Reburial potential and survivability of the striped venus clam (Chamelea gallina) in hydraulic dredge fisheries

Abstract

The striped venus clam (Chamelea gallina) is the main edible bivalve living in Italian waters. According to Regulation (EU) 2020/2237, undersized specimens (total length of the shell, < 22 mm) must be returned to the sea. C. gallina specimens of different size classes that had undergone hydraulic dredging and mechanized sorting were analysed for reburial ability in a laboratory tank and for survivability in the laboratory (135 clams, 21 days) and at sea (320 clams, 15 days). In the tank experiments, the reburial times (T50 and T90) and the upper (+) and lower (−) confidence intervals (CIs) of the whole sample were about 4 h (CI+ 4.4, CI− 3.6) and 8 h (CI+ 8.2, CI− 7.7), respectively, and were significantly shorter for the medium-sized clams (22–24.9 mm) than for the smallest (< 21.9 mm) and the largest (> 25 mm) specimens. For the field survivability experiments, clams under and above the minimum conservation reference size were placed in separate metal cages. Survival rates were 94.8% and 96.2% respectively in the laboratory and at sea, without significant differences between the two experiments or among size classes. These findings conclusively demonstrate that C. gallina specimens returned to the sea have a very high survival probability and that they can contribute to mitigate the overexploitation of natural populations.

Chamelea gallina reproductive biology and Minimum Conservation Reference Size: implications for fishery management in the Adriatic Sea

“…Italian clam fishery is the sole fishery where the number of vessels and operators has not declined in the past four decades [ 6 ]. The biological and management factors that allowed the clam fishing stocks to withstand the high fishing effort include: the high reproductive potential (clams of 22 mm TL produce 1.4 × 10 5 oocytes/female, a fairly high fecundity whose order of magnitude is shared with 25 mm clams) and the multiple spawning events occurring within the same reproductive season; the early maturation, since all clams > 15 mm TL are sexually mature within the first year of life; the closure of the area within 0.3 NM of the coast (Regulation (EC) 1967/2006 [ 10 ]) to dredging activity; this measure has halved the area previously suitable for clam harvesting and provides a large area (581.7 km 2 ) where a huge amount of breeders contribute to the reproductive output of the population; the daily quota (reduced to 400 kg/vessel from the previous 600 kg/vessel; Delegated Regulation (EC) 2016/2376 [ 12 ]) has strongly reduced the fishing effort, because the boats take less time to achieve the predetermined quota; the two-months fishing closure adopted in summer during the peak of reproduction; the technical measures set for the fishing gear (for both the dredge and the sieve on board) reduce the catch of juveniles and the fraction below 22 mm TL almost to zero [ 73 ]; the setting of restocking areas, entered into force in 2017, where fishing is banned and where fishermen are required to discard undersized specimens harvested elsewhere (Delegated Regulation (EC) 2016/2376 [ 12 ]); the high survival rate of C. gallina (higher than 95% [ 74 ]); the specimens returned to the sea can grow and contribute to the spawning fraction of the population; the seeding and fishing area rotation applied by Management Consortia, the bodies responsible for fishery management, make the exploitation more sustainable and responsive. …”

“…the high survival rate of C. gallina (higher than 95% [ 74 ]); the specimens returned to the sea can grow and contribute to the spawning fraction of the population;…”