Life history and production of the crayfish, Pacifastacus leniusculus trowbridgii (Stimpson), in a small woodland stream Public Deposited

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  • A study was conducted during 1959 and 1960 on the history and productivity of the crayfish, Pacitastacus leniusculus trowbridgii, in Berry Creek, a small woodland stream in western Oregon. This included an estimation of the annual crayfish production, in terms of tissue elaborated, of a 1500 foot controlled section of the stream during 1960. The possible role of crayfish in the economy of the controlled section was explored in the light of natural food habits and feeding tests in the laboratory. Copulation and spawning proceed from mid-September through October. The majority of females over 65 mm. mature and are thought to spawn every year. Initial maturation is possible in the third year of life but the majority of crayfish enter the breeding population in their fourth year. An abrupt change in rate of abdominal width expansion at moult marks the inception of maturity in small females. When the abdominal width exceeds 50 per cent of the cephalothorax length, maturity can be expected the next fall. A correlation was found between female size and ovarian egg count. Pleopod egg counts suggest a 40-50 per cent mortality prior to hatching in late April to early June. Hatching and the associated characteristics of stage one and two young are discussed. The stage two young leave the female parent to take up an independent existence. Sexual differentiation can be detected in stage five and six when the primary gonopods appear in juvenile males. Their development is described. Three age classes of Juvenile crayfish were found to occupy the shallow portions of the stream. At least three age classes of adult crayfish occupy the larger pools, giving a total of at least six age classes present in the population within the controlled section. The annual growth and growth per moult relations of this population were measured and were supplemented by laboratory data. The total crayfish production in the controlled section amounted to nearly 13 kilograms (123 lbs/acre) of which two-thirds originated in the juveniles. In terms of stream bottom type, one-third of the total production was elaborated in the riffles, glides, and small pools while two-thirds were elaborated in the medium-sized and large pools. Exoskeleton manufacture amounted to from 21 per cent to 34 per cent of the total production. There was a shift in feeding habits as the crayfish increased in size. Nearly 65 per cent of the diet of sub-yearlings consisted of animal matter whereas adults over 80 mm. relied chiefly on plant material, less than 10 per cent of their diet being in animal form. Juvenile crayfish fed extensively on mayfly nymphs and Tendipedid larvae. Plant material in the stomachs was almost entirely of direct terrestrial origin. Feeding tests in the laboratory served to supplement the stomach contents analysis. The food basis of crayfish production was explored. It was estimated that nearly 107 kilograms of food was consumed by the crayfish population during 1960, of which 70 per cent was plant material of terrestrial orig1n. Although only 19 per cent of the incoming food was consumed in the shallows by the first two age classes, they accounted for over 31 per cent of the total animal material consumed by the population. By contrast, nearly 90 per cent of the plant material was consumed in the medium-sized and large pools by the older age groups. The minimal contribution of terrestrial primary production to the controlled section was found to be 349 kilograms (dry weight). Of this amount, about six per cent may have been consumed by the crayfish population during 1960. Finally, the productivity of crayfish in aquatic communities and factors of possible influence on crayfish production in the controlled section are discussed.
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