Blue marlin (Makaira nigricans) are iconic, highly migratory pelagic fish that roam the tropical and temperate waters of the Atlantic, Pacific, and Indian Oceans. Answering the question of exactly “how many” remain is a scientific challenge due to their vast range and deep-sea habitat. Scientists cannot conduct a simple physical count of these widely distributed billfish. Instead, population health is determined through complex mathematical models that analyze fishery data, providing the best available estimates of the species’ overall biomass and health.
Current Population Status and Conservation Ranking
Blue marlin global population health is not uniform, requiring separate assessments for Atlantic and Pacific stocks. The Atlantic population, managed as a single unit, is classified as Vulnerable on the International Union for Conservation of Nature (IUCN) Red List of Threatened Species. A 2024 stock assessment from the International Commission for the Conservation of Atlantic Tunas (ICCAT), using data through 2022, indicated that the Atlantic stock is overfished but is currently not subject to overfishing.
The assessment calculated the relative biomass to be approximately 67% of the target level, indicating a depleted state. Statistical modeling suggests there is a 39% probability that the Atlantic stock remains both overfished and subject to overfishing.
The situation in the Pacific Ocean is less concerning, where the stock is considered healthier. Scientific assessments show that the Pacific blue marlin stock is likely not overfished and overfishing is not occurring. In 2019, the spawning stock biomass was estimated to be 17% above the target level needed for maximum sustainable yield. This differentiation illustrates the varied impact of fishing pressure and conservation measures across the world’s oceans.
Logistical Challenges in Estimating Numbers
A precise, individual count of blue marlin is impossible because they are highly migratory and oceanic, living far from coastlines. These fish spend most of their time in the upper layers of the open ocean but make regular vertical excursions, diving up to 800 meters, which makes direct observation impractical. Therefore, scientists must rely on indirect methods to estimate population size and trends.
Stock assessments utilize complex mathematical models, such as Stock Synthesis and JABBA, to integrate various data points and estimate biomass. These models rely heavily on commercial catch data, indices of abundance (such as catch per unit effort, or CPUE), and information gathered from conventional and satellite tagging studies. The inherent uncertainty in these factors, particularly inconsistencies in reported landings and discards, results in a wide margin of error for all population estimates.
Primary Human Impacts on Blue Marlin Populations
The main pressure driving the decline of blue marlin populations is commercial fishing, particularly in high-seas fisheries. Blue marlin are frequently caught as bycatch in high-seas fisheries targeting other species, primarily tuna and swordfish. Longline fishing, which deploys thousands of baited hooks over many miles, is the primary source of this incidental capture and subsequent mortality.
Purse seine nets, especially those set around Fish Aggregating Devices (FADs), also contribute to the bycatch problem by accidentally encircling marlin that congregate near these floating objects. Although some regions, such as the United States, require the release of all billfish caught, the survival rate of these released animals is often low due to injuries sustained during the capture and handling process. The issue is compounded by illegal, unreported, and unregulated (IUU) fishing, which undermines management efforts by failing to document all mortalities.
International Conservation and Management Strategies
The conservation of blue marlin is governed by international agreements overseen by Regional Fisheries Management Organizations (RFMOs) due to the species’ highly migratory nature. In the Atlantic, ICCAT sets regulations, including a current landings limit of 1,670 metric tons for the entire stock. Compliance with these limits has been a recurring challenge, with landings exceeding the established cap in recent years.
Specific regulatory measures focus on reducing bycatch mortality, such as mandating or encouraging the use of non-offset circle hooks instead of traditional J-hooks in longline fisheries. Circle hooks are designed to hook fish in the jaw, increasing the chance of survival if the marlin is released. ICCAT also requires that blue marlin caught alive must be released to maximize post-release survival. On the Pacific side, organizations like the Western and Central Pacific Fisheries Commission (WCPFC) and the Inter-American Tropical Tuna Commission (IATTC) manage their respective stocks through similar catch and gear restrictions.