IMPACT FACTORS OF PURSE SEINE NET AND DIP NET FISHERIES ON A CHUB MACKEREL POPULATION

There are two major kinds of fisheries for the chub mackerel in Japan. The purse seine net fishery exploits young and adult fish during the foraging season, while the dip net fishery exploits fish before and du ring the spawning season. To compare the damage to the chub mackerel s tock caused by purse seine net and dip net fisheries, we introduced im pact factors of the two kinds of fisheries on the population. We used data on the number of chub mackerel born in 1982 caught in each year b y the two kinds of fisheries and estimated number of eggs spawned in 1 982. We defined the impact factor as the ratio of the average reproduc tive value of individuals caught by each fishery to the reproductive v alue at maturity. The number of individuals caught by the purse seine net fishery was 66 times larger than the number caught by the dip net fishery, while the impact factor of the former was 84% of that of the latter. Thus, the total damage caused by the purse seine net fishery w as much higher than the damage caused by the dip net fishery. We can e valuate the damage caused by mortality factors on endangered species u sing this method.