The Ecosim component of Ecopath with Ecosim allows experimental simulations of changes in the balanced Ecopath model as result of a variety of changes in the fisheries or the ecosystem. Of particular interest are the expected effects of planned or proposed changes to the fisheries management regime. Because the simulation is sensitive to both model uncertainty and additional interaction parameters (‘vulnerabilities’) they are usually used to assess policy options in terms of the direction of responses and the potential for drastic unanticipated changes. Ecosim simulations are not usually used as a direct management tool.
The following extreme example simulates a complete closure of all fisheries from years 5 to 15 of a 100 year simulation. The red bar at the bottom represents the relative fishing rate over time and the lines above represent the biomass trajectories of individual functional groups.
By increasing the interaction parameters, known as vulnerabilities, the same simulation produces drastically different results
Scenario: increased effort in the beach seine fisheries
The increased demand for bait by the developing longline fisheries was identified as an emerging management issue in the region. In most countries the bait fish are small coastal pelagic species caught in beach seines. Various simulations were made to assess the expected impacts of this change on the ecosystem. An equilibrium analysis indicated that the target species, small coastal pelagics, were not the most affected group. Both groups of turtles and the coastal predators were negatively affected as bycatch in the increased beach seine effort. The small coastal pelagics actually increased with beach seine effort because of reduced predation by coastal predators was more than sufficient to offset the increased fishing mortality.
Scenario: interactions between flyingfish and dolphinfish fisheries
Because of the close linkages between dolphinfish (predator) and flyingfish (prey) in the LAPE area, simulations were run to assess the impacts of fisheries changes separately and combined.
Increasing only the catches of dolphinfish resulted in an expected decrease in dolphinfish biomass and increase in flyingfish biomass due to reduced predation.(blue bars)
Increasing only the catches of flyingfish resulted in an expected decrease in flyingfish biomass and an unexpected decrease in dolphinfish biomass (red bars). This is apparently due to reduced food supply.
Increasing catches of both species produced similar results to those from simply increasing the flyingfish catches (yellow bars) although with greater impact on dolphinfish and less impact on flyingfish.
It appears that dolphinfish are critically dependent on flyingfish for food and fisheries management plans need to consider this in development of these closey related fisheries.