redfish release

WaterLine file photo

One of the biggest factors determining whether released fish survive is how long they’re kept out of the water.

Marine and freshwater fish support important angling industries that provide substantial benefit to local economies. Although catch and release — both voluntary and mandatory — aims to conserve fish stocks, its effectiveness is dependent on the survival and fitness of the released fish. There are many stressors that affect the condition and fate of released fish. Air exposure is one such stressor.

Fish “breathe” by aerobic respiration, but the way fish use oxygen is different from how we use it. We inhale air, use the oxygen and breathe out carbon dioxide. Fish use gills to breathe by diffusing oxygen from the water into their bodies.

Gills are comprised of many rows of gill filaments, feathery structures that provide a large surface for gas exchange. The tips of each filament contain capillaries. As water passes over the filaments, oxygen from the water binds with blood inside the capillaries. The circulatory system then transports the oxygenated blood throughout the body.

When a fish is removed from the water and exposed to air, its delicate gill filaments collapse, resulting in a greatly reduced surface area for oxygen exchange. The amount of time a fish can survive out of the water depends on several factors including fish species, life stage, body weight, activity level, feeding, environmental temperature, and the amount of dissolved oxygen in the water it is being removed from.

Conditions leading up to a fish being removed from the water are very important. For instance, warmer temperatures tend to be associated with a higher degree of hypoxia (low oxygen). Low oxygen is a stressor for fish. Many researchers have evaluated oxygen requirements and the effects of hypoxia on fish at all life stages.

What we know: Fish eggs in general require very little oxygen until they hatch. Among juvenile and adult fishes, larger fish use more total oxygen per hour than smaller fish because they have greater metabolic demands due to their greater tissue mass. Swimming fish use more oxygen than resting fish. Fish in warmer temperatures generally use more oxygen than fish in cooler environments.

High water temperature, when combined with stress and exercise, often leads to behavioral and functional changes. These changes include a lack of movement or equilibrium loss, increased heart rate and stroke volume, and changes in blood and muscle biochemistry. Research that examined the interaction between air exposure and high water temperature in bluegill showed behavioral disturbances such as increased ventilation and equilibrium problems were greater when both stressors were combined as opposed to when these stressors were evaluated individually.

When held in air, fish respire anaerobically (without oxygen) and recruit white muscle fibers for movement. This produces lactate from the metabolism of glycogen. Resynthesizing the lactate back to glycogen after exercise is energetically costly and can delay recovery in fish. If a fish can’t get its blood chemistry balanced back to pre-stress levels, it may die — possibly as long as 72 hours after the catch.

A study of fat snook (Centropomus parallelus) in a Brazilian estuary determined that air-exposed angled fish and tournament fish exposed to air at weigh-in contained very high lactate levels even after just one minute of air exposure, indicating extreme physiological stress when compared to angled fat snook not exposed to air. And, in a laboratory experiment involving rainbow trout, extended air exposure after exercise caused higher mortality than when air exposure was avoided.

Although most fish species are very sensitive to air exposure, worldwide approximately 400 fish species have evolved specialized respiratory adaptations which allow them to breathe air. These include gill modifications, use of the skin, modified swim bladders or true lungs, and specialized respiratory structures in the mouth and gut.

Walking catfish for example possess specialized gills that inhibit gill collapse and the associated loss of surface area. European eels can utilize air from the atmosphere by diffusing it through their well-vascularized skin. This allows them to migrate short distances on land.

Electric eels have well-vascularized mouths. They must surface approximately every minute for air. Without air they will drown. And tarpon come to the surface and gulp air which is then directed to their highly vascularized swim bladders and used for some gas exchange.

Speaking of tarpon: A study conducted on sub-adult tarpon in Tampa Bay and Charlotte Harbor found that angling with minimal (1-minute) air exposure, such as what might be required if taking a photograph on a fishing trip, did not appear detrimental to sub-adult tarpon recovery and survival under normal angling conditions.

This same study found that fight time was a greater determinant in fish stress and survival than air exposure. The author concluded that anglers could play an important part in tarpon conservation by using appropriate tackle and gear to reduce fight times and handling stress.

Bonefish, like tarpon, have a modified swim bladder which may be used for gas exchange. A study on bonefish determined that they could recover from limited air exposure, but because they tend to move very little while recovering, they are very vulnerable to shark predation. This study concluded that if bonefish can avoid predation for the first few minutes following release, then delayed mortality is relatively insignificant, regardless of handling.

Primary factors that have been indicated as influencing stress and mortality associated with catch-and-release angling are angling fight times, extent of air exposure, angling during periods of extreme water temperatures, angling during reproductive periods, gear type and hook location. In this article I focused on air exposure, but hopefully you can see how it’s inextricably linked to a variety of other stressors.

Minimizing air exposure during the catch and release process is just one best management practice anglers can adopt to help ensure released fish survive. It seems like common sense, but it’s also supported by science.

Betty Staugler is the UF/IFAS Extension Charlotte County agent for the Florida Sea Grant Program. She is active in many areas relating to boating, fishing, and watershed/coastal living. Sea Grant supports research and education activities that help Florida’s shoreline communities, industries and citizens wisely use the state’s coastal and marine resources. Contact her at staugler@ufl.edu or 941-764-4346.

Betty Staugler is the UF/IFAS Extension Charlotte County agent for the Florida Sea Grant Program. She is active in many areas relating to boating, fishing, and watershed/coastal living. Sea Grant supports research and education activities that help Florida’s shoreline communities, industries and citizens wisely use the state’s coastal and marine resources. Contact her at staugler@ufl.edu or 941-764-4346.

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