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