Observations of cod behaviour reveal insights into the function and properties of the swimbladder under pressure

Atlantic cod (Gadus morhua L.) is a highly adaptive boreal species with a broad distribution through different ecosystems in the North Atlantic. Diet, environmental factors and population dynamics vary enormously throughout this range, and behaviours are therefore extremely plastic. However, the behaviour of cod will always be constrained by physiology, be it temperature tolerance, swimming speeds or buoyancy control. Cod, like other gadoids, are physoclists, i.e. they have a closed compliant swimbladder that can be filled or emptied in order to achieve neutral buoyancy at any depth. Pressure changes caused by vertical movements lead to expansion and compression of the swimbladder as individuals ascend or descend respectively. Over time, changes in the volume of the swimbladder will occur so that individuals can maintain neutral buoyancy with the minimum effort. Here, we have used data collected by cod tagged with electronic tags and released in five different regions of the NE Atlantic to investigate the neutrally buoyant descent rates of cod that are moving from shallow residence depths to deeper ones. First, we describe the patterns of vertical movement and rates of descent in the recuperation period following tagging, when the swimbladder is re-inflating to achieve neutral buoyancy at capture depth. Using this recuperation behaviour as an indication of swimbladder inflation during extreme depth changes, we then describe similar patterns of natural behaviour during medium-term transitions from shallow to deeper depths as cod migrate between different areas. Third, we assess the significance of behaviour of this kind, its relationship to environmental variables and how it varies between regions and seasons