All fishing gears have some level of environmental effect. Under the FAO Code of Conduct for Responsible Fishing, the fishing sector is expected to minimize its effects on the environment in order to sustain the resources and environment on which it relies. For bigeye tuna caught in surface and deep waters by a wide variety of fishing methods, bycatch is one of the most observable environmental effects, especially from fishing by longlines, gillnets and by purse seines on floating objects (including fish aggregating devices - FADs). In the case of purse seine vessels setting on FADs, bigeye tuna is not the target species and juvenile bigeye tuna are species are themselves regarded as bycatch (see - Sustainability).

Air and water pollution from fishing and fish processing are other environment concerns.

Ocean climate and global warming affect the distribution and catchability of bigeye stocks.

EFFECTS OF FISHING ON OTHER SPECIES

Bigeye tuna are caught by purse-seine, longline, pole-and-line and troll line in the Western and Central Pacific Ocean (WCPO), and by gill nets, longline, purse seine, pole and line, handline and troll line in the Indian Ocean (IO).

Fishing gear used to catch bigeye tuna does not come into contact with the seafloor and so does not directly affect the benthic environment.

Longline fishing targeting bigeye tuna (and albacore tuna) has a substantial bycatch of non-tuna species that can make up from more than half to two-thirds of the total catch, depending on the type of longline fishing. The main fish bycatch species in the WCPO include sharks, billfish, pelagic stingrays, and other finfish including moonfish (opah, Lampris guttatus), although their frequency in catches varies across the region.

Some longline vessels target sharks, especially when tunas are scarce. Sharks are targeted for their fins and meat. Longline crews often significantly top up their income from the sales of shark fins. Each year in the WCPO and IO, tens of thousands of individual sharks are caught and finned. In the WCPO, common bycatch sharks are: blue shark (Prionace glauca), silky shark (Carcharhinus falciformis), oceanic whitetip shark (Carcharhinus longimanus), mako sharks (Isurus spp), thresher sharks (Alopias spp), porbeagle shark (Lamna nasus), winghead hammerhead (Eusphyra blochii), scalloped hammerhead (Sphyrna corona), great hammerhead (Sphyrna mokarran) and smooth hammerhead (Sphyrna zygaena). Many shark species have experienced steep declines in catch in recent years, most likely due to high fishing mortality.

Conversely, sharks cause considerable damage to tuna on hooks in the longline fishery, and can reduce the value of the catch.

To reduce the catch and mortality of sharks, changes in longline gear are being used or investigated. These include changes in hook type and hook position in the water, replacing wire traces with nylon leaders and improving survival by better handling practices.

In the case of sea turtles, the loggerhead turtle is of most concern as bycatch in longline operations in the WCPO, although all species are at some risk. In the IO, all species of sea turtles are at risk of being caught by longlines.

Many species of seabirds are also caught by longlines in the WCPO, including especially large albatrosses in the higher latitudes. In the IO, breeding areas of species of albatross and petrel overlap extensively with the longline fishing grounds but records of bird catches are incomplete for many fleets. Mitigation methods are based on mapping the areas of greatest overlap of seabirds and fishing. Setting lines at night to avoid birds, weighting the lines for faster sinking and attaching streamers to keep the birds away are being encouraged to reduce bird mortality but more research is required to determine the most effective methods.

Marine mammal catches on longlines targeting bigeye tuna are not significant in most areas of the WCPO. In the IO, fishing for bigeye tuna in the northwest causes significant mortality of marine mammals.

Of the surface fisheries that catch juvenile bigeye, gill nets, used extensively in the IO, are of greatest concern for bycatch. This fishing is poorly monitored and traps a wide range of non-target species.

For purse-seine vessels targeting skipjack tuna, but which also catch juvenile bigeye tuna, the percentage of the catch comprised of non-tuna species is relatively low: 1.6% for nets set around FADs and 0.4% for sets on free-swimming schools not associated with FADs. In the WCPO purse seine fisheries, the bycatch of non-tuna species includes mainly silky shark, mackerel scad, mahi mahi (dolphin fish, Coryphaena hippurus), frigate mackerel, oceanic triggerfish and rainbow runner.

The bycatch from pole-and-line and troll fisheries is minimal. However, in the past when pole-and-line fishing was much more common than it is today, the stocks of the small pelagic fish species used as live bait were considered at risk of local depletion in some Pacific Island countries.

The Western and Central Pacific Fisheries Commission (WCPFC) and the Indian Ocean Tuna Commission (IOTC) have conservation and monitoring measures for bycatch mitigation and monitoring. Specific conservation and management measure address bycatch issues for sea turtles, sharks (including finning), sea birds, cetaceans, other finfish and reporting provisions to support bycatch research and monitoring. The International Seafood Sustainability Foundation (ISSF) conducts regionally specific by-catch mitigation training workshops for purse seine vessel skippers.

IMPACTS ON AIR AND WATER

All tuna fishing vessels rely on fossil fuel; their exhaust fumes and refrigerant gases contribute to greenhouse gases (GHG) emissions and thus global warming.

The estimated total carbon footprint of purse seine-caught tuna (all species) in 2009 was approximately 1,530 kg CO² per tonne of tuna landed. The GHG emissions associated with catching tuna by purse-seine vessels, storing it on board and delivering whole fish to processing plants, are three times greater than the emissions stemming from the processing, packaging and transport of the resulting products.

The amount of fuel used to catch a tonne of tuna is greater for longline vessels than for purse-seine vessels. The carbon footprint of longline tuna products in increased further by the air freight required to deliver sashimi-grade tuna and other fresh tuna products to markets because GHG emissions are much higher for airfreight than for sea freight.

Bigeye tuna is a sought after species for sashimi and fresh rather than frozen product receives a premium price. Frozen and canned tuna transported by truck or container vessel have a lower carbon footprint than do fresh, chilled tuna transported by air.

For bigeye that are canned, the water, energy use and greenhouse gas emissions issues common to other canned tuna apply.

EFFECTS OF ENVIRONMENT ON BIGEYE

The life cycles of all tuna species depend on oceanic circulation: currents determine the location of spawning grounds, the dispersal and successful survival and growth of larvae, juveniles and adults, and the distribution of their prey. The availability of the nutrients in the tuna food web, and the location of water at suitable temperatures and levels of dissolved oxygen determine the tuna distribution and abundance.

In the WCPO, the El Niño Southern Oscillation (ENSO) is an oscillation between a warm (El Niño) and a cooler (La Niña) state. ENSO events have significant effects on bigeye tuna catches by surface and deep water fishing gears.

In the surface fisheries, catchability of bigeye tuna is lower during La Niña events when the thermocline is deeper and the surface layer of water is greater in volume, making bigeye less vulnerable to surface fishing gear. Bigeye tuna can tolerate lower levels of dissolved oxygen (O2) than other tunas and can feed at depths greater than 500 m. Longline catch rates of bigeye are also lower during La Nina events. The reverse occurs during El Niño events when the thermocline is shallower and surface temperatures higher. Surface fishery catch rates are higher during El Niño episodes and longline catch rates of bigeye tuna are also increased.

In the Eastern Indian Ocean off Java, ENSO events appear to have similar effects on bigeye tuna catch rates to those in the WCPO. In the IO, the Indian Ocean Index is a climate oscillation between a warmer and cooler state that better predicts warm and cold events in the Western Indian Ocean than do ENSO events. For bigeye longline catches in the IO, bigeye tuna catches tended to increase during warm events, although catch rates are affected by targeting practices.

EFFECTS OF CLIMATE CHANGE ON BIGEYE

Because of their mobility, bigeye tuna are likely to respond to increased sea surface temperatures and lower ambient O2 caused by the increased temperature by moving to areas within their preferred temperature ranges, both for spawning and feeding. Models that use projected changes in temperatures, currents, and food chains in the open ocean predict that future concentrations of bigeye are likely to be found further to the east than they are today. In the western equatorial Pacific, bigeye are likely to lose spawning grounds due to the projected high temperatures. The possible loss of equatorial spawning habitat may be compensated for by increased larval survival in the sub-tropics. Juvenile and adult fish, however, may have to contend with inferior habitat as sea surface temperatures rise and increase stratification of the water column, reducing availability of nutrients and prey, and as lower O2 concentrations occur near the surface.

Overall, by 2035, bigeye tuna catches are projected to decrease by a small amount (usually <5%), depending on the assumptions, in 17 of the 22 Pacific Island countries’ territories. By 2100, in some countries, larger declines of up to 30% are projected under some scenarios.

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GUIDE TO FURTHER READING

Note: Details of all sources are given in References below

For tuna bycatch and discards in the IO tuna fisheries, see David Ardill and others (2011).

Shelley Clarke and others (2014) provided a global overview, by ocean, of longline bycatch and mitigation measures for sharks, sea turtles, seabirds, marine mammals and non-tuna finfish. For the WCPO, Shelton Harley and others (2014) reported on bycatch and non-tuna catch.

On shark catches in the WCPO tuna fisheries, see Shelley Clarke (2011), Shelley Clarke and others (2011) and Tim Lawson (2011). Mike McCoy & Bob Gillett (2005) provided insights into the shark targeting and importance of shark catches for crews on Chinese vessels. Makoto Peter Miyake and others (2010) reported on shark damage to tuna on longlines. The WCPFC’s Conservation and Management Measure (CMM) 2010-07 lists shark species of concern. For information on fishing gear modifications to reduce longline shark catch and bycatch see Don Bromhead and others (2013), Pew Marine Environment Group (2011), and Peter Ward and others (2007).

On baitfish for pole and line fishing in Solomon Islands and Fiji, see Steve Blaber and others (1993). For bycatch conservation and monitoring measures of the WCPFC, see https://www.wcpfc.int/conservation-and-management-measures, for IOTC see http://www.iotc.org/cmms; for a recent compendium of measures, see ISSF (2014).

Peter Tyedmers and Robert Parker (2011) provide a preliminary assessment of the impacts of bigeye tuna fishing and supply chain on air and water. The report from UNEP (2000) provided information on the environment impacts of tuna processing.

Effects of climate change on the foodwebs that support bigeye tuna and other species of tuna are reviewed by Robert Le Borgne and others (2011). For effects of climate change on bigeye tuna fish stocks see Patrick Lehodey and others (2011), who also projected catches under two future scenarios for greenhouse gas emissions. For ENSO effects on bigeye tuna catches in the EIO, see Mega Syamsuddin and others (2013), and for IO F. Ménard and colleagues.

REFERENCES

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• Blaber, SJM, DA Milton & NJF Rawlinson. 1993. Tuna Baitfish in Fiji and Solomon Islands, proceedings of a workshop, Suva, Fiji, 17-18 Aug. 1993. Canberra: Australian Council for International Agricultural Research (ACIAR) Proceedings 52.
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