The Use of Catch-Effort and Tagging Data in Estimating a Flatfish Population

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Abstract
By a modification of the DeLury method an estimate is made of the stock of lemon sole (Parophrys vetulus) on the fishing grounds in Hecate Strait, British Columbia. The method is based on (1) the trend in catch of tagged fish per unit of effort in relation to accumulated catch of tagged fish, and (2) the trend in catch of untagged fish per unit of effort in relation to accumulated catch of untagged fish. At the start of the experiment 4.72 million pounds are estimated to have been present, while during the experiment 3.74 million pounds entered the area of fishing, 3.26 million pounds emigrated from it, and 2.54 million pounds were caught.A Petersen-type estimate based on the ratio of tagged to untagged fish was 4.70 million pounds present at the start of the experiment—practically identical with the one derived from catch-effort information. Total population for the year (1950) is estimated as 9.8–12.2 million pounds, including catch and possible emigration prior to the experiment, the stock on the fishing grounds at the start of the experiment, and the immigration subsequently.The average annual survival rate of age VII – age IX lemon soles in Hecate Strait has decreased from 0.770 to 0.614 during the period of growth of the fishery, 1944–1951. From this difference the average rate of exploitation is estimated as 16 to 20 per cent and the average population as 8.9 to 11.5 million pounds. Since this range is nearly the same as the range described above, it is concluded that almost the whole of the Hecate Strait stock was accessible to fishing in 1950, which was a year of unusually high production.

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