4.2. Control physiology prior to transport
In the present study, we used the dipnet procedure to quickly
sample small fish from large tanks, net pens and live-holds to assess
their physiological state. Our main challenge in providing a representative
fish sample was to minimize stress to both the fish being
sampled (and prevent an over-estimate of transport stress) and those
fish remaining that were eligible for subsequent sampling. Capture by
dipnet and handling, like chasing and confinement, triggers a stress
response in fish [14,46–53]. Thus, studies that consider primary stress
responses such as catecholamine and cortisol release can easily be
confounded by dipnet capture, which causes a primary stress response
even in control fish [54–57]. Time from dipnetting to anaesthesia was
typically 1–2 min, but did range in a few instances from a few seconds
(tanks at Dalrymple) to 5 min (vessel holds in rough seas). Fortunately,
peak post-stress plasma concentration is reached after 30–60 min [58].
Our sampling regime therefore precluded much of the acute effects of
sampling on cortisol release and provided a reliable indication of the
longer-term effects of transport. Plasma lactate, which is released more
rapidly from tissues, may have begun to show the effects of sampling
stress, although the peak response has a similar time course as plasma
cortisol [58].