Application of yield models to the New Zealand rock lobster fishery

Abstract

Two general types of yield model, stock production and dynamic pool (yield-per-recruit), were used to analyse available data for the New Zealand rock lobster, Jasus edwardsii (Hutton). An exponential stock production model was applied to commercial catch and effort data from 1945 to 1975 for the North and South Islands combined. The results suggest a maximum sustainable yield of 4227 t with an estimated optimal level of fishing effort of 784 vessels, about 60% of the 1975 number. Empirical and von Bertalanffy growth equations were derived from limited tag-recapture data from the Gisborne area. Both equations indicate that males grow faster than females. The empirical growth equation, based on moult increment and frequency as functions of size, is considered the more realistic. Two yield-per-recruit models provided different results, especially at low levels of natural and fishing mortality. One model (Beverton-Holt) incorporated the von Bertalanffy growth equation and the other (empirical model) an empirical growth equation; the latter model is considered more realistic. The empirical model suggests that a reduction in fishing mortality below the estimated present values will not result in a substantial increase in yield-per-recruit. No strong evidence for changing the minimum size at first capture emerged from the results of the empirical model. A need is evident for more data, spanning a greater size range, to provide better estimates of growth and natural mortality, because the yield-per-recruit models were found to be sensitive to both. INTRODUCTIONThe reck lobster, Jasus edwardsii (Hutton), is of great economic significance, and is now the single most valuable renewable resource taken by the New Zealand fishing industry. The fishery began to expand rapidly with the development of export markets in the latf 1940s. The latest statistics show that about 1979 t, [valued at approximately N.Z. $23 667 000 were exported in 1977 (Anon. 1978), compared with 1799 t ^valued at N.Z. $4 619 000 in 1966. Thus, it seems prudent to develop management plans as quickly as possible to ensure sustained high productivity cf this resource and to avoid the results of over-fishing.Some information on the geographic distribution, breeding biology, and development of this species is availab , e (Kensler 1967, Sorensen 1969. Street 1970Street , 1973a has provided important information on growth, moulting, movements, reproductior, and predation, and estimates of exploitation rates from field observations and marking studies. Kenslet (1968) examined the size-fecundity relationship ofj a limited number of females.Streejt (1973a, b) described some of the management measures being used and their probable significance, but management regulations should be reexamined in the light of recent information. Since 1974 a tagging programme has been carried out by personnel of the Fisheries Research Division using the western rock lobster tag (Chittleborough 1974), which is retained through ecdysis. In addition, catch sampling, analysis of fishi...

Growth of rock lobsters (Jasus edwardsii) in the Gisborne region, New Zealand

“…The von Bertalanffy parameters calculated in this study differ from those calculated for males by Saila et al (1979) s. 12.…”

“…These areas support a substantial rock lobster fishery, based mainly on thd port of Gisborne, which has been the subject of investigations since 1974. Estimates of yield per recfuit in this fishery were made by Saila et al (1979) Using preliminary growth and mortality data, and mortality estimates for the area, based on tagging and size-frequency distribution data, were made by j\nnala (1979,1980).…”

“…The present study includes much of the growth data used by Saila et al (1979), plus a considerable amount of new data which has allowed some useful comparisons of growth ratds between areas. Growth rate data are also being us£d to predict the time of recruitment into the fishery of rock lobsters less than the legal size (J. D. Booih, unpubl.…”

Fisheries science now and in the future: A personal view

“…These methods seem to be most useful when applied to large, singlespecies resources of low variability. A good example of a relatively static unit fishery to which these techniques have been applied is the New Zealand rock lobster (Jasus edwardsii) fishery (Saila et al 1979). This is a fishery of great economic significance, being the single most valuable stock currently exploited by the New Zealand fishing industry.…”

“…The point is that a fishery such as this is sufficiently stable and understood to permit management with simple goals which can be attained by making decisions which are supported by analytic models. For example, if maximum catch is desired, a stock production model analysis by Saila et al (1979) indicates that a 40% reduction in real fishing effort could result in as much as a 20% increase in total catch, with a resulting doubling of an average vessel's yearly catch.…”