Hidden Mortality: The effects of by-catch

Trawl fisheries

Trawls are large, funnel-shaped nets that are towed behind one or two fishing vessels. A combination of large ‘doors’ on bridles at the sides of the net, weights on the bottom and floats at the top keeps the net open. Caught animals are filtered down to a cod-end that is dumped when the net is hauled aboard. Pelagic, or mid-water, trawl nets target the likes of squid, herring and pollock and typically have low by-catch rates because these species often travel in single-species schools. In addition, the nets make no contact with the sea floor so there is little damage to marine habitats. Fisheries such as mid-water trawling for Alaskan pollock are therefore considered to be among the cleanest in the world.

By-catch can become a concern, however, if the fishery also catches predators with vulnerable life histories that are feeding on the target species. For example, pelagic trawl fisheries for herring occasionally catch the marine mammals eating the small, silvery fish. Similarly, fisheries targeting pelagic squid may inadvertently take the molluscs’ predators, such as crocodile sharks or sharpnose sevengill sharks.

Long-line fisheries

Long-lines are an effective fishing gear that can be employed across various habitats from rivers (where gear such as catfish trotlines may be used) to shallow coastal waters, the open ocean and the deep sea. Unlike trawl and gill nets, long-lines are selective for species that can be lured to take a baited hook, which include most predatory fishes.

A long-line consists of a main line to which a series of branch lines (also called gangions or snoods) is attached. Each branch line terminates in a baited hook. The main line may be any length and contain any number of hooks, but generally it ranges from hundreds of metres long with only 20 or so hooks in some near-shore and deep-sea fisheries to more than 100 kilometres (60 miles) long with over 1,000 hooks in pelagic fisheries. The branch lines may be clipped to the main line during each deployment or sewn permanently into the main line.

Pelagic long-lines are not anchored, but are set adrift and marked with highfliers (floats with a radar reflector and possibly a radio transmitter) to locate the ends. They employ a combination of floats, weighted branch lines and varied branch line lengths to reach the depths of the targeted species. Bottom-set (demersal) long-lines are anchored at both ends and marked by a buoy at one or both ends. Whereas the branch lines for pelagic long-lines may be as much as 10–20 metres (32–64 feet) long, the branch lines for demersal long-lines generally range from only 20 centimetres (eight inches) to three metres (10 feet) long.

Pelagic long-line fisheries are often seen in a negative light by environmental groups because of their by-catch, although they are relatively clean when compared to bottom-trawl and gill-net fisheries in terms of their discard-to-catch ratio. In US pelagic long-line fisheries, for example, only 3–15% of the catch is discarded. However, as in trawl fisheries, the disparate life histories of target and by-catch species are a concern in long-line fisheries. Whereas targeted pelagic fishes such as tunas and mahi mahi mature early and produce many offspring, resulting in population doubling times in the order of two years, the by-catch often comprises charismatic species such as pelagic sharks and sea turtles that have conservative life histories and population doubling times that may be much greater than those of the targeted species.

Sharks are often the dominant by-catch in pelagic long-line fisheries. In the tuna fishery in the Western Tropical Pacific, shark by-catch has been shown to be relatively high – approximately one shark for every two tunas caught. However, a comparison of many pelagic long-line fisheries has suggested that shark by-catch rates were lowest in high-seas fisheries targeting tunas (such as those of Japan, Fiji and Hawaii) and highest in fisheries in shallower water and closer to shore (such as Chile’s mahi mahi and Hawaii and Chile’s swordfish fisheries).

Although the number of individual sharks caught may be very large, relatively few shark species are affected by pelagic in comparison to bottom-trawl fisheries. Globally, the blue shark is the dominant species caught in pelagic long-line fisheries, followed by the silky shark and the oceanic whitetip. The proportion of sharks taken as by-catch being kept and sold as opposed to being discarded at sea varies greatly from one fishery to the next, depending on regulations and the markets available. It has been shown that most of the sharks that end up in the international fin trade are by-catch in pelagic long-line fisheries and there is concern that, while the targeted stocks of pelagic bony fishes are harvested sustainably, these shark species may become depleted.

In most pelagic long-line fisheries, the blue, silky and oceanic whitetip sharks are reported being discarded alive, but post-release mortality has not been estimated and may be high. Marketable species such as the shortfin mako and the three thresher sharks are typically kept to be sold and, given their conservative life histories, they are of great management concern. It is also important to recognise that some pelagic fisheries retain all the sharks taken as by-catch. For example, the swordfish fishery off Uruguay, which has an extremely high rate of shark by-catch, typically markets more than 95% of the blue sharks caught.

Several mitigation measures to reduce shark by-catch on pelagic long-lines have been assessed, though their effectiveness often depends on the species involved. The use of monofilament instead of steel leaders and of squid bait instead of fish bait has been shown to reduce the number of sharks caught, with relatively little – or even a positive – effect on the catch rate of the targeted tunas and swordfish.

Purse-seine fisheries

The largest fisheries in the world are purse-seine fisheries that target small pelagic fishes such as anchovies, herring and menhaden. Historically these fisheries have been relatively clean in terms of by-catch as they typically target dense single-species schools of fish. Industrialised fishing for larger pelagic fishes such as tunas has developed relatively recently – during the latter half of the 20th century – and now purse-seine nets are also responsible for catching more tunas than any other form of fishing.

Three types of fishing gear are primarily used for harvesting tunas: bait boats with hand lines, pelagic long-lines and purse-seine nets. Globally, the three types resulted in similar tuna landings until the early 1980s, but since then purse-seine landings have increased nearly 10-fold whereas long-line and hand-line landings have remained relatively stable. Purse-seine fisheries are now responsible for about 70% of all tuna landings worldwide, although the pattern differs from one species to another. Most albacore tuna are still landed in long-line, hand-line and troll fisheries, but 75–90% of global skipjack, yellowfin and bigeye tunas are caught in purse-seine nets, which are also the dominant gear type used to harvest Atlantic bluefin tuna.

Purse-seine nets are deployed in the open ocean over deep water. Those used for tunas are typically 1,000–2,000 metres (3,280–6,560 feet) long (about 300–650 metres, or 985–2,130 feet, in diameter) and usually at a depth of about 200 metres (650 feet). Very simply, the seine has a float line and a lead line and it is positioned to encircle a school of fish, with one or two boats pulling it from the larger harvest vessel. Once the float line circle is closed, the opening at the bottom of the net is closed, or pursed, by cinching the lead line and thus preventing any fish from escaping. The net volume is reduced and the catch is hauled aboard the harvest vessel, removed from the net and placed in the ship’s hold.

Four methods are used to locate the tuna. Visual spotters on the vessels or in aircraft look for free schools of tuna feeding or swimming at the surface. This is the most challenging and least productive method, as free schools are constantly moving and difficult to locate and the tuna are more likely to be startled and dive deep before the net has been deployed. However, free schools tend to be dominated by the targeted tuna species and there is therefore very little by-catch.

Tunas are often not close enough to the surface to be spotted, but they are known to associate with other species – primarily dolphins, but also whales and whale sharks – that do spend more time at the surface. In the Eastern Pacific, if a large pod of feeding dolphins is located, there is often a substantial school of yellowfin tuna below it – an association that is far less common in other parts of the world. By-catch in dolphin-associated purse-seine fisheries is mainly silky sharks, but rates are quite low.

Floating objects that drift around oceanic gyres (sometimes for years) tend to attract and hold large communities of organisms that seek refuge in the dangerous open ocean. These objects may be natural, such as trees or logs that have been swept into the sea from rivers, or man-made, like lost fishing nets or pieces of wrecked vessels. According to one study, more than 300 fish species are associated with such refuges in the open ocean. Pelagic predators such as tunas and sharks in turn are attracted to these floating objects and the potential prey they harbour. Many tons of pelagic fishes may aggregate at a relatively small floating object and, not surprisingly, by-catch rates on floating-object sets are much higher than on free-school or dolphin-associated sets. The most vulnerable component of this by-catch is pelagic sharks.

The fourth method to locate tunas is a variation on the floating object theme. Fishers exploit the tendency for tunas to gather at floating objects by deploying purpose-built fish aggregating devices (FADs) to attract them. These FADs may be floating or anchored and as simple as palm fronds tied together or as complex as large structures with radio or satellite locator beacons and integrated sonars that enable fishers to estimate the biomass of aggregated fish. FADs attract diverse communities of animals and the by-catch rates of the associated fishery are therefore very high. Pelagic sharks, big and small pelagic fishes and large amounts of undersize tuna discards make up most of the by-catch.

The evolution of tuna purse-seine fisheries in the Eastern Tropical Pacific with their associated by-catch issues is an interesting case study in the complexities of fisheries management – and perhaps in the effects of well-meaning but misguided environmental activism. This was the subject of many years of research by Dr Martin Hall of the Inter-American Tropical Tuna Commission. The targeting of dolphin-associated tunas became an environmental issue in the late 1960s that generated significant controversy due to the high rate of dolphin mortality and estimates of rapidly declining dolphin populations. However, by the late 1980s dolphin mortality began to decrease dramatically after the fishery developed its own technique – called a ‘backdown’ – to reduce dolphin by-catch. After encircling a pod of dolphins and the associated tunas, the fishers would reverse their vessel and pull the back of the net, submerging the floats and allowing the dolphins to escape before being hauled into the net.