Predictors of Weight Gain in the Rehabilitation of Eastern North Atlantic Grey Seals (Halichoerus grypus)

Abstract

As large adult body size is subjected to strong selective pressures, other processes can be compromised before growth when there is resource limitation or stress during the growth phase. Stress has been linked with both weight loss and weight gain in a variety of taxa, and some stressors, such as physical disturbance, are known to affect the development of important physiological processes, e.g., diving physiology in marine mammals. With 40% of the world’s grey seal (Halichoerus grypus) population on UK shores, grey seal pups are frequently admitted to rehabilitation centres within their first four weeks – a crucial growth phase – across the UK. Common rehabilitation practices (e.g., change in environment, social grouping, and disturbance) are known to cause stress to individuals in other contexts. Within grey seal rehabilitation, there are known factors outside the control of the centre that affect weight gain and survival, e.g., being underweight upon admittance. Yet, there are management practices, such as movement of individuals between pools (requires handling, movement, and regrouping) that may cause stress. The effects of these management practices on weight gain and mortality is unknown. By studying the behaviour of individuals, it can provide insights into how or why there are any changes in weight gain and mortality. This study investigates impacts of factors known to affect rehabilitation outcomes (e.g., admittance reason and intake weight) on weight gain and mortality in one rehabilitation centre over six pupping seasons. In a case study year, where data were available, this study also investigates impacts of unexplored factors: pool design and movement between pools, on individual-level weight gain (<70 seals) and behaviour (<39 seals). No evidence was found that management practices (pool characteristics, pool movements) affected weight gain, which was explained by duration of rehabilitation, and admittance reasons: underweight and injured. Pools varied in movement, rest, and head elevated behaviour (may indicate disturbance), suggesting some characteristics such as the visual blockade, might modify stress exposure or energetic costs during growth. No clear pattern across pools with respect to weight emerged, suggesting that although there are indicators of disturbance it might not be affecting weight gain.