128
THE ESSEX NATURALIST
Figure 4 Summary of the data collected at Decoy Point:
'A': Density of gobies per nr. The line represents 2 per m2.
'B': The % of Sand Gobies in the total catch. The line represents 33‰ or 1 Sand
Goby per 2 Common Gobies in the catch.
'C: Water temperature, the line at 10°C.
'D': Salinity of the water collected when fishing. The line at 20‰.
'E': Estimated height of the tide at low-water above chart-datum (Admiralty
Tide Tables). The line at 0.5 m.
The year and start of each month is marked at the bottom of the figure.
migrations into deeper water in winter. Sand Gobies migrate to deep water
off the coast, whereas the Common Goby will only move over short distan-
ces. The Common Goby population is therefore more susceptible to sharp
drops in temperature. Indeed, in the severe winter of 1962, the Common
Goby suffered an almost 100% mortality in the Dutch Wadden Sea
(Fonds, 1973) from which the population took several years to recover.
The Sand Goby may readily move back into the estuaries to feed in
winter when conditions are not severe. The proportion of Sand Gobies
caught altered constantly throughout the year. Often drops in numbers of
gobies caught were associated with fewer Sand Gobies. For example this
occurred in May and November 1974 and in March 1975. The low number
of gobies at the beginning of February 1975, however, was due to very
poor fishing conditions. The river was in spate due to heavy rain and much
plant debris had been carried into the estuary from inland. The current
was sufficient to make the net almost unworkable. The low salinities
occurring at this time did not dispel completely the Sand Gobies from the
area. Probably temperatures sufficiently low to force all the Sand Gobies
into deep water have not been experienced in recent winters.