As mentioned earlier, associated species are those impacted species that are not part of the landed catch. Fisheries for straddling fish stocks, highly migratory fish stocks, and high seas fish stocks, impact other species as a result of: (1) discards, (2) physical contact of fishing gear with organisms (and habitat) that are not caught, and (3) indirect processes. Discards are considered in section 6.1 while physical contact and indirect processes are considered together in section 6.2.
Much more is known about discards than the other mechanisms through which fisheries impact associated species, although information is still limited. The most recent global information on discards is in an FAO report by Kelleher (2005). It estimates that the rate of discards is about 8 percent for all marine fisheries combined (EEZ and high seas), with large differences by countries, gear types, target species and statistical areas.
Shrimp trawling has the highest estimated average discard rate (62.3 percent), but the rates vary widely between fisheries (from 0 to 96 percent). Most shrimp trawling is on stocks confined to the EEZ, although there are some straddling or other high seas stocks of shrimp that are fished. These are likely to be fisheries in relatively deep water for cold-water species, such as the fishery for Pandalus shrimp on the Flemish Cap off Newfoundland and off Labrador in the northwest Atlantic (FAO statistical area 21). The aggregate discard rate for cold/deep-water shrimp fisheries is 39 percent, but where use of bycatch reduction devices (BRDs) is mandated (e.g. as in the Greenland, Labrador and Flemish Cap fishery), the discard rate is relatively low, on the order of 5 percent. There are a variety of finfish and invertebrate species caught, including juveniles of target species of many fisheries. Concern about the impact of shrimp trawling discards on finfish fisheries (particularly for flatfish) motivated mandatory use of BRDs in some fisheries.
Following shrimp trawling, longline fishing for highly migratory species (primarily tuna and tuna-like species) has the highest discard rate (averaging 28 percent with a range of 0 to 40 percent). Other fisheries for highly migratory species have much lower discard rates: 5 percent for tuna purse seines, 0.4 percent for tuna pole and line fishing. The total discards by these highly migratory species fisheries is estimated as about 700 000 tonnes annually. The portion discarded by high seas fisheries is unknown, but it is likely to be substantial.
The most common discard species from longlines is the blue shark. Other sharks, target species damaged by sharks and marine mammals, frigate tuna, kawakawa, Indo-Pacific king mackerel, and narrow-barred Spanish mackerel are also taken and discarded. Longtail tuna (Thunnus tonggol), slender tuna (Allothunnus fallai), the butterfly kingfish (Gasterochisma melampus), the wahoo (Acanthocybium solandri) and Spanish mackerel (Scomberomorus spp.) are here considered associated species as well as the flying fish (Exocoetidae, genera Exocoetus, Cypselurus, Hirundichthys, Cheilopogon and Prognichthys) the sunfish or headfish of the family Molidae, the snake mackerel (Gempylus serpens), escolar (Lepidocybium flavobrunneum) and oilfish (Ruvettus pretiosus) of the Gempylidae family, are species which are caught close inshore but migrate far offshore. They are all part of the regular bycatch of the tuna longliners together with the lancetfish (Alepisaurus ferox and A. brevirostris). Albatross, petrels and other seabirds are also caught by longlines.
For tuna purse seines, some of the discarded species are bonito, dogtooth tuna, rainbow runner, dolphinfish, jacks, sharks, billfish, mantas and undersize target species (i.e. skipjack and yellowfin tuna). Dolphin are also encircled by purse seines in some areas (see discussion below). As noted above, discards from pole and line fishing are minor.
Fisheries for straddling fish stocks and high seas fish stocks are primarily conducted with bottom trawls. The estimated discard rate for trawlers targeting demersal finfish is 9.6 percent (for all fisheries). There is no basis to judge if the rate is likely to be higher or lower for straddling fish stocks and high seas fish stocks than for stocks entirely within EEZs. However, the catch from stocks entirely within EEZs accounts for most of the total catch, such that these EEZ fisheries must account for most of the 1.7 million tonnes of estimated discards by bottom trawlers targeting demersal finfish. Many species are discarded depending on the target species (typically the species composition differs between flatfish and roundfish fisheries), geographic area, and depth. Discards of juveniles of the target species are common, as well as species with low commercial value, such as horse mackerel, long jawed mackerel (Rastrelliger spp.), elasmobranchs (e.g. dogfish and skates), arrowtooth flounders and flathead sole. Many benthic invertebrates are discarded, such as molluscs, echinoderms (e.g. urchins and starfish), crabs, rajids and whelks. Deep-water trawling (often at 1 000 m or more) results in discards of additional species, such as grenadiers, whiptails, rabbitfish, oreos, chondrichthyans (e.g. birdbeak dogfish), batoids and chimaeroids, and cold-water corals (Lophelia sp.).
In addition to bottom trawlers, demersal longlining is an important form of fishing in the Southern Ocean in the CCAMLR area (FAO statistical areas 48, 58, 88). The discard rate for this type of fishing is estimated as 7.5 percent (ranging from 0.5 to 57 percent). The overall discard rate in the CCAMLR area is estimated as 12.7 percent, resulting in about 2 000 tonnes annually.
Most discards are of finfish and invertebrate species that are abundant and there is little risk that "...their reproduction may become seriously threatened..." (according to Article 5(e) of the FSA, this is one of the standards that apply to species associated with straddling fish stocks and highly migratory fish stocks). However, there are some species with abundance so low that even small increases in mortality may raise the risk of being threatened with extinction. In addition, there are species which significant segments of society want protected regardless of their abundance. They are referred to as "charismatic species". Some charismatic species also have a significant extinction risk. Marine mammals, sea turtles and sea birds have long standing status as charismatic species and/or species at risk (of extinction). More recently, cold-water corals (Lophelia sp.) have gained public attention such that they might also be regarded as charismatic. Some species of cold-water corals might have extremely small geographic ranges (e.g. on the top of a single seamount), which means they may be vulnerable to localized depletion and possibly extinction, as well as being charismatic. However, apparent localized geographic distributions might also reflect under-sampling in other areas where the species might be present.
Charismatic species and species at risk of extinction are known bycatch of fisheries for highly migratory fish stocks, straddling fish stocks and high seas fish stocks. Sea turtles and sea birds are a well documented bycatch in longline fisheries for tuna and tuna-like species. Sea birds are also taken by longline fisheries for tunas (e.g. as in the southern bluefin tuna fishery) and for demersal species, such as the Southern cean demersal longline fishery for toothfish. Concern about sea bird mortality from longline fisheries led the FAO Committee on Fisheries (FAO, 2003d) to adopt an International Plan of Action for Reducing Incidental Catch of Seabirds in Longline Fisheries (FAO, 2003b). Measures have been introduced in several fisheries to reduce seabird bycatch. For example the seabird bycatch reduction programme of CCAMLR has reduced seabird mortality by 80 percent or more (CCAMLR, http://www.ccamlr. org/pu/E/e_pubs/am/man-ant/p4.htm).
Concern about longline bycatch of turtles in fisheries for highly migratory fish stocks prompted FAO to hold both an Expert Consultation (FAO, 2004a) and a Technical Consultation (FAO, 2004b) to consider ways of reducing mortality. Recent experiments aimed at reducing sea turtle bycatch and mortality are promising. For example, changes in hook shape and bait type reduced the catch rate of loggerhead turtles and leatherback turtles by 90 percent and 75 percent in the northwest Atlantic. Some of the Regional Fisheries Bodies with jurisdiction over fisheries that interact with sea turtles have begun to adopt measures to reduce interactions (see the reports of the Expert and Technical Consultations mentioned above). In general, the impact of sea turtle bycatch by longline fisheries for highly migratory fish stocks is unknown, but it could jeopardize some turtle populations that are severely depleted, even if the longline fisheries were not the primary cause of the depletion.
Bycatch of marine mammals is known to occur in some trawl fisheries (particularly large high speed pelagic trawls) and to a lesser extent on longlines. It is unclear to what degree marine mammal bycatch by trawlers and longliners occurs in high seas fisheries, but there is probably some. In the case of purse seine fishing for tuna in the eastern tropical Pacific, dolphin are intentionally encircled in the nets since they are an indicator of the location of schools of tuna. This practice has resulted in a cumulative mortality of several million dolphins since the 1960, jeopardizing some dolphin species. This led to the negotiation of the Agreement on the International Dolphin Conservation Program (AICDP), which entered into force in 1999, and whose Secretariat is provided by the Inter-American Tropical Tuna Commission (IATTC). he Program reduced drastically the mortality from 132 000 dolphins in 1986 to about 1 500 in 2003. In spite of this success, dolphin populations appear to have been slow to recover (see http://www.iattc.org/DolphinSafeENG.htm and http://swfsc.nmfs.noaa.gov/PRD/for more information about the Program and the slow recovery of some dolphin populations).
The recent expansion of trawl fisheries to deep water (often at much more than 1 000 m) into areas previously unfished has resulted in the bycatch of cold-water corals (Lophelia sp.), sometimes as boulder size pieces. Rarely has the impact of expanding deep-water trawl fisheries been documented from the initiation of fishing, but for the fishery for orange roughy on the South Tasman Rise straddling the Australian EEZ south of Tasmania, observers estimated in the first year of the fishery that 10 tonnes of coral per tow were brought up. This extrapolates to 10 000 tonnes of coral associated with a catch of about 4 000 tonnes of orange roughy (Anderson and Clark, 2003; Gianni, 2004).
Trawling is the primary type of fishing that results in physical contact between fishing gear and associated species and their habitat. The bycatch of cold-water corals is probably a symptom of a larger impact of trawling as reefs are damaged more extensively than indicated by the corals hauled up in nets. Trawls also come in physical contact with the bottom in areas where reefs are not present and here the affects are less obvious, but ecosystems are altered and species of benthic organisms will be differently affected.
Indirect processes affect the growth, survival and reproduction of species that are the target of fisheries, as well as associated species. When fisheries remove fish from populations, food webs are altered. Some species may suffer from the loss of prey; others may benefit from removal of their predators. Species that compete will be affected differently with cascading impacts on other dependent species.
Alteration of the sea bottom resulting from physical contact by fishing gear probably changes habitat suitability thus indirectly effecting associated species. For example, some species depend on complex "three dimensional" biogenic structures, such as reefs, for shelter from predators. When such structures are destroyed, the species may disappear.
Impacts through indirect processes are hard to detect, and even harder to predict, however, they must occur. The International Council for Exploration of the Sea is a good source of information on the ecosystem effects of fishing (for example http://www.ices.dk/pubs/crr/crr272/CRR272.pdf). Also, the US National Research Council published a report on the effects of trawling on the seafloor (http://www.nap.edu/catalog/10323.html).