Intrinsic rate of increase for Skipjack Tuna is estimated to be 0.40 (PFMC 2003). Age at maturity is 1.5 years (Fromentin and Fonteneau 2001). Maximum age is 8-12 years (PFMC 2003).

Skipjack Tuna aggregate around drifting flotsam. Skipjack Tuna fisheries take advantage of this behavior by setting their nets around floating objects, as well as by using artificial fish aggregation devices (FADs) to attract schools of Skipjack Tuna (ICCAT 2004).

Recruitment (i.e., the addition of juvenile fish to the fishable population) to the Skipjack Tuna population in the Western and Central Pacific is higher after El Nino events (e.g., during 1990-91 and 1997-98). Also, decadal changes in biological productivity in the Pacific Ocean, known as the Pacific Decadal Oscillation, may have caused an increase in recruitment in the mid-1980s (Langley et al. 2002).

Since the majority of Skipjack Tuna available in U.S. markets is caught in the Pacific and Indian Oceans (NMFS 2005), we chose to subtract points here.

Skipjack Tuna are highly fecund; females produce 100,000 to 2 million ova. They spawn year-round in tropical waters and seasonally in subtropical waters. Spawning can occur as often as every 1.2 days (PFMC 2003).

Skipjack Tuna are found worldwide in tropical, sub-tropical, and warm-temperate seas (PFMC 2003).

The abundance of Skipjack Tuna in the Atlantic is unknown. Certain aspects of Skipjack Tuna’s biology (e.g., continuous recruitment throughout the year, but varying in time and area, and variable growth rates) prevent fishery managers from making estimates of biomass (i.e., population size) or from assessing the rate that fish are caught in the fishery (i.e., fishing mortality). Consequently, fishery managers have not set targets for sustainable levels of fishing, such as maximum sustainable yield (MSY). However, there is evidence that overfishing may be occurring in the Eastern Equatorial region where purse-seine vessels use fish aggregating devices to attract Skipjack Tuna (ICCAT 2004).

Skipjack Tuna biomass in the Eastern Pacific Ocean is 60% of the population size estimated if there were no fishing occurring. Biomass of the Eastern Pacific population has fluctuated greatly over time due to changes in recruitment (i.e., the addition of young fish to the breeding population) and fishing exploitation. Managers monitor the size of the breeding Skipjack Tuna population to quantify changes in overall abundance. In 2003, the ratio of spawning biomass to that of a hypothetically unfished population was considered to be high (about 0.61; IATTC 2004a). For comparison, the spawning biomass ratio in 2001 was 0.23 (IATTC 2002). High recruitment of young fish to the spawning population in 2002-2003 likely contributed to the high level of abundance measured in 2003 (IATTC 2004a).

In the Western and Central Pacific, Skipjack Tuna abundance was at a 30-year high in 2002 (Hampton 2002). Managers estimate, however, that in recent years fishing has reduced the population size by 20-25% of its unfished size. Still, biomass of Skipjack Tuna shows considerable variation from year to year. Fishing mortality has increased greatly in recent years, but managers believe the impact of fisheries on the population remains relatively low (Langley et al. 2002).

In the Indian Ocean, Skipjack Tuna abundance is unknown. In recent years, managers have recorded increasing catches due to increasing fishing effort. In their last report, they recommend that catches be monitored closely (IOTC 2003).

No points were subtracted here because trends are either uncertain or they vary according to region.

The Eastern Atlantic population of Skipjack Tuna, for example, shows divergent trends in abundance, depending on the area assessed. This divergence may be because Skipjack Tuna populations appear to be “viscous”, meaning that there is little mixing between areas. A viscous population has the potential for local declines in a portion of the population, which may not affect abundance in other areas. Population viscosity also suggests that few fish are making large-scale migrations. Trends indicate possible local overfishing in the Equatorial area since the mid-1990s (Gaertner et al. 2001) where fishing effort by purse seiners using fish aggregating devices (FADs) is concentrated. The Western Atlantic population appears to be stable (ICCAT 2004).

In the Eastern Pacific, recruitment (i.e., the addition of young fish to the fishable population) is highly variable over time. As such, population biomass also fluctuates greatly, due both to changes in recruitment and levels of fishing exploitation. For example, in 2003, the number of spawning Skipjack Tuna in the Eastern Pacific was almost three times as high as that measured in 2002. Managers believe that this relatively high level of biomass is attributable to strong recruitment of young fish into the fishable population in 2002-2003. More recently, recruitment to the fishable population has been of an average magnitude, which may decrease abundance levels and catches in upcoming years (IATTC 2004a).

In the Western and Central Pacific, recruitment to the Skipjack Tuna population is at one of the highest levels observed since the 1970s (SCTB 2004). Strong El Nino events, such as the ones in the 1990s, appear to have benefited Skipjack Tuna recruitment. Because recruitment drives trends in biomass, biomass has increased since the mid-1980s, peaked from 1998-2000, and remains high (SCTB 2004; Hampton 2002). Catch-per-unit-effort (CPUE; e.g., the number of fish captured per 10 purse-seine sets) was stable from 1988-1997. Managers note that recent increases in CPUE in purse-seine sets on tuna schools unassociated with floating objects may have resulted from adoption of better detection technology in the fleets. For purse-seine sets on floating objects (logs and fish aggregating devices), CPUE increased from 1998 to 2002. This increase may indicate that the Skipjack Tuna population grew over that period or effective fishing effort increased (Langley et al. 2002).

In the Indian Ocean, CPUE rates increased until reaching an all-time high in 2002 (IOTC 2003). However, this increase in CPUE likely does not indicate that the population abundance was increasing, but that effective fishing effort (which is not accounted for by traditional CPUE measures) increased. Managers attribute this to a large increase in catches by purse-seine gear set around floating objects. Indeed, in three major areas, off Somalia, the Western Seychelles, and the Mozambique Channel, CPUE trends have been stable since the late 1980s (IOTC 2003). Overall, Skipjack Tuna in the Indian Ocean have likely been fully exploited since the end of the 1990s (Gaertner et al. 2001).

In the late 1990s, there were remarkable decreases in the mean sizes and weights of Skipjack Tuna caught by East Atlantic fleets. The mean weight has increased since, however (ICCAT 2003).

Juvenile Skipjack Tuna in the Western and Central Pacific are considered by fishery managers to be “very lightly exploited,” and fishing mortality rates are lower than natural mortality rates in the corresponding age classes (Langley et al. 2002).

In the Indian Ocean, mean weight of Skipjack Tuna catches has been stable (IOTC 2003).

We chose not to subtract or add for this factor because there is little evidence for the impacts of fishing on the age, size, or sex distributions of Pacific populations, which supply the majority of Skipjack Tuna to U.S. markets (NMFS 2005). Also, the impact of increased catches of juvenile Skipjack Tuna on their populations may be moderate due to the high natural mortality of juveniles of this species (Hampton 2000).

Skipjack Tuna are not listed as overfished by any of the agencies that monitor their populations (IATTC 2004a; ICCAT 2004; SCTB 2004; IOTC 2003).

The majority of Skipjack Tuna catches are from purse-seine fisheries (including baitboats), with a limited contribution from pole-and-line fisheries. These gear types fish near the surface and are likely to have a low impact on habitat.

The pelagic habitat of Skipjack Tuna is likely robust enough to support healthy populations.

Habitat effects of purse-seine gear are likely minimal.

Habitat effects of purse-seine gear are likely 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 maintaining Skipjack Tuna biomass 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 only 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 the Pacific tuna fisheries that supply the majority of Skipjack Tuna (in both whole and canned forms) to the U.S. market (NMFS 2005). In 2002, 89% of Pacific Ocean catches of Skipjack Tuna were taken by fisheries in the Western and Central Pacific, which remain largely unregulated (Langley et al. 2002). NEED SENTENCE HERE. And, in the Indian Ocean, Skipjack Tuna fisheries are also not managed with catch limits or other measures (IOTC 2003). While Atlantic and East Pacific agencies have implemented measures to regulate fishing by tuna purse-seine vessels (e.g., 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), these measures cover only a small proportion of the tuna fisheries that supply Skipjack Tuna to the US market. Therefore, we choose to award the low score of 1.00 point here. This score will be reconsidered when agencies in the Western and Central Pacific implement control rules for the tuna purse-seine fisheries.

The last population assessment of Skipjack Tuna in the Atlantic was performed in 1999. There are no estimates of biomass, or management targets for biomass for Skipjack Tuna in the Atlantic. Although managers consider there to be two populations of Skipjack Tuna, they are unsure as to what degree of mixing occurs between the populations. Also catches are likely under-reported, due to the discarding of small Skipjack Tuna by purse-seine vessels, including those that use baitboats (ICCAT 2004).

Managers of the Eastern Pacific tuna fisheries do the best job of monitoring Skipjack Tuna. They have population size estimates that reach back to 1975, and they continue to collect data on catches, discards, fishing effort, and size compositions of catches in mixed-tuna fisheries that catch Skipjack Tuna (IATTC 2004a).

Despite tuna fisheries being largely unregulated in the Western and Central Pacific, scientific monitoring of the effects of fishing on Skipjack Tuna populations is adequate (Langley et al. 2002; SCTB 2004). The abundance of Skipjack Tuna in the Indian Ocean is unknown, as are the effects of fishing on the population (IOTC 2003).

Since the majority of the U.S.’s supply of Skipjack Tuna is from Japanese, Philippine, Mexican, Panamanian, and Ecuadorian tuna fisheries (NMFS 2005), we chose to add here. Monitoring of Skipjack Tuna populations in the Pacific is adequate and better than in the Atlantic and Indian Oceans.

Skipjack 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.

Atlantic tuna managers placed a moratorium on setting purse-seine gear on floating objects in the eastern Atlantic from November to January to protect juvenile Bigeye Tuna. This moratorium was first implemented in 1997 and is still in effect today. Since, large decreases in capacity (-29%) and the average annual Skipjack Tuna catch (-41%) have occurred in the European purse-seine fleet. However, landings by Ghanaian purse seiners have increased greatly during this period (ICCAT 2003). The overall effect of the moratorium on the Skipjack Tuna population is not known (Gaertner, pers. comm., 2005).

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). Along with a decrease in fishing effort, the moratorium has resulted in a lower Skipjack Tuna catches (IATTC 2004a).

In the Western and Central Pacific Ocean, no action is being taken to control capacity in the tuna fleets that catch Skipjack Tuna (SCTB 2004). In 2001, Indian Ocean tuna boat owners participated in a voluntary restriction on catching Skipjack Tuna due to an oversupply of the species in the world market. This restriction caused a temporary reduction in the number of purse-seine gear set using fish aggregating devices (IOTC 2003).

Since efforts to control capacity in tuna fisheries are not in place in the Western and Central Pacific, we chose to neither add nor subtract for this factor.

Purse-seine fisheries that catch Skipjack Tuna have varying levels of bycatch associated with them, depending on what type of schools they target and where they fish. Schools of Skipjack Tuna can be associated with floating objects, associated with dolphins (only in the Eastern Pacific), or associated with other fish. Most Skipjack Tuna are caught in purse-seine nets set around floating objects (e.g., Fish Aggregating Devices).

Bycatch in tuna fisheries that target Skipjack Tuna can include Endangered or Threatened sea turtles (IATTC 2004a; SCTB 2004). However, in recent years, the mortality of sea turtles in purse-seine nets in the Eastern Pacific has been greatly reduced. Last year, there were less than 30 observed deaths in the Eastern Pacific fishery. Most of these sea turtles were captured by tuna purse-seine vessels that were using with Fish Aggregating Devices. In the future, Eastern Pacific fishery managers are hoping to prohibit the use of webbing on FADs, in which sea turtles get entangled, which ought to greatly reduce that source of mortality for sea turtles (Hall, pers. comm., 2005).

Bycatch in purse-seine operations that set around floating objects is much higher than bycatch in dolphin-associated fisheries. Hall (1998) estimated that saving 1 dolphin by fishing around floating objects in the Eastern Pacific “costs” 16,000 small tunas, 380 Mahimahi, 190 Wahoo, 20 sharks and rays, 1200 Triggerfish and other small fish, 1 marlin, and other animals.

Despite public opposition to fishing for tuna associated with dolphins, the number of sets on dolphins decreased only moderately in the 1990s and in 2003 was the highest on record (IATTC 2004a). Dolphins, including the Eastern Spinner and Offshore Spotted Dolphins, are still killed in these fisheries, although mortality is much lower than it was in the late 1980s (greater than 100,000 killed in 1989 compared to 3000 killed in 1997; Gosliner 1999). That being said, dolphin populations have yet to recover, and researchers believe the stress inherent to getting caught and released in “dolphin-safe” purse-seine fisheries may cause separation and loss of juvenile dolphins, miscarriages, and other problems (Gerrodette and Forcada 2005).

In the Western Central Pacific, where tuna do not school with dolphins, bycatch is poorly documented. A recent review of the U.S. tuna purse-seine fishery in the Western Central Pacific 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 the period from 1997 to 2002, 98% 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). Between 1997 and 1999, bycatch of billfishes constituted less than 0.021% of the total tuna catch and less than 10% of the total catches of billfishes reported for the eastern Atlantic during that period (Gaertner et al. 2002).

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 Skipjack Tuna supply, which arrives in fresh, frozen, and canned forms (NMFS 2005). Canned “light-meat” tuna generally contains Yellowfin and Skipjack Tuna, along with Bigeye and Tongol Tuna to a lesser extent (Donley, pers. comm., 2005). 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 bycatch rates in purse-seine fisheries and the degree to which these fisheries impact threatened, endangered, or protected species. As a result of these considerations, we chose to award a medium score of 2.00 points here.

Skipjack Tuna are discarded by purse-seine fisheries that target tunas. From 1993-2003, an average of 11.5% of the annual Skipjack Tuna catch was discarded at sea in the Eastern Pacific (IATTC 2004a). In the Western and Central Pacific, purse-seine fisheries discard large numbers of Skipjack Tuna (Ito and Hamm 2004).

Purse-seine fisheries that catch Skipjack Tuna interact with declining species of dolphins, sea turtles, and billfishes, as well as catch other tuna species from declining populations.

The incidental catch of Endangered and Threatened sea turtles, including Olive Ridley, Green, Leatherback, Hawksbill, and Loggerhead sea turtles, in Eastern (IATTC 2004a) and Western Central Pacific (SCTB 2004) purse seines is not likely as detrimental to their populations as their interactions with longlines. However, the level of their interaction with purse seines has not been adequately quantified or addressed. We therefore chose not to add here but plan to re-visit this factor when there are indications that fishery managers are making needed, large-scale efforts to protect declining populations of sea turtles from stress, injury, and death in interactions with purse-seine operations.

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 Skipjack Tuna in the U.S. originates (NMFS 2005) is poorly documented but also likely low (SCTB 2004; IOTC 2003). That being said, from 1993-2003, an average of 11.5% of the annual Skipjack Tuna catch was discarded at sea in the Eastern Pacific (IATTC 2004a). In the Western and Central Pacific, purse-seine fisheries discard large numbers of Skipjack Tuna (Ito and Hamm 2004). Because of the high level of Skipjack Tuna discarding, we chose to not add here.

There is currently a moratorium on fishing with FADs in the Atlantic. The moratorium has reportedly reduced catches of juvenile Bigeye Tuna in the purse seine fishery, which has complied with the regulation. However, other fisheries such as the baitboat fishery that operates in the Gulf of Guinea and assists purse seiners setting on FADs, have not fully complied with the regulation. Better compliance with this regulation is expected to reduce juvenile Bigeye Tuna mortality (ICCAT 2004).

There are some restrictions on fishing with FADs in the Eastern Pacific. During certain months, FAD fishing is prohibited. And in December, purse seine vessels are prohibited from fishing in the Eastern Pacific (IATTC 2002b).

We chose not to add here until bycatch is better documented in the Western Pacific and Indian Oceans and there is better compliance with regulations in the Atlantic Ocean.