FISH STATISTICS.

Otago Daily Times, Issue 19906, 28 September 1926, Page 7

FISH STATISTICS.

INTERESTING FACTS OF MARINE RESEARCH. The fluctuations from year to year in the available supplies of the economically important species of fish have long baffled fishermen and investigators. Recently the veteran Norwegian worker, Dr J. Hjort, has summarised the researches carried out over a prolonged period, and has brought a solution of the problem appreciably nearer by introducing the methods of the science of Vital Statistics. In other words, he has studied the birth rate, age distribution, and migration of selected groups of cod and he: ng, which may be taken as representative of the general population of these kinds of fish. He found that the fish caught in any one year were made up of a number of yearclasses. Thus the spawning shoals of both species may contain individuals from some five, six, or seven years up to over 0 years of age—i.e., more than lo* 15 different classes. The age of a fish is ascertained by markings on the scales, much in the same way as the rings on a tree trunk indicate the number of years of growth. It was furthermore ascertained mat both for herring and cod the numbers hatched out and surviving were amazingly great in the year 1904, for from 1908 to 1914 inclusive by far the greater part or the fish caught began life in 1904. Apparently the conditions governing the survival of the young fish were exceptionally favourable that year. Similar results have been obtained with other fish, thus Mr H. Thompson has shown that seventenths of the market supplies of haddock in 1923 belonged to the year class 1920. Several theories have been put forward as to why certain years are pecially favourable to the young of some species of fish. Unlike whales, fish do not suckle their young, and the presence or absence of suitable and adequate infant food for the fish is regarded by many as a factor of prime importance. The subject, which is of great practical importance to our fishermen, is at present receiving much attention in laboratories devoted to marine research.