Revised estimates of natural mortality for the Foveaux Strait oyster (Ostrea chilensis)

We present revised estimates of the instantaneous natural mortality rate of recruited (legal sized) Foveaux Strait dredge oysters (Ostrea chilensis, P hilippi, 1845) for the years 1974-86. A mark-return experiment was analysed , assuming constant rate of fishing (and hence sampling) over 6 months of t he year (i.e., the period of the fishing season). The natural mortality bet ween 1974 and 1986 of recruited oysters increased from 0.02 to 0.19 year(-1 ) for a sample of oysters released in 1974, and from 0.01 to 0.20 year(-1) for those released in 1973. We explored possible causes for the observed in creases including the impact of indirect fishing mortality, rising disease mortality ahead of the Bonamia sp. epidemic of 1986, and systematic misrepo rting in returns of living and dead tagged oysters. We fmd no direct eviden ce for any of these, and hence speculate that the observed increase in natu ral mortality of the tagged individuals is associated with senescence.