Last time around I promised to write about growth.
In his recent fine book, ‘Mismanagement of marine Fisheries: (Cambridge University Press, 2010) Dr Alan Longhurst discusses the significance of the little-appreciated fact that fishes differ from terrestrial and aerial vertebrates by growing throughout their lives. That actually provides the rationale for the Theory of Fishing that concentrates on the fate of cohorts of fish that are exploited by fishing gears during some period of their natural lives, with individuals increasing in weight as they age, and dying at various rates from a number of natural causes. Through the years of commercial whaling the myth has grown that whales do not grow much, if at all, once they have reached the size at which they become liable to being harpooned, what we call ‘recruited’. In fact they all do grow, and I think the evidence is that they grow throughout their lives (though of course at a diminishing rate). The vertebrates that are somehow bound by gravity – terrestrial mammals, bats and birds for example - can continue to function only by limiting their growth, whereas the fishes and whales defy most of the restrictions of gravity. (I’m not sure what to think about frogs and dinosaurs!)
Scientists studying whales and whaling have misled themselves by looking nearly always at the animal’s length – which is understandable considering the practical problem of weighing an animal that might reach up to one hundred, even 150, tons. The problem is in the perception of cubes. A whale that has increased in length from one time to another by, say 4% - which may be barely noticeable – has increased in weight by nearly 13%, which would not only be noticeable if you tried to lift her, but is a difference to be valued in tons and dollars, or pounds or Yen as the case may be. I mentioned in an earlier piece that the Bureau of International Whaling Statistics (BIWS) regularly warned the IWC that the average lengths of catches of baleen whale species were steadily declining. In the years before the application of the discipline of population dynamics to whales this was considered to be evidence of possible ‘over-fishing’; now it would more likely be taken as an indication that fishing/whaling was measurably affecting the size of the ‘stock’.
There is a widespread misunderstanding among lay writers about fisheries problems, and even some less-numerate scientists (thankfully a diminishing crowd) that the progressive reduction in average sizes of landed animals is caused by fishermen selectively catching the biggest individuals. That is rarely what happens. Anglers might want the biggest possible individual but commercial fishermen want the biggest possible weight of total catch so long as the smaller individuals in their catches fetch a reasonable price or are at least marketable. The observed ‘shrinkage’ normally arises from the fact that the exploitation is relatively unselective in itself, but the exploited population is replenished every year by a cohort of initially small individuals, thereby reducing the average size of fish – or whales – in the catch the following year.
During the 1970s, when sperm whales were still being caught in large numbers, by Soviet pelagic expeditions as well as by land-based whalers in Western Australia, Spain, Chile and others, I decided to look at the sperm whale catch statistics. In this species size is doubly important. The males are two to three times heavier than the females; among the baleen whales the females are a bit bigger than the females, which is very unusual among the vertebrates. The male sperm whales migrate deeper into the Antarctic than the females. But the sperm whales feed also in the warmer waters further north, and do that by diving deeply – often very deeply – to eat squids and deep-water fishes. ‘Recruitment’ had seemed to be occurring at a smaller size relative to the size they would attain in the very long-term, than in baleen whales. I was interested in particular in what might be the implications of setting, for the sperm whales, a management objective of Maximum Sustainable Yield (MSY) by weight instead of by the ‘traditional’ maximum number. The results astonished me.
But first let us look briefly at the sperm-whaling industry. Between the two World Wars, after the collapse of the once huge and global ‘Yankee’ whaling industry of the 19th century (caused by the new use of mineral oil for city street lighting rather than the collapse of the stocks of whales), not many sperm whales were killed; edible oil from baleen whales was far more profitable. But after WWII, especially during the ‘cold war’, sperm oil became valuable again, this time as an industrial lubricant with strategic significance. A total of about 30 thousand tons of sperm oil was produced during the 1930s, nearly half of it from the southern hemisphere by ‘pelagic’ factory-ship operations (they were not prohibited – as was baleen whaling – from operating outside polar waters.) From 1949 to 1978 sperm oil production totaled nearly half a million tons, less than a quarter of it by pelagic operations, and most of that by the USSR. The land station catches were by many nations in both hemispheres (including some, such as Portugal, that were not at the time members of the IWC) , feeding a global oil market centred in Germany and supplying especially the USA. Tooth ivory was a nice little by-product earner. A moratorium on all sperm whaling was declared by the IWC in 1981 after several years of very difficult negotiations. Until then half a million sperm whales had been killed, post-WWII.
What I found, using the IWC’s preferred population model of the time. was that a successful MSY(weight) management aim would result in 3-4% more sperm oil produced every year and, more importantly, would take one third less effort (measured as days-work by catcher boats in the season) to produce. The bad news was that to allow the greatly depleted females to increase from the presumed MSY(number) level to the better MSY(weight) level would take more than 50 years. I would not now stand by the detail of those numbers (which I published in 1976) but I think the general picture is correct. So much for the fetishistic ‘sustainable use’ so beloved by whalers but also, unfortunately, by some ‘environmentalists’. More about that next time.