Zusammenfassung: Sea surface height may be measured by a satellite-borne altimeter and its along-track slope used to infer geostrophic currents. A major difficulty is that, in general, the local geoid and satellite orbit are not known to the accuracy desired. Thus, comparison is often made between repeat flights of an altimeter along fixed ground tracks in order to infer the changes in the currents. In practice, it is convenient to calculate a mean height profile from many repeat passes and use this as a reference, so that individual altimetric profiles yield variations about this mean. It is thus important to derive a high-quality reliable estimate of the altimetric mean in order to minimize the errors in the inferred flows for the individual repeats. This work examines various methods for deriving the mean profile.

Zusammenfassung: Several new models are developed to estimate the velocity and diffusion of a population from tagging data. The new estimators apply the inverse principle to the individual trajectories of recovered tags rather than to their local abundance. These models require fewer assumptions and less information than do published abundance-based methods. Techniques are presented for a variety of circumstances, and both discrete and continuous parameterizations of the velocity field are included. The sensitivity of the estimators to violations of the assumptions was examined numerically by using stochastic simulations. The results suggest that the estimators are fairly robust but may fail under certain conditions. Extensions to accommodate these situations are discussed.

Zusammenfassung: Die Fischfauna der als Feuchtgebiet internationaler Bedeutung ausgewiesenen Flussaue ist durch Verschlammung und Eutrophierung gefaehrdet. Insbesondere die im Kies laichenden Arten Rapfen und Zope verlieren Lebensraum. Blei und Guester sind die einzigen Arten mit positiver Bestandstendenz. Charakteristische Arten wie Schleie, Karausche und Hecht nehmen ab. Das Flussneunauge ist ausgestorben. Hilfsmassnahmen sind erforderlich, nach weiteren vorbereitenden Untersuchungen muessen sie zusammen mit Fischern umgesetzt werden.