6. Maintenance and controls

This chapter illustrates the main types of damage that can occur on a surface net- pen farm site, the source of the problems, and the maintenance steps necessary for good farm management. Structural damage can cause major economic losses, through material costs, time for qualified workers to rectify any problems, and above all, loss of inventory owing to escaped fish.

The first rule to be followed by those who want to undertake net cage culture is to pay scrupulous attention to the installation and maintenance of all the farm site's components. Moreover, small imperfections or irregularities in components must not be ignored, and instead must be quickly remedied. The failure of a single component can have repercussions on the entire structure.
The marine environment has a high number of physical, chemical and biological forces at work, a high number of variables involved, and it is constantly in motion. This motion affects everything submerged or on the water surface.

The physical forces that affect a net cage are grouped into two general types:
? Static forces include gravity (which pushes downwards according to the mass) and buoyancy (which pushes upwards, according to the density) of the site's components.
? Dynamic forces are mainly horizontal, and depend on the wind, the waves and the current.
In both cases, these forces, acting together, affect the whole farm structure, which is thereby constantly stressed in accordance with the action-reaction principle. These processes can cause abrasion and breakage of the various components.
Net cages are also exposed to chemical and biological processes, including:
Oxidation (rust) of metallic parts (chains, shackles, rings or plates), which corrodes the materials, making them thin and weak.
• Chemical degradation from the saline environment, which attacks the plastic polymers of ropes and nets, reducing their breaking load.
? Biological activity, especially marine organisms and biofouling (mussels, barnacles, worms, etc.). Biofouling adds considerably to the structures' weight and, in the case of the netting, by increasing resistance to the movement of the currents.