Size, movements, distribution and gonadal stage of the western king prawn (Penaeus latisulcatus) in a temperate estuary and local marine waters

Abstract

Penaeus latisulcatus were collected monthly over three years from the large Peel‐Harvey Estuary and nearby shallow and deeper marine waters in south‐western Australia. The relationships between catch rates obtained by seine net, and various environmental variables and sampling locations in the estuary, strongly indicate that salinity and distance from estuary mouth exert a marked influence over the distribution and abundance of prawns within the estuary. Since commercial catches of prawns in the Peel‐Harvey were very low during the 1970s, when the macroalga Cladophora montagneana coated much of the substratum of the estuary, the extent of the preferred sandy substratum of P. latisulcatus is also assumed to affect abundance. The modal carapace lengths (CL) of males and females caught by beam tide nets during their migration out of the estuary were sharply defined and in each year remained similar during the migratory period (typically February to July), when freshwater discharge, salinity and temperature were changing markedly. Although this provides strong evidence that the time when king prawns leave the estuary is at least partly related to factors associated with the attainment of a certain body size, it is also relevant that the decline in catch rate was greatest in the year when salinity fell most rapidly. Migration reached a maximum between March and May and in each month peaked just after the full moon. Comparisons between length‐frequency data for emigrating prawns and prawns caught commercially by trawling in marine waters indicate that, after leaving the estuary, many P. latisulcatus migrate a few kilometres northwards and remain there for a few months. The months when the incidence of mature ovaries is relatively high in the larger of the trawled female prawns (CL > 30 mm) is consistent with the time of immigration of new recruits (2‐3 mm CL) into the estuary (December‐April). Penaeus latisulcatus start migrating into the Peel‐Harvey Estuary later than into inshore marine waters (November or December vs. September) and subsequently emigrate from this alternative nursery area at a larger size (carapace length > 20 mm vs. < 18 mm CL).