Growth Retardation in Farmed Chinook Salmon Oncorhynchus tshawytscha: Are Coping Strategies Involved as a Causal Mechanism?

Abstract

A form of growth retardation known as 'late runting' currently affects ~7% of chinook salmon (Oncorhynchus tshawytscha) farmed by the New Zealand King Salmon Company (NZKS Co. Ltd). Remedial action is a high priority, as these runts incur considerable costs to the company through loss of production. Anecdotal evidence suggests that an individual's stress responsiveness and adopted stress coping strategy may be associated with late runting, as no major pathologies have been found. Chronic activation of the physiological stress response, and thus prolonged elevations in circulating cortisol, could potentially decrease growth and fitness due to reduced feeding efficiency. Many salmonid species exhibit a high or low cortisol response to stressors, which are typically associated with a set of physiological and behavioural traits thought to characterise either a proactive (active) or reactive (passive) coping strategy. Our initial hypothesis was therefore based on the proposition that late runts would adopt a reactive coping strategy identified by low-risk behaviour, low standard metabolic rate (SMR) and high stress responsiveness. A range of experiments were thus performed in different life-stages of O. tshawytscha to resolve whether i) juveniles from families with high incidences of late runting are associated with one particular coping strategy (i.e. reactive) ii) experimental removal of social stress in sea cages would allow growth recovery in late runts iii) late runts and normal growing fish exhibit different levels of stress responsiveness in sea cages. Contrary to our initial expectations, evidence from the current study indicates that late runts might actually adopt a proactive coping strategy as revealed low cortisol stress responses and faster rates of recovery from stress events. There was however no evidence of any difference in risk taking behaviour and/or metabolism between high and low runting families at the hatchery. Growth retardation may therefore result through an inability to dominate and change feeding routines in the sea cage, highlighting the importance of environment on fish performance. Potential ameliorative measures include selectively breeding for desired traits, or perhaps more successfully, modifying the sea cage environment to encompass all coping style preferences which may help maximize the performance of all individuals within the population. However, further research is needed to substantiate the evidence from the current study, allowing for the implementation of actions of benefit to NZKS Co. Ltd, and the New Zealand salmon farming industry as whole.