Mussel production management: Raft culture without thinning-out

Abstract

Mussel raft culture as traditionally practiced in Galicia (Spain) represents an extreme case of aggregation, where culture density along growth ropes is maximized to achieve greater commercial yields. However, to maintain high growth rates during the entire cultivation period the initial density must be reduced through a process called “thinning-out” when rope weight increases by a factor of 10.In raft culture, the annual commercial cycle is superimposed on a production cycle that can last up to 18months. For this reason, the thinned-out mussel ropes can only occupy 75% of the usable surface area of the raft. Additionally, the rope handling process has a pronounced impact on production costs, affecting both labor and material costs.With the objective of optimizing economic yields in mussel raft culture, we conducted a study to compare a new technique without thinning-out with the traditional method, and we evaluated the effect of density on mussel growth for both techniques.For this purpose, three different densities were prepared for the no thinning-out ropes (800n, 1000n and 1200n mussels/m), and the other three for the thinning-out ropes (400y, 500y and 600y mussels/m).At the end of the experiment, average length and average live weight for mussels cultured without thinning-out were significantly lower than values observed for mussels cultured with thinning-out. However, the no thinning-out technique did not result in a lower total biomass production.The technique without thinning-out has a production cycle of 12months, which coincides with annual production. On the other hand, the culture with thinning-out technique has a 15-month cycle (seeding and thinning-out processes); thus annual production was 20% lower than total production. Under these conditions, production at the two highest densities (1000n and 1200n) was significantly higher than production at all densities with thinning-out.If we consider economic yield per raft per year, which also takes into account the higher price of mussels with increasing size, the difference between the two culture methods is smaller. Only the 1200n ropes still produced a higher economic yield than the 600y ropes. On the other hand, the shortening of the cultivation time and the elimination of the thinning-out reduce the production costs.In conclusion, the results obtained with the new culture technique without thinning-out at high seed densities enable substantial improvements in terms of biomass, economic yields, and operating costs for mussel production.