摘要:

Cowichan Lake is 34 kilometres long, up to 4 kilometres wide and up to 150 metres deep, with an area of 62 square kilometres. Its average depth is 51 metres. Its shores are of precipitous rock for over half their length, the remainder being boulders or gravel, with a very little marsh.Summer surface temperatures are usually close to 20 °C., while the winter minimum in 1938 was 5.0°. Oxygen is generally abundant in the lake, the lowest value observed being 3.4 cc. per litre, just before the fall overturn (November 24, 1939). The reaction of the water is alkaline (pH 7.0–7.8), and its bicarbonate content was 18–22 mg. per litre (as CaCO3).Lists are presented of organisms collected in the lake. The plankton is reasonably rich in variety but was poor in quantity, in 1940 at least. Bottom organisms also are not abundant. Two species of lampreys and three fishes were found in the lake, plus a number of Salmonidae whose occurrence is described elsewhere.

ISSN:0706-652X    

DOI:10.1139/f52-022

出版商:NRC Research Press    

年代:1953

数据来源: NRC

摘要:

In pink salmon (Oncorhynchus gorbuscha) the survival during the freshwater phases of the life-cycle has been found to vary from about 1 to 24 per cent of available eggs. Natural survival during marine existence is considered to average about 5 per cent. In the central region of the British Columbia coast the annual catch averages about 60 per cent of the adult fish. Populations maturing in "even" and "odd" years vary in size independently. Fluctuations in level of stock originate mainly in fresh water.Population levels and changes are determined by the combined effects of three types of mortality: (a) mortality which becomes relatively heavier as populations increase in density (compensatory); (b) mortality which becomes relatively heavier as populations decrease in density (depensatory); (c) mortality which is independent of density (extrapensatory). Compensatory mortality is especially identified with the, period of spawning and incubation. Depensatory mortality is considered to occur mainly during the period of fry migration and to be due to predation. Extrapensatory mortality may occur at any stage; it is most variable during the period between entrance of the adults into fresh water and emergence of the free-swimming fry.Populations of chum salmon (O.keta) are controlled by similar influences. Effects are modified by higher egg-production and a less rigid life-span than in the pink salmon.

ISSN:0706-652X    

DOI:10.1139/f52-023

出版商:NRC Research Press    

年代:1953

数据来源: NRC