Fishing Industry, Effects of (marine mammals)


The fishing industry probably represents the single area » of human activity that has the most profound effects on * marine mammals. These effects can be categorized broadly as “operational effects” and “biological effects.”

Operational effects include the accidental capture of marine mammals in fishing gear, a problem that has brought more than one marine mammal population to the brink of extinction. Although accidental capture usually results in the death of the animal concerned, there are also instances where marine mammals are injured or affected in some way during fishing operations so that their survival probability or reproductive potential is compromised. Not all operational interactions have a negative effect on marine mammals. In some cases the effect of the fishing operations may be positive for the marine mammal where, for example, they feed on discarded fish or take fish that have been caught before these can be retrieved onto the fishing vessel. In a few cases there are even mutually beneficial collaborative efforts between fishermen and marine mammals, with marine mammals assisting in fish capture and being rewarded with a portion of the catch.

Biological effects encompass all the consequences of the large-scale removal of animal biomass from the marine ecosystem through fishing activities, including, although not limited to, possible competition for resources between fisheries and marine mammals. Competitive interactions can be direct or indirect. Direct competition occurs where the mammal and the fishery are both taking the same kind of fish. Indirect competition includes situations where the fishery and the marine mammal population are taking two different types of fish, but where the removal of one of these fish influences the availability of the other through some competitive or predatory link. Indirect interactions need not be competitive, and sometimes the effect of die fishing industry may be to increase the abundance of marine mammal prey items through indirect ecological interactions. Sometimes fisheries may physically alter a habitat and so change the composition and abundance of the fish community to the detriment or advantage of marine mammals and other predators.

I. Operational Effects

Operational effects cover interactions between fisheries and marine mammals that relate to the mechanical process of fishing. Several fisheries have well-documented problems with unwanted entrapment of marine mammals. In some cases the numbers of animals involved are large enough to seriously endanger the marine mammal populations concerned, and in one or two instances, species have been brought to the verge of extinction through such accidental effects. Examples considered cover gill net fisheries, pelagic trawls, and purse seine fisheries.

A. Gill Net Fisheries

Gill nets are a widely used fishing gear with a long history of use in many parts of the world. Their use has become more widespread since the 1950s or 1960s with the introduction of nylon as a netting material during the 1950s. They represent a fuel-efficient means of fishing and, when set on the seabed, provide a fishing method that can be used to exploit areas of rough ground that cannot be fished easily by towed gear. When used in surface waters, they are usually left to drift with the wind and tide and are effective in targeting dispersed fish schools. They are usually left to fish unattended, and fishing times may range from a few hours to several weeks, but 24 hours would be a typical soak time. It has been suggested that in contrast to the traditional nets that were made of cotton and other natural fibres, the use of stronger nylon twines has contributed to an increased rate of marine mammal entanglement. This, coupled with a dramatic increase in their usage since the 1950s, has led to some serious conservation and animal welfare problems with respect to marine mammals.

Small cetaceans, such as porpoises (Phocoenidae), and some species of seals seem especially prone to becoming entangled in gill nets. In some instances, this does not present a conservation problem. In Britain, for example, gray seals (Halichoerus grypus) are frequently caught and drowned accidentally in gill net and tangle net fisheries (see Fig. 1). In a seal tagging program run by the Sea Mammal Research Unit in the North Sea, over 20,000 gray seals have been tagged soon after birth since the 1950s. Returns of tags by fishermen indicate that at least 10% of known subsequent mortalities are due to net entanglement, and at least 1.5% of all pups tagged were recovered dead in fishing nets. Tag loss over the months and years after tagging and failure to return tags from entangled seals are two reasons why this latter figure must be an underestimate of total mortality rates due to entanglement. Despite such mortality in fishing gear, gray seal numbers are increasing in British waters.

In other cases, accidental catches in fishing gear can lead to conservation problems. Concerns have been expressed over the numbers of harbor porpoises (Phocoena phocoena) that become entangled in gill nets throughout much of this species’ range. Numbers are known to have declined in some areas such as the Baltic Sea, possibly as a result of entanglements. In other areas, including the North Sea, the Celtic Sea, and the Gulf of Maine, the total numbers of porpoises entangled annually are thought to be unsustainable and are likely to result in long-term population declines.

Young gray seal caught and drowned in a skate tangle net set in the English North Sea.

Figure 1 Young gray seal caught and drowned in a skate tangle net set in the English North Sea.

A close relative of the harbor poipoise. the vaquita (Phocoena sinus), is threatened with extinction through accidental catches in gill nets. In this case, the species has a restricted range, in the upper Gulf of California in Mexico, where there are large numbers of small boats using gill nets and tangle nets to catch a wide variety of fishes. Although the population may number no more than about 600. one estimate suggests that at least 40 animals drown in gill nets every year. This is clearly an unsustainable rate of mortality and this species’ future therefore seems bleak.

The North Atlantic right whale (Eubalaena glacialis), once one of the more commonlv seen whales in the North Atlantic, is now reduced to a population of around 300 animals in that area. The population is declining and it is estimated that at current rates it will be extinct within 200 years. Most of this population migrate along the eastern seaboard of the United States every year, where they too are vulnerable to entanglement in gill nets and also in lobster pot lines. Lobster pots are usually set in “strings” of several pots in a line, with each end of the string marked by a surface-floating buoy attached bv a line to the pot string on the seabed. Although only three right whale deaths have been attributed directly to this cause since 1970. evidence of entanglement-related scars on live animals has been identified in around 60% of the population. It has been suggested that some entangled whales die and are dragged to the seabed without being recovered, others inav suffer injuries that lead to subsequent death by other causes, while at least two right whales, encumbered with fishing gear, have been fatally wounded in collisions with ships. These high levels of entanglement may therefore be a major factor in the decline in this population of right whales, even though the vast majority of the entanglements are not immediately fatal.

Marine mammal entanglements in gill nets may occur for a variety of possible reasons. Some people maintain that the animals do not detect the netting and swim into it before realizing it is there. It may be that the animals do detect the netting but that they do not recognize it as something dangerous and attempt to swim through it as though it were some natural obstacle. such as seaweed. Another possibility is that the animals are fully aware of the netting, and the danger it poses, but they simply make mistakes and become entangled while feeding close to the net due to inattention.

There has been much attention given to means of reducing the numbers of marine mammals that become caught in gill nets and tangle nets because of the conservation problems that such entanglements represent. So far, the only effective means of reducing bycatch that has been found is the use of pingers. Pingers are small battery-powered devices that emit a brief high-pitched noise every few seconds. They are attached to the float line or lead line of the gill net and are of similar size and shape as a net float so as to avoid tangling the net when it is being set or hauled. They are effective in reducing the entanglement rates of several marine mammal species in gill nets, although exactly why they work is not clear. They have been developed in collaboration with the fishing industry and are currently being used in several major gill net fisheries around the world.

B. Pelagic Trawls

Pelagic trawling is another fishing method that has increased enormously in recent decades. Although trawling dates back for more than a century, this was initially performed with low-opening nets dragged along the seabed. As various aspects of technology have improved, so trawling techniques have been refined, and trawls have been used to catch fish above the seabed and even near the surface of the water. The development in the 1950s of acoustic fish finders and net sounders that enable the skipper to control the position of the net with respect to that of a fish school has been a key technological development.

Initially, during the 1950s, pelagic trawls were used to catch fish like herring that form dense schools. Typical nets might have had an opening of around 2000 m2. perhaps 50 m wide and 40 m high. Since then, both nets and trawlers have grown in size, as other pelagic fish species have been targeted, some of which may form much more dispersed schools. Net openings at least 10 times this are now common, with horizontal and vertical openings of up to 200 m. There are numerous records of marine mammals becoming caught in pelagic trawls, sometimes in large enough numbers to present a conservation problem, although the nature and scale of the problem remain obscure in most fishing areas.

Hooker’s sea lion (Phocarctos hookeri) is endemic to the sub-Antarctic islands of New Zealand, around which a pelagic trawl fishery for squid developed in the 1980s. Observations of fishing activity between 1988 and 1995 suggested annual mortalities of between 20 and 140 Hooker’s sea lions, at a rate of about 1 animal every 340 trawl tows. The total population of this species was only around 13,000 animals in the mid-1990s, and die accidental catches were therefore considered significant. There is currently a quota system in operation in this fishery to limit accidental catches.

Dolphins have also been reported caught in pelagic trawl fisheries in several areas, including the United States, Europe, and New Zealand. The capture of dolphins in pelagic trawls appears to be very variable, depending on the area and probably the type of trawl being used. In one U.S. pair-trawl fishery common bottlenose (Tursiops truncatus), short-beaked common (Delphinus delphis), and Risso’s dolphins (Grampus griseus) were observed accidentally caught in an average of one in every 5 trawl tows, although this fishery has subsequently been closed. In one New Zealand midwater trawl fishery, rates of common and bottlenose dolphin catches averaged about one animal in every 9 trawl tows observed. In European Atlantic waters, one study recorded common and Atlantic white-sided dolphins (Lagenorhynchus acutus) caught in sea bass (Dicen-trarchus labrax) and albacore tuna (Thunnus alalunga) pelagic trawl fisheries at a rate of one in every 10 to one in ever)’ 15 trawl tows. In other pelagic trawl fisheries for anchovy (En-graulus encrasicolus), pilchard (Sardina pilchardus), and mackerel (Scomber scombrus) in the same area, no dolphin deaths were recorded.

The reasons why some pelagic trawl fisheries have relatively high levels of accidental dolphin catches and others have low or zero levels are not yet clear. It is probable that such factors as the dimensions of the net, the towing speed and duration, and the foraging activity of dolphins around the nets are important. There are several accounts of dolphins taking advantage of trawling activity by feeding on fish escaping through the meshes of the trawl. Such behavior may increase chances of dolphin entanglement. The trend toward increasing net sizes may also have increased the numbers of dolphins caught in some pelagic trawl fisheries in the past few decades, but these interactions remain relatively poorly investigated,

C. Tuna Purse Seine Fisheries

As with gill nets and pelagic trawls, technical innovations during the 1950s enabled the development and expansion of purse seine fisheries around the world. Purse seines are used to catch pelagic fish and work by first encircling a school of fish with a long net, hanging from the surface down to depths sometimes of several hundred meters. The bottom edge of the net can be pursed so as to prevent any escape under the netting once it has been set in a circle. The major technical innovations that allowed this fishery to develop globally were the introduction of nylon as a netting material, enabling much larger and stronger nets to be constructed, and the development of the power block, with which large amounts of netting could be hoisted up out of the water.

American fishermen working in the eastern tropical Pacific Ocean during the late 1950s worked out a way of using dolphin schools in conjunction with purse seine nets to catch yellowfin tuna (Thunnus albacares). They discovered that tuna would aggregate under dolphin schools, even if the dolphins were chased. This meant that by using speedboats to corral dolphins they could exploit this behavioral characteristic of the fish to round up tuna schools that would otherwise be invisible below the surface. By corralling a dolphin school and setting a purse seine net around it, a school of tuna would normally also be encircled. The dolphins were not intentionally killed in this activity, but many died as nets were being hauled in and fish were being brought on board. The scale of the fishery meant that in some years hundreds of thousands of dolphins drowned as a result of this fishing activity.

The eastern spinner dolphin stock (Stenella longirostris ori-entalis) was depleted to 44% of its original (pre-1959) level, whereas an offshore pantropical spotted dolphin stock (Stenella attenuata) was reduced to about 20% of its pre-exploitation level. From the 1970s onward, in the face of public concerns over the issue, the fishery developed and implemented means of reducing this toll. By encouraging the dolphins out of the net before trying to remove the fish, annual mortalities were reduced to a few thousand per year.

D. Other Types of Fishing Activity

Although tuna purse seines, pelagic trawls, and gill nets have been highlighted as fishing methods where operational interactions have been a cause for concern, many other fishing methods can, under the wrong circumstances, have an impact on marine mammals. Marine mammals have been reported ensnared in the most unlikely types of fishing gear, including the lobster pot lines referred to earlier. In most cases, such interactions occur rarely and the number of animals involved is so small that there is no cause for concern over unsustainable levels of catch. Occasionally, however, circumstances may conspire to cause a problem. At other times the use of certain types of fishing gear may even result in a benefit to the marine mammals and cause a loss to the fishery.

In several parts of the world cetaceans, often killer whales (Orcinus orca) and false killer whales (Pseudorca crassidens), have been reported to remove fish from hooks during long line fishing operations. This can make fishing in certain areas with long lines unprofitable, and many methods have been tried to eliminate such behavior. There has been remarkably little success reported in trying to prevent such predation, and sometimes the boats involved have had to switch gear or move to other areas.

Normally, interactions between marine mammals and hook and line fisheries have few negative impacts on the marine mammals. In the case of the baiji or Chinese river dolphin (Lipotes vexillifer), however, the situation is different. The baiji is endemic to the Yangtze River system in China; the population probably numbers no more than a hundred and is in decline. One of the most important causes of mortality is due to animals becoming ensnared in “rolling hooks.” This type of fishing involves using many sharp unbaited hooks on a line set on the bottom of the river to snag bottom-dwelling fish. Dolphins foraging near such hooks sometimes become snagged too, occasionally causing death. Over 50% of recovered river dolphin carcasses had died as a result of such entanglements. The small population size and the restricted distribution of this species mean that even uncommon occurrences such as these can have a highly significant impact on a species.

Other types of fishery where marine mammals actively benefit from fishing activities include trawling, purse seining, and lobster potting. Several species, including bottlenose and white-beaked dolphins (Lagenorhynchus albirostris), gray seals, and South African fur seals (Arotocephalus pusillus), have been reported to remove fish from fishing gear. Dolphins typically take undersized fish as they come through the cod end of a trawl. Fur seals swim into purse seine nets by climbing over the float line once a school of fish has been encircled and make a meal of the trapped fish. Gray seals have been observed removing bait from baited lobster pots. There are numerous other examples of marine mammals taking advantage of fishing activities in similar ways, often taking marketable fish, which may provoke considerable resentment on the part of the fishing crews.

In a few places in the world, including Burma (Myanmar), Mauritania, and Brazil, fishermen and dolphins have learned to collaborate in the capture of fish, usually by dolphins driving fish toward fishermen waiting with nets. Some of the catch is then given back to the dolphins as a reward.

II. Biological Effects

The more widespread but less well-understood interactions between fisheries and marine mammals are ecosystem-level effects, where fisheries may cause fundamental changes to the species composition of the marine environment.

Every year the fishing industry removes about over 90 million ton of fish and other marine organisms from the world s oceans. Another 20-30 million ton of unwanted animal biomass may be caught but discarded prior to landing every year. It has been suggested that fisheries may account for 8% of the global primary productivity of the oceans, and in some of the more heavily exploited areas as much as 35% of local primary productivity may be required to sustain fishery catch levels. It is clear that such levels of fishing activity are likely to have profound effects on marine ecosystems, especially on the top predators, such as marine mammals, as fish populations are reduced and restructured on a very large scale. In theory, therefore, fisheries may compete with marine mammals by depleting their food.

There is of course another side to this concern. Whereas fisheries may cause a depletion of the food resource for marine mammals, marine mammals are often accused by fishery bodies of consuming large amounts of fish, thereby reducing the amount of food available for people to eat. As a result of this latter concern, in some parts of the world there are frequent calls for marine mammals to be culled as unwanted competitors.

In both cases, however, it has proved extremely difficult to demonstrate any clear competitive interaction between marine mammals and fisheries. This is mainly because of the complexities of the marine ecosystem, which make it very hard to predict how changes in one fish stock will effect either their predators or their prey. Some brief examples will illustrate this point.

A. Increased Food Availability

The North Sea and adjacent areas are among the most heavily fished sea areas of the world, with annual landings of all species of around 2.5 million metric tons. One of the most numerous marine mammals in this region is the gray seal (Halichoerus grypus), which feeds on a range of fish species, including Atlantic cod, haddock, whiting, saithe, sandeels, sole, plaice, Atlantic herring, and sprats (Gadus morhua, Melanogrammus aeglefinus, Merlangius merlangus, Pollachius virens, Ammodytes spp., Solea solea, Pleuronectes platessa, Clupea harengus, and Sprattus sprattus). Fisheries also target all of these species, and most of the fish stocks concerned are designated as fully exploited or overexploited. Despite these facts, the gray seal population has expanded at an apparently steady rate over several decades during which fishing pressure in the region has been intense, to reach 108,500 animals in 1994. There are at least two good reasons for this apparent paradox.

First, the most important food of the gray seal is the sand eel (or “sandlance” in the United States). The sand eel fishery takes over 1 million metric tons of sand eels per year and is the single largest fishery (in terms of the amount landed) in the North Sea. Despite fishing pressure, sand eels are still extremely abundant and appear to have increased in abundance since the 1950s. The proposed reason for this is that sand eel numbers have increased in response to massive declines in the abundance of Atlantic herring and mackerel as a result of intense herring and mackerel fishing, especially in the 1960s and 1970s. Although sand eels are an oily fish (and well suited to the seals diet), unlike herring they are not considered useful for human consumption, and those that are fished are generally reduced to fishmeal. This means that they are a low-value species and have so far escaped the intense levels of fishing pressure applied to others stocks. Being a fairly sedentary species that spends much of its time buried in the sand, they also appear to be well suited to gray seal foraging habits.

Alongside this, although gray seals and fisheries consume other more marketable species in common, gray seals typically consume smaller-sized individuals of around 15 to 20 cm in length, whereas commercial fisheries generally concentrate on fish of 30 cm and larger. As commercial fishing pressure intensifies, larger fish have become scarcer, but smaller fish of the same species may not be affected (or at least not until there are two few large fish left to generate sufficient eggs to replenish the stock). Indeed, in many of the commercially useful gadid (cod family) fishes, small fish are consumed in large numbers by bigger fish of the same species. A reduction of the numbers of larger fish may actually boost the numbers of smaller fish.

It would seem that although gray seals are feeding in an area that is very heavily fished because they exploit a niche that is not directly in competition with fisheries, they are able to thrive. Indeed their population expansion may have been assisted by fishery-induced changes to the species and size structure of the system they inhabit.

B. Decreased Food Availability

Although pinniped numbers may have increased with increasing fishing pressure in one area, this is by no means the norm. In the Gulf of Alaska and the Bering Sea, several pinniped populations have undergone dramatic declines over periods when fishing activity has been increasing. The Pribilof population of Northern fur seals (Callorhinus ursinus) declined from 1.25 million animals in 1974 to around 877,000 animals in 1983. At around the same time, harbor seal (Phoca vitidina richardii) numbers in the Gulf of Alaska and southeastern

Bering Sea declined, with one major haul-out site at Tugidak Island recording an 85% reduction in numbers between 1976 and 1988. Populations of both of these species now seem to be stable or increasing, but numbers of Steller sea lions (Eu-matopias jubatus) in western Alaska started to decline in the 1970s and numbers are still declining. Declines of up to 80% have been recorded in some areas.

The reasons for the declines of these three species over much the same time period are not known, but there is general agreement that food availability, especially for younger animals, seems to be a key issue. For all of these three species, Alaska pollock (Theragra chalcogramina) is an important prey item. Alaska pollock is also the target of one of the largest single species fisheries in the world. The fishery for Alaska pollock increased greatly during the 1970s with over a million tons being landed annually from the eastern Bering Sea alone throughout most of the 1980s and 1990s.

An obvious explanation is that the fishery has deprived the pinnipeds of their food. A closer inspection, however, reveals that the situation is more complex. Overall, the pollock biomass has stayed remarkably buoyant throughout this time period. Numbers of pollock, especially numbers of the larger or older age classes that are the target of the fishery, have not declined until relatively recently. Smaller pollock are consumed by the pinnipeds and are not targeted by the fishery, but are cannibalized by the larger fish. Numbers of smaller pollock have declined. Furthermore, for Steller sea lions at least, pollock does not appear to be a favored food item. In other parts of the Steller sea lion’s range, such as southeastern Alaska, where the population is not in decline, pollock makes a smaller contribution to the diet, after oily fish such as herring.

It may be that the three pinniped species have all suffered from some change in the relative abundance of their preferred diet items. It has not yet been possible to determine whether such ecosystem level changes have been the result of long-term oscillations in oceanographic conditions or whether the pollock fishery has in some way altered the abundance of the pinniped’s preferred prey items through the cascading effects of restructuring the pollock population.

C. Other Effects

The effects of fisheries on marine mammals do not necessarily have to be mediated through changes in their food supply. Changes to predation on marine mammals could also arise through the effects of fishing. On the Atlantic coast of Canada, also a heavily fished region, gray seal numbers have also been increasing steadily for more than two decades for unknown reasons. One suggested reason has been that fisheries may have greatly reduced the number of large sharks in coastal waters of Atlantic Canada, and as gray seals are known to be subject to predation by certain sharks, such a reduction might be one of the factors contributing to the increase in gray seal numbers.

Conversely, recent increases in predation on sea otters (En-hydra lutris) in the Aleutian Islands have been attributed to behavior changes in killer whales in the region, some of which now seem to be preying more heavily on sea otters than in previous decades. The possibility has been raised that this change in behavior has been caused by a decline in other more favored food items, such as sea lions. Such declines could, as has been suggested earlier, be at least partly due to the effects of fishing on sea lion food items.

All of these hypotheses demonstrate the complex ways in which fishery-induced changes to the marine ecosystem may affect marine mammals, although none has yet proved testable. In almost all cases where some form of competition is perceived, any closer scrutiny of the situation reveals that the complex predatory interrelations of the marine ecosystem make it extremely difficult to predict the results of any proposed management action. The extent to which the fishing industry competes with marine mammals is therefore still very much an open question.

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