Øyvind’s PhD research is within the SubCage Biology project. In addition to research investigating the tolerances of both salmon to submergence described earlier, specific experiments are aimed at determining safe limits for raising and sinking Atlantic cod in submerged cages.

Background
Farmed cod forced rapidly to the surface from 20 m depth in sea-cages experience severe barotrauma, indicated by ballooning of the swim bladder and leading to poor welfare and, on occassion, mortality. In the relatively new cod farming industry, a distinct trend is to use deeper nets to provide greater volume and allow cod to access deeper waters where environmental fluctuations, such as temperature, are less frequent. If submersible cages are to become a common farming method, the rate of lowering and lifting of the cage will be a critical factor for farming of fish with closed (physoclistous) swim bladders.

In contrast to the open swim bladder of salmon, the closed swim bladder system in cod causes limitations for rapid vertical movement. The time needed for cod to double the pressure in their swim bladder increases with increasing pressure, from a few hours near the surface, to several days at hundreds of metres. The rate of gas secretion also increases with temperature.

Deflating the swim bladder is a faster process; a 50% pressure reduction takes 3-4 hours (Harden Jones & Scholes 1985, Strand et al. 2005). Buoyancy challenges related to pressure reduction are greatest near the surface where the largest relative change in pressure occurs. A fast ascent corresponding to 60-70 % pressure reduction caused swim bladder rupture in cod kept in a pressure chamber (Tytler and Blaxter, 1973). Righton et al. (2001) followed individual cod in the sea, and suggested 25% reduction and 50% increase in pressure as limits for cod to maintain proper buoyancy control. Based on observed maximum pressure reduction chosen by free swimming DST-tagged cod in a 35 m deep cage, a pressure reduction of 40% is suggested by Kristiansen et al. (2006) as the upper limit for handling farmed cod. In this study we aimed to make a safe procedure for lifting of Atlantic cod kept in sea cages.

Material and methods
We investigated whether a 40% pressure reduction relative to neutral buoyancy depth was a safe limit for lifting of cod, if this was independent of depth and temperature (16°C and 4°C), and how long an acclimation time was necessary to acquire neutral buoyancy at the new depth. A submerged sea-cage (5x5x2.5 m3) containing 100 cod (~1200 g) with neutral buoyancy at start depth, were lifted to 40% pressure reduction from three depths (10, 20, 30 m; Figure 1). In the same experiment, we also estimated the swim bladder secretion rate and time to neutral buoyancy after a rapid descent near the surface to 10, 20 or 30 m (Figure 1). Swimming behaviour (tilt angle, swimming speed, tail beat frequency) based on video observations was used to judge positive or negative buoyancy and coping ability. A feeding test was used to evaluate stress levels and behavioural control at fixed intervals after the pressure changes.

Figure 1 Submergence and lifting steps for Atlantic cod in the experiment.

Results and discussion
In general, cod coped with all of the vertical lifting and sinking steps in this regime through behavioural adaptation. Submergence from the surface to 20 and 30 m caused negative buoyancy, indicated by tilted swimming (head up) and extended periods where cod lay upon the net bottom, especially during dusk and dawn. After each lifting step, positive buoyancy was indicated by increased swimming speeds and ‘yo-yo’ swimming where cod drifted upwards without swimming until they reached the top of the cage and then swam downwards to the bottom of the cage with powerful tail beats. A duration of 2-4 hours was normally sufficient for cod to return to a good appetite response and more relaxed swimming behaviour. Neutrally buoyant cod therefore appear capable of being rapidly lifted vertically in sea-cages for distances equivalent to a 40% pressure reduction, independent of depth. Rapid descent of neutrally buoyant cod from the surface to 30 m sometimes resulted in large amounts of cod resting upon the net bottom, which may have implications for both cod welfare and loadings on net cages.