Aspects of the reproductive biology of murray cod, Maccullochella peelii peelii

Abstract

Aspects of reproduction in Murray cod, Maccullochella peelii peelii, in NSW tributaries of the Murray-Darling river system were studied. Murray cod has a distinct seasonal cycle; monthly gonadosomatic indices (GSI) and mean oocyte diameters were low between December and March, increased rapidly from June, presumably due to vitellogenesis, and were highest in October. Oocytes underwent group-synchronous development, but only one batch of yolky oocytes was spawned during the 4-5 week breeding season. Absolute fecundity ranged from 6,800 (total length, TL 480mm, weight 2.1kg) to 86,600 (1050mm, 22.7kg) eggs, and relative fecundity ranged from 3.2 to 7.6 eggs/g. Spawning was induced by a rise in water temperature to or above 20°C in spring. The presence of many consecutive year classes suggests that Murray cod spawn annually; however, between 1977 and 1980 relatively strong year classes were formed only in rivers that were at, or near flood levels during the breeding season. No females and a few males had matured by 3 years of age. At 4 years of age, 77% of females (> 480mm, 2.1kg) and 72% of males (> 530mm, 2.3kg) were mature. All females and most males were mature at 5 years. In rivers, all cod larger than 590mm and 3.9kg were mature, but the smallest sexually-mature cod sampled from the impoundment, Lake Mulwala, was 610mm and 5.0kg. Management restrictions on the recreational and commercial fisheries for Murray cod, based on the results of this and previous studies are outlined.