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Four different surplus yield models (namely Schaefer, GENPROD, PRODFIT and delayed recruitment) were applied to the data from the fishery for the western rock lobster. The four models all provided similar estimates of maximum yield and optimum effort. These estimates were 8.0-8.6 × 106 kg at a level of effort of 5.6-5.9 × 106 units. Variation in catches was shown to increase with increasing effort although this could not be attributed to increased variation in catch per unit of effort. It was considered that while the four surplus yield models provided good descriptions of the relationship between total catch and equilibrium effort up to the level of effort so far encountered in the fishery, not as much confidence could be placed in their ability to predict catches at levels of effort outside the already observed range.

Daily locomotor activity rhythms in individual P. longipes cygnus were shown to be of a non- crepuscular nature with peak activity occurring immediately after the onset of darkness. Mean activity of animals in an intermoult stage increased with increasing temperature between 17 and 25C but then declined, whilst for animals in a premoult stage there was no significant trend with temperature, activity rates remaining at a low level. Individual variation of response of intermoult animals also increased with temperature and is suggested as a contributing factor to high daily variation in commercial catch rates during months of high water temperature.

Breakdowns in the assumptions made by the Jolly (1965) method of estimating population size were detected in mark-recapture experiments with the western rock lobster. Using information gathered from sampling by both baited traps and by diving it was shown that, in particular, the assumptions of (1) a single population and (2) equal probability of capture of marked and unmarked rock lobsters, were not valid at all times in the 'population' studied, and this resulted in underestimates of population density. However, it was found possible to make suitable corrections to the raw data to arrive at apparently unbiased estimates of population density. In the cases studied, the calculated unbiased estimates of population density agreed well with unbiased estimates calculated from information gathered by marking and recapturing by different methods. Single census and the De Lury (1958) methods also grossly underestimated population density.

The catchability coefficient, q, of a population of the western rock lobster P. cygnus was calculated monthly from November 1969 to January 1973 using measurements of catch, effort, population density, and area of reef. The catchability coefficient varied significantly during this time and was positively correlated with water temperature and water salinity, and negatively correlated with the percentage of rock lobsters in a premoult condition. A linear combination of these three factors was highly correlated with the catchability coefficient (P < 0.001) and appears to offer a method of adjusting the catchability coefficient and so leading to an improved stock assessment for this species.

Samples of egg-bearing females of P. longipes cygnus (George) were taken from the Abrolhos Islands, Fremantle, and Jurien Bay areas and divided into two groups (viz. "newly spawned" or "late stage") depending on the extent of development which had taken place in the eggs. The number of eggs carried by each individual was then counted. Fecundity is found not to vary significantly (at the 10% level) between the three localities, and so all localities are combined to give pooled estimates of fecundity for newly spawned and late stage eggs. A small amount of egg loss occurs during incubation, but this is not significant at the 10% level. The percentage of infertile eggs is shown to be independent of carapace length. A linear relationship between carapace length and number of eggs produced is found for P. longipes cygnus, and this type of relationship appears to hold for many of the members of the family Palinuridae in contrast to the non-linear relationship found in the Homaridae.