Intrinsic rate of increase (r) is 0.20 when the population is at a level of abundance that produces the maximum sustainable yield (a management benchmark) of Yellowfin Tuna. Age at 50% maturity for Yellowfin Tuna females is 2.5 years. Maximum age is 8 years (PFMC 2003).

Yellowfin Tuna are attracted to areas in the oceans that are rich in nutrients and support thriving ecosystems, such as seamounts and areas of upwelling. They also aggregate around drifting debris, including floating logs, and around anchored buoys and large marine mammals (e.g., dolphins in the Eastern Pacific). It is not clear whether these areas provide important habitat to the tuna or if they are merely advantageous places to forage. Fishers exploit this aggregative behavior by setting their nets around floating objects, marine mammals, and artificial fish aggregating devices (FADs), which they set out to attract tunas (IATTC 2004a; ICCAT 2004; PFMC 2003).

Yellowfin Tuna spawn frequently, and females can spawn millions of ova per spawning event. Such events can occur daily, if environmental conditions are good (i.e., in areas of high prey abundance; PFMC 2003).

Yellowfin Tuna are highly migratory and distributed worldwide in tropical and sub-tropical seas. In the Pacific, they are found between 40 °N and 40 °S (PFMC 2003). In the Atlantic, they are found between 45 °N and 45 °S (Brown, pers. comm., 2005), except for the Mediterranean Sea, where they are not found (Fishbase 2004).

In general, there has been a remarkable decline in large predatory fish populations worldwide (Myers and Worm 2003). Most Yellowfin Tuna populations are not doing poorly relative to fishery managers’ target abundance sizes, but their populations are depleted compared to historic levels of abundance. For instance, the population of Yellowfin Tuna in the Western Central Pacific is estimated to be 20 to 50% smaller than its previous, unfished size, but compared to managers’ benchmark size, its abundance is very high (SCTB 2004).

In the Atlantic, estimates of Yellowfin Tuna abundance hover around the level that fishery managers set as a target population size. This target size is the biomass needed to produce maximum sustainable yield in the fishery (BMSY), which is a common benchmark used in fisheries management. The most recent estimates of Yellowfin Tuna abundance in the Atlantic range from 73 to 110% of the BMSY. Similarly, estimates of fishing mortality bracket the target mortality rate (estimates range from 87 to 146% of the target). Thus, managers are concerned that the level of fishing pressure may be too high for the population to sustain (ICCAT 2004).

In the Eastern Pacific Ocean, the abundance of Yellowfin Tuna has also oscillated lately around the level needed to achieve maximum sustainable yield. To evaluate the status of this population, managers monitor changes in the ratio of spawning adults alive today to the estimated number of spawning adults alive in an unfished population. Models show that in 2003 this spawning biomass ratio was most likely lower than the level needed to produce maximum sustainable yield. Scientists predicted that Yellowfin Tuna biomass would increase in 2004, then subsequently decrease (IATTC 2004a). It is too early to determine whether these predictions have occurred.

For the Western Central Pacific Ocean population of Yellowfin Tuna, estimates of abundance are high, ranging from 175 to 246% of the target size set by fishery managers. Managers are concerned, however, that if the level of fishing pressure continues to increase, the population could quickly become overfished (SCTB 2004).

Like Atlantic Yellowfin Tuna, the Indian Ocean population is either slightly above or below the target size. In 2003, managers were concerned that overfishing may have occurred, but the data are not yet available to confirm their suspicion (IOTC 2003).

We chose a score of 2.00 points, because several populations of Yellowfin Tuna appear to be at a medium level of abundance.

There are signs of decline in the catch-per-unit-effort data (e.g., the number of fish caught per 1000 deployed hooks) for Atlantic Yellowfin Tuna fisheries (Brown, pers. comm., 2005). In the Eastern Pacific, the population has declined slightly in recent years, and high fishing pressure is expected to cause a continued decline in 2005. That being said, biomass remains higher than it was in 1983, when it was at its lowest recorded level (IATTC 2004a). In the Indian Ocean, Yellowfin Tuna have declined by 40% since the 1980s, when large-scale tuna fisheries in that ocean first appeared (IOTC 2003).

Worldwide, fisheries are catching high numbers of juvenile Yellowfin Tuna (IATTC 2004a; ICCAT 2004; SCTB 2004; IOTC 2003). For example, in the Atlantic during 1997-2001, 54 to 72% of the Yellowfin Tuna caught by purse-seine vessels each year were small (individuals that weigh less than 3.2 kg), as were 63 to 82% of the Yellowfin Tuna caught by baitboats (ICCAT 2004). Continued high levels of fishing pressure on young Yellowfin Tuna could cause problems for their populations if not enough fish are able to reproduce before being caught.

High capture rates of juveniles are probably affecting the age and size distributions of many Yellowfin Tuna populations. For example, data are available in the Eastern Pacific that indicate that the average weight of Yellowfin Tuna caught by fishers is below the "Critical Weight," a reference point with which managers evaluate the impact of fisheries on the size distribution of Yellowfin Tuna. Consequently, Yellowfin Tuna are considered to be overfished in the Eastern Pacific from a "yield-per-recruit" standpoint (IATTC 2004a).

Few data are available that show conclusively that fishing is affecting the size and age distributions of other Yellowfin Tuna populations. We chose to subtract points here, however, because of the well-documented, large catches of juvenile Yellowfin Tuna worldwide and the high probability that many populations are experiencing changes in their size distributions similar to the Eastern Pacific population.

Yellowfin Tuna are not depleted (i.e., classified as "overfished" by fishery managers) in the Atlantic Ocean. However, overfishing may be occurring (ICCAT 2004). They are also not overfished in the Pacific and Indian Oceans, but managers believe that some populations are fully exploited (IATTC 2004a; SCTB 2004; IOTC 2003).

Purse-seine fisheries account for the majority of Yellowfin Tuna catches, with smaller contributions from longline, pole-and-line, and troll vessels (IATTC 2004a; ICCAT 2004; IOTC 2003). All of these gear types fish near the surface and are likely to have a low impact on habitat.

The Gulf of Guinea and the coastal waters off of Africa serve as major spawning and nursery grounds for Yellowfin Tuna in the Atlantic Ocean. In an attempt to decrease high mortality levels of juveniles in purse-seine fisheries in these areas, managers set a moratorium in 1997 on fishing around floating objects (e.g., fish aggregating devices), which attract high numbers of juveniles. Unfortunately, this measure has not reduced mortality rates of Yellowfin Tuna. Since 1997, mortality of juvenile Yellowfin Tuna has increased substantially (ICCAT 2004).

Oceanic habitat is likely healthy enough to support Yellowfin Tuna populations.

Effects of longline gear on habitat are likely to be minimal.

Effects of longline gear on habitat are likely to be minimal.

Many international agencies are involved in the research of tuna species and management of the fisheries that target them. Overall, these agencies have succeeded in stabilizing Yellowfin Tuna at moderate levels of abundance compared to historic levels. Several of these agencies have mandates to recommend management measures to member countries. They are the International Commission for the Conservation of Atlantic Tunas (ICCAT), the Inter-American Tropical Tuna Commission (IATTC) in the Eastern Pacific, the Commission for the Conservation and Management of Highly Migratory Fish Stocks in the Western and Central Pacific Ocean, and the Indian Ocean Tuna Commission (IOTC). Other organizations conduct research and population assessments and include the Forum Fisheries Agency in the South Pacific and the Standing Committee on Tuna and Billfish (SCTB) also in the Pacific.

Despite the abundance of tuna commissions, overall there are few management measures in place for Yellowfin Tuna. Fisheries in the Western and Central Pacific and Indian Oceans, which take a substantial proportion of catches, remain largely unregulated. While these agencies have implemented a few measures (e.g., minimum sizes, fishing effort reduction, and a moratorium on fishing with fish aggregating devices (FADs) in the Atlantic; observer programs in the Atlantic and Pacific; restrictions on FAD fishing, quotas, and time/area closures for purse-seine fisheries in the Eastern Pacific; IATTC 2004a, b; ICCAT 2004), we consider these measures to be minimal compared to the scale of tuna fisheries worldwide.

In the Atlantic, a 1973 minimum size regulation, which prohibits catches that consist of more than 15% “small” fish (less than 3.2 kg), is neither complied with nor enforced. From 1997 to 2001, undersized fish comprised between 54% and 72% of yearly catches in Atlantic purse-seine fisheries. A moratorium on setting purse-seine nets around Fish Aggregating Devices, which attract small fish, did not reduce juvenile Yellowfin Tuna mortality. Indeed, juvenile mortality increased substantially in moratorium years. It is unclear whether this was caused by the moratorium (ICCAT 2004).

There is little information about the status of Yellowfin Tuna populations in the Indian Ocean, where fishing pressure is high (IOTC 2003). Also, fishing mortality of juvenile Yellowfin Tuna in the Atlantic has increased in recent years for unknown reasons (ICCAT 2004).

Yellowfin Tuna populations are not classified as overfished in the Atlantic, Pacific, or Indian Ocean (IATTC 2004a; ICCAT 2004; SCTB 2004; IOTC 2003). Therefore, managers do not consider recovery plans to be necessary.

Worldwide, longline fisheries for tunas deployed an estimated 1.2 billion hooks in 2000 (Lewison et al. 2004). These hooks capture and kill a diversity of non-targeted species, including Vulnerable, Threatened, and Endangered species of seabirds, sea turtles, and marine mammals. In the Atlantic, ten skate and ray, 55 shark, 67 fish (including billfishes), 3 sea turtle, 8 seabird, and 6 marine mammal species have been caught in longline gear targeting tunas (ICCAT 2004). A 1995 study documented that the percentage of discards in Western Atlantic longline fisheries ranged from 40 to 65% of total catches (Hoey 1995). Also, most sharks caught in Northwest Atlantic longline fisheries are declining and at risk of large-scale extirpation from their historic ranges. Bycatch of these species is likely to be contributing substantially to their decline. Canadian longline vessels often catch as many Blue Sharks, for example, as targeted fish (Baum et al. 2004; Crowder and Myers 2001).

Atlantic tuna longlines also capture and kill Leatherback and Loggerhead sea turtles (Lewison et al. 2004), both of which are listed as Endangered (IUCN 2004). Although mortality of sea turtles captured in the Atlantic longline fishery rarely occurs immediately, post-release mortality may be high enough to be having population-level impacts on Leatherback sea turtles (Brown, pers. comm., 2005).

Data on bycatch for West Pacific longline fisheries are limited, but available data indicate that more than 50 species of sharks and fish are captured as bycatch. Like Atlantic longline vessels, West Pacific longliners often catch as many unwanted sharks as tunas (Bailey et al. 1996). American Samoan longline fisheries, which mainly target Albacore Tuna but also land Yellowfin Tuna, discarded 12% of the fish they captured in 2003. Bycatch species included Bigeye and Skipjack Tuna, Blue Marlin, Mahimahi, Oilfish, Wahoo, and Blue and Oceanic Whitetip Sharks (Ito and Hamm 2004). Also, sea turtle bycatch in the Pacific is having population-level effects on Loggerhead, Leatherback, and Olive Ridley sea turtles (Lewison et al. 2004; Crowder and Myers 2001).

Longline fisheries around the world kill hundreds of thousands of seabirds a year (Gilman et al. 2005). Of the 61 species of seabirds affected by longlines in the world, 25 species, 17 of which are albatrosses, are at dangerously low levels of abundance and threatened with extinction (IUCN 2005).

Sea turtle bycatch in the Pacific is having population-level effects on Loggerhead, Leatherback, and Olive Ridley sea turtles (Lewison et al. 2004; Crowder and Myers 2001), all of which are listed as Endangered (IUCN 2004). Atlantic tuna longlines also capture and kill Leatherback and Loggerhead sea turtles (Lewison et al. 2004). Of the 61 species of seabirds affected by longlines in the world, 25 species are at dangerously low levels of abundance and may become extinct. In general, fishery managers worldwide are not taking substantive steps to abate mortality of sea turtles and sea birds in longline gear (Gilman and Friefeld 2003).

In the United States, time/area closures and gear changes in the tuna longline fishery have been instrumental in reducing bycatch mortality. However, Yellowfin Tuna caught in U.S. waters constitute only a small proportion of U.S. supply (NMFS 2005), so we chose to subtract for this factor.

Tuna longline fisheries catch high numbers of undersized Swordfish and other tunas, as well as other fish and sharks (especially Blue Sharks). Bycatch is often discarded dead and sometimes includes marine mammals, sea turtles and seabirds. In U.S. Atlantic and Pacific waters, there are several areas closed to longlining to help alleviate bycatch issues. The goals of each area closure include reducing sea turtle interactions, decreasing bycatch of Bluefin Tuna, and reducing overall bycatch (NMFS 2004). We chose to subtract points here because the vast majority of Yellowfin Tuna available in U.S. markets is caught in international waters (NMFS 2005), where effective measures to reduce bycatch are not being taken (Baum et al. 2004; Lewison et al. 2004; Gilman and Friefeld 2003).

Populations of almost all sharks caught by pelagic longlines in the Northwest Atlantic are declining (e.g., Hammerheads, Blue Sharks, and Porbeagle Sharks; Baum et al. 2004; Crowder and Myers 2001).

Bycatch of Yellowfin Tuna in other fisheries is low. For example, Eastern Pacific tuna purse-seine fisheries discarded only 2% of the Yellowfin Tuna they captured in 2001 (IATTC 2002).