Feeding and Control of Toheroa (Amphidesma ventricosum Gray) (Eulamellibranchiata) Populations in New Zealand.

Abstract

Surveys of toheroa stocks on the North Island beaches of New Zealand are described and the causes of natural and epidemic mortality discussed. The evidence for previous resurgent populations of tuatua (Amphidesma subtriangulatum Wood) alternating with toheroa is given. Analyses of water samples show that a source of sea-water rich in nutrient salts produces the great quantities of diatoms on which the shellfish populations feed. A quantitative estimation is made of the littoral phytoplankton and the dominant species are given for each beach. Chaetoceros armatus is found on all beaches and, in the cold months, when blooming occurs, comprises in some places 100 per cent. of the diatoms. The greatest quantity recorded is 1500 g dry weight per cu. m. of water. Each beach has a characteristic flora. The rate at which toheroa feed is determined from the diatom consumption. Methods are suggested by which more stable toheroa stocks might be developed and consequently a greater production achieved.