DISTRIBUTION OF THE CRAYFISH IN BRITISH ISLES 351
facing upstream. Stones or rocks in front of the net were slowly
moved aside by hand and crayfishes concealed beneath these
were picked up and placed into a receptacle. Those that escaped
being caught in this manner were usually carried into the net by
the water current. We collected for one hour over the whole
area of this section by this method, usually on every seventh
day and for a period of thirteen months. Our collecting efficiency
was checked on three occasions by re-collecting over the same
area or by employing two inexperienced persons to collect in our
section. Under these conditions very few crayfishes were
caught.
After each collecting period carapace lengths of crayfishes
caught were measured from rostral apex to the posterior median
edge of the carapace. Specimens with carapace lengths greater
than 10 mm were sexed, the smaller individuals being difficult to
determine with the naked eye. The catch was then returned to
the river and we ensured that these returned specimens had be-
come concealed beneath stones or rocks before we left the
river.
Fluctuations in the total Catches for each month
In December 1963 and in January 1964 the mean water tem-
peratures were 2.5°C and 2.0°C respectively. In those months
most of the captured specimens were taken in the deepest water
of our section and our low catches probably indicate that at least
half of our population had migrated into deeper waters beyond
our section.
In February our catch numbers showed a significant increase
that can be attributed to a return of specimens from deeper
waters to our section due probably to a marked rise in the river
temperature.
Unfortunately, in March the river again flooded and did not
return to its normal level until mid-April. Our catch figures for
March and April are probably not representative for the popula-
tion present at those times. The catch in May indicated that our
section was now becoming re-populated with further crayfishes
from the deeper parts of the river, probably due to a progressive
rise in water temperature.
In June 1964 the river was restocked with numerous brown
trout which are natural predators of crayfishes. Heavy predation
of the population probably explains the low June catches. In
addition there was a near total absence of ovigerous females.
Explorations of the river bed in the deep water beyond our sec-
tion by diving, revealed that many ovigerous females were now
to be found in these parts and in waters of some 1-1.5 m in
depth. Females collected in this manner are not included in
the data presented in Figure 1. Throughout July and August the
numbers of these larger females gradually increased in the study
area following the hatching of their young in mid-June and their
subsequent migration from the deeper waters.