Recent studies have demonstrated the great advantages of marine reserves in solving bycatch problems by maintaining the persistence of endangered species without sacrificing the fisheries yields of target species. However, transient phenomena rather than equilibrium states of population dynamics still require further research. Here, with a simple and general model, the transient dynamics of strong stock fish species are investigated under the condition of weak stock persistence. A surprising and counter-intuitive finding is that fisheries yields can strongly fluctuate even if population density both inside and outside marine reserve only slightly varies, leading to transient inconsistency between the population densities and fisheries yields. This finding suggests that population density dynamics cannot be used to predict the transient phenomena of fisheries yields (or vice versa) in fisheries management. These results will deepen our understanding of the transient phenomenon in marine ecology.