We love eating fish! However, it doesn’t mean we need to keep every big fish we catch. A quick measurement, a couple of pictures, and a healthy release preserve the memory and allow the fish to fight another day. Trophy replicas can be made from pictures and measurements.

If you practice catch and release, there are a few things you can do to increase the odds of survival. These are based upon scientific research, experience across the angling community, and common sense, but that doesn’t mean they are without controversy. You will hear arguments for and against several of these recommendations. My goal is only to share them with you. What you do with this information is your choice.

• Depth – ‘barotrauma’ is the term used to describe the negative impacts caused when an organism is brought up rapidly from deep water. Think of “the bends” in scuba divers. A study published by the Minnesota DNR showed a relationship between depth and mortality – fish caught at ~30’ had an 8% mortality rate, at ~40’ it went up to 18%, and at ~50’ it was 35%. Similar results have been found in studies of crappie. A good rule of thumb – if you plan to release fish, target them no deeper than ~30’. Note – some species of fish, due to their anatomy and physiology, appear to be less sensitive to barotrauma. A few examples are lake trout and whitefish.
• Water temperature – you know that feeling when you jump in a lake and hit the cold layer underneath the warm surface water? That is the thermocline, an area of rapid temperature change. There can often be a swing of 20-40 degrees during the heat of summer. Dramatic shifts in temperature can cause physiological stress on fish. This is less of an issue in the spring and fall when surface temperatures are lower.
• Time out of the water – Long duration out of the water is perhaps the single greatest cause of mortality in released fish. Get those fish back fast. If possible, consider unhooking the fish in the net, without removing them from the water.
• Duration of the fight – fighting a fish to the point of near exhaustion can push it past the point of no return—their muscles fatigue just like ours. Consider increasing the size of your fishing line, which allows you to put more pressure on the fish, bringing it to the boat more quickly.
• Hands – before touching a fish, wet your hands. This decreases the risk of damaging their protective slime coats.
• Avoid the gills – if holding a fish by the operculum (gill cover), make sure you keep your fingers away from the gills, they are extremely delicate. A fish bleeding from the mouth due to hooking may not be a big deal, but a fish bleeding from the gills has a much greater risk of dying.
• Don’t ‘stringer’ fish – stringers and live wells should be one-way streets. Don’t sort fish throughout the day.
• Ice fishing – don’t let the fish roll around in the snow and become a powdered donut. Also, if fishing in extremely cold temperatures, be aware that the eyes and fins of a fish can freeze very quickly. Send that fish back down the hole ASAP.
• Hold big fish horizontally – this topic usually stirs a strong debate among anglers. I don’t exactly know what the right answer is. However, there is evidence that holding a fish vertically can cause damage to its internal organs and/or its delicate jaw and gill structures. The risk increases with the size of the fish. Remember, this might be the first time the fish feels its own weight. Their bodies were not designed to be lifted by the jaw or gill plate. It is better to hold the fish horizontally, with one hand under its belly. This supports the internal organs and distributes the weight more evenly, taking pressure off the jaw. Never, ever hold a fish from its eye sockets.   
• Lip gripping device – most of my clients want a picture with their fish, but are not comfortable holding it. Putting their fingers in a mouth full of teeth is not an option. Teaching them how to put their fingers under the gill plate, without damaging the gills, is tricky. An easier option is to use a gripping tool that latches onto the lower jaw. I use a plastic model from Rapala. To further decrease damage to the fish, I cut a gap between the jaws, so they don’t close as tightly. They still hold the fish adequately, but don’t pinch the soft tissue of the lower jaw. Remember to still hold big fish horizontally and support their underbelly.
• Measure, don’t weigh – the days of hanging a fish by its mouth to determine its weight are disappearing. Measuring a fish’s length is less damaging and has become more accepted across the angling community. Length-to-weight calculators for each species are available and can provide fairly accurate approximations. If you really want to weigh a fish, weigh bags are available.
• Net material – rubber nets are thought to be less damaging to the slime coat of fish. They also decrease the chance of a fish getting wrapped in the net as they flop around.
• Use single and/or barbless hooks – consider ditching the treble hooks and/or crimping down the barbs. Both make it easier to unhook a fish.
• Live bait versus artificial – a published study by the Minnesota DNR compared the survival of walleye caught with live leeches versus artificial baits. Mortality was significantly greater in fish caught with leeches, due to deeper and more frequent hook sets in the gut and gills. Fish caught on artificial lures were hooked primarily in the lips. If you prefer live bait, try using circle hooks.
• Attended versus unattended lines – a good example is the use of tip-ups for ice fishing. Generally, live bait is suspended in the water column and left to do its thing. A strike is indicated by a flag popping up. The time between the strike and the response by the angler gives the fish a greater opportunity to swallow the hook. That same minnow fished on a rig that is actively attended decreases the chance of it being swallowed.
• The release – don’t toss the fish overboard. If possible, gently lower it closer to the water. If the fish is exhausted, hold it horizontally in the water until you feel it regain its strength. It will slowly swim away when it is ready. A common misconception is that pumping the fish back and forth will help. I remember my Ichthyology professor saying this was bad for two reasons: 1) a strong current backward across the gills can cause damage, 2) gills work via a countercurrent system, meaning the blood in the gills travels in the opposite direction as the flow of water. By pulling the fish backwards, you are working against this process.