Mechanical clam dredging in Venice lagoon: ecosystem effects evaluated with a trophic mass-balance model

摘要

Harvesting of the invasive Manila clam, Tapesphilippinarum, is the main exploitative activity in theVenice lagoon, but the mechanical dredges used in thisfree-access regime produce a considerable disturbance ofthe lagoon ecosystem. An ecosystem approach to studythe complex effects of clam harvesting was implementedusing a trophic mass-balance model. The trophic relationsin the ecosystem were quantified with a mixedtrophic impact analysis and further evaluated by consideringdifferent explanations for the ‘‘Tapes paradox’’,which consists of the apparent population enhancementof Manila clams by dredging and the apparent nutritionaladvantages that this species receives from re-suspendedorganic matter. The key-role played by thisintroduced species is highlighted by a network analysisthat indicates a ‘‘wasp-waist control’’ of the system byManila clams. The model constructed to characterise thepresent state of the Venice lagoon ecosystem is comparedwith models produced for a reconstructed pastlagoon and a projected future lagoon. The future modelwas obtained by simulating the elimination of clamdredging in 10 years. The three different models werecompared using thermodynamic and informationalindices. Simulating the elimination of clam dredgingproduced a 33% increase in artisanal fishery catches,carried out by means of static gears, even with no changein fishing effort. These simulations also forecast anincrease in the mean trophic level of the artisanal fisherycatches as a positive effect of eliminating mechanicalclam harvesting.