Prediction, Chaos and Ecological Perspective.

As defined, the modeling procedure is quite broad. For example, the chosen compartments may contain a single organism, a population of organisms, or an ensemble of populations. A population compartment, in turn, could be homogeneous or possess structure in size or age. Likewise, the mathematical statements may be deterministic or probabilistic in nature, linear or nonlinear, autonomous or able to possess memory. Examples of all types appear in the literature. In practice, however, ecosystem modelers have focused upon particular types of model constructions. Most analyses seem to treat compartments which are nonsegregated (populations or trophic levels) and homogeneous. The accompanying mathematics is, for the most part, deterministic and autonomous. Despite the enormous effort which has gone into such ecosystem modeling, there remains a paucity of models which meets the rigorous &! validation criteria which might be applied to a model of a mechanical system. Most ecosystem models are short on prediction ability. Even some classical examples, such as the Lotka-Volterra predator-prey scheme, have not spawned validated examples.

Verlängerungen in Fischgesellschaften - mit welchem Schleppnetz kann man sie nachweisen!

A great number of offshore wind energy generators are planned to be erected in the Baltic Sea in the near future. The question arises which impact these constructions may have on the living environment, of which fish are the most prominent candidates. Before these questions are tackled, however, tests need to be done to find out which bottom trawl is the most appropriate one to investigate possible effects. For this purpose four different trawls have been compared. As a result a number of key parameters for the design of an appropriate bottom otter trawl are given.

Weitere Selektionsuntersuchungen an Schleppnetzsteerten für den Plattfischfang in der Ostsee

The presentation includes investigations and results regarding Baltic flatfish selectivity with different trawl codend constructions. These investigations were carried out 1999/ 2000 on board of FRV “Clupea” and a commercial 17 m cutter. As could be shown, both an enlarged mesh size and another mesh form than rhombic can improve the selectivity of these fish. As flatfish catches with trawls mostly also cover cod, this mixed fishery at the southern Baltic coast should in future be managed by differentiated technical regulations for an improved selectivity. The results, also based on UW observations, current measurements, and wind tunnel tests with models, demonstrate some possible technical solutions.

Neues aus der Selektionsforschung am Ostseedorsch

In account of the decision of the IBSFC (International Baltic Sea Fisheries Commission) to admit alternative codend constructions to the presently legal of diamond meshes with 120 mm mesh opening, since seven years a vivid research activity is encouraged at the fishing technology institutions around the Baltic. Though a center of gravity in this field of research lies in the EUproject BACOMA (participants Finland Sweden, Denmark) also the corresponding national institutes of other Baltic countries (Germany, Poland, Russia) have been active with similar investigations. German experiments in 1999 dealt with the effect of the use of netting made of double instead of single yarn on the selectivity. Further research was done on modifications of codends made from netting turned 90° from its original orientation and on simplified versions of the so-called Multipanel codend which consists of three equal diamond and three square mesh panels. The experiments demonstrated on the one hand a definite deterioration of the selective properties in a codend made from double instead of single yarn. Constructional changes in codends made from netting turned 90° on the other hand brought a further improvement of the selectivity of such codends. Finally, the investigations on the Multipanel codend revealed the overwhelming influence of the square mesh panels on the selectivity success. If in a conventional diamond codend three windows are cut out starting 50 cm from the real end of the codend in such a way that the remaining parts have the same breadth as the square mesh panels to insert, the selective effect of such a codend is remarkably improved. According to further preliminary tests this seems to be nearly independent from the type of material used.

Flexibles Gitter für die Größenselektion von Ostseehering

In the Institute for Fishery Technology in Hamburg a new sorting grid for herring trawl fishery was developed which is more adapted to the commercial fishery than existing constructions. Therefore a flexible design was choosen with braided snoods as grid elements and rubber rings for shape stabilization. First tests were carried out with twin trawlers in the Western Baltic in February 1999. They proved principally the serviceability of this grid construction under commercial fishing conditions. The percentage of small sized herring was essentially reduced. Due to only few hauls more detailed quantitative statements are not possible yet.

Fangtechnische Möglichkeiten zur Reduzierung des Beifangs von Meeresenten in der Dorschfischerei mit stationären Fanggeräten

The present contribution summarizes results of fishery investigations with gillnets and longlines in the winter cod fishery which had been carried out in the western Baltic region from 1996 to 1999 by the Bundesforschungsanstalt für Fischerei with financial aid of the country Schleswig-Holstein. The main goal of these investigations was to point out technological possibilities for a reduction of unwanted bycatches of sea ducks in gillnetting. As it is obviously impossible to reduce the local and temporal bycatch problem by means of different gillnet constructions or tactical measurements, only the temporary avoidance of fishing grounds with high abundance of ducks or the change to longlining as catch method with reduced duck bycatches may be an effective solution.

A description of certain indigenous fishing methods from Southern Dahomey

A survey of certain Dahomean fishing methods showed them to be specifically adapted to the exploitation of floodplains and shallow water lagoons. Several types of fish-parks are described, ranging from small installations thaI funclion as refuge traps, to larger constructions that act as a form of fish culture. Ponds are also dug in the floodplains in order to conserve fish isolated during the dry season. These methods all give high yields and are worthy of consideration for use elsewhere on the African continent.