Variation of Growth with Latitude in two Species of Rockfish (Sebastes pinniger and S. diploproa) from the Northeast Pacific Ocean

Abstract

Growth of two Sebastes species was measured as fish length versus age, determined from otoliths. S. pinnigerand S. diploproa were collected extensively over their wide latitudinal ranges in the northeast Pacific Ocean. In both species, females grew faster and larger than males. Growth of S. pinnigerdid not vary with latitude. For S. diploproa, however, there was a cline of increasing growth with higher latitude. In the habitat of the adult fish, which are demersal on the continental shelf and upper slope, latitudinal variation in environmental factors, such as temperature and oxygen concentration, appears insufficient to account for the observed differences in growth. Thus two other possible causes of the growth cline are discussed. First, S. diploproa has been exploited by fisheries more in the north than in the south, possibly resulting in more available prey and thus increased growth in the north (short term density-dependent response). The second possible cause is a density-independent, evolutionary response at the population level characterized by differences in reproductive strategy. Reproductive success in the southern part of the range of this species is probably less predictable due to the effects of oceanographic currents on the survival of epipelagic larvae and prejuveniles; reproductive effort in the south would therefore be greater. The relative importance of the suggested mechanisms for growth differences requires further elucidation from population genetics and studies of density effects and fishing pressure.