Decadal changes in reproduction of walleye pollock (Theragra chalcogramma) off the Pacific coast of northern Japan

We examined stock fecundity, abundance of eggs and larvae, and catch per haul of age 0 walleye Pollock, Theragra chalcogramma, and considered the relationship between reproduction, recruitment and the ocean environment off the Pacific coast of northern Japan for the period between 1985 and 1996. To calculate stock fecundity, we estimated age at maturity and fecundity, and expanded estimates to the expected age composition of the population as estimated by Virtual Population Analysis. Eggs and larvae were collected by research cruises in the main spawning ground in and around Funka Bay. Catch in number of age 0 per haul were obtained from catch statistics. Annual stock fecundity decreased in the late 1980s, from 97 billion eggs in 1986 to 74 billion in 1990, and increased in the 1990s, to 137 billion in 1995. Abundance of eggs and larvae, and catch of age 0 per haul, were high in the late 1980s; however, they were low or intermediate in the 1990s. The steepness of the spawner recruitment relationship in the 1990s was different from the late 1980s, based on positive relationships between stock fecundity and catch of age 0 per haul in each decade. Sea surface temperature in the main spawning ground around Funka Bay was low in the late 1980s, but high in 1990s. We conclude that the decadal-scale change in the ocean environment around the main spawning ground between the 1980s and the 1990s caused a change in the reproduction and recruitment of walleye pollock.