The reproductive strategy of red cod, Pseudophycis bachus, a key prey species for high trophic-level predators

Abstract

The reproductive strategy of red cod, Pseudophycis bachus, was investigated as a contribution towards understanding the population dynamics of this important prey species in the shelf waters of Australia. Gonad organization and development revealed that red cod gametes are produced annually, and fertilization occurs externally. Histological observations revealed that imminent spawning was characterized by yolked, hydrated and migrated nucleus stage oocytes in females and an abundance of spermatozoa in males. Seasonal changes in gamete development and somatic indices for females and males were asynchronous. Seasonal changes in reproductive development together with somatic indices revealed that the main spawning period for red cod was during spring (September to November). While individuals in spawning condition were observed as early as April, spawning activity was most prevalent during September. Individuals in spawning condition were observed in the shelf waters of Bass Strait off the mainland ports of Queenscliff, San Remo and Lakes Entrance. The condition of all individuals collected from the shallow waters of Port Phillip Bay and Western Port was immature or undergoing maturation. Red cod spawn in shelf and slope waters, larvae are recruiting to coastal environments, and early juveniles are found in both bay and coastal environments. Early juvenile red cod were present in Western Port as early as October, and were present within the bay for a substantial part of the first year of growth. Mean size at sexual maturity was 315 and 340mm total length for females and males, respectively, resulting in an age at sexual maturity of 1+. This suggests that movement of this species from bay and coastal environments to deeper waters during the first year of growth may depend substantially on reproduction. Red cod have a high annual fecundity (between 0.363 million and 5.059 million oocytes per fish) which was positively correlated with total length and gutted weight. Predation pressure is important in structuring ecosystem trophic interactions, and consumption of red cod by a range of high trophic level predators may be significant in shaping the dynamics in red cod populations. A reproductive strategy that is characterized by early maturity and high fecundity, coupled with a particularly fast growth rate, suggests that red cod populations are quite resilient to increased predation pressure.