Estimation of fishing and natural mortalities from tag experiments with exact or grouped times at liberty

A simple probabilistic model that incorporates fishing mortality, natural m ortality, tag shedding, and differing reporting rates is developed, a likel ihood function for estimating its parameters is derived, and a set of taggi ng data on newborn school shark, Galeorhinus galeus, is analysed to estimat e their instantaneous natural and fishing mortalities. For a truncated tagg ing experimental period of 92 days, the instantaneous tate of natural morta lity of newborn school sharks was estimated at 9.0427 x 10(-3) (+/-2.4553 x 10(-2)).day(-1) or 3.3028 (+/-8.9680).year(-1) and is equal to a survival rate (from natural death only) of e(-0.0090427) = 99.10%.day(-1) or e(-3.30 28) = 3.68%.year(-1). From this estimate and its corresponding estimates of catchability coefficients, we also calculated their instantaneous rate of fishing mortality in the study area from longlines, gill-nets, or both, and their survival rate, all as a function of time. This work extends previous work by incorporating fish tag shedding rates and reporting rates and by g iving a detailed derivation of the likelihood function.