Fishery-independent survey of the breeding stock and migration of the western rock lobster (Panulirus cygnus)
Department of Primary Industries and Regional Development (DPIRD) WA
Chris F. Chubb
1. To use independent spawning stock survey techniques to validate spawning stock indices derived from commercial fisheries data and to examine specific impacts of the current management package over its full term. Specific impacts include trends in egg production and measurement of effective effort creep by comparison of fishery independent and fishery dependent spawninf stock indices.
2. To undertake pre-season tagging of juveniles in the shallow water of the limited entry fishery to obtain more detailed information on the migration and growth of these lobsters to aid in the understanding of the effects of distributing catch more evenly throughout the season.
The fishery for western lobster (Panulirus cygnus) has supported an annual catch of about 10,500 tonnes per annum over the last twenty years and is worth between $200-300 million per annum. The fishery has been experiencing increasing exploitation rates over time and estimates in the early 1990s were suggesting that the brood stock had declined to between 15-20% of unfished levels. These low levels were considered to pose a serious risk to future recruitment and resulted in a number of management measures being introduced in the 1993/94 season aimed at \raising the levels of the brood stock.
In the past, the state of egg production in the stock has been estimated using data obtained from the commercial fishery. Data from this source can introduce possible bias, in that it is possible for fishers to avoid certain areas where there are large numbers of female animals in a breeding state and which under the new management measures are now required to be returned to the sea Furthermore, the effect of increases in fishing power on commercial fishing effort due to changes in gear technology, can lead to the spawning stock index being over estimated if valid measures of the increases in effectiveness are.pot available.
The only way of avoiding the potential biases of using commercial data, is by conducting a sampling programme independent from commercial fishing data. Such a pilot programme was undertaken at Fremantle and the Abrolhos Islands in 1991, was expanded to include Dongara and Jurien in 1992 and with the assistance of FRDC funding was continued and expanded to include Lancelin and Kalbarri from 1993 to 1996 (FRDC project 93/091). The project was further extended under FRDC funding from 1996 to 1998 (this project), in order to increase the confidence of the results and to examine the breeding stock indices over the full term of the current management package. The results from this latter survey form the basis of this report.
Commercial lobster fishing boats were chartered to do research fishing in five areas on the coast and a research vessel was assigned to the Abrolhos Islands. Fishing took place at each of these areas over ten days during the last new moon prior to the start of the commercial fishing season in mid-November. Standard commercial pots were set on the same GPS positions each year in areas that had previously been identified as localities which consistently yielded large numbers of spawning animals. All lobsters caught were measured, sexed and in the case of females particular attention was paid to their reproductive state. Environmental parameters (bottom and surface temperature, salinity, swell size) were recorded daily in each area. Egg production indices (expressed as the mean number of eggs per pot lift) were calculated annually for each area separately and for the areas combined, based on the number of mature female animals in the catch. Analysis of the results at all the coastal sites showed significant increases in egg production since the surveys first commenced. All the survey areas have shown an upward trend in egg production since 1993 when the management changes came into effect.
Other analyses showed that there were substantial inter-annual differences in swell size as well as bottom temperature. These environmental factors did not significantly increase or decrease the egg production indices in anyone year, but the analysis did suggest that swell size has an influence on the index.Key Words: mud crab, Scylla serrata, stress indicators, handling stress, survival, emersion, live storage, transport, mortality