Publication Date

3-1999

Series Number

113

Publisher

Fisheries Western Australia

City

Perth, Western Australia

ISBN

0 7309 8425 7

ISSN

1035 - 4549

Abstract

Over the last few years, there has been considerable interest on the part of commercial fishers to move away from using gill nets, the traditional method for fishing for blue swimmer crabs in Western Australia (WA), to fishing with traps. Blue swimmer crab fisheries in WA are regulated by effort controls and most gill net fishers are restricted to hauling 1,000 metres (1,200 metres in the case of Cockburn Sound) of gill net per day.

The fieldwork in this study was initiated in March 1994 in Cockburn Sound and in January 1996 in the Peel-Harvey Estuary. It was aimed at establishing gill net to trap conversion ratios that would allow fishers to change from one gear type to the other without a resulting increase in the catching capacity of the gear types.

In a comparison of catches made per net length and per trap-day by two commercial fishers in Cockburn Sound, each fishing with a different gear-type in March 1994, the mean legal-sized crab catch made over a 24-hour period was 139.42 kg for 1,200 m of net and 1.24 kg/trap/day for traps. Based on these figures it would seem that an estimated 112 traps would be required to take a trap catch equivalent to 1,200 m of gill net in Cockburn Sound.

A similar comparison using the results of two net and five trap fishers between January and March 1996 in the Peel-Harvey Estuary, showed that the catch rate declined significantly (p = 0.0096) over the three month period. The net to trap conversion ratio for January was 50, compared with the other two months, both or which had conversion ratios of 40 traps/1,000 m of net.

Aside from net to trap conversion ratios calculated from the fishing trials mentioned above, conversion ratios were also calculated for Cockburn Sound and Peel-Harvey Estuary using monthly compulsory catch and effort returns supplied by commercial trap fishers. These data showed that, over the five year period 1994-98 in Cockburn Sound, there were significant variations in catch rate from year-to-year (p = 0.0128) and from month-to-month (p = 0.0001), but that these factors were not significant over the shorter (three year) period 1996-98 for which data were available for Peel-Harvey Estuary. Based on these data, conversion ratios of 67 traps per 1,200 m of net and 55 traps per 1,000 m of net were established for Cockburn Sound and Peel-Harvey Estuary respectively.

Commercial blue swimmer crab landings reported in WA over the last 20 years, have shown a dramatic increase since the introduction of traps into this fishery. Based on the results of the compulsory catch and effort data presented above, there is good reason to believe that, in the case of Cockburn Sound the increase in catch has been due, in part, to an overestimated net to trap conversion ratio introduced into that fishery. It is, however, considered unlikely that this explanation alone has accounted for the more than doubling of the commercial catch in Cockburn Sound that has occurred over the last five years.

Examination has shown in both Peel-Harvey Estuary and Cockburn Sound, that when fishers change their gear from nets to traps that they fish substantially more days (in both areas approximately double) per year. Trap fishers utilise all their gear each day more often than net fishers, who occasionally choose through convenience to work with less than their allowable net allocation. Finally, there is evidence to indicate that traps maintain catch rates over a longer period through the year than do nets, particularly in the critical (winter and spring) periods when catch rates are low for both methods.

This paper has provided an analysis of considerations that should be taken into account for fisheries management purposes in the future when considering revisions to current net to trap conversion ratios, or when implementing these ratios to other blue swimmer crab fisheries that have yet to change from nets to traps.

Number of Pages

24

Keywords

net to trap conversion ratios, catching capacity, commercial fishing, compulsory catch and effort returns, catch rates, Department of Primary Industries and Regional Development

Disciplines

Aquaculture and Fisheries | Natural Resources Management and Policy | Research Methods in Life Sciences

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