Skipjack Tuna, purse-seine-caught

Katsuwonus pelamis

Sometimes known as Aku.

This species is wild-caught.

Summary

Skipjack Tuna have high fecundity and relatively fast growth rates, life history characteristics that have the potential to buffer fish populations from overfishing. But with catches accounting for half the annual global tuna supply, they are only at moderate levels of abundance. Skipjack Tuna school with juvenile Yellowfin and Bigeye Tunas, and fishing vessels target these schools together. Tuna purse-seine fisheries interact with sea turtles and marine mammals. Pole-and-line-caught Skipjack Tuna is a better alternative, because this gear has no bycatch problems.

Criterion Points
Life History 3.00
Abundance 2.00
Habitat Quality and Fishing Gear Impacts 3.75
Management 1.50
Bycatch 1.50
Final Score (average of criteria) 2.35
Color
Final Score Color
2.60 - 4.00
2.20 - 2.59
1.80 - 2.19
1.40 - 1.79
0.00 - 1.39

Last updated January 11, 2008.

Life History

Core Points (only one selection allowed)

If a value for intrinsic rate of increase (‘r’) is known, assign the score below based on this value. If no r-value is available, assign the score below for the correct age at 50% maturity for females if specified, or for the correct value of growth rate ('k'). If no estimates of r, age at 50% maturity, or k are available, assign the score below based on maximum age.

1.00
Intrinsic rate of increase <0.05; OR age at 50% maturity >10 years; OR growth rate <0.15; OR maximum age >30 years.
2.00
Intrinsic rate of increase = 0.05-0.15; OR age at 50% maturity = 5-10 years; OR a growth rate = 0.16–0.30; OR maximum age = 11-30 years.
3.00
Intrinsic rate of increase >0.16; OR age at 50% maturity = 1-5 years; OR growth rate >0.30; OR maximum age <11 years.

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).

Points of Adjustment (multiple selections allowed)

-0.25
Species has special behaviors that make it especially vulnerable to fishing pressure (e.g., spawning aggregations; site fidelity; segregation by sex; migratory bottlenecks; unusual attraction to gear; etc.).

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).

-0.25
Species has a strategy for sexual development that makes it especially vulnerable to fishing pressure (e.g., age at 50% maturity >20 years; sequential hermaphrodites; extremely low fecundity).
-0.25
Species has a small or restricted range (e.g., endemism; numerous evolutionarily significant units; restricted to one coastline; e.g., American lobster; striped bass; endemic reef fishes).
-0.25
Species exhibits high natural population variability driven by broad–scale environmental change (e.g., El Nino; decadal oscillations).

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.

+0.25
Species does not have special behaviors that increase ease or population consequences of capture OR has special behaviors that make it less vulnerable to fishing pressure (e.g., species is widely dispersed during spawning).
+0.25
Species has a strategy for sexual development that makes it especially resilient to fishing pressure (e.g., age at 50% maturity <1 year; extremely high fecundity).

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).

+0.25
Species is distributed over a very wide range (e.g., throughout an entire hemisphere or ocean basin; e.g., swordfish; tuna; Patagonian toothfish).

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

+0.25
Species does not exhibit high natural population variability driven by broad-scale environmental change (e.g., El Nino; decadal oscillations).
3.00
Points for Life History

Abundance

Core Points (only one selection allowed)

Compared to natural or un-fished level, the species population is:

1.00
Low: Abundance or biomass is <75% of BMSY or similar proxy (e.g., spawning potential ratio).
2.00
Medium: Abundance or biomass is 75–125% of BMSY or similar proxy; OR population is approaching or recovering from an overfished condition; OR adequate information on abundance or biomass is not available.

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).

3.00
High: Abundance or biomass is >125% of BMSY or similar proxy.

Points of Adjustment (multiple selections allowed)

-0.25
The population is declining over a generational time scale (as indicated by biomass estimates or standardized CPUE).

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).

-0.25
Age, size or sex distribution is skewed relative to the natural condition (e.g., truncated size/age structure or anomalous sex distribution).

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).

-0.25
Species is listed as “overfished” OR species is listed as “depleted”, “endangered”, or “threatened” by recognized national or international bodies.

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

-0.25
Current levels of abundance are likely to jeopardize the availability of food for other species or cause substantial change in the structure of the associated food web.
+0.25
The population is increasing over a generational time scale (as indicated by biomass estimates or standardized CPUE).
+0.25
Age, size or sex distribution is functionally normal.
+0.25
Species is close to virgin biomass.
+0.25
Current levels of abundance provide adequate food for other predators or are not known to affect the structure of the associated food web.
2.00
Points for Abundance

Habitat Quality and Fishing Gear Impacts

Core Points (only one selection allowed)

Select the option that most accurately describes the effect of the fishing method upon the habitat that it affects.

1.00
The fishing method causes great damage to physical and biogenic habitats (e.g., cyanide; blasting; bottom trawling; dredging).
2.00
The fishing method does moderate damage to physical and biogenic habitats (e.g., bottom gillnets; traps and pots; bottom longlines).
3.00
The fishing method does little damage to physical or biogenic habitats (e.g., hand picking; hand raking; hook and line; pelagic long lines; mid-water trawl or gillnet; purse seines).

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.

Points of Adjustment (multiple selections allowed)

-0.25
Habitat for this species is so compromised from non-fishery impacts that the ability of the habitat to support this species is substantially reduced (e.g., dams; pollution; coastal development).
-0.25
Critical habitat areas (e.g., spawning areas) for this species are not protected by management using time/area closures, marine reserves, etc.
-0.25
No efforts are being made to minimize damage from existing gear types OR new or modified gear is increasing habitat damage (e.g., fitting trawls with roller rigs or rockhopping gear; more robust gear for deep-sea fisheries).
-0.25
If gear impacts are substantial, resilience of affected habitats is very slow (e.g., deep water corals; rocky bottoms).
+0.25
Habitat for this species remains robust and viable and is capable of supporting this species.

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

+0.25
Critical habitat areas (e.g., spawning areas) for this species are protected by management using time/area closures, marine reserves, etc.
+0.25
Gear innovations are being implemented over a majority of the fishing area to minimize damage from gear types OR no innovations necessary because gear effects are minimal.

Habitat effects of purse-seine gear are likely minimal.

+0.25
If gear impacts are substantial, resilience of affected habitats is fast (e.g., mud or sandy bottoms) OR gear effects are minimal.

Habitat effects of purse-seine gear are likely minimal.

3.75
Points for Habitat Quality and Fishing Gear Impacts

Management

Core Points (only one selection allowed)

Select the option that most accurately describes the current management of the fisheries of this species.

1.00
Regulations are ineffective (e.g., illegal fishing or overfishing is occurring) OR the fishery is unregulated (i.e., no control rules are in effect).

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.

2.00
Management measures are in place over a major portion over the species’ range but implementation has not met conservation goals OR management measures are in place but have not been in place long enough to determine if they are likely to achieve conservation and sustainability goals.
3.00
Substantial management measures are in place over a large portion of the species range and have demonstrated success in achieving conservation and sustainability goals.

Points of Adjustment (multiple selections allowed)

-0.25
There is inadequate scientific monitoring of stock status, catch or fishing effort.
-0.25
Management does not explicitly address fishery effects on habitat, food webs, and ecosystems.
-0.25
This species is overfished and no recovery plan or an ineffective recovery plan is in place.
-0.25
Management has failed to reduce excess capacity in this fishery or implements subsidies that result in excess capacity in this fishery.
+0.25
There is adequate scientific monitoring, analysis and interpretation of stock status, catch and fishing effort.

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.

+0.25
Management explicitly and effectively addresses fishery effects on habitat, food webs, and ecosystems.
+0.25
This species is overfished and there is a recovery plan (including benchmarks, timetables and methods to evaluate success) in place that is showing signs of success OR recovery plan is not needed.

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.

+0.25
Management has taken action to control excess capacity or reduce subsidies that result in excess capacity OR no measures are necessary because fishery is not overcapitalized.

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.

1.50
Points for Management

Bycatch

Core Points (only one selection allowed)

Select the option that most accurately describes the current level of bycatch and the consequences that result from fishing this species.

The term, "bycatch” used in this document excludes incidental catch of a species for which an adequate management framework exists.

The terms, “endangered, threatened, or protected,” used in this document refer to species status that is determined by national legislation such as the U.S. Endangered Species Act, the U.S. Marine Mammal Protection Act (or another nation's equivalent), the IUCN Red List, or a credible scientific body such as the American Fisheries Society.

1.00
Bycatch in this fishery is high (>100% of targeted landings), OR regularly includes a “threatened, endangered or protected species.”
2.00
Bycatch in this fishery is moderate (10-99% of targeted landings) AND does not regularly include “threatened, endangered or protected species” OR level of bycatch is unknown.

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.

3.00
Bycatch in this fishery is low (<10% of targeted landings) and does not regularly include "threatened, endangered or protected species."

Points of Adjustment (multiple selections allowed)

-0.25
Bycatch in this fishery is a contributing factor to the decline of “threatened, endangered, or protected species" and no effective measures are being taken to reduce it.
-0.25
Bycatch of targeted or non-targeted species (e.g., undersize individuals) in this fishery is high and no measures are being taken to reduce it.
-0.25
Bycatch of this species (e.g., undersize individuals) in other fisheries is high OR bycatch of this species in other fisheries inhibits its recovery, and no measures are being taken to reduce it.

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).

-0.25
The continued removal of the bycatch species contributes to its decline.

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.

+0.25
Measures taken over a major portion of the species range have been shown to reduce bycatch of “threatened, endangered, or protected species” or bycatch rates are no longer deemed to affect the abundance of the “protected” bycatch species OR no measures needed because fishery is highly selective (e.g., harpoon; spear).

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.

+0.25
There is bycatch of targeted (e.g., undersize individuals) or non-targeted species in this fishery and measures (e.g., gear modifications) have been implemented that have been shown to reduce bycatch over a large portion of the species range OR no measures are needed because fishery is highly selective (e.g., harpoon; spear).

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.

+0.25
Bycatch of this species in other fisheries is low OR bycatch of this species in other fisheries inhibits its recovery, but effective measures are being taken to reduce it over a large portion of the range.
+0.25
The continued removal of the bycatch species in the targeted fishery has had or will likely have little or no impact on populations of the bycatch species OR there are no significant bycatch concerns because the fishery is highly selective (e.g., harpoon; spear).
1.50
Points for Bycatch

References

Allen, R. 2005. Personal Communication. IATTC.

Brown, C. 2004. Personal Communication. NMFS.

Donley, P. 2005. Personal Communication. Southwest Fisheries Science Center.

Goujan, M. 2004. Informations sur les captures accessoires des thoniers senneurs gérés par les armements français d’après les observations faites par les observateurs embarqués pendant les plan de protection des thonidés de l’Atlantique de 1997 à 2002. In: Collective Volume of Scientific Papers, 56. ICCAT.

Gaertner, D. 2005. Personal Communication. ICCAT.

Gaertner, D., F. Ménard, C. Develter , J. Ariz and A. Delgado de Molina. 2002. By-catch of billfishes by the European tuna purse seine fishery in the Atlantic Ocean. Fisheries Bulletin of the US 100: 683-689.

Gaertner, D., A. Fonteneau, and F. Laloë. 2001. Approximate estimate of the maximum sustainable yield from catch data without detailed effort information: application to tuna fisheries. Aquatic Living Resources 14: 1-9.

Gosliner, M.L. 1999. The Tuna-Dolphin Controversy. In: Twiss, J.R., Jr. and R.R. Reeves. Conservation and Management of Marine Mammals. Smithsonian Institution Press, Washington, D.C.

Fromentin, J.-M. and A. Fontenau. 2001. Fishing effects and life history traits: A case study comparing tropical verses temperate tunas. Fisheries Research 53: 133-150.

Hall, M.A. 2005. Personal Communication. IATTC.

Hall, M.A. 1998. An Ecological View of the Tuna-Dolphin Problem: Impacts and Trade-offs. In: Reviews in Fish Biology and Fisheries 8: 1-34.

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