Tarpon Physiology

Fish get stressed just like humans do. And with human influence on natural systems ever increasing, the more productive fishing grounds become even more crowded due to electronic bulletin boards, cell phones, magazines, television and so on. Tackle also evolves as does advances in communications and navigation. Pressure specifically on tarpon populations is no exemption and that does affect tarpon behavior.

We need to learn more about the subtle, sub-lethal effects of angling as well as at each life-history stage. The trauma of being caught can cause physical and physiological damage to tarpon. Physical damage includes hook wounds, excessive bleeding and torn tissue. Over-handling or holding the fish out of water can cause injury. Through all this, a tarpon must change its internal chemistry and body systems to adapt to the ordeal of being caught. The resulting stress can be acute (intense and short-term) or chronic (long-term).

 The ultimate negative effect of an angling battle is of course death. A long fight may spell the end of a tarpon as some studies show such a correlation. Even if a tarpon survives, it may not get back to normal. In time, subtle changes could affect the tarpon population by reducing growth and maturation rates, reducing reproductive success or affecting overall fitness

Large tarpon over 75 pounds are caught throughout Florida as a seasonal fishery that targets sexually mature fish in saltwater environments. This is especially true when they feed in shallow water in the spring prior to spawning, when milling or daisy-chaining along beaches or gathering in passes and under bridges before going offshore to spawn.  

Fishing pressure in Florida on large tarpon is heaviest from May to July, which is their peak spawning season. The largest tarpon landed (over 200 pounds) tend to be ripe females. At that time of year tarpon put more energy into reproduction and their lower energy reserves can result in more difficulty recovering from the stress of a fight. Warmer summer and early fall water temperatures have also been shown to cause more pronounced stress than cooler temperatures. 

Smaller and sexually immature sub-adult tarpon reside in inshore creeks, canals and ponds and can be caught year-round. Catch-and-release mortality studies using sonic telemetry indicated that large adult tarpon recover well from angling if not attacked by large predators and handled with care. However, smaller adult tarpon may be more likely to die after release due to being easier to handle and lift, thus keeping them out of the water longer.  

The stress effects of angling on Atlantic tarpon are unknown and may differ at various sizes. Studies show that the stress response of an animal may be related to its body size or age. Females take 10 to 12 years to reach sexual maturity and a tarpon that age is about four feet long – a decent-size fish to target even though still considered a sub-adult.  

We need to learn more about catch-and-release effects on all sizes of tarpon to make sound management decisions and their conservation. To achieve this, Bonefish and Tarpon Trust (BTT) has partnered with the federal Wallop-Breaux Sportfish Restoration Fund and the Florida’s tarpon-possession permit program to fund a new tarpon-research program. This study evaluates catch-and-release stress by analyzing blood chemistry.  

The methodology included sampling sub-adult and adult tarpon at rest to establish control values and then doing the same after stressful angling events. Blood samples were drawn from the caudal (tail) vessel. Additional sub-adult tarpon were caught and bled, but prior to drawing blood these tarpon were held in the air for 60 seconds – about the time needed for a photo or tagging. Half of the tarpon were held vertically by the jaw and the other half held horizontally.  

Hematocrit (the ratio of red blood cells to plasma or packed cell volume) and hemoglobin content were measured on whole blood at the time drawn. Hemoglobin is the red blood cell pigment that carries oxygen to tissues. The remaining whole blood was spun immediately to separate red blood cells from plasma, which was then frozen until ready for processing. Using the plasma, we measured glucose, lactate, the stress hormone cortisol and electrolytes (salts). 

Electrolytes are an important concern. To be able to move freely between saltwater and brackish or fresh water, a fish controls the amount of electrolytes in its system by a process called osmotic or ionic regulation. These mechanisms help fish maintain a relatively consistent level of salt in the blood and in the cells.  

Marine fish such as tarpon naturally ingest a lot of seawater (freshwater fish don’t need to do this.) When stressed, marine fish drink even more to balance the amount of water they lose across the gills. Such ionic regulation must take place when a tarpon is stressed because the internal salt balance of the fish is disrupted. If the fish cannot self-regulate back to equilibrium with its environment, routine biological functions of the fish can be disturbed enough to cause harm.  

It’s thought that during a fight, tarpon may switch from aerobic respiration (breathing through the gills) to anaerobic respiration, in which energy stored in the white muscles is used to enable faster swimming bursts. In turn, lactate levels increase and blood pH drops. Levels of enzymes, glucose and stress hormones in the blood will also be affected by the change in activity. By evaluating the magnitude of response in the tarpon’s blood chemistry, biologists can deduce how tarpon of different sizes are affected by the exhaustive exercise of the fight, exposure to air and being handled.  

We hope the analysis now taking place will answer questions such as:

Do smaller tarpon have a more extreme response to physiological stress than adult tarpon?

Are there size-related differences in the blood composition of tarpon at rest?

Are there size-related differences in the blood composition of tarpon after the stress of being caught?

Does tarpon blood chemistry show a significant difference in the way sub-adults and adults respond to the stress of angling?

Do fight time, water temperature, dissolved oxygen and body size have an effect on the stress response of angled tarpon?

Do sub-adult tarpon show stress effects from prolonged exposure to air?

Do sub-adult tarpon show stress effects from vertical and horizontal handling?

Are sub-adult tarpon able to survive the stress of exhaustive exercise?  

BTT members should take pride in supporting research that determines the best practices to ensure that tarpon can survive the all stresses – the results of the studies will be disclosed to you soon. 

For more information contact Kathy Guindon  This e-mail address is being protected from spambots. You need JavaScript enabled to view it