Inferring the Summer Distribution of Migratory Pacific Hake (Merluccius productus) from Latitudinal Variation in Mean Lengths-at-Age and Length Frequency Distributions

Abstract

Average summer distribution of abundance and biomass for the migratory population of Pacific hake (Merluccius productus) is described by age, length, and latitude, based on a model proposing that (1) changes in mean length-at-age with latitude and (2) the nonnormal (skewed to the right) appearance of length-at-age distributions near southwest Vancouver Island could be caused by length-dependent migration velocities and changing migration velocity with latitude. Our model uses mean length-at-age data collected annually near four latitudes from California to Vancouver Island, length frequency data collected annually near southern Vancouver Island from 1978 until 1988, and hydroacoustic data collected triennialiy from 1977 until 1986. We conclude that (1) hake slow their northward migration from the spawning grounds off southern California as they approach the productive feeding grounds off southwestern Vancouver Island, (2) older (larger) hake migrate further north than younger (smaller) hake, and (3) older hake are more abundant in Canadian waters than younger hake. Although all three conclusions have previously been partly substantiated, we show that seemingly undramatic patterns in the mean, variance, and distribution of lengths-at-age can be remarkably revealing about the distribution of a migrating fish population.