Home : Features : Saltwater Fish : Tarpon

The Physiological Effects of Body-Size and Catch-and-Release Angling in Tarpon

Anyone who has fought a tarpon, large or small, knows they exert a tremendous amount of effort during an angling event. The study described here attempts to measure physiological effects experienced by caught and released Atlantic tarpon.

---

Ever hooked a tarpon that suddenly had a burst of energy or two (or six)?  Changes in the way a tarpon uses its stored energy reserves allow it, and other fish, to exhibit such bursts when on the hook.  By evaluating the magnitude of response of those bursts using the tarpon’s blood chemistry, biologists can start to deduce any potential effects, both non-lethal and lethal, of the stress of angling on tarpon.  

Some studies have shown that fish which undergo the stress of an exhaustive exercise such as tangling with an angler may experience elevated post-release mortality. Other studies have shown no effect of exhaustive exercise on mortality. Research does indicate that the stress response and recovery of fish after exercise may be further affected by the size of the fish, air exposure during handling time, the water temperature at the time of capture, the life-history stage of the fish being caught, its ability to swim while recovering, and how “fit” or well trained a fish is to exercise.

Limited work in these studies has been performed on saltwater species (e.g. cod, flatfish, tunas, sharks) but is expanding.  The Atlantic tarpon is world renowned and sought after because of its fighting ability on various tackle and provides an excellent model to evaluate a primitive, pelagic (open sea) species’ physiological response to exhaustive exercise at different life-history stages.

Large tarpon in excess of 70 pounds are caught throughout Florida in a seasonal fishery that targets sexually mature fish in salt water environments before, during, and after their spawning season.  Sub-adult tarpon (sexually immature, meaning too young to bear offspring) that are much smaller (5 to 30 pounds) can be targeted year-round in backwater, estuarine, and pond environments.  Understanding how tarpon of all sizes react to different catch-and-release activities can provide useful information for anglers, scientists, and managers to develop suggested conservative methods for handling before release. 
  
We propose to evaluate size-related differences in the physiology of resting tarpon and then specifically compare the stress response of adult (more than 70 pounds) and sub-adult (less than 30 pounds) tarpon to catch-and-release angling using blood chemistry.  We then propose to evaluate the influence of air exposure and handling on sub-adult tarpon since they are likely more easily handled than large tarpon when caught by anglers.  Results will be used in developing guidelines for anglers and managers to promote safe handling for minimizing stress and promoting survival of tarpon.

Photo by Charlie Gardner
Adult tarpon were collected from Tampa Bay, then transported first by boat, then by flatbed trailer to a 30,000 gallon holding tank at the FWC Stock Enhancement Research Facility (SERF).

What is the plan?

Objective 1:  Describe the blood composition of adult and sub-adult Atlantic tarpon at rest (control levels) and any size-related, variation for select blood parameters.

 What blood parameters will be measured?

Baseline levels (controls) of blood chemical parameters hematocrit, hemoglobin, metabolites (glucose and lactate), stress hormones (cortisol), electrolytes (Na+, K+, Ca2+, and Cl-), and osmolality (total salt content) will be measured from tarpon in a resting state.  Sub-adult and adult tarpon will be captured and placed in captivity at the Florida Fish and Wildlife Conservation Commission’s Fish and Wildlife Research Institute’s (FWC-FWRI) Stock Enhancement Research Facility (SERF) at Port Manatee.  Each tank is a self-contained, filtered, and treated seawater system. Temperature, salinity, dissolved oxygen, and pH will be monitored daily. After an adequate time has passed for the fish to become acclimated to its new surroundings, a few tarpon will be quickly and humanely euthanized and immediately sampled for baseline levels of blood parameters.  Initial experiments will be performed to compare and explore alternative ways to measure baseline levels of blood in resting fish.  However, we believe that four tarpon from each experimental group should be enough to establish baseline controls through sacrificial methods.

How will biologists take blood from a tarpon? 

A small sample of blood will be withdrawn from the caudal (tail) vessel with a sterile syringe, just like when you take your dog or cat to the vet. Hematocrit (a ratio of red blood cells to plasma or packed cell volume) will be measured on whole blood at the time of withdrawal.  A small sample of whole blood will be used to measure hemoglobin (red blood cell pigment) content.  The remaining whole blood will then be spun immediately to separate the plasma from red blood cells.  Plasma samples will be immediately frozen and stored in liquid nitrogen until ready for processing. 

Blood is drawn from the area near the anal fin of the tarpon using a Vacutainer® tube. The blood is then processsed at our mobile laboratory and prepared for submission to an independent analytical lab.

Who will process the blood samples? 

Metabolite (energy fuels), electrolyte (salt), and hormone levels will be measured on the plasma at an analytical blood lab.  This ensures the same procedures and standards will be used for each sample. Unused plasma will be stored at -76°C (-104.8°F). 

Why do biologists care about electrolytes (salts) in tarpon?
 
A fish can control or regulate the amount of electrolytes (salt) in its system to be able to move freely from saltwater into brackish or freshwater and back again.  

It is through a process called osmotic or ionic regulation.  These mechanisms help fish control and maintain a relatively consistent plasma (blood) salt content and cell volume.  Organs involved in such osmotic regulation include the gills, kidney, rectal gland and intestine.  This ionic regulation must take place when a fish is stressed because the internal salt balance of the fish is disrupted.  If the fish can not self-regulate back to equilibrium with its environment, biological and healthy functions of the fish can be interrupted and these can have lethal or harmful effects to the fish.

Marine fish, like tarpon, ingest or "drink" a lot of seawater. It would be like a human drinking 35 liters of water per day.  (Freshwater fish do not do this!)  Marine fish drink to balance out the amount of water they lose across the gills, as fish need water to survive just like humans and all animals.  A saltwater fish can move 30-60 percent of its body weight per day across the gills.  However, all this excessive drinking of salt water increases the amount of water in its body and adds to a net salt influx which must then be excreted. The salt influx is more severe when under stress.  

Fish can actively excrete sodium (Na+) and chloride (Cl-) back into the seawater.  There are special cells called chloride cells in the gills of a fish where this happens.  These cells are unique to fish.  The physiology of how these cells help regulate ion flow in and out of the fish also differs for freshwater and saltwater fish.  Ingested or swallowed seawater that gets past the gills is further diluted in the esophagus or throat of the fish. Salt (nutrients for the fish) and water are also absorbed into the digestive system.  The gut of marine fish will absorb 60-70 percent of the salts in the seawater it drinks. The remaining bulk of the now diluted (less salty) water gets absorbed in the small intestine.  All of the excess nutrients (calcium, potassium, magnesium, sulfates, excess water) not needed by the fish for nutritional purposes or for biological function are excreted as waste (kidney and rectal gland). Urine flow is low but very concentrated.

Will anything more than blood be used from each tarpon, especially from euthanized fish?

Length and girth measurements of each tarpon will be measured and each fish will be weighed, or weights will be estimated using a weight-length-girth relationship.  On our control fish, a full necropsy (dissection) will be performed to evaluate overall tarpon health, parasites, baseline brevetoxin, or red tide, levels for healthy tarpon, sex and maturity stages, and age.

Are tarpon “at rest” in the lab the same as tarpon “at rest” in the wild? 

Tarpon swim all the time and are therefore always active to some extent.  This “normal” swimming activity combined with aerobic respiration (routine breathing) is taken into consideration when biologists speak of measuring a tarpon’s baseline or standard metabolic rate; “normal” resting circumstances.

In the wild, we will attempt to acquire control “at rest” levels of the chosen array of blood parameters by quickly capturing adult and sub-adult tarpon and sacrificing them instantaneously; samples must be taken within a minute of capture.  There are many logistical challenges associated with quickly capturing, restraining and sampling tarpon and we are going to start with the sub-adult tarpon first to see if their blood chemistry differs in the lab versus in the wild.  

Objective 2:  Describe the physiological response to catch-and-release angling in adult and sub-adult tarpon using blood chemistry and compare for any size-related, intra-species variation.

Where will the FWC-FWRI obtain angled (exercised) tarpon for blood samples?

Sub-adult tarpon that have become landlocked will be angled in a small pond or areas where they are available in the wild. For each landed tarpon less than 30 pounds, fight time, length, girth, water temperature, salinity, and dissolved oxygen will be recorded.  A blood sample will be taken immediately and processed as previously outlined.  These tarpon will be kept in the water while being handled and sampled.  Tarpon will be dart-tagged to avoid repeat measures and then released. Subsequent mortalities of sampled tarpon will be noted. 

Adult tarpon will be caught in the Tampa Bay or Charlotte Harbor areas. These are areas where several tarpon can be caught and sampled in a relatively small window of time.  We may need your help.  If you are a tarpon angler willing to permit us to draw blood from a tarpon you fought, please feel free to contact us so we can add you to the list of willing participants.  FWRI Staff will keep you updated with sampling plans. For each tarpon, fight time, length, girth, water temperature, salinity, dissolved oxygen will be recorded.  Fish will be handled at the side of the Marine Research vessel in a sling while a blood sample is taken and processed as previously outlined.  Fish may or may not be dart-tagged, but DNA samples will be taken that can serve to identify individual fish if recaptured in the future.   

What are you trying to learn about these fish?
 
We wish to determine if fight time, water temperature, dissolved oxygen, or the size of a tarpon has a significant effect on changes observed in the blood chemistry of adult and sub-adult tarpon.  In addition, we hope to evaluate the effect of air exposure and handling a fish vertically and horizontally on the physiological stress response in tarpon less than 30 pounds.

How will the effect of air exposure on the stress response in sub-adult tarpon using blood chemistry be evaluated if all fish are to be handled in the water?

Objective 3: Additional sub-adult tarpon will be angled in ponds and the same field and blood variables will be recorded as previously described. Prior to drawing blood, however, these tarpon will be exposed to air for 60 seconds, a moderate time for a tarpon to be held; perhaps for a photo.  Half of the tarpon being held in the air will be held vertically by the jaw and the other half will be held horizontally with one hand on the jaw and one hand supporting the belly. Fish will be tagged and then released.  Any subsequent mortality will be noted.

Is the FWC-FWRI working with any other agencies or groups on this project? 

Yes, this project is being coordinated by the FWC-FWRI, in conjunction with the University of South Florida (USF) and granting organization Bonefish and Tarpon Unlimited (BTU) and FCF Flats Fishing Alliance.  It is proposed that the project will last two years; from April 2007 through March 2009.

What is expected to be done with the results?

The preparation of any progress reports for funding groups, final report and manuscript(s) to peer-reviewed journal(s), dissertation chapter, and educational brochure or list of suggestions for BTU will be conducted by USF graduate student, USF faculty and FWRI staff. Reports would be submitted to BTU, USF, and FWC-FWRI.  

This group of tarpon researchers and the FWC-FWRI will then coordinate with Dr. Steve Cooke (bonefish research) to coordinate education/outreach activities aimed at producing a pamphlet describing suggested handling tactics that will minimize stress and maximize survival in tarpon and bonefish fisheries. 

Presentations of findings will also be made at professional meetings and conferences and articles for Web sites and magazines may be written.

Where is the funding coming from for this study?

A private grant was obtained from Bonefish and Tarpon Unlimited.  Matching costs and in kind contributions are able to be provided as a result of an existing research infrastructure funded by the Federal Wallop-Breaux Sportfish Restoration Fund and the state of Florida tarpon permit program.

Photo credit: Excepted as noted, Florida Fish and Wildlife Conservation Commission