Impacts of non-native species in a shallow lake: a simulation modeling approach for restoration and management
Tim W. Stewart
There is a long use of models in fisheries and ecology. These models have varying assumptions of how underlying processes occur. Chapter 2 found that violating the assumption of continuous fishing could have consequences for managing harvest of fish populations. In particular, as more fish are harvested in a discrete rather than continuous fashion, parameter estimates of biomass dynamics models assuming continuous harvest can be biased. Accounting for discrete harvest using a semi-discrete biomass dynamics model reduced parameter bias and should be used when discrete harvest is occurring.
Extending the exponential semi-discrete biomass dynamics model to an actual population of common carp revealed that the population was increasing. Current levels of commercial harvest were insufficient to suppress biomass. Additionally, the rate of biomass increase was rapid, doubling in approximately three years in the absences of harvest. The effect of rapid biomass increase was further demonstrated by a sensitivity analysis of the model to unintentional underharvest. Sensitivity analyses results suggest the potential for a strong fisherman effect on common carp populations. A framework to assess the minimum amount of harvest needed to maintain common carp biomass was presented.
The effects of common carp and zebra mussels in aquatic food webs investigated in Chapter 4 revealed a potential impediment to lake restoration. In particular, zooplankton predation by age 0 yellow bass may limit top down regulation of phytoplankton biomass. Based on mass-balance estimates, age 0 yellow bass were estimated to consumer up to 50% of zooplankton production. The recent invasion of zebra mussel was shown to have an effect on phytoplankton populations, likely compensating for reduced zooplankton abundance. Common carp food web impacts were lower than expected, due to abundant benthic food resources.
The potential common carp and zebra mussel ecosystem impacts in the context of ongoing restoration were evaluated in Chapter 5. Simulated changes in common carp biomass had dramatic effects on water quality and recreational fishery yield. This effect was due to increased suspended sediment reducing water transparency, which in turn limited phytoplankton production. This also limited any effect of zebra mussels on water quality by limiting food resources (i.e., edible phytoplankton). In scenarios simulating baseline common carp biomass, increasing or decreasing zebra mussel biomass had positive and negative effects on water transparency respectively. Overall, zebra mussel impacts, positive or negative were limited to baseline common carp biomass scenario classes. Additionally, macrophytes biomass showed a dramatic increase when commercial fishing increased in scenarios of baseline common carp biomass, however this may lead to nuisance levels. Unexpectedly, macrophyte increases in response to increased commercial fishing mortality also stimulated common carp increases as an increased food resource. This in turn reduced simulated water clarity and macrophyte biomass.
Results of this study suggest that in-lake processes are a significant component to water quality and recreational fishery yield. Controlling common carp biomass will critical to achieve water quality goals and minimize adverse effects on recreational fishery yield. The recent invasion of zebra mussels to the system will likely positively affect water clarity; however this will be limited by common carp. With common carp biomass controlled, zebra mussels can be expected to clear the water column; however this will reduce pelagic primary production, and require consumers to shift to feeding within benthic food web portions.