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

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.

Phytoplankton–Macrophyte Interaction in the Lagoon of Venice (Northern Adriatic Sea, Italy)

“…From 1998 until 2003, the three investigated areas were almost uncolonized by macrophytes. It was extensively reported that in this period, the disappearance of submerged vegetation was at least partly due to the significant illegal clam fishing [69]. This activity strongly interacted the benthic compartment and contributed to the process of sediment resuspension and erosion, causing a decrease in water transparency and light availability.…”

Modeling species invasions using thermal and trophic niche dynamics under climate change

“…Successfully invading thermophilic species are expected to take advantage of increased average temperatures (Dulčić and Grbec, 2000; UNEP -MAP -RAC/SPA, 2008; Dulčić et al, 2010Dulčić et al, , 2011Brotz and Pauly, 2012;Zenetos et al, 2012;Pecarevič et al, 2013) but they also need to either find or make trophic niche space, such as by outcompeting native species (e.g., Pranovi et al, 2003). Meanwhile, some local native species might be negatively affected by higher temperatures, but may be able to exploit new trophic opportunities as a trade-off.…”

Lagoon Resident Fish Species of Conservation Interest According to the Habitat Directive (92/43/CEE): A Review on Their Potential Use as Ecological Indicator Species

“…On the fishing management plans, the activities impacting the bottoms have to be carefully assessed to avoid the sediment resuspension and the nest destruction. As an example, in the late 1990s, in the Venice lagoon, the clam harvesting activity with hydraulic and mechanical dredging systems that dug 10-30 cm deep furrows dramatically affected the biological equilibrium and the benthic communities [98,99].…”