Life in Lakes and Rivers by T. T. Macan

Author:T. T. Macan [T. T. Macan and E. B. Worthington ]
Language: eng
Format: epub
ISBN: 9780007406135
Publisher: HarperCollins Publishers

Table 6. Production of Salmo trutta in the Horokiwi Stream (Allen, 1952)

The trout is a particularly suitable animal for a study of this kind. It is large, it lays large eggs whose numbers can be ascertained with accuracy, the eggs hatch within a short period, there is only one generation a year, and there was little movement up or downstream in the Horokiwi. It is not difficult to understand why data comparable to that for the trout are scarce. The eggs of invertebrates are very small by comparison, and few can be identified. It is known that some hatch over a long period, even though the oviposition time is short, which means that in any sample of the young that does not include the eggs, and samples rarely do, it is impossible to know how much loss any reduction in numbers really represents; the gap may have been partly filled by larvae from eggs unhatched at the time of the first sampling.

A similar complication makes calculation of production by planktonic Cladocera difficult, because reproduction is fast and continuous, and sexual maturity is reached after a short period. It is extremely difficult to culture the animals from the open water of a big lake in captivity in conditions that might make information about frequency of reproduction, number of young per birth, and age of young at sexual maturity applicable to a wild population.

Up to this point production has been discussed in terms of weight. It is more accurate to discuss it in terms of energy. This sometimes appears to be an unnecessary complication to the layman, but the housewife would have no difficulty in explaining one good reason for doing it. At the butcher’s she knows that a pound of meat may be a pound of edible flesh or may include a weight of bone which, even though the dog may enjoy it, consists largely of calcium salts, which the body requires in small amounts only. ‘Ecological energetics’, as the current jargon has it, has been much studied in recent years, and in a book of that title, J. Phillipson (1966) has set out to describe it clearly, simply, and shortly.

Production, as will have been evident from the foregoing, is used here to mean the production of living matter in a limited period of time. Within a year the population of a large mammal will have done no more than produce enough offspring to make good relatively small losses; a pond-snail, which dies after laying eggs, will have produced the whole population except that portion which the preceding generation contributed to the egg; smaller animals may have reproduced themselves many times over. The element of time in the definition is important because in the long run loss balances gain and the energy that runs through the living organisms of the world keeps the system working without loss or gain. This must obviously be so or the whole world would have been overpopulated or depopulated years ago.

Energy in the form of heat from the sun pours onto the earth at the rate of about 15.

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