The intrinsic rate of increase for Skipjack Tuna is estimated to be 0.40 (PFMC 2003a). Age at maturity is between one and two years (ICCAT 2008). Based on the age-structured assessment model, the average age to which 1% of recruits to the fishery would survive in the absence of fishing is around 3.5 years (Hampton, 2009).

Skipjack Tuna aggregate around drifting flotsam. This biological feature has been exploited by fishers through the use of Fish Aggregating Devices (FADs) in conjunction with purse seine gear (ICCAT 2008b). The attraction of Skipjack Tuna is strong and FAD use is believed to have changed stock structure and migratory patterns (ICCAT 2008c). Pole and line gear is not used in conjunction with FADs. Nonetheless, since the effects of FADs are far reaching and potentially impact all Skipjack Tuna, points are subtracted.

Recruitment to the Skipjack Tuna population in the Western and Central Pacific increases following strong El Nino events like those in 1997-98 and 2004-2005 (WCPFC 2008). Evidence suggests that El Nino impacts on juvenile recruitment are greater when El Nino rapidly follows a period of La Nina (DEH 2005). Changes in the biological productivity of the Pacific Ocean, referred to as the Pacific Decadal Oscillation, may have caused an increase in recruitment in the mid-1980s (Langley et al. 2002). The majority of the Skipjack Tuna available in U.S. market is caught in the Pacific and Indian Oceans (NMFS 2005).

It is unclear whether recruitment rates to Skipjack Tuna populations vary with El Nino Southern Oscillation events in the Atlantic and Indian Ocean as they do in the Pacific Ocean (WCPFC 2008). However, both the Atlantic and Indian Ocean are known to experience cyclical fluctuations in ocean temperature.

Skipjack Tuna are highly fecund, with females producing between 100,000 and 2 million eggs per spawning event. Skipjack Tuna spawn year-round in tropical waters and seasonally in subtropical waters. Eggs hatch as soon as one day after fertilization. Spawning can occur as often as every day. (SOEST 2000)

Skipjack Tuna are found in tropical, sub-tropical, and warm-temperate waters of every ocean. Skipjack Tuna are commercially fished for in the Atlantic, Indians and Pacific Oceans. In the Pacific Ocean, for example, Skipjack Tuna inhabit the waters between 45ºN and 45ºS in the Western Pacific, although this range can change due to El Nino Southern Oscillation events. Skipjack Tuna in the Eastern region of the Pacific Ocean (e.g., the United States, Mexican, Central American, and South American coastlines) inhabit the region between 40ºN and 40ºS. During strong El Nino events, the Eastern Pacific boundary can extend as far north as 50ºN (PFMC 2003b).

The biomass of Skipjack Tuna in the Pacific Ocean is believed to be >125% of BMSY (WCPFC 2008). Biomass of the Pacific population can differ from year to year because of regional variations in abundance, recruitment variations due to El Nino events, and variable spawning rates. For example, populations in the Eastern Equatorial Region were higher than usual in 2004-2005 due to strong El Nino events that caused a higher recruitment rate. Conversely, recruitment was lower in 2001-2003 due to La Nina events. Biomass is estimated to be significantly higher in certain regions of the Western Pacific Ocean where the pole-and-line method is favored (WCPFC 2008). Abundance of Skipjack Tuna in these regions may skew biomass estimates (WCPFC 2008). Despite wide scale Skipjack Tuna fishing operations in the Pacific Ocean, Skipjack Tuna are still abundant; the ratio of spawning biomass (SBR) to that of a hypothetical unfished stock was 0.61 in 2003 (IATTC 2008). Catches have increased due to an increase in the scale of fishing operations (SCTB 2004), however there is no indication that stocks have been depleted from increased fishing effort (WCPFC 2008).

The abundance of Skipjack Tuna in the Indian Ocean is believed to be stable (IOTC 2008), though there has been a slight downward trend in Indian Ocean Skipjack Tuna catch. This trend, however, may be a result of lower efforts near Somalia due to piracy and an increase in fuel prices (IOTC 2008a). Since exploitation rates are low in the Indian Ocean (believed to be at less than 20%), it is unlikely that the decrease in catch is due to a lack of abundance caused by fishing effort (IOTC 2008a).

The abundance of Skipjack Tuna in the Atlantic Ocean is also believed to be stable (ICCAT 2008a). BMSY is estimated at 1.66 and has been stable for the last 15 years. Biomass has fallen over the last several decades. Due to high recruitment, however, populations are still considered healthy (ICCAT 2008a).

In the Indian Ocean, catch rates have followed a consistently upward trend until 2007, when catches of Skipjack Tuna by purse seine fleets dropped to their lowest levels since 1998 (IOTC 2008a). Since there has been no population assessment it is unclear, although unlikely, if the drop in catch rates correlates to a lack of abundance. Mean catch weights have not varied since the 1990s, therefore it is unlikely that the decline in catch is due to overfishing. It is more likely that the decline is due to an increase in fuel prices for fishing fleets. Future catch records or Catch Per Unit Effort (CPUE) data are needed before a population decline can be confirmed.

In the Atlantic Ocean, the CPUE has been mostly stable for the last 15 years. However, CPUE and landings have fallen significantly since 1993 in some regions of the Eastern Atlantic Ocean, suggesting a possible decline in population (ICCAT 2008a). Decreased catch in some areas may be due to decreased effort or moratoria (NMFS 2005b). Nonetheless, landings have increased in other regions, notably in the Western Atlantic (NMFS 2008). CPUE has risen in the Dakar fishery, which is one of the largest Skipjack Tuna purse seine fisheries (ICCAT 2008a).

Since CPUE in both the Atlantic and Indian Oceans are mostly stable, points are neither added nor subtracted.

The average weight of catch in the Pacific Ocean has decreased in recent years (IATTC 2008). Additionally, use of drifting fish aggregation devices by the purse-seine industry has greatly increased juvenile mortality (DEH 2005). Nonetheless, as of 2002 Skipjack Tuna mortality rates were lower than natural mortality rates in corresponding age classes (Langley et al. 2002).

Age and size distribution in the Eastern Atlantic is bimodal, meaning that there are more large and small sized Skipjack than there are intermediate (ICCAT 2008a). The stock structure in the Western Atlantic is not well known (NMFS 2005b). High recruitment and high natural mortality makes accurate estimates of age and size distribution difficult.

Because catch rates of juvenile fish have increased and average weight has decreased in the Pacific Ocean, where much Skipjack Tuna is caught, points are subtracted.

Skipjack Tuna are not listed as overfished, depleted, endangered, or threatened by any of the agencies that monitor their populations (IATTC 2008; ICCAT 2008)

The majority of Skipjack Tuna are caught by purse seine fleets. A significant minority (15% of catch) are caught by pole and line gear (Langley et al. 2002). Pole and line gear is frequently used in coastal and offshore waters of Brazil, Maldives, Japan, Indonesia, and the Solomon Islands (WCPFC 2007). Pole and line gear, which fishes close to the surface, is believed to have little impact on habitat.

The pelagic zones of the Atlantic, Indian and Pacific Oceans are likely robust enough to support healthy populations of Skipjack Tuna.

Effects of pole and line gear on Skipjack Tuna habitat are minimal.

Effects of pole and line gear on Skipjack Tuna habitat are minimal.

Several international agencies are involved in the research and management of Skipjack Tuna fisheries: the Inter-American Tropical Tuna Commission or IATTC for the Eastern Pacific Ocean; the Commission for the Conservation and Management of Highly Migratory Fish Stocks in the Western and Central Pacific Ocean; the International Convention for the Conservation of Atlantic Tuna (ICCAT); and the Indian Ocean Tuna Commission (IOTC).

In the Pacific Ocean, the IATTC and the Commission for the Conservation and Management of Highly Migratory Fish Stocks have mandates to recommend management measures to member countries. Other agencies such as the Standing Committee on Tuna and Billfish only conduct research and stock assessments. Since it is widely believed that there is no 'immediate concern about the status of Skipjack Tuna' in the Pacific Ocean (AFMA 2007) there are few management measures in place. The IATTC has implemented measures to regulate the fishing of Skipjack Tuna by purse seine vessels (e.g., observer programs, quotas and time/area closures for purse seine fisheries; IATTC 2004b). Overall, management objectives are vague (Maunder et al. 2006). Existing measures apply only to a small proportion of the Skipjack Tuna fisheries that supply U.S markets with tuna.

In the Atlantic Ocean, the ICCAT releases frequent stock assessments, management recommendations, and scientific studies on Skipjack Tuna (ICCAT 2008a; ICCAT 2008b). Additionally, the National Marine Fisheries Services (NMFS) releases yearly status reports and recommendations to ICCAT (NMFS 2008a). French and Spanish regulatory bodies monitor European Skipjack Tuna fisheries and have on several occasions closed the fishery early (to limit the incidental catch of juvenile Bigeye Tuna) (NMFS 2005b). No regulations specifically protecting have been deemed necessary by either organization. ICCAT, however, has made efforts to curb FAD use and this will likely affect Skipjack stocks (ICCAT 2008b). Skipjack Tuna fisheries in the Atlantic Ocean do not neatly fit into any category. They are unregulated, yet regulation is not considered necessary. Voluntary moratoria are occasionally placed on the Spanish and French fishery, but ICCAT has not recommended limiting Skipjack Tuna catches (NMFS 2005b). Skipjack Tuna stocks benefit from regulations on FADs and purse seines, but Skipjack Tuna themselves are not protected by any measures.

In the Indian Ocean, the IOTC currently has no regulations in place regarding Skipjack Tuna. Regulations regarding Bigeye and Yellowfin tuna are in place and the IOTC is currently studying whether or not these regulations impact the Skipjack Tuna fishery as well (IOTC 2008b). The IOTC requires a record of all Skipjack Tuna catches in the Indian Ocean (Ibid.). The IOTC also publishes an IUU Vessel list.

Although Skipjack Tuna are common in all three oceans, it is unclear the degree to which these levels can be attributed to the efforts of international agencies. Overall, management measures are often weak or vague, so a core point of 1 is awarded.

For the Indian Ocean, the IOTC carefully monitors Bigeye and Yellowfin Tuna abundance, but information regarding Skipjack Tuna is inadequate. Additionally, information regarding pole-and-line catches in the Indian Ocean is inadequate. CPUE records are mostly inaccessible. Furthermore, since some nations fishing in the Indian Ocean do not cooperate with the IOTC, the data provided by the commission may not be accurate (IOTC 2009). It should be noted that many management agencies, if not all, face this difficulty. Outside of the IOTC, few organizations have available information about the Indian Ocean Skipjack Tuna fishery. The FAO publishes yearly world catch records for the Indian Ocean in Fishstat, a downloadable computer program. The Australian Fisheries Management Authority releases some catch regulations, but no scientific reports regarding Indian Ocean Skipjack stocks.

For the Atlantic Ocean, in contrast, the ICCAT and the NMFS publish frequent and easily accessible reports. ICCAT publishes yearly population assessments, management recommendations, and biennial management reports. The NMFS publishes yearly suggestions to ICCAT that include BMSY estimates and yearly catch records. There is however, some degree of scientific uncertainty regarding the Eastern Atlantic population because of frequent recruitment and high mortality. The uncertainty is not due to scientific oversight, but rather due to the biological characteristics of Skipjack Tuna.

Skipjack Tuna stocks in the Pacific Ocean are well monitored. The last stock assessment in the western and central Pacific Ocean published in 2008 (WCPFC 2008). Skipjack Tuna catch records in the Western and Central Pacific Ocean were last published in 2007 (WCPFC 2007). Despite adequate scientific monitoring, Skipjack Tuna catches in the western and central Pacific ocean are largely unregulated. Eastern Pacific Ocean stocks are also well monitored; population estimates date back to 1975 and data regarding catches, discards, fishing efforts, and catch size are continually collected (ICCAT 2004).

Because, two of three fishing regions have adequate scientific monitoring, points are added.

Skipjack Tuna populations are not considered overfished by the international agencies that monitor them. Therefore, recovery plans are not considered necessary.

In the Western and Central Pacific Ocean, no current action is being taken to control the capacity of tuna fleets catching Skipjack Tuna. In the Eastern Pacific Ocean, the IATTC has instituted a one-and-a-half month moratorium on fishing for Yellowfin, Bigeye, and Skipjack Tuna between the 40N and 40S parallel, the 150W meridian and the American coastline. The moratorium lasted through 2006 and has not been renewed (IATTC 2004b). The moratorium resulted in a decrease in fishing effort and in Skipjack Tuna catch (IATTC 2004a). Since efforts to control the capacity of Skipjack Tuna fisheries in the Western and Central Pacific Ocean are not in place, and since the moratorium on Skipjack Tuna fishing in the Eastern Pacific has not been renewed, points are neither added nor subtracted.

Pole and line gear generally results in low amounts of bycatch. For this reason, quantitative data concerning pole and line bycatch is scarce. Pole and line fisheries are selective when catching tunas and discarded fish likely have a high rate of survival (WPFMC 2002). Estimates of catch composition vary, but in Hawaii’s pole and line fishery, catches are composed of 99.6% Skipjack Tuna (WPFMC 2002). Since bycatch is not considered a problem, we have awarded a core point of 3.

Skipjack Tuna in the Pacific Ocean are caught by purse seine fisheries targeting other species of tuna. According to IATTC, an average of 11% of the annual Skipjack Tuna catch was discarded at sea in the Eastern Pacific Ocean (IATTC 2008). In the Western and Central Pacific, purse-seine fisheries discard large numbers of Skipjack Tuna (Ito and Jamm 2004). In the Atlantic Ocean, Skipjack are also occasionally caught in squid nets (NMFS 2001b). Discard of Skipjack is low, approximately 43 kg per ton (Chassot et al. 2008). Because bycatch of Skipjack Tuna is high in the Pacific Ocean, where Skipjack Tuna are often caught, points are subtracted.

Pole and line fisheries have very low bycatch of threatened, endangered, or protected species. Measures are not considered necessary.

Pole and line fisheries have no significant bycatch concerns and are considered highly selective. Discarded fish, including undersized individuals, are usually in a viable condition (WPFMC 2002).

Pole and line fisheries are a model for selective fisheries.