Mortality estimates for the rock lobster,Jasus edwardsii, near Gisborne, New Zealand

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Abstract
The instantaneous total (Z) and fishing (F) mortality rates were estimated for the New Zealand rock lobster, J. edwardsii (Hutton), along the east coast of the North Island from the Wairoa River to East Cape. Estimates of Z were made from the seasonal size-frequency distributions (SFD) of rock lobsters landed from 5 separate fishing areas for up to 5 fishing seasons using 3 different methods. Z and F were also estimated from the rate of return of tagged rock lobsters released in 4 of the 5 areas. The best estimates of Z and F for each area were considered to be estimates of F based on tag returns grouped quarterly and taken over an 8 or 9 mo. fishing season; estimates of Z based on these estimates of F from tag returns, with the addition of an assumed value for the annual instantaneous natural mortality rate of 0.10; and estimates of Z from the SFD analyses. These estimates were more consistent with the estimated lifespans in the fishery than were the estimates of Z derived from tag returns.

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