industrial fisheries to the total blue marlin harvest and that the landings from these fisheries are not fully accounted for in the current ICCAT database. The Committee expressed its serious concern over this limitation on data for future assessments. Such data limitation impairs any analysis of the current regulations.

Currently, four ICCAT Contracting Parties (Brazil, Canada, Mexico, and the United States) mandate or encourage the use of circle hooks on their pelagic longline fleets. Recent research has demonstrated that in some longline fisheries the use of non-offset circle hooks resulted in a reduction of billfish mortality, while the catch rates of several of the target species remained the same or were greater than the catch rates observed with the use of conventional J hooks or offset circle hooks.

More countries have started reporting data on live releases since 2006. Additional information has come about, for some fleets, regarding the potential for modifying gears to reduce the by-catch and increase the survival of marlins. Such studies have also provided information on the rates of live releases for those fleets. However there is not enough information on the proportion of fish being released alive for all fleets, to evaluate the effectiveness of the ICCAT recommendation relating to the live release of marlins.