Maturation in the female eastern king prawn Penaeus plebejus from coastal waters of eastern Australia, and considerations for quantifying egg production in penaeid prawns

The trawl fishery for eastern king prawns, Penaeus plebejus, is of major commercial significance on the east coast of Australia. This paper describes the reproductive biology of female P. plebejus and presents new information relevant to quantifying egg production in penaeid prawn populations. The study concluded that the gonosomatic index (GSI) covaried with prawn size and was, therefore, likely to be a poor indicator of reproductive activity for penaeid prawn populations. GSI was also found to be not independent of ovary histological stage, moult stage, insemination status and parasitisation by bopyrid isopods. An histological description of ovarian development and the incidence of each stage of development is provided. This includes a description of the ovulation stage, which has not been previously recorded in naturally-occurring populations. Fifty percent of female P. plebejus were classed as mature at 42mm CL. The relationships between GSI, percent mature, and percent inseminated with size suggest that there is a decline in the capacity of large (> 60 mm CL) females to produce and fertilise eggs. The declines in both GSI and percent mature is indicative of ovarian senescence in large(o1d) females, which has not been recorded previously in penaeid prawns. An index which considers the relationships between the percent mature, fecundity and the percent inseminated with size, as well as the influence of mortality on the population, is put forward to determine the relative contribution of different size classes of females to egg production. The index suggests that females >50 mm CL contribute little to egg production, and that the bulk of eggs are produced by 35 to 48 mm CL females