The effect of predation by wintering cormorants Phalacrocorax carbo on grayling Thymallus thymallus and trout (Salmonidae) populations:: two case studies from Swiss rivers.: Comment

1. Using the same data on the predation effect of cormorants Phalacrocorax carbo on the grayling Thymallus thymallus population located in the river Rhine below Lake Constance, Staub et al. (1992) and Suter (1995) reached contradictory conclusions (additional and compensatory mortality, respectively). This comment clarifies the reasons for the differences. 2. The biomass of older fish (age classes 4 and 5-7) was not judged to be decreasing by Suter because of the use of inappropriate (parametric) inferential statistics and no use of power analysis. More powerful (non-parametric) inferential tests show a significant decrease in biomass, 3. Staub cr nl. compared fish numbers per length class in the angling catch (autumn) and in the spawning catch (spring). Differences were thought to be the effect of cormorant predation during winter. Suter converted fish numbers per length class into biomass per age class for the same comparisons. However, this conversion considered neither the length increase of grayling during winter (c. 1 cm) nor the dominance of faster growing males (> 1.5 cm longer than females of the same age) in the spawning catch. Thus, changes of the age pyramid during winter have been masked.