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’ target population 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 purse-seine gear on habitat are likely to be minimal.

Effects of purse-seine 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).

To protect dwindling populations of Yellowfin and Bigeye Tuna in the Eastern Pacific, managers instituted a one-and-a-half month moratorium on fishing for Yellowfin, Bigeye, and Skipjack Tuna within the area bounded by the coastline of the Americas, the 40°N parallel, the 150°W meridian, and the 40°S parallel. This moratorium will occur in late summer or late fall on an annual basis through 2006 (IATTC 2004b). That being said, no action is being taken in the Western Central Pacific or Indian Ocean to control capacity (SCTB 2004; IOTC 2003). We have chosen to subtract here, because trade data indicate that most of the U.S.’s supply of Yellowfin Tuna is from fisheries in these poorly managed regions (NMFS 2005).

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.

Purse-seine fisheries catch Yellowfin Tuna in 3 types of schools: associated with dolphins (which only occurs in the Eastern Pacific), associated with artificial fish aggregating devices (FADs), and associated with other fish. The 3 methods yield varying levels of bycatch. Bycatch species can include Endangered or Threatened sea turtles (ICCAT 2004c; SCTB 1996, 2004), but the mortality of sea turtles in purse-seine nets has been greatly reduced in recent years to less than 30 observed deaths last year. Most sea turtles are captured by purse-seine vessels targeting Skipjack Tuna associated with FADs. Fishery managers are hoping to prohibit the use of webbing on FADs, in which sea turtles get entangled, in the near future, which ought to greatly reduce that source of mortality for sea turtles (Hall, pers. comm., 2005). Since most Yellowfin Tuna are caught in schools associated with dolphins, the bycatch of sea turtles in tuna purse seines is not a major problem for the Yellowfin Tuna fishery, but is something to consider for the Skipjack Tuna fishery.

Fishing for Yellowfin Tuna associated with dolphins in the Eastern Pacific has been and continues to be controversial. Incidental mortality of dolphins in purse-seine nets was very high in the 1960s and 1970s, and, as a result, populations of Spotted and Spinner Dolphins declined. Measures by the U.S. and the Inter-American Tropical Tuna Commission in the 1980s and 1990s (e.g., increased regulations, observer coverage, and scientific studies) eliminated bycatch of dolphins in the U.S. fleet (because U.S. vessels were prohibited from setting their nets around dolphin herds) and led to a 99% decline in dolphin bycatch (from more than 100,000 dolphins a year to less than 2000) in the international purse-seine fleet (Gerrodette and Forcada 2005; IATTC 2004c).

Despite this remarkable reduction in bycatch mortality, dolphin populations have yet to recover. The World Conservation Union lists both of these species as Lower Risk (i.e., their populations are dependent on conservation measures; IUCN 2004). Two depleted populations of dolphins, the northeastern offshore Spotted Dolphin and a subspecies of Spinner Dolphin, have not shown any statistically significant increase (or decrease) in abundance between 1979 and 2000 (Gerrodette and Forcada 2005).

In dolphin-associated purse-seine operations, fishers encircle dolphins and tuna with nets, retrieve most of the net, “back down” to enable the dolphins to escape, and then bring in the rest of the net and catch. During these operations, encircled dolphins die due to net canopies and collapses and from stress in the nets during prolonged back-down operations. Gerrodette and Forcada (2005) presented several hypotheses to explain the lack of recovery by dolphins, including underestimated dolphin mortality due to limitations in observer coverage, unobserved mortality of orphaned calves when their lactating mothers are killed, and the stress caused by being captured and released in purse-seine sets may be causing separation and loss of juvenile dolphins, miscarriages, and other problems.

In order to be labeled “Dolphin-Safe” in the U.S., a can of tuna must only contain tuna caught in purse-seine operations that did not intentionally chase and encircle dolphins (Gerrodette and Forcada 2005). However, these regulations do not prevent all vessels in the fishery from fishing for dolphin-associated tuna, just the vessels that want to sell their tuna to the U.S. Indeed, the number of times purse-seine vessels fished on dolphin-associated schools decreased only moderately in the 1990s and in 2003 was the highest on record (IATTC 2004a). Members of the northeastern offshore population of Spotted Dolphins have been estimated to interact with purse-seine nets 2 to 50 times a year (Perkins and Edwards 1999).

Purse-seine vessels also set nets around floating objects (such as logs and fish aggregating devices) or around schools of fish that are unassociated with objects. Bycatch in purse-seine operations that fish on floating objects is much higher than bycatch in dolphin-associated fisheries. Hall (1998) estimated that saving 1 Eastern Pacific dolphin by fishing around floating objects “costs” 16,000 small tunas, 380 Mahimahi, 190 Wahoo, 20 sharks and rays, 1200 Triggerfish and other small fish, 1 marlin, and other animals. Because of much higher levels of bycatch in purse-seine sets on tuna schools associated with floating objects (including FADs) and on unassociated schools (IATTC 2004a), the Inter-American Tropical Tuna Commission believes that purse-seining for Yellowfin Tuna associated with dolphins is the most responsible method of the three methods (Allen, pers. comm., 2005). In 2004, FAD fisheries only accounted for 10% of the total Eastern Pacific Yellowfin Tuna purse-seine catch (IATTC 2004a).

In the Western Central Pacific, where tuna do not school with dolphins, bycatch is poorly documented. A 1996 study found that bycatch comprised 0.35 to 0.77% of the total catch (by weight) for sets on tuna associated with schools and 3.0 to 7.3% of the total catch for sets on floating logs. Observer coverage, which contributed the data for this study, was very low and restricted to Japanese, U.S., and New Zealand seiners. Therefore, these percentages should be interpreted as low. Bycatch species included Amberjack, Mackerel Scad, Rainbow Runner, Drummer, Mahimahi, Ocean Triggerfish, and Blue Marlin, and the majority of bycatch that is discarded was dead or injured. In some fleets, many species captured incidentally were retained for local sale or consumption. Observers documented incidental capture of sea turtles too, which were generally released alive (SCTB 1996). A recent review of the U.S. tuna purse-seine fishery in the Western Central Pacific also cited bycatch of finfish, billfishes, sharks, and rays but provided few details (Ito and Hamm 2004).

In the Atlantic, tuna purse-seine vessels have low levels of bycatch and do not interact with sea turtles and marine mammals regularly (Brown, pers. comm., 2005). During 1997 to 2002, ninety-eight percent of catches in the French purse-seine fleet consisted of targeted Yellowfin, Skipjack, and Bigeye Tunas, 1.2% were small and other tunas, and the other 0.8% included Wahoo, Common Salmon, Sea Bream, Dolphinfish, Wreckfish, Triggerfish, Horse Mackerel, Pomfret, Marlin, Sailfish, and Silky Shark (Goujan 2004).

To decide which score to award here, with consideration of incidental catches of sea turtles in the Eastern and Western Central Pacific, deleterious effects on dolphins in the Eastern Pacific, and low overall bycatch rates throughout the ocean basins, we looked at trade statistics. The US imports the majority of its Yellowfin Tuna supply, which arrives in fresh, frozen, and canned forms (NMFS 2005). Canned tuna generally contains Yellowfin and Skipjack Tuna, along with Bigeye and Tongol Tuna to a lesser extent (Donley, pers. comm., 2005). It is imported in such large quantities that it likely is the major source of Yellowfin Tuna in U.S. markets. The major countries that export canned tuna to the US are Thailand, Ecuador, Philippines, and Indonesia (NMFS 2005). Except for Ecuador, these countries generally fish in the Western Pacific and Indian Ocean, where little is known about current bycatch rates in purse-seine fisheries. As a result of all of these factors, we chose to award a medium score of 2.00 points here.

In the Eastern Pacific, where interactions between international tuna purse-seine vessels and marine mammals remain common, incidental mortality of dolphins has decreased 99% since the late 1980s. In the US fleet, regulations prohibiting vessels from targeting Yellowfin Tuna associated with dolphins have eliminated dolphin mortality from the fishery (IATTC 2004c).

Although Spinner and Spotted Dolphins are no longer “removed” (i.e., incidentally killed) in high numbers by the Eastern Pacific tuna purse-seine fishery, their populations have yet to recover (see above for hypotheses regarding this; Gerrodette and Forcada 2005; IATTC 2004c). We chose not to add here to account for the continued impacts on dolphin populations, especially the northeastern offshore Spotted Dolphin population and an eastern subspecies of Spinner Dolphin, which are likely caused through their frequent interactions with purse-seining operations in the Eastern Pacific (Gerrodette and Forcada 2005).

Bycatch rates in Eastern Pacific and Atlantic Yellowfin Tuna purse-seine fisheries are generally low (Allen, pers. comm., 2005; Brown, pers. comm., 2005). Bycatch in Western Pacific and Indian Ocean tuna fisheries, where the majority of Yellowfin Tuna in the U.S. originates (NMFS 2005) is poorly documented but also likely low (SCTB 1996, 2004; IOTC 2003).

Bycatch of Yellowfin Tuna in other fisheries is low. Discards of Yellowfin Tuna in high-seas longline fisheries are low. For example, in U.S. Western Central Pacific fisheries, only 5% of captured Yellowfin Tunas were discarded in 2003 (Ito and Hamm 2004).