A stock assessment model for transit stock fisheries with explicit immigration and emigration dynamics: Application to upstream waves of glass eels

Fisheries for catadromous fish migrating through the fishing grounds are conceptualized as transit stock fisheries, whose local population dynamics is completely controlled by immigration and emigration. We present an extension to generalized depletion models to estimate the abundance and natural mortality rate of transit stock fishes using detailed fishery catch and effort data. We apply the model to 13 annual seasons of the Anguilla japonica glass eel fishery in the Shuang River estuary, northeast Taiwan. Estimates of abundance and natural mortality compare well with estimates for other species obtained from experimental studies or from data-rich stock assessment models at similar spatial scales. Parameter estimates are generally robust to biases and imprecision in the fisheries catch and effort data. The size of migration waves arriving at the estuary support a splitting mechanism controlled by eddies during oceanic migration. This extension of generalized depletion model provides a useful tool for the assessment of glass eel fisheries in East Asia, South East Asia, Europe and North America when only fisheries-dependent data are available.