Clam culture in the Venice lagoon: stock assessment of Manila clam (Venerupis philippinarum) populations at a nursery site and management proposals to increase clam farming sustainability

Abstract

-The Venice lagoon is the main Manila clam (Venerupis philippinarum) production site in Europe. Following the unsustainable exploitation of this site (free-access fishery) local authorities have been striving to regulate fishing activities and most clams are produced using mollusc culture techniques based on wild spat harvesting and on its seeding in appropriate, designated culture areas. Therefore, the quality and abundance of wild clam stocks are critical factors that could limit the full development of this activity. The present study aims at assessing Manila clam stocks (spat, adults per square meter) using ordinary kriging, in an area of the Venice lagoon located near the Porto Marghera industrial zone. Previous studies have identified this area as one of the most important natural clam settlement sites in the Venice lagoon. The study area was approximately 1085 ha; 46 sampling stations were examined. Stock assessment (animals were grouped by size and age group), biomass and correlation with sediment texture were evaluated. The results obtained and comparisons with studies from the literature enabled an estimation of clam recruitment potential and an analysis of the conservation status of clam stocks. Thereafter, in view of a full reclamation and an ecological restoration of the study area, different measures are proposed for the sustainable use of this area that include the establishment of a biological protection area and a shared and rational management of the nursery area; these measures could potentially increase the sustainability of clam culture in the Venice lagoon.

Management Models of the Manila Clam (Ruditapes philippinarum) Fisheries in Invaded European Coastal Systems

“…This work (Caill-Milly et al, 2021) also highlighted some difficulties such as the available funds to schedule regular surveys (with a financial contribution supported by the professionals which is the structure carrying out the campaign from funders) and available funds for research programs applied to local issues. All the actions implemented in Arcachon to ensure the sustainability of the Manila clam stock illustrate that this species is not perceived as invasive (Caill-Milly et al, 2021), which is not the case in other countries where this species occurs such as Portugal (Ramajal et al, 2016;Carvalho, 2017), Spain (Juanes et al, 2012; and Italy (Pellizzato and Da Ros, 2005;Vincenzi et al, 2006Vincenzi et al, , 2011Brusà et al, 2013).…”

“…This model resulted in the rational management of this resource exploitation (Brusà et al, 2013). The concessions granting process could still be improved upon by evaluating the productivity potential of different locations in the lagoon and thus improving the transparency of decision-making and the sustainability of the activity (Vincenzi et al, 2011).…”

Sediment Carbon Variations in the Venice Lagoon and Other Transitional Water Systems of the Northern Adriatic Sea

“…Indeed, in the latter TWSs, the availability of Corg was reduced both by (i) high concentration of clams (on average 3-6 kg m −2 [7]) that filtered the organic matter present in the water column near the surface sediment, and (ii) sediment disturbance caused by clam-fishing with hydraulic and mechanical dredges, which mixed the first 10-15 cm of surface sediment releasing nutrients and organic matter in the water column [10]. The above-mentioned impacts are clearly observed by examining the data of carbon distribution obtained in the central basin of the Venice Lagoon, during different times: (a) 1987: The lagoon was dominated by a massive algal growth with a biomass ranging from 5 to 20 kg fwt m −2 in 65 km 2 out of 132 km 2 of the whole basin [35]); (b) 1993: Macroalgal biomass decreased from 4.78 to 0.69 kg m −2 fwt [7,24,35]; (c) 1998: Clam-fishing occurred in the entire central lagoon and the macroalgal biomass decreased further to 0.11 kg fwt m −2 with significant decrease of nutrient concentrations in surface sediments [10]; (d) 2003: Clam-fishing reached its excessive impact and lasted until 1999-2000 when clam production reached 40,000/60,000 tons y −1 [39]; (e) 2011: Clam-fishing declined to ca. 2000 tons y −1 and the lagoon showed a progressive ecological recovery [7]; (f) 2014: Aquatic angiosperms and macroalgae of high ecological value started to recolonize the lagoon [7]; and (g) 2018: The lagoon recovery continually increased and the aquatic angiosperm Ruppia cirrhosa, which disappeared in the open lagoon during the massive macroalgal development [40], started to re-colonize the choked areas.…”

A history of invasion: COI phylogeny of Manila clam Ruditapes philippinarum in Europe

“…In Italy, Manila clam spread occurred rapidly, and quickly populations became naturalised (Pellizzato, 1990) thus its exploitation became the most economically important fishing activity, especially in the Venice Lagoon (see Boscolo Brusà et al, 2013 for a complete list of references). However, due to the initial lack of reliable regulation and unsustainable exploitation of fisheries resources, there has been a constant decrease in clam production (Boscolo Brusà et al, 2013), which determinated a recent transition from clam fishing to clam farming activities, and to the rational management of natural spat (Boscolo Brusà et al, 2013).…”

“…Currently, in the Venice lagoon most clam harvesting is carried out in licensed areas directly managed by farmers (Boscolo Brusà et al, 2013), using seeds derived from natural spat. This system has been already established in other Northern Adriatic Sea lagoons, like Goro lagoon, where the production remained stable for almost 3 decades (Bartoli et al, in press).…”