|Abstract or Summary
- 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