Incidental Catches (marine mammals)


Marine mammals sometimes get caught up and killed in ‘ fishing operations. In many cases these deaths are en-i tirely unintended by the fishermen concerned and are incidental to the main fishing operation. They are therefore referred to as incidental catches. Sometimes they are also referred to as “bycatches,” although this term is also used to described the capture of some species that, while not the main target of a fishery, still have some value and may therefore be landed. Incidental catches are generally unwanted and discarded.

Incidental catches of marine mammals have probably occurred for as long as people have been putting nets and lines into the water. Most species of marine mammal that occur in places that are heavily fished have been recorded caught in at least one type of fishing gear. Most types of fishing gear have been reported to ensnare marine mammals at one time or another. Some captures seem to defy reason. Large whales, for example, may become caught in a single lobster pot line, whereas porpoises get caught in simple fish traps that they are able to find their way into, but not out of. Others catches are easier to comprehend, as when trawls with openings of several hundred meters in circumferences scoop whole schools of dolphins from the sea.

In the past, and indeed in many parts of the world today, unintentional captures of marine mammals might be treated as a bycatch and landed for consumption. During the latter half of the 20th century, however, fishing technology has changed faster and more completely than ever before, which has led to a reappraisal of the issues surrounding bycatch and incidental catch. Nets have become larger and stronger, numerous new fishing techniques have been devised, and fishing intensity throughout the world has increased dramatically, nearly trebling marine fishery landings over a period of just 40 years.

Such developments have had unintended negative impacts on nontarget species, including marine mammals, so that incidental catches have now become a critical issue for some marine mammal populations. Marine mammals generally reproduce slowly, and their populations are not able to withstand much additional nonnatural mortality. The removal of just 1% of the population per year may be more than a marine mammal population can sustain in the longer term. For this reason, many nations now legislate to protect marine mammal populations from deliberate or accidental exploitation, and there are several international agreements with the same aim.

Legislation to protect marine mammals from excessive mortality has resulted from a variety of case studies that have uncovered unsustainable levels of incidental capture. Several of these case have become widely publicized and have generated considerable public attention and debate.

I. Examples

A. Eastern Tropical Pacific Tuna Purse Seine Fishery

The first interaction to be recognized as a serious concern for the conservation of marine mammals was the large-scale capture of pelagic dephinids (mainly Stenella and Delphiniis species) in the U.S. tuna purse seine fishery of the eastern tropical Pacific Ocean (ETP). Tuna boat skippers learned diat diey could catch large tuna by herding dolphin schools with speedboats and then surrounding them with long, deep, purse seine nets. Fishennen were exploiting the curious fact that in the ETP (and some other places), large yellowfin tuna (Thunnus albacares) will school under and follow dolphin schools. Once the dolphins and associated tuna are surrounded, the nets can be “pursed,” whereby the bottom end of the net is closed off, thereby catching the tuna. At this point the dolphins can still surface to breathe within the encircled net and could escape by jumping over the floats. Pelagic del-phinids, however, seem to find it difficult to escape from such an enclosed situation, and many became trapped and died under folds of the surrounding purse seine or simply fainted and died.

This fishing technique was begun in the 1950s, but was not recognized as a potential conservation problem until the early 1970s, when a monitoring program was established. During much of the 1960s and up to 1972. annual mortalities are thought to have ranged between 200,000 and 500,000. Thereafter a variety of efforts were made to reduce the kill, but tens of thousands of dolphins were still being killed annually throughout most of the 1980s. Pantropical spotted dolphins (Stenella attenuata) were the most frequently killed species, and numbers of this species in the ETP were more than halved over the 1960s and 1970s. Populations of other species were also severely impacted.

Largely as a result of public pressure, and the introduction of “dolphin safe” tuna retailing, this practice has now been greatly reduced. New techniques have been devised by the skippers to ensure that a very high proportion of the dolphins used in this way to catch tuna are encouraged to escape from the nets before the fish are removed. Under a training and monitoring scheme run by the Inter-American Tropical Tuna Commission, dolphin mortalities had been reduced to between 2000 and 4000 animals per year by the mid-1990s. Efforts continue to reduce these figures further still.

Throughout the world, since the discovery of the effect of the ETP tuna fishery on dolphin populations, it has become clear that there are numerous other fisheries in which marine mammals are being killed in large numbers. In some cases, populations or species have been threatened with extinction. Two of the most severe cases are those concerning the baiji (Lipotes vexillifer) and the vaquita (Phocoena sinus).

B. The Baiji

The baiji. otherwise known as the Chinese river dolphin, inhabits the middle and lower parts of the Yangtse River system in China. The total population size is not known, but is thought to be a few hundred at most and is declining. The major source of mortality for this species appears to be snagging in “rolling hook” fishing lines. These are lines equipped with many closely set, sharp, unbaited hooks designed to snag fish foraging on the river bed in the same areas as the Baiji. In one study, 45% of all known Baiji deaths were attributed to snagging in rolling hooks.

C. The Vaquita

The vaquita is a species of porpoise restricted to the upper part of the Gulf of California in Mexico. Population studies suggest that only around 600 animals remain, that numbers are declining, and that the species is in critical danger of extinction. Again, the major source of mortality is incidental catches in fishing operations, in this case gill net for sharks and other large fishes. Gill nets are simple long panels of netting that are set to stand vertically in the water with floats along their top and a weighted rope on their bottom. Depending on the amount of weight added, they either sit on the seabed floating upward or they float at the surface hanging down. They are left to ensnare fish that happen to swim into them, but also catch marine mammals by entangling them. Annual vaquita mortalities in gill net fisheries are estimated at around 40-80 per year, which is clearly an unsustainable level of mortality given the size of the population.

D. New Zealand Sea Lion

Another species that has been threatened in a similar way. but by an entirely different sort of fishery, is the New Zealand sea lion (Phocarctos hookeri). These sea lions are restricted to New Zealand’s sub-Antarctic islands, mainly the Auckland Islands. The population is thought to have been much reduced by commercial sealing activities in the 19th century. The total population size was estimated at around 13,000 animals in the mid-1990s, although it has been further reduced by a mass die-off in 1998. A trawl fishery for squid was started around the Auckland Islands in 1979, which subsequently resulted in significant numbers of sea lions being drowned in the large trawl nets, raising concern that the population might be reduced further or even threatened with extinction. The New Zealand government implemented an observer scheme in the late 1980s to monitor the numbers being killed and it now sets incidental catch limits every year in order to prevent unsustainable levels of mortality. Once the annual limit is reached the fishery must stop fishing.

II. Causes for Concern

Although the just-described examples are perhaps the most extreme cases, there are numerous others around the world where significant numbers of marine mammals are killed incidentally in fishing operations. It is usually the smaller species and those that occur in continental shelf waters where most fishing occurs that are impacted most heavily.

Incidental catches do not always impact on entire species. In many instances, marine mammal species may be widespread and in little danger of overall extinction. Nevertheless, incidental catches may be frequent enough to reduce or eliminate a local population (Fig. 1). This is the case for the harbor porpoise (Phocoena phocoena). While they are in no imminent danger of extinction as a species, in several areas, including the Gulf of Maine off the U.S. northeast coast, incidental catch rates are or have been high enough to push local populations into decline.

In other parts of its range, including the English Channel and the Baltic Sea. harbor porpoises have all but disappeared. While the causes of these disappearances are not known, fishing is intense in both areas and incidental catch rates in adjacent areas are blown to be high enough to be unsustainable.

Throughout the world, small inshore species such as the harbor poipoise are known to be victims of incidental catches in fishing operations, but the level of such catches and the likely impact remain unknown. Monitoring incidental catch rates and estimating population sizes are both very expensive. A significant issue in this regard is that there does not need to be a very large number of incidental kills for the total effect to be significant. When a marine mammal population numbers in the hundreds or even the tens of thousands, a few individuals to a few hundred individuals taken per year may be enough to generate a population decline. Furthermore, even when the marine mammal population is much larger, if the fishery is also large, significant catches can occur while still remaining unknown. Generally speaking, incidental catches are rare events. Typically, a capture event only occurs in one or two out of every hundred fishing operations. Such low levels may remain unnoticed. although the aggregate effect over a large number of vessels and operations may be significant. Such low levels of capture also make monitoring more difficult.

Although most attention worldwide has focused on the potential conservation issues that incidental catches of marine mammals raise, animal welfare considerations are also a concern. Whereas some incidental capture of marine mammals in fishing operations is an inevitable consequence of fishing, in some nations at least, any large-scale fatalities of marine mammals are publicly unacceptable regardless of whether they are sustainable at a population level.  

A harbor porpoise entangled in a cod gillnet in the North Sea, one of several thousand dying this way every year in European gillnet fisheries.

Figure 1 A harbor porpoise entangled in a cod gillnet in the North Sea, one of several thousand dying this way every year in European gillnet fisheries.

III. Attempts to Resolve the Problem

Most of the numerically significant incidental catches of marine mammals tend to be in static fishing gear, mainly gill nets. Despite the attention focused on this subject in recent years, it is still not known how or why marine mammals actually become caught in such nets. It is not known, for example, whether mammals are attracted to nets by curiosity or by the presence of trapped fish, whether they do not notice the netting, or whether they simply do not understand the potential consequence of swimming into it. Despite our ignorance, some progress has been made toward resolving the problem.

One potential solution to the problem of marine mammal capture in gill nets has been developed in North America. Pingers, or acoustic beacons, exploit the sensitive hearing of marine mammals by emitting an intermittent, short, high-pitched noise that most fish cannot hear but that appears to repel or warn off marine mammals. Attached at regular intervals along the length of a gill net, these 16-cm-long cylindrical devices have been shown to reduce the numbers of marine mammals (mainly harbor porpoises, but also dolphins and sea lions) caught by up to 90% (Kraus et al., 1997). Pingers were first developed in Canada, and their use is now mandatory in several U.S. fisheries.

Pingers certainly appear to be useful but there are still some concerns about their use. As they are a recent technological innovation, marine mammals may become habituated to their noises and start to ignore them. If, as seems to be the case, pingers displace animals from an area, and if they are used to the very large numbers that would seem to be necessary, then it is also possible that marine mammals may become excluded from parts of their foraging habitats. Pingers rely on batteries, and they relv on people replacing those batteries to ensure that they continue to work. This can be an expensive and time-consuming operation that many people might eventually prefer to forget about. Finally, the pingers themselves are expensive, so that the cost of equipping a net with pingers may exceed the cost of the net. In many less-developed countries, it is unlikely that they will ever become widely used for this reason alone.

Issues with mobile fishing gear are somewhat different. There are or have been several initiatives worldwide that aim to keep marine mammals out of towed fishing gear. In the ETP tuna fishery referred to earlier, special techniques and nets have been developed to help dolphins to escape from the purse seine net once the net has fully encircled the school of tuna. During the “backdown procedure” the skipper reverses the vessel and is able to sink a part of the net floatline under the water, enabling the dolphins to escape. This part of the net is also made up with a smaller meshed panel (the Medina panel), reducing the chances of dolphins becoming entangled as they escape. Similarly, in New Zealand, special marine mammal escape devices have been designed and used in squid trawls. A large grid is placed near the rear of the net, set at a 45° angle to the vertical plane. Fish can pass through the grid, but larger animals such as sea lions are forced upward and out the net through an escape hatch.

In general, the incidental capture of marine mammals is caused by a combination of fishing technique or gear design and the behavior of the marine mammal. Resolving problematic interactions therefore involves some combination of change to fishing gear use or design and the manipulation of marine mammal behavior. Very little is known about the behavior of marine mammals in relation to fishing gear, especially in the context of incidental capture. In part this is because of the difficulties of studying marine mammals underwater, but it is also because of the rarity of such events, which makes observing their occurrence very difficult. Finding solutions to the problem is therefore a slow and arduous process.

Most fishing practices and gear designs have been adopted by fishing communities because they are effective in catching fish, and making changes may therefore reduce the profitability of a fishery. Effective mitigation measures therefore need to be devised in collaboration with the fishing community in order to minimize the adverse impacts on fish catches, but they may also require a legislative approach to ensure compliance or equability within a fishery. In this respect, managing the incidental capture of marine mammals may be seen as part of a much more wide-ranging and ongoing problem of managing a global industry that, in the last 50 years, has outgrown its resource base.

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