Evaluating ecosystem impacts of gear regulations in a data-limited fishery-comparing approaches to estimate predator-prey interactions in Ecopath with Ecosim

Ecosystem models, such as Ecopath with Ecosim (EwE), are useful tools for developing ecosystem-based management strategies. Model development, however, requires data for the estimation of input parameters including time series for calibration. Most small-scale fisheries lack such information, making it difficult to reliably use ecosystem models for management strategy exploration. This is the case for the Chwaka Bay fishery (Zanzibar), where an increase in dragnets has led to concerns for unsustainable development. While EwE could help to explore the impacts of gear regulations in the bay, this is hampered by the lack of time series to estimate the predator-prey interactions (vulnerabilities) in the model. Here, we explore available approaches for estimating the vulnerabilities to simulate the effects of a dragnet prohibition with and without reallocation of fishing effort. Simulations suggest that banning dragnets would be beneficial for the fishing community judged by the increase in biomass of functional groups and fishers' profits, but not if dragnet fishers were to continue fishing in the bay by reallocating to other gears, indicated by the reduced fish biomass and fishers' profits. The overall trend (decreasing or increasing) in the changes of functional groups, ecosystem indicators, and profits were relatively consistent across vulnerability settings, illustrating that an unfitted EwE model can be used for qualitative management strategy exploration.

Automated summary

Ecosystem models like Ecopath with Ecosim (EwE) are important tools for ecosystem-based management strategies. However, lack of data and time series for calibration makes it difficult to use these models, especially for small-scale fisheries like Chwaka Bay, where dragnet fishing is a concern for sustainable development. The study explores different approaches for estimating vulnerabilities and simulates the effects of a dragnet prohibition. The results suggest that banning dragnets would benefit the fishing community in terms of increased biomass and profits, but only if dragnet fishers do not reallocate to other gears.