132 THE ESSEX NATURALIST
Figure 1 Map of Crouch Estuary showing stations I, II, III and IV.
Results
The approximate numbers of the species caught are tabulated in ascend-
ing phylogenetic order in Tables 1-4. Note that no authorities are given
for these scientific names, the names used are those from the standard
texts quoted in the reference list.
Discussion
The trawl sampling carried out in October 1973 and 1974 gives an
indication of the variability of the results. Since the sampling area, gear
and season were all unchanged, the difference in catch reflects the com-
bined effect of the difference in species of animals present and the 'catch-
ability' of those animals. This 'catchability' probably varies with the
behaviour of the animals and the trawl under different conditions of the
tide, disturbance etc. Assuming these changes in behaviour to be relatively
minor within a species, the major differences in animals caught must
reflect seasonal or annual differences in their abundance. However, the
catches of Whiting (Merlangius merlangus) from Station IV on successive
days illustrate one problem. These fish shoal, and results can therefore be
'patchy'. Thus the 60 caught on one day, and the single specimen found
the next day may have no ecological significance.
Comparison of the results with previous surveys is more difficult
because of the catching bias of the gear. Most of the previous work in the
area has entailed a more intensive survey of smaller areas than the present