Simulating fish passage impacts on a fragmented metapopulation of Paddlefish

The persistence of fish population segments that are isolated by dams may be directly related to passage rates (both upstream and downstream) across the dams that isolate them. In anticipation of proposed bypass structures at two dams currently fragmenting the Alabama River, we programmed an individual-based model (Alabama River Paddlefish Passage Model; ARPPAM) to simulate the effects of increasing connectivity between population segments of a migratory fish species on the likelihood of segment extirpation within 100 years. We designed six different realistic passage efficiency scenarios to account for uncertainty in the ultimate efficacy of the bypass structures, and to test the effect of increasing segment connectivity (fish passage) on the overall metapopulation. We used the Paddlefish Polyodon spathula as our model species and programmed detailed life history information and movement dynamics according to measured quantities (although several demographic and movement rates for the metapopulation were uncertain or unknown). We used a simulation to see how a range of values would affect our inference about the influence of increased connectivity. Higher juvenile entrainment probability and associated mortality increased the probability of segment extirpation. These effects were lessened by increasing segment connectivity. Our model has broad applicability and provides a flexible framework to test a variety of hypotheses about metapopulation dynamics of potamodromous species in fragmented systems globally. Furthermore, fish passage facilities are costly, so modeling population consequences at early stages of development can save money and help formulate criteria for their design.