In the late spring of 1961 the trough clam, Mactra sulcataria REEVE, which had excessively propagated on the coast of Tokyo Bay, were thinned out by means of fish pumps. However, 60 to 80 per cent of the collections was found crashed instead of 20 per cent usually damaged with a hand operated dredge. In order to determine a cause most responsible for the unex-pectedly high damage of the collections, three species of shellfish, the trough clam, Mactra sulcataria REEVE, the common clam, Meretrix lusoria (RÖDING), and the short-neckd clam, Tapes (Amysdala) japonica DESHAYES were separately kept in a water tank and subjected to pumping up with the number of impeller revolution varying from 330 to 990 r.p.m. (Plate 1). The fish pumps used were of different models, T, H, and S, each being 100mm in impeller caliber(Plate 2).
The results revealed that an extent of damage on shellfish was nearly proportional to the number of impeller revolution and markedly dependent upon the structure of an impeller (Table 2 and Fig. 1). The trough clam was more vulnerable to pumping impact than the other bivalves under study. Pumping up several individuals together had a greater rate of damagd than pumping up only one individual of shellfish at a time. Most of broken bivalves had damage only on one side of the shell (Plate 3).
Those findings suggest that fish pumps for shellfish collection should have the impeller and its casing coated with rubber or similer materials to reduce pumping damages.
