All fishing gears have some level of environmental effect. For skipjack, bycatch is one of the most noticeable effects (ISSF, 2012), but air and water pollution are other environment concerns. Ocean climate and global warming affect the distribution and catchability of skipjack stocks.

Effects of Fishing on Other Species

In the equatorial regions of the Indian and Western and Central Pacific oceans, the surface fishery for skipjack (purse seine and pole-and-line) also catches the smaller size classes (< 80 cm) of juvenile yellowfin tuna and juvenile bigeye tuna, and, in fishing free schools in the Western and Central Pacific Ocean (Harley et al., 2013), larger (>100cm) yellowfin tuna. Development of FAD fishing in the 1990s (and the later development of anchored or fixed FADs) led to higher fishing efficiencies and catch rates but also higher catches of juveniles and discards (Miyake et al, 2010, Leroy et al., 2012). A set on a FAD school catches many fish other than tuna. Juvenile bigeye are caught by FAD fishing and, because the stock size of bigeye is far smaller than the stock size of the other tunas caught under FADS (yellowfin and skipjack), the effect of the purse seine catch on the bigeye stock is greater than it is on the latter. The two main effects of FAD catches on juvenile bigeye are a reduction in bigeye stock size (also an effect of any other fishery including longline), and the potential reduction in yield (“growth overfishing”) of bigeye in longline fisheries (which target spawning-size bigeye). The effect on yellowfin stock is similar but not as significant.

[Image SKJ-???] Most RFMOs have mitigation measures in place for sea birds and sea turtles. In the Indian Ocean, cormorants, migratory shearwaters (which are common in coastal waters of many IOTC coastal states), sea turtles and sharks, are particularly vulnerable to bycatch in gillnet fisheries (IOTC, 2011c, MRAG, 2012). Sea turtles caught in fishing operations are usually discarded, and about 50% of the turtles are alive when brought in (MRAG, 2012). In the Western and Central Pacific, attempts have also been made to prohibit fishing activities for schools of tuna associated with whale sharks because of the FFA members’ concern about accidental mortality of the sharks (FFA Members, 2010; Pew Environment Group, 2012); the PNA introduced a ban in 2010, supported and extended by the Western and Central Fisheries Commission (WCPFC) in December 2012 (WCPFC, 2012b).

In the case of the pole-and-line fishery for skipjack in the Western and Central Pacific Ocean, the effect on baitfish resources (which are coastal) has a limiting effect on pole-and-line fishing in eastern parts of the Western and Central Pacific Ocean and would be a limiting factor in reviving the pole-and-line fishery which has been in a long decline, despite proposals to integrate community-based baitfishing (Gillett, 2011).

Impacts on Air and Water

Exhaust fumes and refrigerant gases from fishing vessels and processing plants contribute to global warming. The estimated total carbon footprint of purse seine-caught tuna in 2009 was approximately 1,530 kg CO2 per tonne of tuna landed, 75% of which is directly or indirectly (e.g. extraction, processing, and transport) from the consumption of fuel by the fishing vessel (Tyedmers & Parker, 2011).

Modern tuna fishing relies heavily on fossil fuel. Fuel consumption by vessels typically exceeds the energy use and greenhouse gas (GHG) emissions from processing, packaging and transport of resulting products combined (exceptions include when fresh products are transported by air). Compared to purse seine fisheries, pole-and-line fisheries targeting skipjack, and troll fishing, have much higher fuel use intensity (Tyedmers & Parker, 2011).Purse seine vessels fishing for skipjack are more fuel-efficient than purse seiners targeting other tuna species (ibid.).

Tuna processing can have significant environmental effects if not properly conducted and regulated. For skipjack, most initial processing is done in low-cost countries where waste discharge has grown considerably (Havice & Reed, 2012) and where the regulation and monitoring of waste discharge is often weak. Other issues, especially for island states, are the high use of water in all stages of processing, energy use, noise, odour and solid waste (UNEP, 2000).

Effects of Environment on Skipjack

As an oceanic species, ocean climate conditions have pronounced effects on skipjack and its fisheries, especially the El Niño Southern Oscillation (ENSO) in the Western and Central Pacific Ocean. ENSO is a swing between a warmer (El Niño) and cooler (La Niña) ocean conditions across much of the tropical Pacific and that occurs on irregular cycles (2-7 years) due to interactions between the atmosphere and the ocean (Lehodey et al, 2011). ENSO affects skipjack through its affects on ocean currents and temperatures (Ganachaud et al. 2011) that influence where and when fish spawn, and how larvae, juveniles and their prey are dispersed or retained in areas conducive to their growth and survival (Lehodey et al, 2011). In the Western and Central Pacific Ocean, skipjack recruitment is linked to ENSO events; the biomass of fish recruited to the stock is positively correlated with the Southern Oscillation Index value of eight months earlier (Lehodey et al, 2011). In El Niño conditions, skipjack biomass is higher than in La Nina conditions (Lehodey et al. 1997).

ENSO also affects where the larger skipjack catches are taken in the Western and Central Equatorial Pacific. During El Niño (warm) events, higher purse-seine catches are taken in the central Pacific, e.g., Kiribati (Line Islands). Several months after an El Niño is completed, catches increase in the Solomon Islands and PNG, and particularly when an El Niño is followed by a La Niña (Lehodey et al, 2011).

Effects of Climate Change on Skipjack

In the shorter term (by 2035), fisheries for skipjack across the western and central-eastern Pacific are projected to gain from a warming ocean (Lehodey et al. 2011, Bell et al. 2011). However, by mid century, catches of skipjack in the western Pacific are expected to begin to decline, whereas those in the central-eastern Pacific are projected to remain significantly higher than the average catches made between 1980 and 2000 (Lehodey et al. 2011). One reason for these projections is that skipjack tuna are expected to move to areas within their preferred temperature range as  sea surface temperatures (SST) increases. However, once SST in areas of the ocean exceeds the thermal maxima for skipjack, negative effects on skipjack populations will occur. By 2100, the effects of climate change are expected to cause a 5-10% decline in skipjack production relative to 1980-2000 catch levels (Lehodey et al. 2011).

[Image SKJ-020] The main fishing grounds and the catchability of skipjack in the surface fishery in the tropical Pacific are expected to change in a manner akin to the changes observed during present-day El Niño events. Prime skipjack fishing grounds are expected to move eastward along the equator, and towards higher latitudes, changing the location of suitable fishing areas for Pacific Island countries (Lehodey et al; 2011). Where skipjack remains within its preferred temperature range, catch rates of surface schools may increase as the water column becomes more stratified due to increasing SST and decreasing salinity.

All life cycle stages of skipjack are also expected to be affected by the projected changes to the nutrients in ocean surface layers, which affect the natural food available for fish (Le Borgne et al. 2011). Complex climate-related changes could result in trophic cascades, i.e., linked increases or decreases in the availability of prey or predators when the abundance of one or the other changes within the oceanic food web (Griffiths et al, 2010).