Analysing ecosystem and demersal stocks dynamics in the Chilean Patagonia system (41°28.6′S to 57°S) from 1980 to 2020 using food web modelling

Resumen: The Chilean Patagonia system (41°28.6′S to 55° S) is influenced by the Cape Horn Current and exhibits high diversity, endemism, and important demersal stocks. However, food web structure and dynamics, the ecological role of key living components, and the impacts of fishing are poorly understood. In this paper, we model the structure and dynamics of the food web that sustains the demersal fish targeted by the fisheries operating in Chilean Patagonia, which takes place on the Patagonian Shelf and inshore waters. For this, we built a quantitative model for this ecosystem in 1980 using the Ecopath with Ecosim software, with 15 functional groups (from phytoplankton to top predators) including demersal target species such hoki (Macruronus magellanicus), kingklip (Genypterus blacodes), southern blue whiting (Micromesistius australis), skates, and southern hake (Merluccius australis). We split hoki into juvenile and adult stanzas based on their importance as prey (juveniles) and landings (adults). Input data came from studies to estimate abundance/biomass, production, and diets of target species and other key functional groups, while catch data came from official statistics. The model was balanced and fit to biomass and catch series from 1980 to 2020 using fishing mortality (F), vulnerability to predation parameters (v), and primary productivity anomaly (PP) estimated by model that aims to simulate PP trend. Zooplankton and benthos were the most important prey at intermediate trophic level consumers, while small pelagic fish and hoki juveniles were the main prey for higher trophic levels (i.e., hoki adults, southern hake, and southern blue whiting). Predation mortality (M2) of target species was important and variable in time. The dynamics of target species and the whole food web from 1980 to 2000 was explained mostly by fishing mortality, but model fit increased when considering PP and v. We conclude that i) juvenile hoki is a key prey group in the system, especially for hoki adults (cannibalism) and southern hake, ii) southern hake is the main predator among demersal fish, and ii) fishing and bottom-up changes were important drivers in the system likely mediated by trophic interactions. These findings are important in moving forward on the development of ecosystem-based fisheries management in Chilean Patagonia.