Blue whale: Limiting factors |
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As with several other whale species, the main cause of the decline of blue whales is whaling. A minimum of 11 000 blue whales were killed in the North Atlantic between the end of the XIX Century and 1960. Although the commercial hunting of blue whales ended in the North Atlantic in the 1950s, blue whales of the Northwest Atlantic appear to be having trouble making a comeback. Researchers studying the status of blue whales in Canadian waters have identified factors that could potentially limit the recovery of this population*.
Entrapment Predation Shipping traffic Whale watching Fishing Pollution Prey abundance and climate change * Sears, R and J. Calambokidis. 2002. COSEWIC status report on the blue whale from Atlantic and Pacific waters (Balaenoptera musculus). Prepared for the Committee on the Status of Endangered Wildlife in Canada. 43 p.
WhalingWhaling for blue whales in the early XX Century greatly reduced the size of the North Atlantic population. Approximately 1 500 blue whales were taken in eastern Canadian waters from 1898 to 1951, including 80 to 100 taken by a whaling station in Sept-Îles, Quebec from 1911 to 1915. In the period from the late XIX Century until 1960 at least 11 000 blue whales were killed in the North Atlantic. Few blue whales were taken after 1951 in eastern Canadian waters; in fact whaling for this species in the North Atlantic was halted by the IWC in 1955. EntrapmentEntrapment in ice in winter is an occasional cause of blue whale mortality. At least 41 blue whales were reported caught in the ice along the West Coast of Newfoundland between 1869 and 1992. In 77 percent of these cases entrapment proved fatal for the animal. The St. Lawrence blue whale photo-identification programme has demonstrated that many St. Lawrence blue whales sport scars on their backs obviously caused by ice. Some animals remain in the Gulf of St. Lawrence in winter to take advantage of the clouds of plankton that accumulate along the ice edge. Ice, which shifts according to wind and currents, can form mortal traps. PredationThe only known blue whale predator is the killer whale, a cetacean that occasionally hunts other marine mammals as well. Very few blue whales in the St. Lawrence carry the rake-like markings thought to be caused by killer whales and no killer whale attacks on blue whales have yet been reported. Therefore, predation is probably not a major cause of death of Northwest Atlantic blue whales. Shipping trafficThe high level of shipping traffic in the St. Lawrence could be a limiting factor; over 5 000 ships navigate up and down the St. Lawrence every year. Shipping lanes run through areas regularly used by blue whales. Deep wounds and scars—attributable to collisions with the propeller or hull of large vessels—have been observed on 16 percent of all blue whales photo-identified in the St. Lawrence. Though there is little direct evidence of mortality due to ship strikes, the relatively high numbers of blue whales with scars that can be linked to ship strikes indicate that this is likely a serious problem. Whale watchingA major whale-watching industry exists in the St. Lawrence Estuary. A study carried out in St. Lawrence Estuary on fin whales —a species closely related to blue whales—has shown that in the presence of a large number of boats, fin whales shorten their dive time; time that is potentially spent hunting prey. This could have the effect of diminishing the amount of food the whales are able to capture, consequently affecting their capacity to store energy reserves and eventually reducing their chances of survival or reproductive success. A project, launched in 2002, is aimed at evaluating how blue whales react in the presence of boats. FishingTens of thousands of marine mammals die each year caught in fishing gear in all oceans of the world. Gillnets have caused the drowning death of at least three blue whales in the St. Lawrence since 1979. Also, at least five cases of animals entangled, but swimming freely, have been reported since 1990. This limiting factor appears to be less serious than ship strikes, however it does warrant monitoring. PollutionSignificant PCB and pesticide levels have been found in St. Lawrence blue whales. Levels of organochlorine pesticides and PCBs were analyzed in blubber biopsy samples from 38 male and 27 female blue whales sampled from 1992 to 1999 in the Gulf of St. Lawrence. There were significant differences in the concentrations of PCBs and organochlorine pesticides in blubber biopsies from male and female whales, with female whales having lower levels due to maternal transfer to their offspring during nursing. Concentrations of persistent contaminants in the blubber of blue whales are approximately two orders of magnitude lower than contaminant concentrations in beluga whales from the St. Lawrence Estuary. This may be due to the fact that blue whales are more transient inhabitants of the region and feed at a lower level of the food web—they feed mainly on krill—than toothed whales do. Contaminant concentrations in offspring are often similar to their mothers’. This raises concerns over the toxicological impacts of exposure to contaminants during sensitive early life stages. Is there a link between the low number of calves observed in the St. Lawrence and contamination? The investigation continues. Prey abundance and climate changeChanges in plankton abundance off the California coast—linked to higher temperatures in the oceans—have been documented since the 1970s and could provoke profound changes in blue whale distribution in this area. The dramatic decline in the abundance of zooplankton appears to be responsible for a 40 percent decline in the numbers of seabirds in this area, including a 90 percent decline of sooty shearwaters, which are zooplankton predators. For the moment nothing indicates similar phenomena taking place in the St. Lawrence. However, a study by Michel Harvey has demonstrated that krill abundance has dropped by 70 percent over the past 10 years in the Lower St. Lawrence Estuary and that a new species of zooplankton has begun appearing in large numbers. Will these changes affect the distribution, abundance and health of blue whales and other marine species? These are the kinds of questions that keep researchers busy. |
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