A Preliminary survey of 21 Ceylon lakes. 3. Parasites and predators, food of fish, and marginal fauna

Three nematodes, two acanthocephalans and three cestodes were collected by dissecting 15 species of fish. Of these parasites only Hedruris sp. a larval nematode is responsible directly for the death of fish, since it is found in predatory species a reduction of these might prove of considerable consequence to fish, production. Bothriocephalus gowleonensis, a cestode probably introduced from China with Chinese carp, was recorded in an indigenous carp Puntius sarana. This parasite causes heavy mortality in grass carp in China and is a potential danger to local species. A single crustacean parasite was recorded. It is likely that they are widespread. No protozoa were recorded but there can be little doubt that they occur in local freshwater species and may be of considerable importance in fisheries. Several predators (insects, fish and other vertebrates) were recorded and are doubtless of considerable importance. However, the role of parasites and of predators (especially fishes) in relation to the productivity of fisheries can hardly be assessed at present and needs further study. A study of the fauna found at the shallow edges of 21 irrigation reservoirs showed an abundance of fauna in the low-country reservoirs whilst the up-country reservoirs had a poor fauna. The fauna showed insects, crustacea and mollusca in this order of abundance. The presence of insect and molluscs feeding fish in our fauna shows that these rich sources of food are being utilised.

Further investigations on the relative efficiency of different shaped otter boards

Horizontal curved, vertical curved and V-shaped otter boards were studied to compare their relative efficiency under identical fishing conditions. The gear operated with V-shaped otter boards performed well followed by the gear fitted with horizontal curved boards. Vertical curved boards were found to be comparatively less efficient, but with slight modification, can be used advantageously for bottom and off bottom fishes. Fishing at various depths with horizontal curved and V-shaped otter boards at different scope ratios showed better performance of horizontal curved boards at a scope ratio of 1:6 and V-shaped otter boards at a scope ratio of 1:5.

A review of experimental trawling from small and medium sized mechanized vessels off Kanikada

The most suitable otter trawl for small boats was found to be a 10.9 to 15m 2-seam trawl with 100cm x 50cm x 35kg horizontally curved otter boards together with long single sweep line. For operation from medium sized trawls, 18.26m 2-seam 18.3m 4-seam and 29.26m long wing trawl were found suitable. An 18.3m 4-seam trawl was netting a considerable quantity of off-bottom fishes. Shrimps predominated in the catches of the 29.26m trawl. Productive grounds for Cynagris species, Psenus species and Decapterus species within 50 to 100m depth ranges off Kakinada were available for profitable exploitation.

A demersal trawling gear for inshore waters along the west coast

Results of comprehensive efficiency tests of three tested designs of 15m bulged belly, 15.8m six seam and 29.26m longwing type trawls in combination with 114x57cm both rectangular flat and horizontal curved wooden otter boards are reported. Of the possible six combinations, the bulged-belly trawl with flat rectangular otter boards has performed better in landing prawns. Further this study has indicated the selective action of the different designs.