Fishery InteractionsFishery Interactions between marine mammals and fishery are as old as fishery itself but are continuously increasing during the last decades. Growth of human population has lead to a extending demand for marine protein resulting in intensive harvesting of the worlds oceans and expansion of fishery into high seas. Therefore fishery interactions are now under the most serious threats which cetaceans (whales, dolphins, porpoises) face. There are several types of fishery interactions: bycatch, depredation, changes in cetacean behaviour and ecology caused by aquaculture and resource competition between humans and whales particularly in light of the huge number of dramatically decreasing fish populations. Active hunting of whales and dolphins is also a kind of fishery interaction but will be discussed on the whaling web page.
BycatchAnimals of a species which are caught unintentionally in a fishery are called bycatch. Bycatch is a problem which affects many cetaceans e.g. harbour porpoises, Burmeister's porpoises, common dolphins, spinner dolphins, pantropical spotted dolphins, dusky dolphins, pilot whales, killer whales, beaked whales, humpback whales and right whales. Some species are even threatened by extinction mainly due to this problem. Examples are the Vaquita and the Hector's dolphin. Several species as well as many populations will be lost during the next few decades if there wont be a change. In some humpback and right whale populations 50% to at least 70% of the animals become entangled once in their life and 10 to 30% become entangled each year. Global marine bycatch is in the hundreds of thousands of whales, dolphins and porpoises annually and there is still a big knowledge gap about the global dimension of this kind of fishery interaction. Research is usually focused on a certain species in a certain area which is supposed to be affected by a certain type of fishery. So for many regions there are still no information on bycatch rates available. One of those regions is the complete Oceania (South Pacific). If problems were figured out the knowledge usually did not lead to significant changes in fishery until few years ago. Since seas are used by many nations international collaboration in research of fishery interactions is of big importance but was rarely the case in the past. The amount of bycatch highly depends on geographic area and type of fishery. Many marine mammals die in trawl nets. Trawling means a large fishing net is pulled through the water by one or more boats. Common dolphins for example are affected by trawl fishery off New Zealand which is targeted on jack mackerel. Other cetaceans often caught by trawlers are Atlantic white-sided dolphins, long-finned pilot whales, bottlenose dolphins and white-beaked dolphins. An important factor in bycatch caused by trawling is the haulback procedure. When hauling is stopped the entrance of the net collapses. So the cetaceans are trapped and drown if the net stays below the water surface for a long time. Light conditions can have an effect as well since cetacean bycatch seems to be higher during extremely dark conditions when little or no moonlight is available during the night. There are also differences in type of trawling. Mid water trawls are more likely to catch whales than bottom trawls since hauling is faster and therefore the animals have less time to escape the net. Mid water trawling means the net is towed through free water whereas during bottom trawling the net is moved on or close to the sea floor. The major source of bycatch mortality however is the gillnet fishery. Gill nets are walls of netting which can be set at any depth. Mesh size is adapted to the species targeted since fishes are caught while trying to cross the net. They pass their heads through the net but are too big to pass it completely. As soon as the fish tries to escape by swimming backward, it becomes entangled by the gills. That is why the net is called gillnet. In Chilean bottom set nets Burmeister's porpoises and Chilean dolphins are often caught and sometimes Peale's dolphins die in the nets. Dwarf spinner dolphins, Indo-Pacific humpback dolphins and Irrawaddy dolphins end up as bycatch of the gillnet fishery in the Gulf of Thailand. In the Philippines, spinner, spotted and Fraser's dolphins are taken by this kind of fishery, common dolphins as well as bottlenose dolphins die as bycatch in Australian waters. Common dolphins, Indo-Pacific humpback dolphins and bottlenose dolphins are also often caught in the gillnets off South Africa which are targeted on sharks. In the Peruvian waters bycatch in gillnets is a big problem which mainly affects long-beaked common dolphins, dusky dolphins, bottlenose dolphins and Burmeister's porpoises. Large numbers of Guiana dolphins are bycaught in gillnets off the Brazilian coast. These are only few examples for areas and species affected by this fishery. Drift nets have caused death of thousands of cetaceans worldwide. In 1992 the UN adopted a resolution which bans drift nets from international waters but drift netting still continues illegal and in some coastal waters e.g. off Peru, Morocco, Italy, Spain, Tunisia, France, Turkey, Algeria and the US. The countries around the Mediterranean Sea still make up a drift net fleet of more than 500 vessels. In 1994 a study has shown that more than 10,000 cetaceans are bycaught in Mediterranean drift nets every year. According to an article published in 2006 approximately 16,500 striped and common dolphins die annually in Moroccan drift nets alone. Fin whales, minke whales and long-finned pilot whales are also caught in these nets. Drift nets can be up to 50km in length and since they are not anchored to the sea floor they sometimes get lost during storms and drift through the oceans as so called ghost nets. Between 1990 and 2000 about 25,000 dolphins were bycaught in drift nets in the eastern North Atlantic of which about 11,000 were common dolphins and approximately 12,000-13,000 were striped dolphins. False killer whales are often bycatch in the longline fishery which has already lead to serious depletion of their stocks. The interactions between dolphins and the purse seine tuna fishery were what made this fishery so successful but also became one of the major threats for the dolphin species involved. Spinner dolphins, spotted dolphins and common dolphins are often associated with yellow fin tuna. Until the 1990's these species were commonly used to reach higher catch rates of tuna. Therefore speed boats have chased the dolphins to gather them into a tight group. Then the net was set around the dolphin school. Since the tuna stays with the dolphins both dolphins and tuna were caught in the net. If the dolphins would have escaped a portion of the tuna school would have been likely to be missed by the fishermen, too. Usually the dolphins were then released during a small opening of the net but shipping manoeuvres for it are difficult and so fishers did not always succeed. So large numbers of dolphins ended up as bycatch. In the eastern tropical Pacific, which is the main area of purse seine tuna fishery, more than 6 million dolphins were killed since 1960. This is the largest documented cetacean killing in the world. By the end of the 1970's bycatch rate decreased from 500,000 to 20,000 per year but when fleets of Mexico and Latin America extended their tuna fishery the problem became serious again. From 1994 on in the USA only dolphin safe tuna was allowed to be sold which means the tuna had to be caught without encasing the dolphins in the net. This had a big effect on tuna fishery since the USA was one of the main consumer of tuna in the world. In 1999 less than 3000 dolphins were killed in nets of the purse seine tuna fishery.
SolutionsTo reduce the bycatch problem pingers were developed. A pinger produces loud sounds which can be heard by dolphins and porpoises and often keep them away from fishing gear. It is a small device which can easily be attached to nets. Pingers work well for harbour porpoises, short-beaked common dolphins, Hector's dolphins, Franciscana's and at least in some areas like the Balearic Islands also for bottlenose dolphins where they reduced bycatch in set nets by 49%. Bycatch reduction for the short-beaked common dolphins is even higher. However the effectiveness depends on geographic region, dolphin or porpoise species and type of fishery. Additionally up to now no long term studies with pingers have been carried out. That means researchers do not know if the dolphins will habituate to the sounds of the pingers. If so, pingers could become a dinner bell for dolphins and porpoises and thus even attract these animals. Another problem with pingers is that in many coastal regions of the world large areas are covered by gillnet fishery and aquaculture. So the utilization of pingers could lead to an exclusion of dolphins, porpoises and other toothed whales from coastal habitats.There are also several ways to enhance the detectability of gillnets to make it easier for cetaceans to swim around them instead of getting entangled but this works only when the animals are echolocating which is rarely the case during activities other than foraging. Therefore often the only solution is closure of specific areas to gillnetting. Creating whale and dolphin sanctuaries where fishing is permitted or limited is also a possibility which was implemented e.g. in New Zealand in 1988 to reduce bycatch of Hector's dolphins. But rules for these areas are not only to be defined but also have to be enforced. Otherwise the sanctuary does not improve the situation at all.Catch limits have to be set and enforced not only for gillnetting and further research on and monitoring of bycatch is important to achieve a better understanding of the global dimensions of this issue.To find solutions for the bycatch problem is not only important for marine mammals but also for many other marine species which die in fishing gear. For example several species of albatrosses and sea turtles face extinction from bycatch but also many kinds of sharks are seriously affected. DepredationDepredation by cetaceans means that whales or dolphins steal fish and/or bait from fishing gear. This causes both loss of catch and net damage and therefore loss of money for the fishermen. Additionally the presence of dolphins causes the fishes to flee which also results in a smaller catch. In the South Pacific Island region whales strip complete tuna longline catches. Only heads of the fishes are left. In the whole South Pacific false killer whales are known to feed on longline catches and at least in the Coral Sea also short-finned pilot whales are involved. In the Strait of Gibraltar orcas depredate on red tuna and in the Southern Ocean longlines of fishermen which target on Patagonian toothfish get emptied by orcas and sperm whales. Depredation understandably causes anger of the fishermen. So they developed very different methods to avoid loosing catch. Some fishermen e.g. changed the fishing area or time, have modified their fishing gear or played sounds from feeding killer whales which could be harmful to other cetaceans and therefore make them going somewhere else. Unfortunately lots of lethal methods like shooting or harpooning or utilization of explosives and poisons to kill these animals are applied as well. At Iki Island in Japan thousands of bottlenose dolphins and hundreds of Pacific white-sided dolphins, false killer whales and Risso's dolphins were killed between 1976 and 1982 because people wanted to stop the decline of yellow fin tuna stocks in the area by depleting these cetaceans.Depredation is not only a problem for the fishermen but also for the whales themselves since they increase their risk to end up as bycatch by approaching the fishing vessel and gear that close SolutionsAcoustic devices which locate the whales before they localize the fishing vessel are a solution to avoid depredation by whales and dolphins. Some of these devices also reduce the echolocation ability of toothed whales close to the net which makes it harder for them to detect the fishing gear. However more information about these still very new instruments have to be collected to enable conclusions about there long-term efficiency. Another solution is the modification of longline gear which reduced depredation by orcas and sperm whales in the Southern Ocean by up to two thirds. This special kind of modification was introduced to Chilean longline fishery. Another modification type of the longline gear was introduced to the Uruguayan fishery and has reduced depredation from 71% of the catch to 27%. At the same time catch rates increased from 15.53 to 23.03kg/hour. In UK longline fishery off South Georgia (Subantarctic island group) and in a Russian vessel which operates in the SW-Atlantic so called protection or cone nets are used. These nets slide down over the hooks and the catch as soon as the line is hauled. This is a very new technique which needs further examination but first results are promising. Additionally fishing vessels which are operating in the same area should communicate to avoid approaching groups of whales or dolphins. Quieter vessels and bait and/or gear with bad smell could also be solutions. Cetaceans could be distracted by recorded sounds of fishing vessels which are played from different stations in the fishing area. AquacultureAquaculture means the farming of marine and freshwater organisms. Fish and shellfish farms change the behaviour of dolphins which occur in their vicinity. This is because the fishes within the aquaculture are fed by humans and part of the food leaves the farm. This attracts other fishes which approach the farm to feed. These fishes are natural prey of dolphins. So the dolphins forage around fish farms because they have learned that aquaculture's are linked to easy food. These dolphins show decreasing natural feeding activities, changes in distribution and less traveling activity. The dolphins which spend most of the time around the fish cages again sometimes attract tourists which may lead to situations which are dangerous for the dolphins. Additionally small cetaceans get entangled in aquaculture gear. They also cause damage of the gear and occasionally release fishes out of the farms. Another impact of marine aquaculture is that small fishes are used to produce fish meal as food for the animals farmed. These small fishes are not targeted for human consumption but are important prey for many commercially caught fishes. If these small fishes are removed from the ecosystem in high numbers prey of many predators becomes depleted.
SolutionsStrong behavior rules for tourists around fish farms and enforcement of these rules would at least solve one issue. Pingers are tested to keep whales and dolphins away from aquaculture's with controversial results.
Food competition between humans and whalesMainly due to declining resources the question of competition between fishermen and cetaceans raised. There is only one study we know about which researches this issue related to the ecological role of cetaceans and fishery in the food web: the Lenfest Ocean Program. According to preliminary results of this study there is only a small overlap between species targeted by the fisheries and species which whales, dolphins and porpoises feed on. In the tropics and subtropics the amount of fish taken by these animals is much lower than that taken by other predators like different fish species. Additionally in first simulations a reduction in the number of whales would not have an influence on the amount of commercially important fish available in the worlds oceans. For some fish species however there is competition which has effects to both directions. Fishermen catch less fish because cetaceans compete with them but whales and dolphins also get less food because fisheries are overfishing the fish stocks. Since the fishing industry is landing more than 90 million ton of fish and other marine organisms each year it seems to be quite obvious who depletes the fish resources. These dimensions of fish removal from the oceans changes the ecosystem and therefore affect marine mammals as well as other predators. Conservation and threats to whales
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