A STOCHASTIC BIOECONOMIC ANALYSIS OF SILVER EEL FISHERIES

Catadromous species such as the eel often present several distinctive features (high plasticity in somatic growth, size‐dependent sexual maturation, strongly skewed sex ratio, variable recruitment) that make their management quite complex. Fisheries usually operate in lagoons where effort is exerted on the sexually mature individuals migrating back to the sea, or on the young prereproductive individuals feeding in the lagoons, or on both. The aim of this work is to investigate the possibility of improving the economic return of the most typical catadromous fish, the European eel Anguilla anguilla, by exploring which fraction of prereproductive individuals should be caught every year, and whether declining recruitments from sea should be supplemented by restocking young eels (called elvers). Eel population dynamics is described through a demographic model based on a multiple classification of individuals by age and size and which explicitly includes environmental stochasticity and variation in model parameters. The model has been calibrated on the data from the most important Italian eel fishery located in the Comacchio lagoons, where only the migrating, mature eels are traditionally harvested. Different policies in terms of fishing effort, net selectivity, and restocking density have been analyzed by using the simulation model. Optimal management has been assessed with reference to the average net economic return under different discount rates. A Monte Carlo approach has been used to evaluate the variability of the mean economic benefit. Our model predicts that substantial improvement, relative to the traditional harvesting policy, can be achieved by restocking elvers, while catching yellow eels with fyke nets allows only for minor improvements of economic return. A sensitivity analysis shows that these results hold for different discount rates and for realistic variations of harvesting costs, but if the present price gap between yellow and silver eels were to increase, yellow eel fishing would be unprofitable. Also, elver restocking should be performed only in low‐density populations. Otherwise the density‐dependent increment in natural mortality may overcompensate the benefits of restocking. The conceptual framework used for modeling the specific case of European eels demography is very flexible and can be easily extended to a variety of other catadromous species.