A new criterion for multi-purpose platforms siting: Fish endurance to wave motion within offshore farming cages

Abstract

Early studies have described the existence of a critical speed in fish when challenged by unidirectional currents, often calculated as number of body length/second (BL/sec). In salmon farming, the criterion related to the disruption of normal swimming behaviour in cage and the consequent onset of intense aerobic challenging, marked by the start of ram ventilation, has recently been proposed with regard to unidirectional currents under tidal conditions. Unfortunately, this criterion appears only partially satisfactory within the specificity of the marine offshore environment, where wave motion dominates even at relevant depth, as in the case of long-lasting storms and associated waves in long wavelength, not uncommonly over 100 m of length.In the present work, the orbital velocities generated by wave motion at different depths are proposed as additional criteria, besides critical speeds to unidirectional currents, to define the likely limits of exposure of fish stocks farmed in offshore environments and consequently, to set up a range of sea states compatible with fish welfare. This finding can support the definition of an acceptable sea state for the whole Multi-Purpose Platform (MPP) design, where the farmed fish are considered as the weakest link in the platform chain design.