Network analysis suggests changes in food web stability produced by bottom trawl fishery in Patagonia

Abstract

Demersal fisheries are one of the top anthropic stressors in marine environments. In the long term, some species are more vulnerable to fishery impacts than others, which can lead to permanent changes on the food web. The trophic relationships between predator and prey constitute the food web and it represents a network of the energy channels in an ecosystem. In turn, the network structure influences ecosystem diversity and stability. The first aim of this study was to describe for the first time the food web of the San Jorge Gulf (Patagonia Argentina) with high resolution, i.e. to the species level when information is available. The San Jorge Gulf was subject to intense fisheries thus our second aim is to analyse the food web structure with and without fishery to evaluate if the bottom-trawl industrial fishery altered the network structure and stability. We used several network metrics like: mean trophic level, omnivory, modularity and quasi-sign stability. We included these metrics because they are related to stability and can be evaluated using predator diets that can weight the links between predators and prey. The network presented 165 species organized in almost five trophic levels. The inclusion of a fishery node adds 69 new trophic links. All weighted and unweighted metrics showed differences between the two networks, reflecting a decrease in stability when fishery was included in the system. Thus, our results suggested a probable change of state of the system. The observed changes in species abundances since the fishery was established, could represent the state change predicted by network analysis. Our results suggests that changes in the stability of food webs can be used to evaluate the impacts of human activity on ecosystems.

Free shrimp cocktail for everyone! Trophic relationships of 13 elasmobranchs in Central Patagonia, Southwestern Atlantic

“…This increase was promoted by the abundance of P. muelleri, which supported its highest historical catches (Moriondo et al 2017), and G. gregaria, forming big swarms of the pelagic ecotype (Ravalli et al 2013;Diez et al 2016). In agreement with this crustacean dominance, P. muelleri and G. gregaria are key species in the local food web structure, being the two most linked species (Funes et al 2022). In addition to the high availability of P. muelleri, the local bottom-trawl Patagonian shrimp shery inputs individuals of the species as discards in the system.…”

“…: Cristiniani 2016; Tschopp et al 2020;Pasti et al 2021). This consumption of discards promoted signi cant changes in the local food web, increasing omnivory, and decreasing mean trophic level and overall food web stability (Funes et al 2022). According to the theory of optimal foraging (Schoener 1974), benthic-demersal species assessed by this study are expected to feed in a way that maximizes their energy intake and minimizes the energetic costs of foraging.…”

“…Indirect effects of shing activities involve trophic interactions at the community level through selective removal of predator or prey species, removal of competitors, and species replacement (Stevens et al 2000). Reduced abundance of commercial species populations under heavy shing pressure, over shing of juveniles, and bycatch of sh and invertebrate species may change the system food supply and promote marine food web rewiring (Badalamenti et al 2008;Funes et al 2022). These system alterations mainly affect predators at medium and higher trophic levels, forcing organisms such as elasmobranchs to change their feeding habits (Tschopp et al 2020).…”

Top predator sea stars are the benthic equivalent to polar bears of the pelagic realm

“…For example, increasing top-down control of sea stars on bivalves will alter walrus energy acquisition and space use, which in turn will affect polar bears. This assumption is supported by the numerous studies highlighting the worldwide deleterious effect of bottom trawling of benthic species on the pelagic subweb structure and complexity ( 24 ) and suggests that marine conservation initiatives should not overlook the benthic compartment.…”

Primer registro de un depredador natural del bagre marino, Genidens barbus, (Ariidae, Siluriformes) en el estuario del río Negro, Río Negro, Patagonia Argentina

“…La inclusión del efecto de la pesca sugiere cambios esenciales en las interacciones entre las especies, donde la merluza común adquiere relevancia como especie clave. Además, el efecto de la pesca genera cambios en la estructura y funcionamiento del ecosistema: mayor proporción de especies omnívoras, disminución en el nivel trófico medio y menor estabilidad del sistema (Funes et al 2022) (Fig. 2).…”