Publication Date

6-1993

Series Number

100

Publisher

Fisheries Department of Western Australia

City

Perth

ISBN

0 7309 1808 4

ISSN

1035-4549

Abstract

A two year study was undertaken to collect data for the assessment of the impact of the saucer scallop and western king prawn trawl fisheries on the benthic communities in coastal waters off south western Australia, between 31°20'5 and 34°23'5 latitude.

Monitoring of reported commercial landings, on-board sampling of commercial catches, research vessel trawl surveys, experimental and exploratory trawling, and underwater television observations, have enabled the development of an extensive data-base on the fishery and benthic communities of the region.

These data indicate that the existing fishery operates on a number of relatively small, discrete grounds where saucer scallops or prawns are abundant, and affects approximately 2 per cent of the waters shallower than 50 metres, within the fishery management zone. Recorded fishing effort and catches vary significantly from year to year, primarily in response to the abundance and price of saucer scallops and prawns which are the major target species in the southern sector of the fishery. During a year of intensive commercial catch sampling (1990/91), an estimated 354 tonnes of marine fauna were taken by the trawlers, comprising 109 tonnes of the target species, 21 tonnes of retained by-catch and 224 tonnes of discards of which approximately 67 per cent were unlikely to survive.

Research vessel surveys recorded 150 species of teleost (bony) fish, elasmobranchs (sharks and rays) and invertebrates from commercial trawl grounds. Of these species, 39 were recorded at some time in commercial landings, however only 5 species, in addition to the target species of saucer scallops and prawns, were taken in any quantity. Of the top 10 species taken as by-catch, only two species, the blue manna crab and southern school whiting were both abundant and of significant interest to recreational fishers. Other species in the trawl by-catch which were found to be reasonably abundant and also of recreational interest were sand trevally, red mullets, long spined and blue spotted flathead, squid and cuttlefish.

detailed assessment of the stock of southern school whiting, the most recreationally important by-catch species, indicates that the adults are predominantly in offshore waters and more abundant in the northern half of the fishery. Surveys revealed that an extensive stock, in excess of 2000 tonnes, exists in the management area, and that trawl catches together with present recreational boat angling catches are not likely to exceed the estimated sustainable yield from the stock.

Underwater television observations indicated that productive scallop trawl grounds are predominantly sand substrates, and that such substrates dominate the fishery north of Geographe Bay. In contrast, the Geographe Bay sea-floor was found to be largely untrawlable with very limited areas of sand habitat suitable for saucer scallops. Comparisons offish communities present in surveys of commercially trawled and untrawled grounds indicated that commercial trawling

had no significant impact on the benthic communities of existing commercial trawl grounds. Visual observations on these trawled sand substrates suggested that the physical impact of trawling was short lived.

The ground in the Zone D management sector (Comet Bay) was found to be atypical of the remainder of the fishery due to both the target species (western king prawns) and the size composition of the by-catch. The data show that Comet Bay, like other near-shore areas is a nursery area for a number of recreationally and commercially important species. However, only blue manna crabs, southern school whiting, goat fish and sand trevally were in significant numbers in the Zone D by-catch. The impact of trawling on the overall stocks of prawns, blue manna crabs and southern school whiting has been considered in detail.

Number of Pages

98

Keywords

Saucer Scallop, Western King Prawn, Trawl fisheries, Commercial fisheries, South west Western Australia

Disciplines

Aquaculture and Fisheries | Marine Biology | Natural Resource Economics | Natural Resources and Conservation | Natural Resources Management and Policy | Population Biology | Sustainability

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