Dönni, W., Enz, C.

Computer modeling in fish ecology: just for the specialists?

Summary: Ecological systems are often very complex in structure. To draw an analogy we have to develop models. Today‘s computers an modern software allow modeling of dynamic systems quite easily. The advantages and limitations of computer modeling are described using three examples dealing with fish ecology and fisheries.

Under the Swiss law the water temperature of a stream should not change more than 1.5 °C as a result of a discharge. A model was developped to investigate the effects of a discharge of thermally used groundwater into a trout stream. The model simulates the development of the mean water temperature downstream from the discharge place. Several realistic scenarios with different stream flows and temperatures of the discharged groundwater showed that a warming of the groundwater by 12 °C is legal even under extrem conditions (low water flow in the stream, hight temperature of the pumped groundwater).

The simulation model SIMCOR is used to understand and quantify some of the processes that influence the whitefish (Coregonus sp.) stock in Lake Lucerne (Switzerland). SIMCOR simulates the development of the catch and the stock of whitefish using water temperature, phosphorus concentration of the lakewater and the meshsize of the nets used by professional fishermen. The model was validated using data on annual whitefish catch and year-class strength. Different scenarios, simulated by changing the input variables, provided information about the previous and the possible future developments of the stock and the commercial catch of whitefish in Lake Lucerne. Additionally SIMCOR simulates the development of the annual catch according to the individual year-classes.

A population dynamic model was used to evaluate mortality factors determining year-class strength and yield of whitefish (Coregonus sp.) in Lake Hallwil (Switzerland). We used the number of whitefish larvae stocked annually and the mean age of gillnetted whitefish as input variables, while data on annual whitefish yield and year-class strength were used to validate the model. Our modeling results indicated that the weather situation in May negatively influenced year-class strength and yield. Thus, it sustained our hypothesis that gas supersaturation, resulting from intensive oxygen production by algae during sunny weather in spring, caused lethal gas bubble syndrome in Lake Hallwil whitefish larvae. Our modeling results further indicated that high phyllopod densities in May were associated with strong year classes.