Abstract
The effects of mechanical stress in the Venus clam Chamelea gallina during hydraulic dredging were assessed in both laboratory and field studies in order to measure physiological biomarkers at organism level (clearance rate, respiration rate, scope for growth, and survival in air test). In the laboratory, mechanical stress was simulated by shaking clams in a vortex mixer. In the field, clams were collected seasonally at two sites along the northern Adriatic coast (Lido and Jesolo) and four levels of stress were applied: the highest was that used in commercial fishing (i.e. high water pressure and mechanised sorting) and the lowest manual sampling by SCUBA divers. Survival in air was the most sensitive biomarker in evaluating mechanical stress in the laboratory. Clearance rate also decreased significantly when shaking was applied. Field results indicated that high water pressure and mechanised sorting affected clearance, scope for growth and survival in air, all showing decreasing trends as mechanical stress increased at both sampling sites. The detrimental effects of mechanical disturbance may be emphasised depending on season, when exogenous and endogenous stress increases. A potential risk is highlighted mostly for undersized clams that are fished and then discarded.
Highlights
The Venus clam Chamelea gallina is an infaunal filter-feeder that is widespread on well-sorted fine sand in shallow waters (0-10 m depth) and widely distributed throughout the Mediterranean and the Black Sea
Hydraulic dredging of C. gallina has progressively increased since its introduction in the 1970s, reaching a peak in the early 1980s, with 100000 t of clams harvested per year (Froglia, 1989)
The reduced numbers of C. gallina along the northwestern Adriatic coasts are mainly due to intense overexploitation of the resource, the great number of vessels employed, and the high efficiency of the gear used for harvesting, with additional detrimental effects due to the inappropriate and insufficient management procedures adopted in the past to protect clam stocks (Froglia, 2000)
Summary
The Venus clam Chamelea gallina is an infaunal filter-feeder that is widespread on well-sorted fine sand in shallow waters (0-10 m depth) and widely distributed throughout the Mediterranean and the Black Sea. In the North Adriatic, C. gallina is caught using hydraulic dredges, consisting of a cage of steel bars in which the organisms are collected. Commercial-sized clams, larger than 25 mm, are sorted and retained, whereas undersized clams and non-target species are discarded. Hydraulic dredging of C. gallina has progressively increased since its introduction in the 1970s, reaching a peak in the early 1980s, with 100000 t of clams harvested per year (Froglia, 1989). The reduced numbers of C. gallina along the northwestern Adriatic coasts are mainly due to intense overexploitation of the resource, the great number of vessels employed, and the high efficiency of the gear used for harvesting, with additional detrimental effects due to the inappropriate and insufficient management procedures adopted in the past to protect clam stocks (Froglia, 2000)
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