Effect of stocking density on growth and survival of juvenile Manila clams (Ruditapes philippinarum) farmed in suspended lanterns in a North Italian lagoon

To evaluate the effect of stocking density on pre-fattening growth and survival, Manila clam seeds from three different batches (B1, B2, and B3) were farmed in plastic net lanterns at two sites (western and northern) in Sacca degli Scardovari (Italy). Different stocking densities were compared, i.e. EXTRA (50,000 clams m-2), HIGH (30,000), and MEDIUM (20,000) for B1 and B2 clams; HIGH, MEDIUM, and LOW (10,000 clams m-2) for B3 clams. At the end of pre-fattening at the western site, the shell length decreased with increasing stocking density of both B1 (14.2 mm vs. 13.6 mm vs. 12.9 mm at HIGH vs. MEDIUM vs. EXTRA density; P < 0.001) and B2 (14.9 mm vs. 13.6 mm vs. 12.5 mm at MEDIUM vs. HIGH vs. EXTRA density; P < 0.001) clams. At the northern site, the shell length decreased in the following manner: MEDIUM to HIGH to EXTRA density for B1 clams (P < 0.001) and MEDIUM to EXTRA to HIGH density for B2 clams (P < 0.001). The same trend was recorded for B3 clams at both western (16.1 mm vs. 14.3 mm vs. 12.7 mm at LOW vs. MEDIUM vs. HIGH density; P < 0.001) and northern (15.6 mm vs. 13.9 mm vs. 13.2 mm; P < 0.001) sites. The stocking density did not affect the survival rate at the northern site and of the B3 clams, whereas the survival rate significantly decreased from MEDIUM and HIGH to EXTRA density for B1 (84.8 % and 85.4 % vs. 52.8 %; P < 0.05) and B2 (92.5 % and 87.6 % vs. 67.0 %; P < 0.01) clams at the western site where pre-fattening ended one week later than at the northern site. In the present conditions, clams in suspended lanterns reached the minimum sowing size (shell length 11 mm, weight 0.3 g) in 10 weeks. However, an increase in stocking density decreased clam growth and drastically increased mortality when water conditions became less favourable.