反復ネット採集におけるカタクチイワシ卵仔魚採集数の変動

概要

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Six series of ten repeated net hauls were made to examine the variation in catches of eggs and pre-larvae among the repeated hauls. The obtained results show that catches of eggs and pre-larvae of Engraulis japonicus were slightly over-dispersed or randomly distributed. A poisson mixture model is applied to explain these results. Suppose plankton organisms of some definite types, distributed at random with a mean M. Let the mean M be variable and set Q(M)dM be probability that M lies between M and M+dM. Then the probability of catching n organisms isP(n)=∫∞0Q(M)e-MMn/n!dM.Consequently, we can obtain following relation between the variance σn2 and mean μn of catch n; σn2=μn+μn2(σM/μM)2where σM2 and μM are the variance and mean of M respectively. This relation is equivalentto the one of the variance to the mean in a negative binomial model(NBM). But NBM is a special case that Q(M)is Gamma distribution in this model. Apparently, when σM≠0, then σn2>μn. But there is no reason why aggregation always occurred because variation of M is caused from many sources, for example, aggregation, prey-predator relation, transport by current, etc. Additional researches should be directed to-ward the causal factors inducing variation of M.
日本水産學會の論文