Species Replacement of Pelagic Fishes

The sardine (A), chub mackerel (B) and anchovy+Pacific saury+horse mackerel (C) fluctuate thier stock size from decade to decade. If species A is stronger in competitive ability than B, B is stronger than group C, and C is stronger than A, then parmanent oscillation is possible. This is called Cyclic Advantage Hypothesis. If this is true, we can predict the next dominant fish, while we can't predict when the next replacement occurs.

Impact of Fisheries on Chub Mackerel Stock

In 1980's, the stock level of chub mackerel was low and catch in Japan was still large. I evaluated the initial stock level in 1972 and the mortality before recruitment during 1972-1984, and calculated the stock level in 1988 and total catch during 1975-'88 under several fishing policy. The actual fishery was worst in stock conservation and much worse in total catch than optimal policy. There are two types of fisheries for chub mackerel in Japan, dip-net fishery (DF) and purse-seine fishery (PF). Catch by PF is much larger than DF, while DF exploits during spawning season. To calculate age- and season-specific reproductive values, we evaluate impact of each fishery on the chub mackerel. Impact of PF was consistently much larger than that of DF.

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