Using a time-dynamic food web model to compare predation and fishing mortality in Pleuroncodes monodon (Galatheidae: Crustaceae) and other benthic and demersal resource species off central Chile

The stock biomass of the carrot prawn (Pleuroncodes monodon) in the south-central area of Chile (33º–39°S) has decreased progressively in the last 12 years. The depletion and lack of recovery of carrot prawn and other fish and crustacean stocks has been attributed mostly to fishing mortality (F). Predation can be also an important factor for the dynamics of marine species, especially those that are important prey in the ecosystem, like the carrot prawn and other demersal crustaceans. However, predation mortality (M2) has been less studied and quantified in Chile and elsewhere. Therefore, it is important to estimate and compare predation (M2) and fishing (F) mortality to understand their individual and combined effects on the dynamics of fishing stocks. In this study, we analyzed the biomass changes of carrot prawn and other components of the marine food web in south-central Chile from 1992 to 2018. For this, we built a food web model that was later fit to time series of observed biomass and catch data, using observed fishing mortality as forcing factor. We used the Ecopath with Ecosim software as modeling platform. This model provided M2 and F series for carrot prawn and other demersal crustaceans, which were compared to evaluate their relative contribution to total mortality (Z) from 1992 to 2018. The quantitative model considered 29 functional groups, from primary producers to top predators, and the following fishing fleets operating in the study area: artisanal and industrial purse seine fleets, artisanal and industrial hake trawling fleet and demersal prawn trawling fleet. The results indicated that M2 was the main component of Z in adults and juveniles carrot prawns, with Chilean hake as the main predator in both cases. M2 was responsible for 84 % and 86 % of Z of adults and juveniles of carrot prawns, respectively. M2 was also greater than F in yellow prawns (Cervimunida johni), whose main predator was big eye flounder (Hippoglossina macrops). In contrast, F was greater than M2 for the nylon shrimp (Heterocarpus reedi). We conclude that the stock dynamics of the carrot and yellow prawns has been strongly influenced in recent decades by the predation by Chilean hake on pre-recruits of carrot prawn as prey and bigeye flounder on yellow prawn as prey. We recommend that M2 should be considered both in evaluating the past dynamics of these stocks used to establish their status and in future projections used to establish biologically acceptable catch.