Setose and maximum size rules have severely distorted the sex ratios and may have affected the sizes of mature (breeding) animals in the population. Research is necessary to investigate current and future projection of these management measures on fertilisation success in the breeding population.
To effectively manage the brood stock, it is important to have a knowledge of whether eggs from different regions in the fishery differ in ‘quality’ (i.e. would be likely to produce first stage larvae with different survival characteristics). This is particularly important for the Abrolhos Islands, which is estimated to contribute between 45-65% of egg production, but where the sizes of the eggs have previously been recorded as 10 to 15% smaller than at the coast. This project will assess whether the smaller eggs result in lower survival of stage 1 larvae, as an indicator of overall survival
At present no weighting is given to the contribution made by the three zones to egg production in the fishery. It is necessary to more fully understand the spatial distribution of the breeding stock in the zones when assessing effects of management packages.
Final report
Increases in efficiency due to modem electronic equipment, improved fishing vessels and knowledge about the grounds and lobster behaviour, have all led to western rock lobster fishers becoming more efficient at catching western rock lobsters in Western Australia. One of the outcomes of these increases in efficiency has been an increase in pressure on the breeding stock. Managers have responded by introducing measures to protect the breeders. In 1993, this involved the imposition of measures to limit the exploitation of mature female lobsters by introducing a legal maximum size for females as well as the protection of setose (mature) females.
The combination of high exploitation rates and protection of large, mature females, but not males, has since resulted in very distorted sex ratios of mature animals across the fishery, particularly those over the maximum size limit. It is well known from laboratory studies on other species of lobsters, that highly distorted female-dominated sex ratios (such as has been caused in the wild by the setose rule), and large females being forced through lack of choice to mate with small males (such as has been caused in the wild by both the setose and maximum size measures) can lead to sperm limitation effects (fertilisation of eggs but reduced brood sizes). These same management measures could have more positive effects - for instance in some species, the older individuals produce larger eggs, that in tum produce larger larvae that have been shown to have better survival characteristics.
These, and other unknowns, led to the initiation of this research project aimed at investigating these biological issues. The outputs from this research, together with other data, provide an indication of the contribution to egg production of different management zones in the western rock lobster commercial fishery.
