A Model for Optimization of Socio-Economic Fishery Values on the Rio Grande in New Mexico

This report describes a mathematical model which simulates sportfish production, yield, and economic value in the Rio Grande basin of New Mexico. The model links hydrologic, biologic, and economic components into a mathematical representation of fisheries habitat, fishery dynamics, economic benefits, and income generated by sport-fishing in New Mexico. The model has a research version programmed in FORTRAN and a user-friendly version programmed in APL.

The hydrology component is based on time-series correlations of U.S.G.S. data for inflows and outflows of a series of 19 river-basin segments which represent 8 reservoirs and 11 connecting reaches in or near the mainstream Rio Grande. It spans the period 1976 to 1984 and operates on the principal of mass balancing of water and material (suspended solids, total phosphorus, total nitrogen) in each of the 19 river segments. The hydrology component simulates material concentrations, water surface areas, water volumes, discharges, and changes in water elevation at two-week intervals for all 19 segments. These model outputs are accessible to the user and provide inputs for the biologic and economic components. Short-term predictions can be updated with the most recently determined data obtained from U.S.G.S. monitored stations.

The biologic component operates with inputs of area-depth-capacity data, water volume, water discharge and fluctuation, mean total phosphorus, total nitrogen, and total suspended solids from the hydrology submodel. It also requires inputs of mean seasonal solar radiation, water temperature, concentration of allochthonous suspended organic matter in the inflow, carbon-nitrogen ratio of allochthonous suspended solids, fish density and mean biomass per age class for each basin segment, and the fishing effort estimated by the economic component, stocking, history and harvest regulation. The biologic component is basically a process model which simulates productions of phytoplankton, zooplankton, zoobenthos and fish. For reservoirs, fish biomass and density are simulated by size group over any one to five-year sequence selected from the time period simulated (1976 to 1984). Fish dynamics in reservoirs are predicted for nine groups of functionally similar species (guilds): crappie, sunfish, whitebass, blackbas, catfish, walleye, northern pike, carp and suckers, and trout (including salmon). A population model is used which distributes predicted fish-food production among fish guilds according to types of food eaten, habitat preference, and food consumption rates. Total fish production alone is simulated for connecting waters.

Fish biomass for each segment is output from the biologic component that serves as input for the economic component. Also, the water-surface area of reservoirs and the discharge of connecting waters required inputs for the economic component from the hydrology component. The economic component also requires for input basin estimates of travel cost, segment elevation, basin segment access, the alternative water bodies available, and other measures of basin segment quality. The economic component estimates, for each of the 19 segments, the fishing effort in angler days, the economic benefits derived by New Mexico anglers, and the income and employment generated in each of the counties in the basin. Multiple-regression techniques are used to develop an angler demand schedule. The fishing effort estimated in the economic component serves as input for the biologic model where it contributes to the estimation of fish survivorship and production.

Model users have numerous entries to model outputs and for simulating management decisions. In the user-friendly version the user can enter the model and obtain output on reservoir surface area, reservoir volume, water levels, discharge in connecting waters, total plant production, allochthonous loads of suspended organics, zooplankton production, zoobenthos production, total fish production, fish guild density by size class, fish guild yield by size class, fish guild surplus yield by size class, economic benefits to New Mexico anglers, and county income generated. The model users can modify water volume flowing through the basin by month, water stored in each reservoir, water discharged through connecting waters, material concentrations (suspended solids, total phosphorus, total nitrogen) in the water, site access, and site quality. Modifications of the historic record are simulated for periods of 1 to 5 years for individual water segments or combinations of water segments up to all water segments in the basin. Both FORTRAN and APL versions of the model are designed to run on IBM microcomputers or compatible hardware. Linkage with mainframe capability will decrease processing time. Depending on the dimensions of analysis and output desired, running time on the microcomputer is from a few minutes to several hours. Use of a mainframe reduces running time to about 1% of microcomputing time. Advances in microcomputing expected in the next year should reduce running time to speeds comparable to mainframe times.

The appendix includes FORTRAN and APL Programs and a complete mathematical documentation of the model [FORTRAN version].