Food webs and the sustainability of indiscriminate fisheries

Abstract

Here, we introduce a novel theory for multispecies fisheries that exploit fish stocks evenly within and across trophic levels in an entire ecosystem (i.e., fishery comprises all fleets). These “indiscriminate” fisheries may be common in developing countries where fish provide the main source of dietary protein. We show that simple food web modules, motivated by empirical patterns in body size and energy flow, yield general and robust predictions about the fate of such a fishery. Specifically, high and uniform fishing mortality modifies the fish community in a manner that leads to increased productive capacity from a low-diversity assemblage of small-bodied fish with rapid population growth and turnover (the productive monoculture effect). We then argue that catches are relatively indiscriminate in the Tonlé Sap, a highly productive inland fishery in Cambodia that feeds millions, and show consistent qualitative agreement between the theory of indiscriminate fishing and this existing empirical data. As the theory suggests, this indiscriminate fishery appears to be remarkably productive at the community level in the face of high fishing mortality; however, it tends to be unsustainable at the species level as the Tonlé Sap has a much depleted species diversity under its current high fishing mortality. We end by arguing that the reduced diversity of these types of fisheries likely put them at severe risk of being heavily impacted by changing environmental conditions such as climate change and hydroelectric development.

Spatial and temporal changes in fish yields and fish communities in the largest tropical floodplain lake in Asia

“…Later, fishers catch all kind and size class of fish. The catch in the manner that all fish species and size class across the food webs were exploited is called the indiscriminate fishing (McCann et al, 2015). Fishing has a direct influence on alteration of the fisheries production, fish composition and community, and ecosystem functioning (Anderson et al, 2008).…”

“…The Tonle Sap fisheries are indiscriminate in that they exploit all types and size class of fish (McCann et al, 2015). Indiscriminate fishing is common in developing countries where fish provide the main source of dietary protein.…”

High fishery catches through trophic cascades in China

“…Marine coral reef fisheries in Kenya have been shown to produce more catch with increasing effort, but with the result of decreasing functional diversity and trophic complexity (9). Indiscriminate marine fisheries in Cambodia continue to produce high catches despite high fishing mortality, but at the cost of reduced species diversity (10). Because trophic cascades can increase the total harvestable biomass in a system, reversing them in systems where predator removal has already occurred by implementing management that protects predators could result in decreases in catch.…”

Linking freshwater fishery management to global food security and biodiversity conservation

“…Recent analyses of marine fishery sustainability in terms of embodied primary productivity indicate widespread overfishing (35), but lack of systematic data on river ecosystem energetics precludes performing a similar analysis using our map of estimated catches. Instead, shifts in catch size structure and species composition in intensive fisheries of Asian and African rivers have generally been interpreted from a demographic standpoint (15,38). Although data on fish demography and ecosystem energetics are inadequate to support rigorous analyses of the production potential of multispecies subsistence fisheries in rivers globally, models indicate that fastgrowing, small-bodied species can support high yields even under intense fishing pressure (38).…”