A stochastic integrated VPA for herring in the Baltic Sea using acoustic estimates as auxiliary information for estimating natural mortality

Abstract

Acoustic estimates on stock numbers at age for herring in the Central Baltic Sea are used in a stochastic integrated VPA to obtain estimates of mean natural mortality rates by ages for 1981–1987. The estimates of the natural mortality rates are inconsistent with the results of the ICES Multispecies VPA (MSVPA) carried out for the Central Baltic Sea. The high natural mortality rates of the young ages estimated by the MSVPA are not confirmed by the present analysis, where the natural mortality rates are estimated to be the same (0.18 year ⁻¹ ) for ages 1 to 9. It is concluded that the acoustic survey underestimates the abundance of ages 1, 2, and 3. However, the actual natural mortality might lie somewhere between the estimates from the MSVPA and those presented here, because the MSVPA could be wrong by as much as a factor of 2 owing to uncertainties in the consumption data used for cod in the MSVPA.