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By Adam Johnson
Fighting, handling and holding fish
in captivity place severe metabolic demands on brain, muscle, heart,
gill and other organ tissues putting them at considerable
physiological risk. In general terms we call this stress, but the physiological
situation is highly complicated.
The degree of stress fish realize, and the potential for
subsequent recovery, depends on the type and duration of the
physiological stress we place them in and the environment in which
they are allowed to recover. To
gain a better understanding of fishing-related stress, we must first
gain a basic understanding of some of the physiological mechanisms
Metabolism – A Continued Need for Oxygen
The energy used to fuel virtually
all cellular functions is derived from the compound adenosine
triphosphate (ATP). ATP
is needed to make muscles contract, drive brain impulses, allow the
heart to beat, provide oxygen uptake by the gills and on an on.
ATP is made up of adenosine (a compound formed from adenine and
ribose, a carbohydrate) attached to three phosphate groups.
As the last phosphate bond is broken (actually called a
phosphoryl bond by biochemists), chemical energy is released.
The cell converts this chemical energy into mechanical energy
to do the work of the cell. The
by-products remaining after this reaction are adenosine diphosphate
(ADP) and inorganic phosphate (Pi).
In the cell, ADP and Pi can again combine through the work of
complicated metabolic pathways to form ATP, and the energy cycle
continues. The metabolic
reaction looks like this:
ADP + Pi
Most freshwater fish rely heavily on
oxygen in their environment. This
oxygen is used, primarily, to help fuel the biochemical mechanisms
associated with energy production.
Oxygen-associated energy metabolism is highly efficient, and
produces the constant supply of energy that fish (or people, for that
matter) rely upon to support basically all physiological functions.
Aerobic energy metabolism takes place in a cellular organelle
called the mitochondria using a metabolic process known as oxidative
energy synthesis is the sole job of the mitochondria and cells can
hold literally thousands of these tiny power plants.
All types of foods (fuels) are metabolized for aerobic energy
production, including fats, carbohydrates and proteins.
However, not all energy production
relies on oxygen. Cells
have developed mechanisms to keep energy supplied to tissue high
during short bursts of sudden, high-intensity exercise, or for short
periods when oxygen levels are inadequate (a situation known as hypoxia,
or lack of oxygen). Anaerobic
or hypoxic energy metabolism is inefficient and cannot be relied on to
produce enough energy to keep tissues functioning in the long term.
Glucose (a six-carbon sugar) is the primary energy substrate
for anaerobic energy production. Glucose is converted to energy down a non-oxidative metabolic
pathway called glycolysis.
Fish must rely on constant supplies
of energy. This energy is
most often provided by the oxygen utilization metabolic pathways of
oxidative phosphorylation in the mitochondria.
Fish can swim continuously for long
distances without tiring at a broad range of speeds.
This type of swimming, called steady state swimming, is
used by fish during normal cruising, or for long distance travel.
Muscles that are used in this type of exercise have a large
number of mitochondria and use oxygen for energy synthesis. As long as there are constant supplies of oxygen, fish
basically never become tired during this type of exercise.1
Sudden bursts of high-intensity
swimming are called burst swimming.
This type of swimming normally last for only seconds (possibly
minutes) and ends in a physical state of exhaustion.1,2
Burst swimming is critical when fish attack prey, when they
migrate against strong currents or up waterfalls, or when they are
fighting after being hooked. This
high-intensity exercise totally drains fish of energy reserves.
Recovery from such exhaustive exercise may take hours, or
sometimes days, depending on the availability of oxygen following the
exercise, the duration of the exercise and the degree to which energy
substrates are consumed by (or lost from) the tissue.
Energy metabolism during burst swimming is anaerobic.
Associated with the depletion of
energy reserves during burst swimming is an increase in tissue
(including blood) lactic acid (or lactate).
As an acid, lactate produces hydrogen ions that lower the pH of
the tissue and reduce the total energy supply of the cell (called its energy
charge).1,3 It also drains the cell of important metabolites it needs to
recover. Once these
metabolites are exhausted, the fish will not be able to perform
another burst of exercise until they are replenished.
Clearance of lactic acid, and restoration of cellular stasis,
can take anywhere from four to 12 hours.
This allows the fish sufficient time to restore lost
metabolites, but may not be sufficiently long to allow cellular energy
levels to rebound. Factors
such as body size, water temperature, water hardness, water pH and
oxygen availability all play a part in time to recovery.
The following is a list summarizing
the effects of certain factors on the physiology and recovery from
exhaustive exercise in fish:
Why is this important?
Stress associated with catch and release can contribute to
stress related mortality. The
lessons learned from studies investigating the affects of, and
recovery from, exhaustive exercise have the potential to decrease the
number of stress-related deaths and increase fish productivity.
Understanding the energy metabolism of fish, and the factors
that affect energy metabolism, are critical to understanding how fish
must be handled and treated when caught.
Osmoregulation – Maintaining Salt
Balance in Stressed Fish
The regulation of salt balance is
fundamental to life. The
structure and function of cells depend closely on their interactions
with water and its solutes, and few factors affect the viability of an
organism as extensively as osmoregulation.
Thus, fish invest considerable energy in controlling the
composition of intracellular and extracellular fluids.
In fish, osmoregulation typically consumes 25% - 50% of the
total metabolic energy output, possibly the largest energy consumer in
The mechanisms used by fish to
maintain salt balance are highly complicated and extremely energy
dependent. Since every molecule of glucose that is used to generate
energy in a hypoxic environment is less than 1/10 as efficient as it
is in an oxygen rich environment, the energy demand of tissue
osmoregulation cannot be met by anaerobic energy metabolism alone.
A rapid fall in cellular ATP levels causes a slow-down, and
eventual stop, in the cellular pumps used to move sodium, potassium
and other ions across the cell membrane (called Na/K pumps).
A disruption in Na/K pump activity causes the cell to lose ion
homeostasis, and ions are then free to run down their concentration
gradients putting the survival of the cell – and the fish – at
Both fresh- and saltwater fish are
constantly faced with the challenge of ionic and osmotic regulation.
Freshwater fish, in which tissue ion concentrations are much
greater than the water in their surrounding environment, must deal
with osmotic water uptake and loss of ions through permeable
epithelial tissues and via the urine.
The opposite is obviously true in saltwater fish.
Freshwater fish produce copious amounts (up to 20% of the body
weight per day) of highly dilute urine.
Fish kidneys are highly efficient at keeping body salts (ions)
out of the urine, allowing them to stay in the body.
While very small amounts of salt are passed in the urine, most
osmoregulation is managed by cells in the fish’s gills.
Sodium is the primary ion found in
tissue. Transport of
sodium across cell membranes is energy dependent and is facilitated by
an enzyme called Na/K-ATPase. This
enzyme is integral to the cell membrane and uses the energy supplied
by ATP to move sodium in one direction across the cell membrane, while
its counter-ion (potassium or whatever) moves in the other direction.
This process allows muscles to contract, it provides the
electrochemical gradient to stimulate the heartbeat and allows all
manner of neuronal signals to be transmitted.9,10
Osmoregulation occurs in the fish
gill. The enzyme Na/K-ATPase
is found in the basolateral membrane of the gill lamellae.
In freshwater fish, the system works like this:
Ammonia is produced as a waste
product of fish metabolism. Unlike
higher animals, fish do not excrete ammonia in the urine, although
some urea, another nitrogenous waste, is excreted that way.
Instead, ammonia (and most of the urea) diffuses through the
gill epithelium. About
80% - 90% of the nitrogenous waste of fish metabolism are excreted via
Ammonia from the blood is exchanged
across the gill cell membrane for sodium.
This removes the ammonia from the blood and increases the
ammonia concentration in the epithelial cell.
In turn, the plasma sodium concentration is increased and the
sodium concentration in the gill cell goes down.
To restore osmotic balance in the gill cell, the cell then
passes the ammonia out of the cell into the environment and exchanges
it for another sodium ion from the water.
In like fashion, chloride ions from the water are exchanged for
bicarbonate (formed as CO2 from cellular respiration
combines with water in the cell).
Chloride ions move into the cell and bicarbonate moves out of
the cell and into the environment.11
Hydrogen ions can also be exchanged for sodium in this manner,
helping to control blood pH (Figure 1).
These two mechanisms for ion
exchange, absorption and secretion, occur within two cell types of the
fish gill, respiratory cells and chloride cells.
Given that chloride cells are used to eliminate salts,
they are larger and more highly developed in saltwater species and not
well established in freshwater species.
These two cell types are found in two anatomically distinct
locations within the gill with very different exposures to blood flow.
Respiratory cells are supplied by arterial flow, while chloride
cells are found in portions of the gill supplied with venous blood.
These positional differences support the ion exchange
mechanisms described here. Respiratory
cells, which are involved in gas exchange, nitrogenous waste removal
and acid-base balance, are supplied by arterial flow and they exchange
sodium and chloride for ammonia and bicarbonate, respectively.
Chloride cells, which deal strictly with salt excretion, are
located away from the respiratory cells.
Again, the most important point to
remember is that the exchange of ions that takes place in these cells
is highly dependent on energy availability.
If there is not sufficient energy to drive the Na/K pumps,
these exchanges cannot occur and water will flood the cells by
control of salt and water balance is vital and requires considerable
metabolic energy to power it. Energy
is the key constituent.
1: Diagram of the model
of ion transport in the teleost gill
Consequences of Oxygen Starvation on
After only a few minutes of hypoxia
there is a general depolarization of the membrane surrounding brain
cells causing a release of neurotransmitters (particularly amino acids
and most importantly glutamic acid) that speed up calcium influx into
the cell. The increased
level of calcium in the cells triggers a number of degenerative
processes that can lead to neurological damage or death.
These processes involve the degradation of DNA, proteins and
the cell membrane itself, and besides calcium and glutamate involve
enzymes that eat away at cell membrane constituents (called lytic
enzymes), free radicals and nitric oxide.12,13
The important events of this anoxic catastrophe are common to
most vertebrate brains. Similar events occur in other tissues, like liver14-16,
muscle7, blood cells17 and heart9.
Once calcium has invaded the cells it takes much more energy to
remove it, via the ATP-dependent calcium pumps.
Another consequence of anoxia is the
release of pituitary hormones, notably prolactin, in most fish
species, fresh- and saltwater. The
release of this hormone affects the permeability of osmoregulatory
surfaces (gill, skin, kidney, intestine and urinary bladder) and ion
transport mechanisms. Prolactin
release regulates water-ion balance by decreasing water uptake and
promoting ion retention (especially Na+ and Cl-).
In doing so, prolactin helps to maintain the salt balance of
the blood and tissues and keeps the fish from swelling with water.11
The main threat to freshwater fish
is the loss of ions by diffusion into the external hypoosmotic
environment, rather than the elimination of excess water.
Even though the regulation of water balance may be important,
it is secondary to the importance of ion retention.
However, the effect of prolactin on water permeability should
not be dismissed as inconsequential.
Prolactin decreases the osmotic permeability of the gills,
retaining ions and excluding water.
It also increases gill mucous secretion, contributing to
ion-water balance by impeding the passage of molecules across the
So, which is more important, salt or
oxygen? The answer is
clear. In fish that have
been stressed by sudden bursts of high-intensity exercise, energy
deprivation is a vital concern. Tissues
use up and become almost totally depleted of energy, and it takes
several hours (or perhaps days) for them to recover.
Anaerobic energy metabolism cannot keep pace with cellular
demand and large amounts of oxygen are required to drive the oxidative
pathways of energy metabolism in cellular mitochondria.
Oxygen deprivation will not allow these pathways to function
efficiently, if at all.
Salt balance, no matter the
osmolarity of the livewell environment, cannot be maintained without
large amounts energy to fuel the process.
And, while the importance of maintaining ion homeostasis cannot
be over emphasized, the first consideration must be providing stressed
fish the energy they need to turn on the osmoregulatory processes.
How Much Oxygen is Enough?
Oxygen, not temperature or salt level, is
the main culprit in fish death in the livewell or in catch-related
stress. However, livewell
water temperature is a main determinant of how much oxygen can be made
available to fish and how quickly they will utilize what’s
The maximum amount of dissolved oxygen in water
is called its saturation level. Saturation
level decrease as the temperature of the water increases. For example, at 70 degrees, water saturates at 8.9 parts per
million (ppm). At 80
degrees, saturation is achieved at 8.0 ppm, and at 90 degrees only 7.3
ppm. At higher
temperatures, fish metabolism also increases and they use oxygen
faster. Therefore, at 80
degrees, oxygen concentrations below 5.0 ppm may prove quickly fatal.
Here is an example used by Hal Schramm, noted
fisheries biologist, that will put this temperature/metabolism/oxygen
relationship into perspective. Ten
pounds of bass in a 15-gallon livewell will reduce the oxygen
concentration from 75% saturation to stress levels in about eight
minutes at 60 degrees; in seven minutes at 70 degrees; and in only 2.5
minutes at 85 degrees.18
Standard livewell aeration systems simply cannot
keep up with this oxygen demand.
A recirculating aeration system will raise the oxygen level in
a 15-gallon livewell from 3 ppm to 7 ppm in about eight minutes when
the water is 60 degrees. It
will take about 14 minutes at 70 degrees.
At 85 degrees, a standard livewell system simply cannot get to
7 ppm. With several fish
in the livewell, a standard livewell system is not able to keep the
oxygen level above stressful limits that may prove fatal, or will
certainly create stress on the fish that may not be recoverable.
Decreasing the water temperature with ice is one
solution, but remember that too great a change in water temperature
adds its own element of stress. Large
changes in water temperature affect lactic acid clearance and slow
metabolic recovery. In
addition, to lower the water temperature by five degrees for a full
tournament day in temperatures above 85 degrees could require up to
50-pounds of ice.
Supplemental oxygen is required, along with temperature control of the livewell, to supply fish the oxygen they need to recover from metabolic stress and promote osmoregulation. There are no two ways about it; oxygen delivery is the key to helping fish overcome the stress that comes with angling and survival in the livewell.
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