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

Summary Data relating to possible predatory effects of cormorants Phalacrocorax carbo (L.) on grayling Thymallus thymallus (L.) in the river Rhine did not support hypotheses of overfishing, but were consistent with changes in the fish population resulting from variable recruitment and density‐dependent effects (Suter 1995). In a government report, fisheries managers using the same data claimed to have found a causal relationship between cormorant predation and grayling decline (Staub et al. 1992). In their comment, these authors argue that certain data transformations and statistical procedures in my paper were responsible for the different outcome (Staub et al. 1998). However, the authors are unable to quantify any effects of procedures they question, and resort to vague reasoning. Their technical objections are either irrelevant or misleading, since they repeatedly make wrong assumptions or misinterpret parts of my analysis. Different interpretations of predation effects are rather a consequence of the shortcomings of the original report: Staub et al. (1992) had not applied any statistical treatment to the data not tested any hypotheses, but had arrived at their ‘conclusions’ simply by looking at a number of raw graphs depicting grayling catches grouped by fish‐length classes. Moreover, their approach (both in the original report and in the present comment) suffers from a lack of biological understanding, particularly of dynamic processes in fish populations and density‐related effects.