Figure 2.

Environmental variables for the lake Mjøsa (left panel) and its main inlet river Gudbrandsdalslågen (right panel). Left: The water temperature in the 0–10 m depth layer in the lake Mjøsa has been measured at least seven times each year (March–November) over the 1972 to present period and is here presented as the mean. The plotted values resemble the June–October period arithmetic mean values from the same station at the deepest point in the lake that has been surveyed consistently over the period. The brown trout catch data constitute annual catches from a selected panel of local recreational fishermen. Under the assumption of similar behaviour of the trout over the period these catch data can be used as a proxy for the trout density in the lake. Finally, echo sounding surveys have been performed at four occasions over the study period. These data reveals that the prey fish density (mainly smelt and vendace) has varied a lot over the last three decades, but there is no overall trend. The river data, right panel, constitute mean mid day June-to-October water temperature and discharge (m³ s⁻¹) values at the Hunderfossen dam. In order to enable presentation of the variables in the same figure, all values have been standardized so as to mean = 0 and standard deviation = 1 (estimates of means and standard deviations are given in parenthesis in figure legend).