Designing freshwater protected areas (FPAs) for indiscriminate fisheries

Highlights

• Freshwater protected areas (FPAs) provide fisheries benefits similar to MPAs.

• In Cambodia's Tonle Sap, FPAs increase both protein and cash returns to the fishery.

• Tonle Sap fishing communities may have to fish less now to catch more later.

• Bridging funds can help subsistence fishers realize the long-term benefits of FPAs.

• Realizing FPA benefits may require national government and communities acting together.

Abstract

Freshwater protected areas (FPAs) are increasingly important for biodiversity conservation, given the intensive use of these systems for water, energy and food production. However, the fisheries benefits of FPAs are not well understood, particularly for indiscriminate fisheries typical of tropical systems. Here we report the results of a model that tests the fisheries effects of no-take protected areas in conditions unique to indiscriminate riverine/floodplain systems. The model has a generalized form applicable to a wide range of systems. We report the results of the general model, as well as those from a specialized form parameterized for the Tonle Sap lake, Cambodia. Both the general and Tonle Sap versions of the model show that FPAs can pay important fisheries benefits, especially where it is difficult to control fishing mortality through gear restrictions or other means. The harvest and profit benefit response curves have similar shapes, with additional FPAs paying high dividends at less than approximately 50% FPA coverage, and then truncating and declining thereafter. In the specific setting of the Tonle Sap of Cambodia, FPAs would pay a large increase in harvest because current FPA coverage is low. It may be counterintuitive to community fisheries managers in Cambodia that the best way to increase harvest is to restrict fishing, but at very high levels of fishing effort, reducing effort or area fished will improve both harvest and profit. In Cambodia, it may make sense to maximize harvest rather than profit because fishers living in poverty need to maximize protein offtake, but the benefits of FPAs remain. Similar considerations may apply in many freshwater and indiscriminate fisheries.

Introduction

The effects of no-take protected areas on fisheries production and conservation have been studied in marine environments for several decades (Lester et al., 2009; Gaines et al., 2010; Halpern et al., 2009), but with few exceptions, protected areas have largely been ignored in freshwater systems (Hermoso et al., 2016; Saunders et al., 2002; Srinoparatwatana and Hyndes, 2011; Suski and Cooke, 2007). This may be a missed opportunity because many of the conditions that give rise to marine protected area (MPA) success are arguably even more relevant in freshwater systems. In particular, freshwater systems are often marked by high fishing mortality, often combined with indiscriminate fishing (McCann et al., 2015), settings in which freshwater protected areas (FPAs) may offer benefits both to fisheries and to biodiversity conservation (Allan et al., 2005). FPAs are now being tested in a number of freshwater settings (e.g., Srinoparatwatana and Hyndes, 2011) under the premise that they could return fisheries, economic, and ecological benefits. However, their effects on harvest, profits, and conservation are not well understood.

Both theory and empirical studies have shown that when fish stocks are heavily fished, protected areas can act as a substitute for active fishery management in delivering conservation benefits. For example, Hastings and Botsford (2003) showed that MPAs can be as effective as optimized harvest strategies in producing fish yield and fishery-wide conservation, and Lester et al. (2017) showed that MPAs can increase yield and biomass in areas surrounded by open access fishing. These outcomes are a consequence of the buildup of fish biomass inside the protected area, some of which spills out into the adjacent fished area, providing a ‘fish bank’ that delivers dividends over time. Conservation of non-target species often results as well (Lester et al., 2009).

While the conditions under which MPAs deliver positive outcomes in the marine setting are now well-established, almost no research addresses the potential design and fisheries outcomes of protected areas in freshwater settings. Instead, theory and research on FPAs has focused mostly on biodiversity conservation (Suski and Cooke, 2007; Bower et al., 2015). The combined fisheries and biodiversity benefits of FPAs are less often explored. MPA fishery benefits often hinge on the degree to which overfishing is occurring, with more overfished fisheries tending to benefit more from MPAs. The stock recovery and spillover effects noted in some MPAs would be important reasons to consider implementing no-take FPAs, since many freshwater systems are overfished, especially in the tropics. But important differences between freshwater and marine fisheries call into question whether theory and empirical evidence from marine systems can be directly applied to freshwater settings (Bayley, 1995).

Freshwater fisheries are often exploited by small-scale, low-capital fishermen and are protein-maximizing systems, in contrast to many marine fisheries, which are often highly capitalized and may be focused on maximizing profit, rather that subsistence protein (KC et al., 2017). In many freshwater settings in the tropics, profit margins are low and resources for managing the fishery or enforcing gear restrictions or yields are limited (Bayley, 1995). In these settings, fisheries involve many small-scale fishers using equipment of all types and sizes, indiscriminately targeting all available sizes and species of fish (McCann et al., 2015; Allan et al., 2005).

The theory of “indiscriminate fisheries” describes the likely outcomes of offtake in multi-species fisheries (Costello, 2017; McCann et al., 2015). It suggests that indiscriminate fisheries seek to maximize protein production, resulting in systems with higher trophic levels removed, with some systems properties similar to those found in productive monocultures of terrestrial agriculture. These fisheries may be highly sensitive to environmental perturbations, such as climate change, for reasons analogous to those that make crop monocultures vulnerable to drought and disease. These qualities distinguish indiscriminate fisheries from temperate and marine single-species fisheries and call into question whether the theory and practice of MPAs translate directly to these settings.

While freshwater systems cover less than 0.8% of the earth’s area (compared to 71% covered by ocean, about 5% of which is covered by MPAs), they provide protein for tens of millions of people worldwide (Abell et al., 2008; Dudgeon et al., 2006; McIntyre et al., 2016). Because these fisheries emphasize volume of offtake over selection of large individuals or specific species, the context for potential protected area benefits to individual fishers is quite different to the environments in which MPAs have been studied. Analyses targeting indiscriminate freshwater fisheries are needed to understand the potential benefits of protected areas in these settings.

We draw our motivation for this analysis from perhaps the prototypical example of an indiscriminate freshwater fishery (McCann et al., 2015): the Tonle Sap in the heart of Cambodia. The Tonle Sap Lake (TSL) is a complex freshwater system that provides protein to over 2 million people (Lim et al., 1999; Lamberts, 2006). It serves as an excellent test case of the fisheries impact of freshwater protected areas, because it closely approximates the conditions described in indiscriminate fisheries theory (McCann et al., 2015). In particular, the fishery emphasizes high harvest volumes (Hall et al., 2006), in a manner analogous to agricultural monocultures, rather than the high-value offtake of select species and sizes which typifies many marine fisheries.

In this paper we make three discrete contributions. First, we examine the theoretical conditions under which an indiscriminate freshwater fishery could benefit from an FPA. Second, we examine a range of potential design features (size, location, number) for a network of FPAs, and show how these designs give rise to different outcomes. Finally, we apply this model to the Tonle Sap to examine whether FPAs are likely to benefit fisheries in one of the world’s most prolific freshwater fisheries. For the latter contribution, we find that context matters – the results hinge critically on the degree of cooperation in harvest across different fishing villages situated around the lake. To characterize this, and other key model parameters, we use new survey data and other results from a multi-year multi-disciplinary project on the Tonle Sap ecosystem. We believe that all three contributions are novel, and that these findings will help inform freshwater fisheries management and conservation in a variety of tropical settings.