1. School of Mathematics and Computer Science, Yunnan Minzu University, Kunming 650500, P. R. China
2. Hubei International Science and Technology Cooperation Base of Fish Passage, China Three Gorges University, Yichang 443002, P. R. China
3. Laboratory for Industrial and Applied Mathematics, York University, Toronto M3J 1P3, Canada
4. Department of Applied Mathematics, University of Waterloo, Waterloo N2L 3G1, Canada