CURRENT STATUS: Pacific sleeper shark numbers increased dramatically in the North Pacific during the 1980s and 1990s. In areas where few sharks ever were caught before, now fishermen are catching many more sleeper sharks. Many fishermen are reporting more and larger sharks each year in the Gulf of Alaska.
ECOLOGY/CONSERVATION: During the 1970s, temperatures in the North Pacific Ocean increased, followed by a change in species composition in the region. The ecosystem supported great quantities of shrimp and crab before the 1970s, but these species nearly disappeared and were replaced by pollock, cod, halibut and arrowtooth flounder. This species composition change is referred to as a “regime shift.” Other noteworthy and dramatic changes included decreases in sea lions, seals and forage fish (capelin, sandlance and herring) and increases in salmon sharks and Pacific sleeper sharks.
One theory explaining the regime shift involves increased winds, global warming, the Gobi Desert, iron and a group of phytoplankton (small single-celled plants) called “diatoms.” As the Earth warms due to global warming, scientists predicted, and have seen, stronger and more persistent winds. When these winds are especially strong they can sweep across the Gobi Desert of Mongolia carrying tons of dust laden with the element iron. Much of this iron is deposited in the North Pacific Ocean, which promotes the growth of a class of organisms called diatoms. Diatoms are single celled plants and can grow rapidly if the conditions are right. The diatoms use iron in a process that keeps them near the water’s surface and in the euphotic zone where they capture the sun’s energy. When the iron supply is used up, the diatoms sink to the bottom of the ocean. If there is lots of iron and it comes in a steady supply, the diatom population blooms and diatoms stay near the surface of the ocean and promote the surface food web and ecosystem. But if the winds are sporadic the diatoms grow, but soon deplete the iron in the water and sink to the bottom. This may be better for the ocean floor food web and ecosystem. The regime shift of the late 1970s may have been a result of the processes described here. As global warming changes wind patterns and the strength of the wind, we can expect more regime shifts and ones of greater magnitude.
Another theory of reduced marine mammal populations is that great white sharks have become abundant in Alaskan waters. Sharks are secretive by nature and do not readily reveal their presence, making it difficult for scientists to study. Sleeper shark populations are at record highs in the North Pacific, and they are preying on many species of fish and on some marine mammals. Sharks may be exerting an influence on the North Pacific marine ecosystem that will be long-lasting. Some scientists and many fishermen are concerned about what will happen, now that sharks are so common in the North Pacific Ocean. Many people have proposed shark predator-control programs without understanding the consequences.
Mathematical ecosystem models predict that there may be worse consequences if people reduce the shark population than there will be if they do not. Though sharks compete for some of the fish people catch and eat, sharks also reduce large changes in prey population numbers. According to some population models, removing sharks likely will result in increased salmon, black cod and pollock numbers. The increase in these smaller predatory fish would increase predation on smaller but extremely important forage fish such as herring, capelin and sandlance. The predicted outcome of the subsequent declines of the forage fish is for further reductions of seal and sea lion populations. This would be bad for fishermen. It also would be a big concern for those people trying to bring the sea lion back from the brink of extinction.
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