The Effect of Different Balancing Approaches on a Food-web Model of Lake Huron

Models are commonly used by fishery scientists to explain past events or predict future effects. An increasingly popular modeling tool for this purpose is the Ecopath with Ecosim (EwE) software. Ecopath requires a mass-balanced snapshot of a food-web at a particular point in time, which Ecosim then uses to simulate dynamics through time. Attempts to achieve mass-balance in Ecopath are rarely successful based on available estimates of biomass, production, and diets, and thus changes to the parameters are required to balance the model. There has been little discussion of whether these changes to achieve mass balance affect model results. We constructed two EwE models for the offshore community of Lake Huron, balanced in contrasting but realistic ways. One placed more confidence in estimates of consumption; levels of production were increased to achieve mass balance. The other placed more confidence in estimates of production. To assess the effect of balancing approach on model results, we compared ecosystem metrics within Ecopath (ascendency and system omnivory index (SOI)), as well as biomass dynamics within Ecosim. Within Ecosim, we compared predictions given alternative assumptions about the type of control (top-down or bottom-up), under two scenarios of (1) increased fishing mortality or (2) increased environmental production. Ascendency for the first balancing approach was approximately twice that of the second, mostly due to greater production in planktonic groups. SOI showed very little difference between the two approaches. Differences of predicted changes in relative biomass were greatest under scenarios assuming top-down control, but were at most 10% when averaged over all groups in the model. When expressed relative to the overall change in biomass for each scenario, the differences between balancing approaches represented at most 25%, but were for scenarios where biomass changed very little. Our findings suggest that, in general, the way in which Ecopath models are balanced does not have a large effect on model conclusions, and that the magnitude of the effect is greater when top-down control is prevalent in the system.