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Edited on Mon Jun-30-08 01:49 PM by kristopher
- Seafood Consumption - Aquatic Food-Chain Consumption -
Japanese research claimed that whales, dolphins and porpoises consume 500 million tonnes of food a year - 3-6 times the quantity harvested for human consumption. Japanese and Norwegian studies show Minke whales in the northwestern Pacific and northeastern Atlantic fed on Pacific saury, sardine, herring and capelin (99D2). The research was rebutted by Australia and independent marine biologists who say there is little direct competition for food resources between whales and commercial fisheries. Baleen whales, except Bryde's whale, spend winter months in the tropics but travel to the Antarctic to feed for the bulk of their annual energy intake. Bryde's whale, which is a tropical resident, feeds mainly on krill. Partial recovery of some whale populations over the last 3 decades cannot nearly explain the decline in fisheries worldwide. Barbara Whitman, a marine biologist, said the Japanese figures could be misleading because it was 500 million tonnes of sea food, including plankton and krill, and not 500 million tonnes of fish which was said to be consumed by the mammals (99D2).
Minke whales along the Norwegian coast and in the Barents Sea eat about 1.8 tonnes of biomass/year. Its 1996 menu: 600,000 tonnes of krill, 630,000 tonnes of herring, 140,000 tonnes of capelin, 255,000 tonnes of cod, 130,000 tonnes of haddock and 55,000 tonnes of other species such as saithe and sand eels. In other words, harp seals and Minke whales together consume about the same amount of biomass as the Norwegian fishermen land, i.e. about 3 million tonnes (98U7).
Studies by Norwegian Institute of Fisheries and Aquaculture show that seals eat considerable amounts of fish and fish-feed. For instance, the Eastern ice field population of harp seal consumes 1.1-1.7 million tonnes of biomass annually, consisting of 390,000-550,000 tonnes of marine crawfish, 260,000-410,000 tonnes of capelin, 190,000-430,000 tonnes of polar cod, 70,000-205,000 tonnes of herring and 30,000-150,000 tonnes of cod. This consumption is based on the stock size 600,000 - not the most recent 1.5-2 million harp seals (98U7).
The 2.2 million harp seal population of Barents Sea consume as much as 3.3-3.5 million tonnes of biomass/year. The Barents Sea harp seal population's annual diet has been estimated as follows (in tonnes): crustaceans 1,223,800, capelin 807,800, polar cod 605,300, herring 212,400, cod 100,500 and "other fish" 404,200. A very low capelin population impacts upon these figures - harp seal consumption increases slightly: polar cod 876,000, codfish (cod, saithe and haddock) 359,700, herring 392,500, crustaceans 1,204,400 and "other fish" 618,800 (recent research by Norwegian Inst. of Fisheries and Aquaculture) (99H1).
In the USA, NMFS data shows that seals and sea lions on the west coast consume 285,000 tonnes of fish/ year-vs. 217,000 tonnes in 1993. In 2004 their consumption is expected to reach 396,000 tonnes, i.e. about the same amount as caught by fishermen (Aftenposten, 6/4/98, quoting Pacific Fishing Magazine) (98U7).
The Japanese Institute of Cetacean Research (ICR) estimated total food consumption of cetaceans worldwide as 280-500 million tons - 3-6 times the amount taken in marine-capture fisheries <1>. The underlying scientific basis of these claims has been challenged by Environment Australia by CSIRO Marine Research <4>. Below are some of the arguments regarding the ICR contention. (00J1) (Reference citations are given below the arguments.)
(1) Before industrialized fishing began, the marine environment would have been in some sort of steady state (fluctuating environmental factors permitting). Therefore, in the past, when whale populations were far larger, there was obviously enough fish and plankton to sustain them, and enough left over to support the huge global fish stocks that have been exploited by modern day fisheries. The partial recovery of some whale populations over the past 30 years cannot nearly explain the decline in fisheries worldwide." (00J1)
(2) Fisheries rely on relatively few species. Of the 20,000 known species of fish, 9,000 are routinely fished, 22 species are taken in amounts of 100,000 tonnes/ annum while five groups make up 50% of global fisheries. Fisheries discard an estimated 27 million tonnes of fish annually in addition to the landed catch. (00J1) Comments: These facts are probably meant to imply that, statistically, the overlap between the marine species consumed by whales and by humans is likely to be small.
(3) The wide scale industrialization of the whaling fleet and the move towards factory ships after 1926, as opposed to land stations, significantly increased killing and processing efficiency. Currently it is accepted that many whale populations were reduced to less than 10% of their original size. At least 1.5 million were killed in the 50 years following 1925 after industrialization of the whaling fleet (00J1).
(4) Target fish of commercial fisheries do not coincide with those taken by whales. Baleen whales that migrate to the southern ocean in the austral summer depend, for example, largely upon krill for food for a significant part of the year, although some fish are also taken. Over the rest of the year, feeding is greatly reduced in many species (00J1).
(5) Toothed whales prey upon cephalopods to a large extent, and in the case of many whales, such as sperm whales, the major prey groups are considerably spatially separated from targets of commercial fisheries <4>. In the ICR report, total food consumed by sperm whales alone is given as 254 million tonnes (upper limit). Sperm whales feed largely upon cephalopods. So there is no overlap between sperm whale diet and commercial fisheries (00J1).
(6) The greatest predators on fish populations in the majority of ecosystem examples studied, by an order of magnitude, are other predatory fish <4,13>. It has been stated <4> that a qualitative overlap in dietary items between large baleen whales in the Southern Pacific (and predatory fish?) is not supported by evidence. Accordingly, total consumption by sperm whales should be removed from this total, together with the total for baleen whales in the Southern Hemisphere. If this is done, then the ICR's 500 million tonne figure must be reduced by 360 million tonnes. Hence, a new upper bound limit for fish consumption by whales can be set at 140 million tonnes/ year where there is a potential for dietary overlap. Undoubtedly, this figure would fall even further if similar considerations were made for other whales, and dietary overlap with fisheries could be defined for them. Baleen whales feeding in Northern Hemisphere waters also consume substantial quantities of planktonic prey, not just fish species. If the lower bound of 280 million tonnes is treated similarly then the adjusted figure for fish consumption by whales becomes 120 million tonnes/ year (00J1).
(7) In any case, ICR's figure of 500 million tonnes is likely to represent an extreme upper bound <4> because its calculation depends upon an assumed food intake by whales amounting to 3.5% of body weight/ day. In fact, it is likely that the true figure, as an annual average, is 1.2%-1.6% (00J1).
References cited in Ref. (00J1):
<1> Tamura, T; and Ohsumi, S, "Estimation of total food consumption by cetaceans in the world's ocean", The Institute of Cetacean Research, Tokyo, Japan, 16pp (1999).
<4> J. W. Young, (1999) Potential for impact of large whales on commercial fishing in the South Pacific Ocean. Report prepared for Environment Australia, CSIRO Marine Research, Australia, 33pp.
<5> P. Johnston, Santillo, D; Stringer, R; Ashton, J; McKay, B; Verbeek, M; Jackson, E; Landman J; van den Broek, J; Samsom, D; Simmonds, M; (1998). Greenpeace Report on the World's Oceans, Greenpeace International, Amsterdam ISBN 90-73361-45-1. 154pp.
<6> P.B. Best, "Increase rates in severely depleted stocks of baleen whales", ICES Journal of Marine Science. 50 (1993) pp.169-186.
<8> T. D. Smith, "Interactions between marine mammals and fisheries: An unresolved problem for fisheries research". In: A. S. Blix, L. Walloe, O. Ulltang, Whales, Seals, Fish and Man, Elsevier Science (1995) pp.527-37.
<13> M. Earle, "Ecological interactions between cetaceans and fisheries" in Simmonds, MP; Hutchinson, JD The Conservation of Whales and Dolphins, Johns Wiley and Sons, UK. (1996) pp. 167-204.
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