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Population Biology of the Japanese Little-neck Clam, Tapes philippinarum, in Kaneohe Bay, Oahu, Hawaiian Islands

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Title:Population Biology of the Japanese Little-neck Clam, Tapes philippinarum, in Kaneohe Bay, Oahu, Hawaiian Islands
Authors:Yap, Wilfredo G.
Date Issued:Jul 1977
Publisher:University of Hawaii Press
Citation:Yap WG. 1977. Population biology of the Japanese little-neck clam, Tapes philippinarum, in Kaneohe Bay, Oahu, Hawaiian Islands. Pac Sci 31(3): 223-244.
Abstract:The Japanese little-neck clam, Tapes philippinarum, an introduced
species in Kaneohe Bay, Oahu, Hawaiian Islands, has a thriving population
only in a U5-hectare mud flat after heavy fishing triggered depletion in six
other beds within the bay. Monthly gonad examination of the clams suggested
that spawning occurs at a low level throughout the year with a peak from January
to February. This observation is corroborated by the appearance of new
recruits in the monthly sample from April to June and by their presence at low
levels at other times of the year. Size-specific fecundity, determined indirectly
from differences in the length: dry weight relationships of ripe and spent clams,
ranges from 432,000 eggs in a 20-mm clam, increasing exponentially to 1.35 x
106 eggs in a 40-mm clam.
Estimates of the population of clams 11 mm and larger, which were 3.09 x
106 in 1970 and 3.4 x 106 in 1972, show a growth of 5 percent per year during
the 2-year period; monthly quantitative sampling showed no evidence of population
growth after 1972. A survivorship curve obtained from the monthly
samples gave a total instantaneous mortality of z = 0.2005. The age-specific
mortality agrees with the age-frequency of the empty shells collected from the
bed, with a correlation coefficient of 0.9345 with 4 d.f. The condition of the
empty shells indicated that 57 percent of the mortality is attributable to crab
predation, mainly by Thalamita crenata, which constitutes 70 percent of the
experimental crab catch in the clam bed. Sixty percent of the broken shells
were 19.5 to 30.4 mm in length; in experiments with predation by T. crenata,
96 percent of those eaten fell within the 14.5 to 30.4 mm size range. The
difference between the lower limits of the size ranges can be attributed to
the size structure of the clams during the survey period. The experimental
population had an artificially maintained size structure. Experimental exclusion
of predators over a limited area suggested that crab predation regulates clam
size structure but not clam density.
Appears in Collections: Pacific Science Volume 31, Number 3, 1977

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