Zusammenfassung: Biases in estimating population sizes of the plus group are examined for bluefin tuna (Thunnus ) and swordfish (Xiphias gladius ). It is shown that the VPA estimates used in the 1990 ICCAT assessment are sensitive to the systematic ageing errors. The biases in the estimates of population sizes of the plus group might increase exponentially with tracing back to the earlier year.

Zusammenfassung: Length-based population analyses have had a fairly long but not too successful history. These methods are required over a range of situations from when aging is impossible to when it is difficult or prohibitively expensive. There have been 3 sorts of approaches: 1) modal analysis to convert length frequencies to ages 2) age-length key enhancement and 3) imbedded growth models. The last group is what is generally known as length-based methods and the focus of this review. At the 1991 ICES Methods Working Group, 5 techniques were compared and these results are summarized. Two of these techniques, cohort slicing and length-based ADAPT, are applied to swordfish (Xiphias gladius ) data. The imbedded growth model for the former technique is the mean size at age while the latter uses the distribution of size at age. A new method is also presented which begins with cohort slicing then estimates numbers at age and enhances an inferred age-length key at each step of an iterative process.

Zusammenfassung: Schaefer's model on the exploitation of a single species fish community obeying the logistic law of growth has been modified by adopting an alternative functional form for the harvest rate. The stability of the nontrivial steady state is discussed. The existence of a bionomic equilibrium is proved. The problem for optimal exploitation of the fishery is studied by using variational calculus. Some results are numerically computed to make a comparative study of Schaefer's model and the present one.

Zusammenfassung: Ócsa bog is one of the last remnants of an ancient moorland situated between the rivers Danube and Tisza in Hungary. During the years between 1984 and 1986, the food composition of five species of fish living in the ponds and channels of a former peat mine were examined: pike (Esox lucius), mud minnow (Umbra krameri), crucian carp (Carassisu carassius), catfish (Ictalurus nebulosus) and pumpkinseed (Lepomis gibbosus). Most of the data were collected during the summer months. Overlapping food ranges of the different species were described with the Renkonen index. Much overlap was found between species Carassius carassius, Ictalurus nebulosus and Lepomis gibbosus.Das Ócsa-Feuchtgebiet liegt ca. 30 km südwestlich von Budapest, östlich des ungarischen Teils des Donautals. Ócsa ist ein Naturschutzgebiet und enthält einen der letzten und wichtigsten Reste des Marschlandes, das sich einst zwischen den Flüssen Donau und Tisza erstreckte. Die Erforschung der Vegetation dieses Gebietes begann im 19. Jahrhundert (Kerner 1867-1879)Im 20. Jahrhundert wurde das Marschland intensiv erforscht (Boros 1936, Zólyomi 1951, Komlódi 1957, Siklósi 1984). Noch heute wird das Gebiet sehr genau botanisch untersucht; nur wenige Wissenschaftler haben jedoch die Fischfauna bearbeitet. Botta et al. (1980, 1984) veröffentlichten über die Gewässer dieser Region, gaben jedoch ihr Untersuchungsgebiet nicht genau an. Andrikovics & Peer (1990) analysierten die Nahrungszusammensetzung einiger Fischarten im Ócsa-Gebiet, vor allem die von Lepomis gibbosus und Ictalurus sp.