Hooking, time out of the water, and handling, all cause great stress in fish. Cortisol, an enzyme indicating stress, quickly increases and can remain elevated for a day. There can be increased heart rate, unnatural behaviour, and the fish are less likely to be able to reproduce, may suffer permanent tissue damage, and are more likely to die.
UK GOVERNMENT REPORT
"Almost all fish live the whole of their lives in water and show a maximal emergency response when removed from water, even for a very short period. This response includes changes in heart rate, increased production of adrenaline, noradrenaline and cortisol and vigorous muscle contractions which could result in escape and return to water. Some parts of the short-term emergency responses are shown in other disturbing circumstances." ... These changes, "often indicate fear in the fish."
"All of the scientific evidence concerning such effects makes it clear that the term stress is certainly relevant to fish and that the means by which stress effects are mediated are very similar to those in mammals." ²
“Catch-and-release studies have consistently shown that the duration of the angling event correlates positively with the magnitude of physiological disturbance and time required for recovery.” ¹
In Finland (Laitinen, 1994), it was found that transferring trout between tanks, followed by restraining them for only 5 minutes, caused their heart rates and breathing to increase for up to 3 to 4 days. Their overall activity was reduced for up to 2 days.
American and Canadian scientists (Cooke, 2000) found in experiments, that even after being caught briefly, the heart beat of bass doubled. The researchers reported that stress rises in bass when they are exposed to air, and increases their chances of dying, especially if they have been exhausted in their fight not to be caught. The scientists were particularly concerned about tournament weigh-ins, where fish are repeatedly handled and kept out of water. Fish health remains affected when they returned to water, and are likely to be less able to reproduce.
Salmon in Norway (Thorstad, 2003) were released by anglers in another experiment. Handling, exposure to air, and harm done by hooks, led to increased stress levels, unnatural behaviour and poor condition. The scientists added that these factors have previously been shown to increase the chance of death in fish.
For fish, the stress of being out of water increases with time, as was shown by a report for the Brazilian Government (Reiss, 2003).
In Alaska, researchers (Mekaa, 2005) discovered that angling caused cortisol, an enzyme indicating stress, to rise after only 2 minutes. Cortisol continued to rise, and took 24 hours to return to normal. The stress of repeated angling adds to the harm done to fish health. In this study, a third of salmon caught bore at least one scar from angling. Angling places enormous stress on the fish, and the longer they are on the hook, the greater their chances are of dying as a result.
Largemouth bass were hooked by anglers in an experiment in Canada (Thompson, 2008). They were placed in a tank on a boat, and blood samples were taken. They were then exposed to air, ranging from 1 second to 15 minutes, mimicking what they may be subjected to by anglers. The fish were afterwards released with a radio tracking device.
After being exposed to air, sub-lethal physiological disturbances were found, together with impairment of their behaviour. AST¹ was significantly elevated in the fish. The enzyme AST is a reliable indicator of permanent tissue damage. Subsequent to their release, the fish that had been exposed longer to air, showed abnormal behaviour and remained longer at the release site.
The scientists said that nesting males were extremely vulnerable to capture, as they care for their young, defend them against predators, and fan their eggs to provide extra oxygen and keep them free of silt.
After rock bass were removed from the water by anglers, their heart beat went up severely. After just 30 seconds, the fish took 2 hours to recover. When they were taken out for 3 minutes, it took a full 4 hours for them to recover.