Fish Mortality – Catch And Release

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Catch-and-release (CR) angling is a hot-button topic for many fisherman, with as many viewpoints as there are flies in my fly box. After a recent debate with a fellow fisherman, I was interested in learning more about the potential impacts of CR. My ultimate goal was to better equip myself and other anglers with real, scientific data so that we can make informed decisions on how we actually go about conducting CR.

As anglers, we must follow sport fishing regulations, which are (usually) based on scientific data and set for us by fisheries managers. Regulations are legally binding and can restrict our use of a fishery by way of seasonal closures, area closures, bait bans, obligatory CR and harvest limits, such as slot sizes and bag limits.

However, there exists a large grey area in which our actions as fisherman are not dictated by law. Should I use bait or artificial flies if both are permitted? How many fish is it okay to accidently foul hook before changing to a smaller lure? Is it sporting to use a fish finder? These are the types of questions that spur heated debates between fishermen on shorelines, pubs and online forums across North America. The reason these questions can be so inflammatory is because there are no right answers, just opinions that are the products of our own personal code or ethics. Ethics are largely based on subjective data or perceptions, such as observations made during our own experiences, opinions and stories from fellow, respected fishermen and personal views on animal welfare and resource use. Our ethics may also incorporate objective data, like the findings of scientific studies.

CR is one of the most controversial topics. There is a broad spectrum of how CR actually plays out in the real world. For example, a fisherman may:

  • only CR a few fish – does not fish out each hole or pool
  • CR all available fish
  • CR until all legally harvestable fish have been taken and then stop fishing, and
  • continue CR after all legally harvestable fish have been taken

All forms of CR can be lethal to fish or impact the sustainability of a fishery because of cryptic mortality. Cryptic mortality refers to fish that are released alive, but die soon after because of stress or physical damage inflicted during hooking, retrieval or release. Some fishermen feel very strongly about which form of CR is acceptable for users of a fishery. If debated, tempers can flare high if the fishery is naturally small, in decline, has high fishing pressure or if there is a strong personal attachment to a certain river, lake or species of fish.

Another source of contention is the differing perceptions on how much CR actually impacts a fishery.

Fortunately, it is relatively easy to take a critical look at the consequences of your actions as a fisherman. Your direct impact on fish abundance can be boiled down to the following equation:

Number of fish harvested + (number of fish released X probability of cryptic mortality) = number of fish killed

I purposefully excluded the importance of which fish are killed – different ages, gender, etc. and possible negative effects of CR on fish behaviour to keep this discussion simple.

It is clear that mothballing your rod and reel is the only guaranteed way to eliminate your impact on a fishery due to fishing. If, like me, you think that response is extreme, you can voluntarily limit the number of fish you kill by reducing one, or a combination, of the components in the above equation. The obvious approach is to harvest fewer or no fish, or catch fewer fish throughout the day. Alternatively, you can reduce the probability of cryptic mortality, meaning the chance that an individual fish will die after you release it. An estimate I often hear is that cryptic mortality is around 10 per cent, meaning that if I release 100 rainbow trout, 10 of them will eventually die from stress or trauma. However, cryptic mortality rates actually vary widely and can be much higher or lower. The estimate of 10 per cent does not take into account important differences in fishing techniques, gear, species or environment, such as water temperature.

Fortunately, there have been numerous scientific studies that have investigated the effects of these factors and many are available for free on the Internet. The methods used in these studies are fairly consistent. Typically, biologists catch a particular fish species using a particular type or types of gear, record water temperature, handling time, where the hook pierced the body and note any bleeding or physical damage. Fish are then kept in pens to observe any mortality within hours to days after release, or fish may be implanted with radio transmitters so their survival can be monitored over the following weeks to months.

I reviewed 16 CR studies to seek out scientifically-tested methods that reduce cryptic mortality. Specifically, I only reviewed studies that investigated the effects of single versus treble hooks, barbed versus barbless hooks, flies versus bait and water temperature. I excluded studies that took place during competitive fishing tournaments. I focused on three popular sportfish species in western Canada: rainbow trout, walleye and northern pike. Gear type may impact these species differently because they vary in body shape, such as the size of mouth, and behaviour, such as aggressiveness when feeding. This is by no means an exhaustive review. Mortality rates of other species, or these species in different environments, may be different than what is discussed here.


Rainbow trout

Rainbow trout are among the top three most common sportfish species caught in British Columbia and Alberta (Survey of Recreational Fishing in Canada 2010). They are stocked extensively across western Canada to provide opportunities to catch fish closer to home, catch trophy-sized fish and to diversify the angling experience. Researchers investigating rainbow trout survival found that mortality of fish caught using flies ranged from four per cent to 10 per cent and mortality of fish caught using natural baits ranged from 32 per cent to 64 per cent (Schisler and Bergersen 1996; Stringer 1967; Shetter and Allison 1955). Trout often swallow bait hooks deeper than flies, resulting in greater damage to sensitive areas such as gills, gill arches and the throat.

Additionally, mortality when using single hooks (10 per cent) has been found to be greater than when using treble hooks (five per cent), although at low temperatures mortality rates were similar (Stringer 1967; Klein 1965). Single hooks may result in greater internal damage than treble hooks, because treble hooks often do not fit into the small mouths of rainbow trout. In contrast, DuBois and Dubielzig (1965) observed little difference in mortality between single and treble hooks, and no significant difference when using barbed and barbless hooks (five per cent mortality for barbed treble hooks, five per cent for barbless treble hooks, three per cent for barbed single hooks, and two per cent for barbless single hooks). It is also reported that mortality of trout played to exhaustion increases as water temperatures increase (Dotson 1982).



Walleye are the most predominant freshwater species caught and harvested in Canada (Survey of Recreational Fishing in Canada 2010). Walleye hooking mortality is generally lower than that experienced by other species (Muoneke and Childress 1994). For example, one study reported no delayed mortality for walleye caught using single hook, artificial lures and natural baits (Parks and Kraai 1991). Similarly, Payer (1989) observed no mortality for walleye caught using artificial lures, although 10 per cent of fish caught using leeches died soon after release because of hooking damage to the throat and gut.

As with rainbow trout, Schaefer (1989) found no significant difference in mortality when using barbed versus barbless hooks.

Environmental conditions also influence mortality rates of walleye. One study reported no mortality when water temperatures were less than 20 degrees Celsius, but up to 12 per cent mortality when temperatures were greater than 20 degrees Celsius (Reeves and Bruesewitz 2007). Catching walleye at water depths over 30 metres may also increase mortality due to distended or damaged swim bladders (Fletcher 1987).


Northern pike

Northern pike is a popular sportfish species across western Canada. They are usually fairly easy to attract and capture because of their aggressive feeding behaviour. Previous studies have found no statistical difference in mortality when pike are captured using different gear types (Arlinghaus and others 2008; Burkholder 1992). For example, mortality of pike caught with single hooks was zero per cent, large treble hooks was three per cent and small treble hooks was five per cent (Burkholder 1992). Other studies have also reported mortality rates less than three per cent (Arlinghaus and others 2008; Dubois and others 1994). In contrast, Falk and Gillman (1975) reported CR mortalities of five per cent and 11 per cent for pike caught on barbed and barbless hooks, respectively. I was unable to find any studies investigating the effects of water temperature on survival.

Some general patterns emerge from this review. Using bait increases the chance of cryptic mortality, but by different degrees depending on species. Barbed and barbless hooks result in similar mortality rates. Lastly, high temperatures can vastly increase the number of fish that die after release.

What does this actually mean to you, the CR fisherman?

  • Be aware that fish released alive may die later due to stress and injury.
  • You can reduce how many fish you kill by lowering your effort on hot days and changing gear type (while following regulations, of course).
  • Although not reviewed here, practising good handling techniques, such as using de-hooking tools and not touching the gills, may reduce mortality.
  • In many cases, reducing cryptic mortality is voluntary – for example, using artificial flies instead of bait. Fisheries managers are well aware of the impacts of CR and do their best to incorporate these effects into management models and regulations.

Is it necessary to voluntarily reduce the number of fish you kill? This question dips back into the realm of subjectivity, and so will be left up to you. The only personal thought I will share here is that I am happy to be part of a large community of anglers that care about fish conservation, regardless of their views on CR fishing.

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