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Tonle Sap Inland Fishery

Evaluating the Biological and Economic Effects of No-Take Zones

The Tonle Sap hydrologic system in Cambodia consists of Tonle Sap Lake (one of the largest lakes in Asia), the Tonle Sap River and the Mekong River. Many local residents rely on the complex ecosystem of this watershed for sustenance, as it accounts for roughly 60% of the total inland catch in the country. The hydrology of the Tonle Sap region is unique in that Tonle Sap Lake increases more than five times in size from the dry season to the wet season. The large fluctuation is driven by an unusual phenomenon; when heavy rainfall and ice-melt raise the water level of the Mekong River, it causes the Tonle Sap River to reverse direction and flow into the lake. This dynamic system is able to support several distinct fish life histories, including species that: (1) stay primarily in the rivers, (2) migrate between the lake and the rivers, and (3) stay in the lake year-round. Although there is a lack of consistent catch data for the system, it is widely accepted among fishers and Fisheries Department officials that fishing effort has increased over the past few decades, followed by a decline in the quantity and average size of fish caught in recent years.

 

 

 

 

 

 

 

 

 

 

 

 

 

One of SFG’s roles in the project is to determine the ecological and economic effects of implementing no-take zones (NTZ)— areas where fishing is prohibited— on the Tonle Sap watershed. To evaluate these effects, we are developing a multi-species, spatially-explicit bio-economic model to simulate different levels of fishing pressure under several distinct NTZ management plans. The model will include two hypothetical species: (1) migrating fish that move between the lake and rivers and (2) sedentary fish that stay in the lake year-round. Accounting for multiple species will allow us to assess the distribution of harvest between the species under different NTZ scenarios. Through this modeling effort, we will elucidate the costs and benefits of implementing NTZs in the Tonle Sap system, and measure how changes in management regimes will influence the catch of each species.

SFG will also investigate the impact NTZs would have on the Tonle Sap region if property rights were reinstated as part of the fisheries management plan. Historically, villages were entitled to property rights for sections of the lake, which they managed and monitored carefully. In recent years, fisheries management has moved away from property rights towards an open access system, where there is a greater incentive to overexploit rather than sustainably manage the shared food resources. Our team aims to determine if overfishing problems could potentially be resolved by transitioning back to property rights in the Tonle Sap watershed, and if implementing NTZs would even be necessary.

Modeling Optimal Harvest Strategies for Indiscriminate Fisheries

The Tonle Sap ecosystem contains a diverse food web which includes fish species of varying trophic levels. A secondary component of the project aims to link the human side of fishery management with the ecological dynamics of “indiscriminate” fisheries, or fisheries that target fish of all sizes and trophic levels. Since large fish— generally predators—are more sensitive to fishing, their numbers are often reduced early in the harvest, causing predator release (when prey species experience a population boom in the absence of pressure from consumers). The increase in prey species can negate or even surpass the loss in predator biomass, resulting in higher total biomass.

SFG is working closely with researchers from the University of Guelph to determine what harvest levels and management strategies would maximize biomass and profit. We will model the effects of harvest methods and intensity at each trophic level in order to maximize harvested biomass or profits. Since large fish are usually more valuable than small fish even when total biomass is equivalent, we hypothesize that optimizing for different goals will drive divergent harvest strategies. Through this effort, we intend to illuminate the effects of indiscriminate fishing on the Tonle Sap ecosystem, and define ways to use it to maximize harvest or profit.

Photo Credits: Floating Village - Alamy for The Telegraph UK, Net Fishing - AP File Photo