Abstract:
Snapper (Pagrus auratus) is harvested extensively by recreational anglers throughout New Zealand but knowledge of discard mortality and sub-lethal stress is essential for sustainable management. Reflex impairment (RI) is a method which uses the presence or absence of reflexes in captured animals to produce a condition score. RI is thought to correlate with delayed mortality and other sub-lethal endpoints so could potentially be used as a simple in field tool to assess the condition and predict the fate of discards. In this thesis, a RI index for snapper was developed using reflexes consistently present in 20 unstressed snapper. Experiments were then carried out during summer and winter, where snapper were exposed to simulated angling as graded durations of chasing and air exposure, and then assessed for the presence or absence of reflexes. Snapper then had blood sampled for measurement of physiological stress (N = 160), or were monitored for mortality over 20 days (N = 180). RI was found to be correlated with the intensity of simulated angling, in particular with the duration of chasing. A physiological stress response was apparent in all fish exposed to simulated angling, but was only correlated with the duration of chasing and appeared to be more severe in summer. RI was therefore correlated with some measures of physiological stress, in particular those related to exhaustive exercise stress (muscle pH and plasma lactate). Despite increased RI and physiological stress, only two snapper monitored for mortality died. RI could not therefore be correlated with mortality. The findings of this thesis suggest that most angled sub-legal snapper are likely to be released with minimal RI and suffer low mortality in a recreational fishery. RI was correlated with the intensity of capture stress and some indices of physiological stress, and may therefore provide a simple and non-invasive method of measuring fish vitality in field studies.
