Influence of a deep artificial entrance channel on the biological characteristics of the blue swimmer crab Portunus pelagicus in a large microtidal estuary

Abstract

The opening of a deep and permanent artificial entrance channel into the Peel–Harvey Estuary in southwestern Australia in 1994 provided an excellent opportunity to determine the biological characteristics of the blue swimmer crab Portunus pelagicus in a large microtidal estuary, after tidal exchange with the ocean had increased markedly and salinities recovered more rapidly from their winter minima. P. pelagicus was collected from the original entrance channel, which is located 15 km to the north of the artificial channel, the two large basins (Peel Inlet and Harvey Estuary) and the saline lower reaches of a major tributary at regular intervals in 1995–1998. The resultant information on densities, size and age compositions, growth rates and reproductive biology were compared with those derived from data recorded by Potter et al. [Mar. Biol. 78 (1983) 75] for P. pelagicus in the Peel–Harvey Estuary in 1980 and 1981. This led to the following conclusions regarding the ways in which this portunid has responded to the impacts of the construction of an artificial entrance channel in this estuary. (i) Late 0+ crabs enter earlier than previously the Peel Inlet and both the Harvey Estuary, which now opens directly to the sea, and a tributary river in which salinities likewise rise more rapidly in the spring. (ii) Female crabs become ovigerous earlier, in concert with the earlier restoration of high salinities in the spring. (iii) Ovigerous crabs emigrate to the ocean sooner, presumably in response to the greater stimulus provided for emigration by the marked increase that has occurred in tidal flow. (iv) The growth rate during the first few months of life has increased, possibly as a consequence of a decline in the density of crabs within the estuary during those months. (v) A cohort of small 0+ crabs, not previously found in the estuary and almost certainly derived from population(s) spawning further south, is present for a few months but then disappears from the estuary. Although large crabs do not now appear to grow towards their maximum size as rapidly as previously, this is an artefact brought about by the culling of large crabs through the marked increase that has occurred in fishing pressure during recent years.