Modeling population dynamics and small-scale fisheries yields of fish farming escapes in Mediterranean coastal areas

Abstract

Coastal ecosystems put up a number of impacts from human activities in the sea; the most recent is fish farming, interacting synergistically with the other impacts and with the natural structure and dynamics of the coastal ecosystem. In the Mediterranean Sea, the number of fish farms has increased dramatically from early ‘80s in coastal waters, releasing a substantial amount of organic matter, modifying the habitat and communities beneath cages and changing the spatio-temporal distribution of species. Among all the effects derived from fish farming, escape events of cultured fish are a relevant issue for management given their potential impact over wild counterparts in terms of habitat and food competition, genetic flow, biodiversity, spread diseases or parasites, and interaction with local fisheries, decreasing the price of the catches. This paper shows the first approach to model the temporal trends of biomass and yields of escapes from aquaculture by means of an EwE model. Three levels of escapes (×1, ordinary level: 5000 ind year−1 fish farm−1, corresponding to 1.31 t year−1; massive event: ordinary level × 91; total destruction of fish farm: ordinary level × 1800) and four levels of fishing effort (ordinary E, ×2, ×5, ×10) were modelled as mechanism to recapture escapees. Temporal variation of biomass and yield is used to define how long should be maintained the effort to catch escapees. The total destruction of a fish farm generates the higher increase of escapees' landings, dissapearing in less than 6 months if fishing effort reach 10-fold the ordinary fishing effort. Differences among revenues from recaptures and derived expenses were always negative but tending to be greater for low levels of fishing effort and/or levels of escapes which means no fishery can be expected to be maintained by escapes. In general, fleets benefit from escapees in terms of yield but gains will depend on how escapees affect either positively or negatively the value of the catch. Simulations using EwE models may result a useful tool to design suitable recapture plans of escapees.