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					                               FISH BIOLOGY AND HUSBANDRY
       superclass: agnatha (jawless) [hagfish, lampreys]
       superclass: gnathostomata (jawed)
                class: chondrichthyes (cartilagenous)
                         elasmobranchii (sharks, skates, rays)
                         holocephali (chimaera)
                class: osteichthyes (bony)
                         dipnoi (lungfish)
                         crossopterygii (coelocanths)
                         actinopterygii (ray finned)
                                  chondrostei (sturgeon, paddlefish)
                                  holostei (gar, bowfish)
                                  teleosti (most bony fish, higher bony fish
Two types of teleosts
       1) soft rayed
                fins supported by soft rays
                more primitive
                cycloid scales (rounded, smooth)
                physostomous swim bladder (connected to esophagous)
                pectoral fins caudal on body
                ex. salmon, eels, goldfish, carp
       2)    spiny rayed
                some fins supported by bony spines
                more evolutionarily advanced
                ctenoid scales (toothlike extensions on caudal border)
                physoclistic swim bladder (no connection to esophagous)
                pectoral fins high on body wall
                pelvic fins more cranial than on soft rayed and can be below pectorals
                ex. angelfish, flatfish, catfish, bluegill, crappie, bass


Fins are supported by rays which are connected to the musculoskeletal system.
There are two types of rays:
        1) Soft: flexible rays made of dermal bone segments arranged end to end in a line
        2) Spines: stiff, unjointed, mineralized rays. These are found on dorsal, anal, and
             pelvic fins of higher fish
Unpaired Fins:
        Dorsal: 1 or more. In higher fish the anterior portion (or first fin) is supported by spines.
        The dorsal fin(s) are used for rapid changes of direction
        Caudal: Primary fin used for locomation; shape varies with motion type used by species
Paired Fins:
        Pectorals: soft rayed only. They are attached to the pectoral girdle at the posterior
        border of the gill cavity. They are used to stabilize and change direction
        Pelvic: variable location –thoracic or abdominal. Higher fish have thoracic pelvic fins.
        They are used to stabilize and brake
          Simple stratified squamous, but nonkeratinized surface layer
          Mucous glands: unicellular, produce mucin for cuticle
          Alarm (club) cells: produce pheromones which cause other fish to flee; no connection to
          skin surface, so pheromones only released when tissues damaged
          Cuticle: a protective substance which coats the skin. It consists of mucin,
          immunoglobulins, etc, and is anti-pathogenic. Handling of fish should be gentle and
          minimal—wear wet latex-like gloves.
         CT, vessles, nerves
         Scales: protective calcified plates which originate in the dermis; usually covered with
                  Placoid scales: a plate beneath the skin with a raised, exposed portion; includes
                  a pulp cavity and dentin (sharks)
                  Ganoid scales: rhomboid shape, overlapping (gar)
                  Cycloid scales: ovoid, smooth edges (lower teleosts)
                  Ctenoid scales: comblike with minute spikes on caudal edge (higher teleosts)
                  Absent in some species (agnatha, catfish)
         Lateral line: A sensory mechanism which consists of a series of pores running along both
         sides of the body. The pores lead to mechanoreceptors which transmit information about
         water pressures, currents, and sound.
         Spongy CT and adipose tisssue; contains melanocytes
Skin factors to consider during surgery:
         Incision should be cranial-caudal, as tension lines run dorsal-ventral
         Nonabsorbable suture recommended because wet absorbable can pull in bacteria
         Close with minimal tension and make knots far from incision, because fish tissue breaks
         Specialized skin cells allow amazing variety in fish coloration. Chromatophores contain
pigments (ex. Melanophores contain melanin) and iridophores contain reflective substances (ex
crystals). Rapid color change is produced by movemant of pigment within the dendritic
chromatophores, while long term changes are produced by changes in the number of cells.
Control is hormonal (ex melanocyte stimulating hormone) and neural. Coloration is used for
mimicry, species distinction, and sexual dimorphism.

        Oxygen concentration is much lower in water than it is in air. Also, water is 800 times
more dense than air, making it harder to pump through the respiratory system. These factors
combine to make efficient respiration challenging to fish.
        The gill is the main respiratory organ of the fish. Gills have a large surface area (10-60
times greater than the external body surface area). Gills are bilateral and covered on each side
by an operculum which opens to the environment on its caudal edge. Gills have three levels of
        1) Gill arches: Most fish have 4 gill arches on each side of the pharynx. The gill arches
            are supported by a bony skeleton. These are the halfcircles that can be seen under
            the operculum.
        2) Primary lamallae (filaments): Each gill arch has 2 rows of filaments pointing caudally
            to form a V-shape.
        3) Secondary lamellae: The dorsal and ventral surfaces of the primary lamellae are
            covered with secondary lamellae. These are semicircular folds of epithelium mounted
            perpendicular to the long axis of the primary lamellae. The secondary lamellae of
            adjacent primary lamellae interdigitate to form a mesh through which water passes.
            These secondary lamellae consist of capillaries covered by a thin epithelium, and are
            the site of gas exchange. The flow of water is counter (opposite) to the blood flow in
           the secondary lamellae, allowing the maximum concentration of oxygen to diffuse
           into the blood (up to 80%).
        Water is constantly moved across the gills by one of two mechanisms:
        1) RAM ventilation: Some fish swim constantly with their mouths open to passively
           move water.
        2) Branchial pump: Some fish actively move water. First they expand their oral cavity
           to pull water into that cavity. Then the mouth closes and they expand the opercular
           cavity to pull water across the gills and into that cavity. Water leaves via the caudal
           opercular opening.

Heart The heart is cranioventral, behind the gills, and a septum separates the gills and heart
from the abdominal cavity. The heart has 2 true chambers, but 4 sections are named.
         1) sinus venosus: thin walls with little muscle, receives venous blood
         2) atrium: chamber; weak pump with moderate cardiac muscle
         3) ventricle: chamber; thick walls with 2 layers of cardiac muscle, the main pump
         4) bulbus arteriosus: fibroelastic with valves to prevent backflow to ventricle; functions
             as a pressure regulator to protect gill vasculature
Circulation: Blood flow is similar in principle to that of mammals; heart to respiratory system for
gas exchange to body. Deoxygenated blood flows from the heart to the ventral aorta which
leads to the gills. Oxygenated blood flows from the gills to the dorsal aorta which leads to the
body. Venous return employs two systems. Venous blood from the tail region travels through
the kidneys on its way to the heart (renal portal system). Venous blood from the gut travels
through the liver on its way to the heart (hepatic portal system). Lymphatic circulation originates
as blind capillaries and empties into veins.
Blood Cells:
         Red Blood Cells: Fish have no bone marrow, and the primary source of RBCs is the head
         kidney, with contributions by the spleen and diffuse lymphomyeloid tissue in the
         mesenteries. Fish RBCs are elliptical and nucleated. Blood volume is 2-4 ml/kg for
         teleosts. I have found references for PCV of 20-30%, 30-50%. Male PCV may be higher
         than female.
         White Blood Cells: The lymphoid tissue consists of the thymus, spleen, and head kidney.
         The thymus and head kidney are the primary sources of WBCs, and the spleen and head
         kidney are the primary sites of antibody production. Fish WBCs are similar to mammalian
         WBCs, consisting of neutrophils, eosinophils, basophils, lymphocytes, and macrophages.
         However, lymphocytes tend to out number neutrophils. Fish have no lymph nodes or
         GALT. Melanomacrophage centers are islands of macrophages and melanocytes found in
         the liver, spleen, and kidneys. They contain melanin, lipofuscin, and phagocytized
         material, and undergo hyperplasia and hypertrophy during disease, starvation, or a
         polluted environment.

Gills: Gills are responsible for the excretion of CO2 and ammonia, the primary nitrogenous waste
product of fish.
Kidneys: Fish kidneys are long, spanning the length of the abdominal cavity, and retroperitoneal
against the vertebral column. Each kidney consists of two sections; the head (anterior) kidney,
and the tail (posterior) kidney. While all portions of the kidney have hemopoietic,
reticuloendothelial, endocrine, and exocrine tissue, the head kidney is predominantly hemopoietic
and the tail kidney is predominantly excretory.
Kidney excretory structure depends on whether the fish is a freshwater or saltwater species.
 Freshwater species’ kidneys , producing lots of dilute urine, have lots of large glomeruli, a high
GFR, and active reabsorption of ions.
 Saltwater species’ kidneys, producing small amounts of concentrated urine, have few or no
glomeruli, and active secretion of ions. Marine fish kidneys tend to lack distal tubules.
Aglomerular kidneys also lack the glomerulus, neck, and upper proximal tubule.
The kidney collecting ducts lead to ureters, which fuse caudally, sometimes enlarging to form a
bladder before leading to the vent. The bladder sometimes has an osmoregulatory function.
Most fish, however, have no bladder, and the fused ureters lead directly to the vent. Urea is the
main waste product in fish urine.

         Maintaining electrolyte balance in an aquatic environment is another challenge fish face
and their coping mechanisms depend on whether they are freshwater or saltwater species.
Freshwater: Freshwater fish are hypertonic in relation to their environment, thus water tends to
diffuse in and ions tend to diffuse out (mainly via the gills). Freshwater fish drink very little and
produce lots of dilute urine. They actively reabsorb ions in the kidneys and across the gills.
 Saltwater: Saltwater fish are hypotonic in relation to their environment, thus water tends to
diffuse out and ions tend to diffuse in (mainly via the gills). Marine fish drink a lot and produce
small amounts of concentrated urine. They excrete ions in the kidneys, gills and the gut.
Elasmobranchs have a rectal gland, which secretes excess sodium and empties into the rectal
intestine. Gills contain chloride cells that secrete sodium and chloride with ATPase pumps.

         The swim bladder (or air bladder) is a gas filled organ used for maintaining buoyancy.
The volume of the organ is adjusted to control buoyancy. In bottom dwellers or fast swimming
fish the swim bladder may be reduced or absent. It originates embryologically as an alimentary
canal outgrowth.
Physoclistic fish (such as the spiny-rayed fish) have no connection between the esophagus and
the swim bladder. Gas must enter and exit the organ via a capillary network on its cranioventral
Physostomatous fish (such as the soft-rayed fish) have a pneumatic duct which connects the
swim bladder to the esophagus, through which gas can enter or exit.
The swim bladder also has some secondary functions, including sound reception , sound
production, and water pressure detection.

         In general, the fish digestive system is similar to the mammalian system. The length of
the digestive tract varies with the species, but, as in mammals, herbivores have longer digestive
tracts than carnivores or omnivores.
Stomach: some fish have a stomach, some do not
Intestines: The fish intestinal tract is not as distinctly separated as the mammalian tract, and the
areas are referred to as pyloric, middle, and rectal. The spiral colon exists in primitive bony fish
and sharks, and consists of a corkscrew ridge within the distal intestine (increases surface area).
Pyloric cecae: blind sacs near the pylorus which release digestive enzymes and absorb nutrients;
species vary in number, from 2 to multiple.
Liver: May be over or around the stomach; 1-3 lobes (usually 2); usually has a gall bladder; the
liver frequently has a high lipid content, and may appear yellowish.
Pancreas: usually diffuse in/around liver (hepatopancreas if tissues completely interfused); some
have discrete pancreas (sharks and skates)
Nutrition: The nutritional requirements vary with species. In general, the primary energy
sources of fish are proteins and lipids. Most fish have a limited ability to metabolize

Forebrain: main structure = olfactory bulbs, which are well developed in muddy water fish
Midbrain: main structure = optic lobes, which are well developed in fast, clear water hunters
Cerebellum: well developed
        Pituitary: similar to mammmals; reproduction and osmoregulation
        Thyroid; most fish have follicles associated with surface of heart, aorta, and arteries,
        some have discrete; similar to mammals plus osmoregulation
        Interrenal tissue: associated with head kidney; adrenocorticosteroids
        Suprarenal tissue (chromaffin tissue): distributed along posterior cardinal vein and
        kidney and with interrenal tissue; adrenalin and noradrenalin
        Ultimobranchial gland: similar to parathyroid; cords of cells near sinus venosus;
        Corpusles of Stannius: within or near kidney; secretes renin like substance; hypocalcin
        decreases serum Ca levels; osmoregulation?
        Pancreatic islets: diffuse tissue around intestine and pyloric cecae
        Caudal neurosecretory system (urophysyis): ventral extension of caudal end of spinal
        cord; enlarged neurons which release hormones (urotensins); osmoregulation?

         Some fish (electric eel, ray, and catfish) possess organs which produce and detect
electrical fields. The producing organ is made of stacks of flat cells derived from muscle cells.
Stimulation causes a reverse of polarity on one side of the cells, to create a battery system with a
voltage produced. Electrorecepter organs are sensory cells within pores. The system is used to
stun prey, in defense, and for electrolocation.

         Some fish are able to illuminate themselves or objects close to them. Most are self-
luminous, using the enzyme luciferase which interacts with the substance luciferin in a special
cell, the photophore. Some fish require symbiotic luminous bacteria to inhabit gland like
structures under their skin. Biolumination is used for concealment (from below), display
(attracting prey or mates), and to illuminate prey close to the mouth.

There are three types of environments or water systems/aquariums for fish.
        3) Freshwater: Freshwater environments have a specific gravity of 1.000, and a Ph
            range of 7.0-7.6.
        4) Brackish water/Estuaries: In these environments the specific gravity varies with tides
            and seasons, but brackish aquariums are usually maintained close to 1.010. Ph is in
            the range of 7.0-7.6.
        5) Saltwater: Saltwater environments have a specific gravity of 1.021-1.025, and a Ph
            range of 8.1-8.3. In aquariums, salinity is produced by mixing synthetic sea salts to

         The most important factor in fish disease and mortality in an aquarium is water quality.
Poor water quality can cause sickness directly or indirectly, by leading to stress which lowers
resistance to disease. An aquarium is not a natural environment; it is a closed system, and
unless carefully controlled, can accumulate pathogens, toxins, and organic nutrients which
promote the growth of bacteria and algae.
Fish, because they utilize oxygen and produce waste, constitute a primary source of water quality
The bioload is, therefore, important to control. Safe stocking densities are up to 1 inch of
fish/gallon (nonaerated tank) up to 3 inches of fish (aerated tank) for freshwater. Saltwater
tanks require a lower bioload, starting at 0.5 inches/gallon as a safe maximum. All
measurements exclude the tail fin.

           The most important waste product in most aquariums is ammonia, because it is toxic to
fish and decreases the oxygen carrying capacity of the blood. The major source of ammonia is
the metabolic waste of the fish themselves, so even with an appropriate bioload, controlling
metabolic waste is vital. This can be accomplished by bacteria which transform ammonia to less
toxic products. Bacteria of the genus Nitrosomas oxidize ammonia to nitrite, which is less toxic
than ammonia. Then bacteria of the genus Nitrobacter oxidize nitrite to nitrate, which is even
less toxic than nitrite. This process is known as nitrification.
            In order for nitrification to occur, the bacteria must be present and healthy. Thus a new
tank requires either store bought cultures or ―seeding‖(a handful of substrate) from an
established tank to introduce the bacteria. Then a conditioning period is necessary, during which
only a few fish are resident to provide ammonia for the bacterial population to establish. After
about a month more fish and a few plants can be added.
           Nitrate can be toxic in high concentrations, so it too must be removed from the system.
It is utilized, and thus reduced, by living plants. Regular, partial water changes (25% every 2
weeks) are also essential.

        The Ph of the water needs to be maintained within a specific range. The tendency is for
the Ph to decrease over time due to organic acids and compounds from the decay of fish waste,
food, etc.
Decreased Ph:
        Effects: direct health problems for fish
                 inhibits Nitrosomas and Nitrobacter and, therefore, nitrification
        Control: check bioload and remove excess fish, food, dead plants, dead fish
                 regular, partial water changes (25% every 2 weeks)
                 use calcium rich substrate
                 temporary solutions-sodium bicarbonate
Increased Ph: (uncommon)
        Effects: direct health problems for fish
                 ammonia is more toxic (less in the less toxic ionized ammonium state)
        Control: check substrates for a calcium rich one
                 temporary solutions- acetic acid, peatmoss

          The oxygen concentration of water is much lower than that of air, because it is much less
soluble in water. Air is 21% oxygen, water is about 0.7% oxygen. Also, as temperature or
salinity increases, oxygen becomes even less soluble. Low dissolved oxygen in the water leads to
respiratory distress, and fish are frequently seen gasping at the water surface. Low oxygen can
also inhibit Nitrosomas and Nitrobacter.
          Oxygen is replenished by:
                      1) air: water interface-it is desirable to have a long tank with a large
                          surface area.
                      2) aeration- provided by aerating filter systems, air stones, or foam
          Oxygen must also be circulated in the water; it will not evenly distribute in stagnant
water. Most aerating systems also circulate the oxygen.

         Most fish are poikilotherms, meaning that their body temperature varies passively with
that of the surrounding water. Some fish are called ectotherms, defined as fish which obtain heat
from their environment while a few are ―endothermic‖ because they can maintain a body
temperature 10 – 20 degrees higher than their environmental temperature (fast swimming, large
fish like tuna and sharks). Howeve, all fish have physiologically preferred ranges. The ideal
water temperature range for fish is species dependent, and can vary from 0° to 45° C. For
example, tropical fish tend to prefer warmer water, while most stream living fish prefer colder
           Problems associated with temperature tend to occur when the water temperature
changes too rapidly and causes stress and can be fatal. Temperature changes should not be
more that 1° C/ 2 minutes. Nor should fish be transferred from environments that differ by more
than 2°-3° C.

         Consistent lighting schedules are necessary to prevent stress, and fish need 8-10 hours of
darkness during the 24 hour day to remain healthy.
         The type of lighting system used varies. Fluorescent lighting is the most common choice,
with 2.4 watts/ gallon and in the natural spectrum. Incandescent lighting is not recommended as
it can increase the water temperature. Direct sunlight is not recommended either, because it
increases the water temperature and encourages algae growth. Reef systems require special
lighting systems which provide high intensity light to the photosynthetic organisms.

         These compounds are routinely put into municipal tap water, but they can be lethal to
fish. Chlorine should be removed from a new tank by letting it sit for 3 to 5 days with occasional
agitation. For small water changes (10%) chemical dechlorinators like sodium thiosulfate can be
used. Charcoal filters will also bind some chlorine. Test kits are available to check chlorine levels
in water.

        Heavy metals dissolved in water can be toxic to living organisms. These can leach into
water from metal pipes, tank frames, or decorations. Saltwater, especially, corrodes metal, and
no metal objects should be placed in saltwater tanks.

         Filtration is necessary to remove contaminants from a closed aquarium system. These
contaminants include suspended particles and dissolved chemicals created from metabolic waste
and decay of organic matter.
Three filtration systems are necessary in an aquarium: mechanical, chemical, and biological
         1)Mechanical Filtration: physically traps small suspended particles and parasites, and
         maintains water clarity. Water flows through media such as sand, gravel, floss, plastic
         blocks or balls, or diatemaceous earth.
         2)Chemical Filtration: chemically removes dissolved compounds and elements. The most
         common medium is activated carbon, which binds and traps chemicals (copper, ozone,
         chlorine, medications). Another common chemical filter used in saltwater tanks is protein
         skimming/foam skimming, in which air is vigorously mixed through the water, creating
         bubbles. As the bubbles rise, compounds adhere to them, creating a foam on the water
         surface which is skimmed off. Other media include UV light, ion exchange resins, peat
         moss, ozone, dolomite limestone, and zeolites.
         3)Biological Filtration: living organisms transform toxic compounds to less toxic
         compounds, as in nitrification. Media include anything which provides a substrate for
         bacteria to grow on, such as the plastic blocks, balls, or gravel in trickle filters or under
         gravel filters, live rock reefs, or decorative objects. It is important to remember that the
         bacteria are susceptible to changes in Ph or oxygen levels, and medications.
         White lobulated organs suspended in mesenteries, beside swim bladder. Ovaries are
usually continuous with oviduct, but some have open bursa similar to mammals; oviduct leads to
pore between anus and urinary pore. Male reproductive system usually empties separately from
urinary system, but some fuse as urogenital sinus.
         Control is hormonal, with environmental cues important (photoperiod, temperature,
water flow). Spawning may be induced with mammal gonadotropins. Spawning is seasonal.
         Synchrony of ovulation varies with species. In some fish all eggs are synchonous and fish
spawn once/season. Some fish spawn more than once/season and have more than one set of
synchronous eggs. Some fish have asynchronous eggs in all stages of development.
         Oviparous: usually external fertilization after release of eggs. Eggs may be buoyant and
float at different depths or demersal (heavier than water and sink). They may be deposited in
clumps or singly, or attached to substrates, put in nests, or held on the body. Fish may spawn in
mass or in pairs. Oviparous egg retainers use internal fertilization but incubate the eggs only
briefly and quickly release them.
         Ovoviviparous: internal fertilization and incubation, but no additional nourishment from
mother while inside. Eggs hatch internally, with newly hatched live young released.
         Viviparous: internal fertilization, with eggs retained and nourished in oviduct by maternal
secrtetions (pseudoplacenta) Live young are released.
         Genetics: Most species have homogametic females, some have homogametic males, and
some have males with one sex chromosome (X) and females with 2 (XX).
         Sexes: Most species have separate females and males throughout life, but there are
many cases of hermaphroditic fish.
                  Hermaphroditic unisexuality: females reproduce only female offspring(sperm of
                  related fish stimulate eggs to divide, but male pronucleus degenerates)
                  Synchronous hermaphroditism: 1 individual has reproductive tissues of both
                  sexes; cross fertilize or self fertilize.
                  Asynchronous hermaphroditism: 1 individual transforms from one sex to the
                  other with age. Protoandrous hermaphroditic (male to female) or protogynous
                  hermaphrodites (female to male).

Fish behavior is complex and species specific. There are some general behaviors that one should
consider for the best husbandry conditions and fish health and reproduction.
Geotaxis: posture responsive to gravity
Phototaxis: swim towards or from light
Electrotaxis: navigate by electrical fields
Thygmotaxis: seek or avoid contact with other fish or objects
Rheotaxis: reaction to current flow
Chemotaxis: respond to chemicals in water
Social behaviors; some are solitary, some school
Territory: fight or hide
Interspecies relationships: symbiotic or aggressive or indifferent
Feeding behavior: bottom or surface feeders, dusk and dawn or midday feeders
Reproductive behavior: nonguarders-- open spawners or brood hiders (need substrate); guarders
– use substrate or build nests (need material); bearers—internal or external

          Fish should be fasted 24-48 hours before anesthesia. Anesthesia requires an anesthetic
tank and a recovery tank (water identical to preanesthetic tank). For short procedures the fish
may be induced, taken out for the procedure, and placed in the recovery tank. It is necessary to
keep the fish moist and oxygenated, however, so longer procedures require a recirculationg
anesthesia tank/table. The fish is held in a V shaped trough. The anesthetic in water is passed
into the mouth of the fish and through the gills, then pumped back into the delivery system to
recirculate. The water should be well aerated. Recovery involves placing the fish in
nonanesthetic water. The fish may be moved head first through the water to move water over
the gills to speed recovery or assist an overanesthetized fish.

      Stage I: Induction
              Erratic swimming, disorientation, increased respiration, loss of tactile response,
              excitement phase, some loss of equilibrium, reduced activity
      Stage II: Sedation
              Loss of equilibrium, slow swimming without direction, decreasesd responses
      Stage III: Anesthesia
              Plane 1: complete loss of equilibrium, swimming and respiratory activity slowed,
              still responsive to stimuli
              Plane 2: Surgical plane—unable to swim, respiration shallow, no response to
              Plane 3: cessation of opercular movements
      Stage IV: Premortem
              Spasmodic overdistension of operculum and cardiac failure

     MS222 solution(Finquel/ Tricaine methane sulfonate)
     The most commonly used. Induction 1-5 min, recovery 10-15 min. Dose is species
     specific, with averages of 20-50 mg/L sedation and 50-100 mg/L anesthesia.
     Quinaldine and Quinaldine Sulfate solution
     Wider margin of safety than MS222 but poor analgesia, not good for surgery
     Benzocaine solution
     Metomidate solution
     Ketamine, ketamine-xylazine IM
     Induction 10-20 min, duration of surgical anesthesia 10-20 min; especially good in sharks
     to avoid muscle spasms

     Fish do respond to noxious stimuli. Bony fish produce opioid like hormones, so opioids
     are probably effective; little research

         2000 AVMA panel on euthanasia approved: buffered MS222 at 500 mg/L, benzocaine
hydrochloride, barbiturates, inhalant anesthesia, CO2, 2-phenoxy ethanol.
         AVMA panel conditionally approved (needs scientific justification in research): stunning by
blow followed by decapitation and pithing brain, decapitation and pithing brain.
Diagnosis: in diagnosing a diseased fish it is essential to examine the fish and the environment-
check the water quality.
Therapy: there are two options for treating fish; water treatment or treat the fish internally.
1) Water treatment:
        a) Dissolving soluble chemicals into the primary tank.
        Advantages: easy, low stress for fish, eliminates pathogens in tank, such as external
        Disadvantages: damages biological filter (can inhibit or kill bacteria of nitrification),
        medications are absorbed by chemical filter, medication must be soluble

        b) Bath or Dip: the fish is placed in a more concentrated solution in a different,
        treatment tank for minutes to hours (bath) or seconds to minutes (dip).
        Advantages: fast, doesn’t alter the primary tank
        Disadvantages: stresses fish, more toxic levels used, pathogens in tank are not
        Examples: Briefly placing a saltwater fish into freshwater or a freshwater fish into
        saltwater can be utilized as a dip, and can effectively remove external parasites.

2) Internal treatment:
        a) Oral medications : in food, usually in a gelatin or powdered form
        Advantages: easy, less stress
        Disadvantages: Fish must eat, amount ingested varies
        b) Injections
        Advantages: certain of the amount
        Disadvantages: difficulty, stress, cost
        Intraperitoneal (IP) - best method in fish, in caudal abdomen
        Intravenous (IV) - caudal tail vein, on ventral midline of tail caudal to vent
        Intramuscular (IM) - poorly vascularized, needle tends to make channels in muscle which
        allow drug to leak out.

There are approximately 30,000 species of fish existing in a wide range of environmental niches.
Because of this diversity, many physiological adaptations have evolved which offer researchers a
variety of experimental models. Agnathous fish, chondrichthyes, and osteichthyes have been used
in research. Fish are easy to maintain in large numbers and reproduce fairly rapidly.
Growth and regeneration: ex. nephron neogenesis after toxin induced injury
Embryology: fish reproduce readily in large numbers with observable embryos
Genetics: gene cloning, transgenics (large, external eggs)
Neoplasia: fish models tend to have lower spontaneous neoplasia rates than mammals, hence
require smaller n for studies.
Ocular: retina and neural connections
Physiology: ex. aglomerular fish

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