Fish Mortality and Angling Methods

Fish Mortality and Angling Methods. A rainbow hen about to be returned to the Ohau Canal. How likely is it to survive the experience?
A rainbow hen about to be returned to the Ohau Canal. How likely is it to survive the experience?

Fish Mortality and Angling Methods – Fly Fishing Versus Spin Fishing

by Don Jellyman

Some time ago I inadvertently became involved in a spirited discussion about the merits of fly fishing versus spin fishing. Fish Mortality and Angling Methods.

One of the most vocal claims of the fly anglers was that spinning lures with treble hooks were much more damaging to fish than were flies. Hence, undersized fish caught by spinning and then released would have a much higher mortality rate than would equivalent fly-caught fish.

As I was in danger of becoming the meat-in-the-sandwich in the discussion, I professed my ignorance but determined to do some investigation. I have since collected eight North American scientific papers (1970-1986) on this and related topics and the following information is gleaned from those sources.

No-kill fisheries

The concept of no-kill (catch and release) trout fisheries is widely established in North America. It obviously works, as evidenced by the fact that in part of Yellowstone River, USA, cutthroat trout are caught an average of 9.7 times per year (see Freshwater Catch 32). Similarly, in parts of Colorado, 50-75% of all trout in catch and release areas have visible hooking scars. So, is one method of capture to be preferred over another?

Interestingly, the consensus in the five studies which dealt with hooking mortality was that there is no significant difference between fish caught by fly, lure (spinner), or worms – provided that the bait was not swallowed. Further, there are no differences between barbed and barbless flies and single and treble hooks.

For example, from a sample of 570 cutthroat trout, mortalities by fishing method within 10 days of capture were: barbed fly 4.0%, barbless fly 3.3%, barbed treble 2.7%, barbless treble 6.0%, worm not swallowed 8.2%, worm swallowed 73.0%. controls (electric-fished) 4.8%. The obvious ‘misfit’ in these results is for swallowed worms.

A study of landlocked Atlantic salmon also gave a figure of 73% mortality within l4 days for deeply hooked fish; if the hook was removed only 10% survived, while if the trace was cut and the hook was left in, 43% survived. An analysis of hook penetration of treble hooks showed that 74% of all fish caught had only one hook penetrating, 22% had two hooks, and 4% had all three hooks. For the majority of fish caught then, a treble hook acts similarly to a single one.

Stress and Fish Mortality

Getting caught is obviously a stressful experience for fish. Biologists have investigated ‘stress’ in relation to fish size, spawning and non-spawning condition, water temperature, and the length of time the fish are played before landing. Stress has been variously measured as the mortality rate, or as physiological stress as seen in blood chemistry.

Overall, the results from these investigations are more variable than those from the hooking experiments, mainly because different degrees of exhaustion were induced. For example, two similar but separate studies on rainbow trout gave 87% and 7% mortality respectively.

However, various studies have shown the following:

  • Physiological stress is greater in hatchery fish than in wild fish and is also greater in larger than in smaller fish. Up to three days are required for blood chemistry to equilibrate.
  • Higher mortalities occur at higher temperatures.
  • An experiment on playing time for cutthroat trout showed no differences in mortality for fish played for 5 minutes or 10 minutes. In another study on cutthroat trout, where fish were played until unable to maintain their normal attitude in the water, mortality over 30 days varied from 0-9% according to temperature.
  • Reproductively mature cutthroat trout were found to be as equally resistant to hooking and handling as were non-mature fish.

Mortality also varies with species but comparisons between experiments are difiicult because of differences in technique. Two studies. however, included both brown and rainbow trout and in both studies the mortality rate for rainbows was several times greater than for browns.


So what does all this mean for the New Zealand angler?

There is no evidence that mortalities of spinner-caught fish are any greater than fly-caught fish. Similarly, whether the hooks are barbed or barbless makes no difference. except that fish caught on barbless hooks could presumably be returned more quickly to the water. From the point of view of hooking mortality, there is no basis for reserving reaches of waterways as ‘fly only’.

As rainbow trout are more vigorous fighters than brown trout, the mortality of released rainbow trout will exceed that of browns, and increasing temperatures will accentuate this trend.

Deeply hooked fish, or fish which bleed profusely, especially from the gills, will almost invariably die if released.

Tips to reduce fish mortality

So angler, if you want to return that fish, don’t play it to complete exhaustion; make sure your hands are yet before handling it; hold firmly but don’t squeeze and don‘t hold it through the gills; return it gently and if it can’t maintain its own position hold its head into the current until it can if in a lake it may be necessary to ‘swim’ the fish by walking along with it, but don’t do it in the warm shallow margins.

Let’s not fool ourselves, wild trout have substantial recuperative powers and survive some horrendous injuries – eel and shag bites, missing eyes, misshapen jaws, deformed fins, and so on. They also withstand the rigours of an environment which may change within hours from a gentle flow to a raging flood. Still, trout are not indestructible. The old gift-shop maxim comes to mind “handle with care”.

This article first appeared in Freshwater Catch, April 1988, No 35. At that time, Don Jellyman was a scientist with MAFFish at Christchurch.

There is no evidence that mortalities of spinner-caught fish are any greater than fly-caught fish.
There is no evidence that mortalities of spinner-caught fish are any greater than fly-caught fish.

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