No abstract

The energy flows of two habitats are quantified and compared by using a trophic network model. The habitats are representative of two typical subsystems of the Venice Lagoon: the seagrass meadows, which represent a mature stage in the ecological succession of lagoon environments, and the Tapes philippinarum fishing grounds, which are subjected to mechanical clam harvesting. The results obtained for the`seagrass habitat' are consistent with other field studies, confirming the`key-stone' role played by phanerogams in a lagoon environment. The results for the`Tapes habitat' show that the energy flow from the sediment to the water column is extremely important. Indeed, the high T. philippinarum biomass cannot be supported solely by the primary production of the local phytoplankton. Instead, this biomass could be supported by a supply provided by the sediment resuspension caused by intensive clam fishing. These findings suggest the presence of a positive feedback between fishing effort and T. philippinarum standing crop. ProblemOver the last decades the Venice Lagoon has been the target of numerous studies and research projects that address a wide range of questions and belong to many different scientific fields (Lasserre & Marzollo, 1996; web site Sistema lagunare veneziano, 1994). Nevertheless, these mainly bio-ecological studies were often limited spatiotemporally and lacked a more global ecosystemic approach such as the one addressed P.S

No abstract

Harvesting of the invasive Manila clam, Tapes philippinarum, is the main exploitative activity in the Venice lagoon, but the mechanical dredges used in this free-access regime produce a considerable disturbance of the lagoon ecosystem. An ecosystem approach to study the complex effects of clam harvesting was implemented using a trophic mass-balance model. The trophic relations in the ecosystem were quantified with a mixed trophic impact analysis and further evaluated by considering different explanations for the ''Tapes paradox'', which consists of the apparent population enhancement of Manila clams by dredging and the apparent nutritional advantages that this species receives from re-suspended organic matter. The key-role played by this introduced species is highlighted by a network analysis that indicates a ''wasp-waist control'' of the system by Manila clams. The model constructed to characterise the present state of the Venice lagoon ecosystem is compared with models produced for a reconstructed past lagoon and a projected future lagoon. The future model was obtained by simulating the elimination of clam dredging in 10 years. The three different models were compared using thermodynamic and informational indices. Simulating the elimination of clam dredging produced a 33% increase in artisanal fishery catches, carried out by means of static gears, even with no change in fishing effort. These simulations also forecast an increase in the mean trophic level of the artisanal fishery catches as a positive effect of eliminating mechanical clam harvesting.

Monthly changes in marginal increments, determined from alternation of macroscopic opaque and translucent bands, of Zosterisessor ophiocephalus sagittae were examined. The time of annulus completion proved to be annual, occurring around June both for females and males, and coinciding with the end of the breeding season and a period of larger body condition. The translucent band is likely to start depositing in summer. Since Sr/Ca peaks coincided with annulus completion, the most likely hypothesis is that the Sr/Ca ratio is influenced by the reproductive season.KEY WORDS: Zosterisessor ophiocephalus -Otolith -Marginal increments analysis -Sr/Ca ratio -Venetian Lagoon. ACKNOWLEDGEMENTS

Standard-Nutzungsbedingungen:Die Dokumente auf EconStor dürfen zu eigenen wissenschaftlichen Zwecken und zum Privatgebrauch gespeichert und kopiert werden.Sie dürfen die Dokumente nicht für öffentliche oder kommerzielle Zwecke vervielfältigen, öffentlich ausstellen, öffentlich zugänglich machen, vertreiben oder anderweitig nutzen.Sofern die Verfasser die Dokumente unter Open-Content-Lizenzen (insbesondere CC-Lizenzen) zur Verfügung gestellt haben sollten, gelten abweichend von diesen Nutzungsbedingungen die in der dort genannten Lizenz gewährten Nutzungsrechte. Terms of use: Documents in Comparison between Artisanal Fishery and Manila Clam Harvesting in the Venice Lagoon by Using Ecosystem Indicators: An Ecological Economics Perspective SummaryArtisanal fishery in the Venice lagoon is a multi-target activity with a long tradition. It was the main fishing activity till the late '80s when, after the introduction and spread of the Manila clam (Tapes philippinarum), the mechanical clam harvesting started. A mass-balance model of the lagoon ecosystem was developed using the Ecopath with Ecosim software. 73 scenarios, obtained by changing the fishing effort of the two different types of fishery, were used to explore their impact on the ecosystem. A set of indicators was applied in order to compare the two fishing activities. The results obtained showed that the two activities are strongly interlinked, even through they don't exploit the same resources. The mechanical clam harvesting could reasonably be considered to be the driving force; it is capable of determining the state of lagoon ecosystem. The above mentioned factors create a lot of conflict between the two types of fishery.