CONSERVATION OF THE HALIBUT — THOMPSON 369 



surroundings. Growth in winter may practically stop. Hence sum- 

 mer and winter zones, strikingly similar to' those of trees, are de- 

 veloped. Under the microscope these can be read on the scales like 

 those of a tree. In the halibut the scales are small and difficult to 

 read. A much clearer picture can be obtained by using the otolith, 

 or ear bone. This is a calcareous secretion formed in a sac of the 

 internal ear of the halibut at the base of the semicircular canals, 

 which somewhat resemble those of man. The crystals of calcium 

 carbonate are laid down in definite summer and winter patterns with 

 a differing content of organic matter, so that summer zones are 

 opaque and white, winter zones translucent. In a large halibut this 

 otolith may reach a length of approximately an inch and can readily 

 be preserved to be read under a microscope or lens (pi. 1, fig. 2). 



It was found that although halibut gi"ow to a large size they take 

 long to do it. Males grow much more slowly than females, the 

 largest never exceeding 40 pounds; the largest females commonly 

 reach 150 or 200 pounds and occasionally 350 or 400 pounds. Growth 

 is very different on different banks. In the colder waters to the 

 north and west a fish grows much more slowly. Usually a female 

 is 12 years of age before it attains maturity and becomes a migratory 

 spawning adult. Extreme ages of 25 or 30 years and over are at- 

 tained. In fact, at the greater ages the growth of the otolith is so 

 small that there is difficulty in reading it. 



The Commission was thus able to separate the commercial catches 

 into the different ages, finding that the numbers diminished with 

 great rapidity as the fish grew older. The reason for this became 

 apparent when the tagging experiments were analyzed. The returns 

 from these indicated that the stock of fish was disappearing at about 

 60 percent yearly and that as nearly as could be made out 40 percent 

 was due to the fishery itself. These rates of removal by the fishery 

 and by natural death were very different according to area. It was 

 estimated that on the western grounds about 12 percent annually was 

 removed by the fishery, instead of the 40 percent on the southern 

 grounds. 



The stock, insofar as it was independent, could be affected only 

 by these rates of death or removal and by the rate of growth as a 

 balancing factor. On a depleted bank, with a corresponding excess 

 of food and a temperature that fluctuated but little, growth seemed 

 unlikely to be variable. No evidence of this appeared from the age 

 determinations. The natural death rate might well be fairly con- 

 stant also in a fish the size of the halibut. This would leave as a 

 variable only the intensity of the fishery, with its corresponding 

 fishing mortality. An attempt to connect fishing mortality with the 

 yield of the fishery and the size of the stock left on the bank seemed 

 therefore in order. It succeeded to such an extent that the major 



