GOBIES IN THE UPPER BLACKWATER ESTUARY 127
4 which covers data from July 1973, the abundance of the gobies is re-
corded as the density per square metre. These values may be lower than
the actual numbers present. Using a similar trawl-net to mine, Healey
(1971) estimated the net efficiency to be 50%. It may be, therefore, that
the density of Common and Sand Gobies could reach 12/m2 at Decoy
Point. However, during the autumn, gobies were found at a minimum of
4/m2, often reaching more than 6/m2 (Figure 4). In the winter and early
summer, the density of the population in the shallow water was around
1/m2.
Table 3
Abundance of Gobies at Decoy Point, R. Blackwater
(b.g.: Black Goby; rock: Rock Goby; 2-spot: Two-spot Goby)
The Common Goby has a life span of about 15 months and the Sand
Goby of 27 months. These short life spans mean that there is a very active
breeding season in the summer leading to the high numbers of gobies
found each autumn. In 1974 the density of gobies did not increase until
August and only reached a short peak at the start of October. In 1973 a
similarly high density was maintained from August till mid-November
and was ended by a sharp drop in temperature in December. Temperature
is generally regarded as the controlling factor of Sand and Common Goby