Zusammenfassung: Increasing numbers of students now study natural resource and environmental economics at undergraduate level, and this interest in the subject has been matched by a corresponding expansion in the number of textbooks and other learning material available for use. Unfortunately one very important aspect of natural resource economics which is often not adequately dealt with in the undergraduate literature concerns the question of dynamics: how rapidly will a resource become exploited, and what time path will it follow? These issues are particularly relevant to the management of renewable resources such as fish stocks whose survival depends on factors such as the rate of entry and exit by the exploiting firms, and the ability of the resource to recover from the effects of intense harvesting. The fault of most textbooks stems not from a failure to cover the question of dynamics, but rather that the treatment is often too advanced or mathematical for the majority of students. Computers can make a useful contribution to our understanding of the dynamics of natural resource exploitation, and in this example the case of marine commercial fishery exploited under 'open-access' conditions is considered. The quantitative assessment of environmental contaminants is a complex process. It involves nonlinear models and the characterization of variables, factors, and parameters that are distributed and dependent each other. Assessments based on point estimates are easy to perform, but since they are unreliable, Monte Carlo simulations have become a standard procedure. Simulations pose two challenges: They require the numerical characterization of parameter distributions and they do not account for dependencies between parameters. This paper offers strategies for dealing with both challenges. The first part discusses the characterization of data with the S-distribution. This distribution offers several advantages, which include simplicity of numerical analysis, flexibility in shape, and easy computation of quantiles. The second part outlines how the S-distribution can be used for hierarchical Monte Carlo simulations. In these simulations the selection of parameter values occurs sequentially, and each choice depends on the parameter values selected before. The method is illustrated with preliminary simulation analyses that are concerned with mercury contamination in king mackerel (Scomberomorus cavalla). It is demonstrated that the results of such hierarchical simulations are generally different from those of traditional Monte Carlo simulations.

Zusammenfassung: Nach dem Rückgang der saprobiellen Belastung der Fließgewässer treten andere Belastungsformen, wie strukturelle Defizite oder toxische Belastungen deutlich hervor. Daher legte die Europäische Gemeinschaft den Entwurf einer EU-Richtlinie zur ökologischen Qualität von Gewässern vor, in welcher den Mitgliedstaaten eine Überwachung der ökologischen Qualität von Fließgewässern vorgeschrieben wird. Gute ökologische Qualität wird darin anhand überwiegend biologischer Kriterien nach dem Prinzip der Naturnähe definiert. Ausgehend von dieser Richtlinie entwarf die Landesanstalt für Umweltschutz Baden-Württemberg (LfU) ein Strategiepapier zur ökologischen Fließgewässerbewertung, welches die Grundsätze eines möglichen Verfahrens vorschlägt. Das Ziel der vorliegenden Literaturstudie war es, Informationen über geplante und angewandte Verfahren, vor allem der Länder der EU, auszuwerten und die erfaßten Verfahren dahingehend zu überprüfen, ob sie für die im Strategiepapier der LfU skizzierte “integrierte ökologische Fließgewässerbewertung” neue Aspekte enthalten, die dafür berücksichtigt werden sollten. Zur Durchführung der Literaturstudie wurden verschiedene Datenbanken abgefragt und zahlreiche Experten befragt. Insgesamt wurden über 800 Literaturzitate ermittelt und ca. 500 davon beschafft. Die gefundenen Verfahren zur Bewertung von Fließgewässern wurden nach den Zielsetzungen und zugrunde liegenden Prinzipien geordnet und kurz nach folgendem Katalog characterisiert: Verfahren, Ziel, Region, Prinzip, Bewertungskriterium, Ergebnis, Methodik, Indikatorgruppen bzw. Parameter, Bestimmungsniveau, Möglichkeiten und Grenzen sowie Bemerkungen. Auf diese Übersicht aufbauend wurden anschließend allgemeine Anforderungen an eine ökologische Fließgewässerbewertung formuliert und Empfehlungen für deren Durchführung ausgesprochen.

Zusammenfassung: Studies of predator-prey interactions have been limited to laboratories or relatively small scale field experiments because of logistic difficulties in large scale field experiments. Much evidence has suggested that predictions based on small scale experiments on predator-prey interactions may not be simply extended to large scales, e.g. whole system. This study was designed to examine predation effects on a prey fish assemblage at the whole lake scale. After a year of premanipulation study, northern pike (Esox lucius) were introduced into a small piscivore-free lake inhabited by 14 species of fishes. In the third and fourth years, the lake was divided by a large mesh fence and northern pike were introduced into one half of the lake. Predation effects on individual prey population were estimated for both direct effects (i.e., consumption of prey species) and indirect effects, including habitat changes associated with predator avoidance behavior, increase of emigration rates and changes of population size structure. The relative importance of direct versus indirect effects was examined using randomized intervention analysis along with a model that quantifies the indirect effects. Decreases of prey fish biomass due to increased emigration was at least as great as that due to the direct effect of consumption when prey biomass was high (>40 kg/ha) but not when prey biomass was low (<30 kg/ha). Significant indirect effects included dramatically decreased abundance of the dominant species (northern redbelly dace, Phoxinus eos), increase of some rare species (sunfish, Lepomis spp. and creek chub, Semotilus atromaculatus), and decreased mean sizes of redbelly dace. Indirect effects occurred rapidly and were most apparent immediately following the introduction of northern pike. Community responses to northern pike predation included changes of species composition, species diversity and community stabilities. The fish communities were significantly different among the pre-pike period, pike period and post-pike period. The greatest variance was related to abundances of redbelly dace and sunfish. However, predation effects on the prey fish community depended on the observational scales used to determine community structure. (DBO)

Zusammenfassung: The Desert Fishes Council has moved into the world of electronic publishing on the Internet. Dr. John Rinne (U.S. Forest Service, Flagstaff, Arizona) has allowed the first author to scan his well-known collection of fish photographs into digital format. They have been variously cropped and enhanced, and can now be viewed in different formats or downloaded by anyone in the world with Internet access. The second author is converting the Proceedings (staring with Vol. 24 [1992 meeting]) to hypertext format, so they may be browsed, searched or downloaded. All future DFC Proceedings will be published on WWW at the same time final hard copy is sent to the printer. The World Wide Web (WWW) allows anyone who has obtained free, public domain client software (readily available on Internet), primarily one called Mosaic, to browse effortlessly through multimedia files (graphics, video, sound, formatted text) that are stored on computers around the globe. Multimedia presentations can combine real-time sound, graphics and video with text. Exploration of WWW and the DFC system using Mosaic is facilitated by hypertext links, which offer the simplicity of “point and click” movement through the “cyberspace” of WWW. The Texas Memorial Museum of the University of Texas at Austin is providing storage space for these large DFC files at no charge as a pilot project for other WWW projects at UT, and can likely continue to provide this service for the forseeable future if it receives the usage and growth are predicted. All DFC members are encouraged to provide additional text, graphics and other files for inclusion in the system. Growth of the Internet has been phenomenal, and as more and more people throughout the world obtain access to it, these electronic files will allow DFC to better reach a far larger audience than was formerly possible. DFC members and others using materials provided here will be better equipped to educate the public regarding the plight of desert fishes. The system should also improve communications among ourselves and with others regarding fish conservation efforts throughout the world, and eventually should lead to reduced costs of publishing the Proceedings.

Zusammenfassung: The paper discusses the links between fish stock assessment and economics of fisheries, as perceived by a fisheries biologist. It focuses on the type and format of data required from the biological side as well as the economic side, to merge the biological and the economic methodologies into a bio-economic methodology. The biological methodology in question derives from the analytical approach, which operates with cohorts and size distributions of several stocks exploited by several fleets fishing on several fishing grounds. The economic methodology comprises costs and earnings analysis of the harvesting and the processing sectors, and computations of indicators for the economic performance of the sub-sectors. The paper is based on applications of bio-economics to tropical fisheries (targeting at shrimps). The methodology has been applied during a series of workshops and seminars organized by the Fisheries Department of FA0. The paper addresses both biologists and economists.