These reversals are likely driven by growth compensatory mechanisms that result from food limitation. Relative to the 1980s, yellow perch populations in the Inner Bay are currently at low abundance, have better growth rates, higher survival, and females appear to be maturing at an earlier age. We have developed a discrete age-structured model of abundance that simulates historical changes in abundance. The model is used to evaluate the likely significance of food-limited compensatory mechanisms relative to other processes and how they act on the population. Model results provide information that gives fishery managers a better picture of what regulates this population so that it may be managed more effectively in the future.
Many large rivers are waterways for barges and other commercial vessels.
The propellers of towboats that push barges can entrain adult fish. We combined quantitative trawling and a model of diffusion of entrained water in river channels to estimate entrainment mortality of large riverine fishes in the Mississippi and Illinois rivers. We also conducted ambient bottom trawling to estimate abundance of live fish, and observed additional recently killed gizzard shad, shovelnose sturgeon Scaphyrhynchus platorhynchus , and smallmouth buffalo Ictiobus bubalus . We developed ancillary estimates of entrainment mortality rates of shovelnose sturgeon and smallmouth buffalo based on the estimate for gizzard shad and on the distribution of numbers of killed fish of each species in the combined ambient and entrainment samples. Entrainment cannot be eliminated as an important source of mortality.