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Conserving Your Catch

Community Based Sustainable Fishing Education Project Information Sheet #1

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Conserving Your Catch - FAQ, References and additional resources

Frequently asked Questions

What Is Barotrauma?

Barotrauma is physical damage to body tissues caused by a difference in pressure between an air space inside or beside the body and the surrounding gas or liquid. Barotrauma typically occurs to air spaces within a body when that body moves to or from a higher pressure environment, such as when a SCUBA diver, a free diving diver, an airplane passenger ascends or descends or you bring certain species of fish up from deep water. Boyles Law defines the relationship between the volume of the air space and the ambient pressure. Fish brought to the surface from depths of 20 meters or more are likely to suffer from barotrauma. The susceptibility to this condition and response to treatment varies between species. With humans that suffer from this you can simply put them into a pressure chamber, return them to the pressure they were at and then adjust the pressure slowly to normal ambient pressure. With fish, they must only be returned to the depths they came from and released as they were at that pressure to begin with and that is where they belong. Damage occurs in the tissues around the body's air spaces because gases are compressible and the tissues are not. During increases in ambient pressure when at deep water depths, the internal air space provides the surrounding tissues with little support to resist the higher external pressure. During decreases in ambient pressure when the fish is reeled in from deep water, the higher pressure of the gas inside the air spaces causes damage to the surrounding tissues, organs and muscle structure when that gas becomes trapped. This will cause the “puffed” or “popped” eyes and the expelled stomachs we see from the fish that suffer from this. Remember though, not all symptoms are visible and just because his eyes and stomach may not appear to be “popped” or visible doesn’t mean the fish is OK. There are air bubbles in his muscle system as well as the other organs that can cause him to expire if he is not returned to a proper depth for the pressure to equalized and the effects to be reversed. Until now there has been two methods promoted for dealing with fish that are suffering from barotrauma (although opinions among researchers and experienced fishers differ as to the best method for dealing with this condition).

Venting, or puncturing the swim bladder with a fine hollow needle has been used to allow the expanded air in the swim bladder to escape. Venting methods can vary with species however in most cases the needle is inserted in line with the top of the pectoral fin and below the 4th dorsal spine. Unfortunately, research has shown this method to cause a very high mortality rate in most cases since any intrusion into the body can cause infection.

Weighted line An alternative, less intrusive method, has been to use a weighted line to return the fish to the bottom. The fish is hooked in the jaw with a barbless hook or bent wire design, attached to a weight and lowered to the bottom on a cord. When the cord is "jerked" the hook comes out of the fish.

What is Deep Hooking?

Deep hooking refers to the position where the hook has pierced the fish. This is generally defined as inside the mouth but deeper than the mouth i.e. throat, gills, oesophagus or stomach. A study conducted by Lyle et. al. (2007) found that the survival rate of deep hooked fish (64 %) is significantly lower than shallow hooked fish (100 %) if hooks are forcibly removed. Shill (1996) provided evidence that if the line is cut at the mouth and hook is left intact deep hooked fish have an increased survival rate (34 % more than fish that have had hooks forcibly removed). Fishing with “tight” lines and using circle hooks have been shown to reduce the incidence of deep hooking sand flathead (Lyle et. al. 2007). As flathead have an enlarged mouth relative to there body size it can be inferred that by fishing with tight lines and circle hooks you can greatly reduce the incidence of deep hooking in many other recreationally targeted species. This would result in any released fish having an increased likelihood of survival.

Why is it important to protect fish skin and mucous layer?

The mucous layer of fish is a concoction of many different compounds and has a variety of roles (Hellio et. al. 2002). Some of these roles are: reproduction, excretion, disease resistance, communication, feeding, nest building and protection (Shepherd 1994). If the mucous layer is damaged or removed the fish has an increased risk of pathogen infection and mortality (Svendsen & Bøgwald 1997).

Why is it important to return fish to the water as quickly as possible?

The longer the period a fish is exposed to air the greater the risk of mortality this is especially evident in smaller fish (Davis & Parker 2004). Therefore it is important to try and reduce the potential time a fish is out of the water. This can be done by using suitably sized hooks, circle hooks and barbless hooks which will aid in dehooking fish.

Safely minimizing the fight time can also aid in reducing mortality as lower fight times means that there is lower concentration of cortisol (stress indicator) (Chopin et. al. 1996).

Further reading:

For further information on post release mortality click on the following link:

For information on the complexity of fish mucous and issues with discerning its function click on the following link:


Chopin F.S.; Arimoto T.; Inoue Y. 1996, A comparison of the stress response and mortality of sea bream Pagrus major captured by hook and line and trammel net, Fisheries Research, Volume 28, Number 3, pp. 277-289(13)

Davis MW, Parker SJ (2004) Fish Size and Exposure to Air: Potential Effects on Behavioral Impairment and Mortality Rates in Discarded Sablefish. North American Journal of Fisheries Management: Vol. 24, No. 2 pp. 518–524

Hellio C, Marie Pons AM, Beaupoil C, Bourgougnon N and Le Gal Y, 2002, Antibacterial, antifungal and cytotoxic activities of extracts from fish epidermis and epidermal mucus, International Journal of Antimicrobial Agents, Volume 20, Issue 3, Pages 214-219

Lyle, J.M., Moltschaniwskyj, N.A., Morton, A.J., Brown, I.W. & Mayer, D., (2007). ‘Effects of hooking damage and hook type on post-release survival of sand flathead (Platycephalus bassensis).’ Marine and Freshwater Research, Vol 58: 445-453.

Schill D.J. (1996) Hooking Mortality of Bait-Caught Rainbow Trout in an Idaho Trout Stream and a Hatchery: Implications for Special-Regulation Management. North American Journal of Fisheries Management: Vol. 16, No. 2 pp. 348–356

Shephard, KL 1994, Reviews in Fish Biology and Fisheries. Vol. 4, no. 4, pp. 401-429.

Svendsen, YS; Bogwald, J, 1997, Influence of artificial wound and non-intact mucus layer on mortality of Atlantic salmon (Salmo salar L.) following a bath challenge with Vibrio anguillarum and Aeromonas salmonicida, Fish & Shellfish Immunology. Vol.7, no. 5, pp. 317-325.