Fisheries and Aquaculture Management
Lecture 5:
Fish and its Environment
Introduction
Class : Pisces
 Fishes are poikilothermic (organisms with
fluctuating internal temperatures), aquatic
vertebrates with jaws.
 The body is streamlined.
 It is differentiated into head, trunk and tail.
Between head and trunk, the neck is absent.
 Locomotion is effected by paired and median fins.
 The body has a covering of scales.
 They are of various types like placoid, cycloid,
ctenoid and ganoid scales.

Placoid
cycloid
ctenoid
 The body muscles are arranged into segments
called myotomes.
 The Alimentary canal consists of a definite
stomach and pancreas and terminates into cloaca
or anus.
 Respiration is performed by gills. Gill slits are 5-7
pairs.
 They may be naked or covered by an operculum.
 The heart is two chambered (an auricle and a
ventricle).
Basic Piscine Shapes
 Most fishes fall into one of six broad categories
based on body configuration: fusiform ( rover
predator, piscivores (lie-in wait predator), surface
oriented bodied fish, and eel-like fish.
 The way a fish looks is a good indicator of how it
"makes a living.“
 Body shape, mouth location and size, tail shape
and color can reveal a lot about a fish's lifestyle.
Fusiform:
 Fusiform, or streamlined, fish like the barracuda or jack are
capable of swimming very fast. They usually live in open
water.
 This is the body shape that comes to mind when most
people think of fish: streamlined (fusiform), with a pointed
head ending in a terminal a narrow caudal peduncle
tipped with a forked tail.
 The fins are more or less evenly distributed about the
body providing stability and maneuverability.
 Such fish typically are constantly on the move, searching
out prey, which they capture through pursuit.
Laterally compressed:
 Fish that are laterally compressed (flattened from side to side or
flatfish) usually do not swim rapidly (some schooling fish are an
exception).
 However, they are exceptionally maneuverable.
 Many, like the angelfish, are found near coral reefs.
 Their shape allows them to move about in the cracks and
crevices of the reef.
 A flounder is a laterally compressed(deep-bodied) fish that live
on its side on the bottom.
 In these fish, the eye on the downward side migrates during
development to the upward side, and the mouth often assumes
a peculiar twist to enable bottom feeding.
 Flat fish have the most extreme morphologies of bottom fish.
Depressed:
 Depressed fish (flattened from top to bottom), like stingrays, live
on the bottom.
 Bottom fish possess a wide variety of body shapes, all of them
adapted for a life in nearly continuous contact with the bottom.
 In most such fish, the swim bladder is reduced or absent, and
most are flatten in one direction or another.
 Bottom fish can be types: bottom rovers, bottom clingers,
bottom hiders, flat fish and rattails.
 Bottom rovers have a rover-predator-like body except that the
head tends to be flattened, the back humped, and the pectoral
fin enlarged.
 Bottom clingers are usually small fish with flattened heads, large
pectoral fins and structure (usually modified pelvic fins) that
allow them to adhere to the bottom.
Depressed (Contd’):
 Such arrangements are handy in swift streams or intertidal areas
that have strong current e.g. the gobies.
 Bottom hiders are similar in many respects to the bottom
clingers, but they lack the clinging devices and tend to have more
elongated bodies and smaller heads.
 These forms usually live under rock or in crevices, or lie quietly
on the bottom in still water.
 In contrast, skates and rays are flatten dorso-ventrally
(depressiform) and mostly move about by flapping or undulating
their extremely large pectoral fins.
 Not only is the mouth completely ventral in these fish, but their
main water intakes for respiration (the spiracles) is located on
top of the head.
Eel-like:
 They have elongated bodies, blunt or wedged shape, and
tapering or rounded tails.
 If paired are present they are small, while the dorsal and anal fins
are typically quiet long.
 Scales are small or embedded and even absent.
 In cross section, their bodies can range from compressed to
round.
 Eel-like fish are particularly well adapted for entering small
crevices and holes in reefs and rocky areas, for making their ways
through beds of aquatic plants, and for burrowing into soft
bottoms.
Tails
The shape of the tail can be an indicator of how fast a fish usually
swims.
1. Crescent-shaped:
 Fish with crescent-shaped tails, like swordfish, are fast swimmers
and constantly on the move.
2. Forked:
 Fish with forked tails, like the striped bass, are also fast
swimmers, though they may not swim fast all of the time.
 The deeper the fork, the faster the fish can swim.
3. Rounded:
 Fish with a rounded or flattened tail are generally slow moving,
but are capable of short, accurate bursts of speed.
Mouths
 The location and size of the mouth can be a good indicator
of the food a fish eat and where it lives.
 Fish with large mouths generally eat large food items like
another fish; however, the whale shark eats very small
organisms which it strains from the water with its huge
mouth.
 Fish with small mouths eat small food items: small
crustaceans or molluscs; and, fish with tiny mouths eat tiny
things like zooplankton
Endotherms - animals who derive most or all of their body heat from
their own metabolism e.g. Mammals, birds, some fish, and numerous
insects are endothermic.
A. Endothermic Regulation
Advantages: The animal can remain active at a wide range of
environmental temperatures.
Note: The internal temperature of endotherms does have a range. It
varies from time to time and place to place. However, the great
constancy is the core temperature temperature deep within the body.
The core temperature can vary such as in animals that hibernate.
Disadvantages:
1) Endothermy takes a great deal of energy
2) Birds and mammals can't tolerate much change in their core
temperature.
3) There are body size restrictions to endothermy - the animal can't be
too small.
Ectotherms - animals who warm their body mainly by absorbing
heat from their surroundings e.g. most Invertebrates, fish,
amphibians, and reptiles.
B. Ectothermic Regulation
Advantage: Takes little energy
Disadvantage: In cool weather, activities slow down
 Ectotherms can tolerate a wider range of temperatures than
endotherms.
 Ectotherms generate their body temperature by behaviorabsorbing sun or contact with a warmer surface.
Ecto- versus endothermic organisms
 Body temp of ectotherm (lizard) decreases with decreasing
environmental temp, while the temp of the endotherm remains
constant.
 The latter has to increase its metabolic rate (MR) in response to cold
and hot. MR of ectotherm follows the change in environmental temp.
 Notice the much higher MR of endotherm.
Heat production and conservation in ectotherms
 Heat produced through muscle activity gets lost through the gill.
 Tuna, great white sharks and mackerels all have the ability to trap most
of the heat through a countercurrent heat exchanger which maintains
a constant gradient over a longer distance (incoming arteries are
getting warmed up by outgoing veins) to transfer the heat.
 Why is this beneficial? Greater power output.
Heat production and conservation in ectotherms
Heat conservation: Countercurrent heat exchange
 Air conducts heat poorly and is therefore a good insulator.
Structures that trap air can insulate: the under-fur in mammals and
down feathers in birds.
 Decreased surface area: smaller appendages and larger body size.
 Decreased blood flow reduces heat loss.
 Water is an effective conductor of heat, quickly draining heat away
from an organism. Marine mammals either have fur or blubber.
How do endotherms maintain a constantly high temperature?
Thermoregulation in mammals and birds involves generating
heat, retaining heat, and cooling mechanisms.
1. Thermiogenesis - active generation of heat through
a) Oxidative metabolism
b) Shivering - rapid contraction of opposing muscle- The
conversion of ATP to ADP releases heat.
c) Non-shivering thermiogenesis - occurs in some mammals
and a few birds; increase in metabolism triggered by
hormones
d) Utilization of brown fat - fat with numerous mitochondria
(thus brown color). These mitochondria release heat from
metabolism and do not generate ATP. Brown fat is found in
human infants, bats and hibernating mammals.
How do endotherms maintain a constantly high temperature?
 Brown adipose tissue: Abundant with fat and mitochondria, rich
blood supply.
 A protein called thermogenin uncouples the movement of protons
across membranes from ATP production, burning fuel without
producing ATP but heat is still released.
 Found in newborn humans and many small mammals and
hibernators.
How do endotherms maintain a constantly high temperature?
2. Regulating heat exchange - slowing heat loss
a) Vasoconstriction of vessels close to surface of body prevents heat
loss from blood flowing close to the surface. This also helps to keep
core temperature stable. Costly- the limbs cool down and the
muscles don't work as well.
b) Insulation - fat, boy hair, feathers
3. Regulating heat exchange - cooling
a) Vasodilation of vessels close to surface of body allows heat escape
by radiation from blood vessels.
b) Convection - heat lost by the movement of air across the surface of
the body; evaporative cooling
c) Conduction - the direct transfer of heat by contact to a cooler solid;
such as an animal sitting in a pool of cold water or on a cool rock.
Endotherms also can help to regulate their temperature by behavioral
mechanisms: ceasing activity and finding a cooler environment.
How do endotherms maintain a constantly high temperature?
Controlling thermoregulation
 In humans, the hypothalamus, the body's
thermostat, monitors the temperture of the blood
flowing through it and also receives information from
sensory receptors in the skin.
 In response to temperatures below the normal range,
the thermostat activates thermiogenesis and heatsaving mechanisms.
 In response to warmer temperatures, the thermostat
activates body cooling mechanisms such as
vasodilation, sweating, or panting.
Hydrostatic Pressure
Q.
What Causes Buoyancy? : Pressure!
Recall: The pressure at depth d in a liquid is
where ρ is the liquid’s density, and p0 is the pressure at
the surface of the liquid. Because the fluid is at rest, the
pressure is called the hydrostatic pressure. The fact that
g appears in the equation reminds us that there is a
gravitational contribution to the pressure.
A floating object is in static equilibrium
Buoyancy
 Fishes have two means of maintaining buoyancy
 Neutral buoyance
 Regulation by swimbladder
 Neutral Buoyancy
 Many fish are functionally weightless in water
 This allows them to save energy while staying in a
certain area
Q. What is Required for Neutral Buoyancy?
 Specific gravity must equal that of surroundings
 Fresh water sp. gr. = 1
 Salt water sp. gr. = 1.026
 Different regions may have slight specific gravity
differences due to dissolved materials
Strategies to Maintain Neutral Buoyancy
1. Body made of large quantities of low density
compounds
 Low Density Bodies
 Many fish have large quantities of lipids
 Specific gravity < 1
 Large livers filled with squalene
 Hydrocarbon sp.gr. 0.8
 A few fish have trigliceride oils in bones
Strategies to Maintain Neutral Buoyancy
2. Fins are shaped and angled to generate forward
lift
 Fins Designed for Lift
 Leading edges of fins help maintain
position
 Small amount of energy gives large
amount of lift
 Also, body drag is eliminated by shape of
fins and body
Strategies to Maintain Neutral Buoyancy
3. Reduction of heavy tissues like bone
 Bones are thin
 Living in water does not require as much
support
 Sp. gr. of bone = 2.0
 Cartilage is less dense than bone
 Sp. gr. = 1.1
 Many fish do not have a bony skeleton
Strategies to Maintain Neutral Buoyancy
3. Having a swimbladder filled with an appropriate
amount of air
 Major organ for buoyancy control
 Allow for precise control of total body
specific gravity
 Normally 5% of marine fish body, 7% of
freshwater body
Types of Swimbladders
1. Physostomous Swimbladders
 Fish must swallow air to deliver it to the
swimbladder
 Requires these fish to live in shallow water
 They cannot take in enough air to be buoyant at
deep water and actually move to deep water
2. Physoclistous Swimbladder
 Swimbladder is inflated via circulatory system
 Rete mirabile (wonderful net)
 Gas gland
 Fish are able to live away from the surface
Regulation by Swimbladders
1. Modified Swimbladders
 Some fish have more than one swim bladder
Often fish with great vertical movements
Allows them to gain or lose air more quickly
2. Bottom Dwellers
 Swimbladder is not needed
 Reduced
 Absent
 The normally bottom dwelling sea robin can use
their pectoral fins to produce lift while
swimming.
Regulation by Swimbladders
3. Species Living in Flowing Water
 Usually have reduced swim bladders
 Less buoyancy helps them to maintain a given area
 Their buoyancy requirement is met by other means than
the swimbladder
4. Mola mola
 Has no swimbladder
 Commonly a surface dweller – sometimes floats on the
surface
 Large amounts of body fluid that are about ½ the specific
gravity of seawater
 The cartilaginous fish (e.g. sharks and rays) and lobed
finned fish do not have swim bladders. They can control
their depth only by swimming (using dynamic lift)
Uptake of oxygen in and carbon dioxide release from the fish
 During respiration fish, like other animals, take in oxygen
and give out carbon dioxide.
 The process is done by using gills in almost all fish
although some can also use the skin and some have lung
like structures used in addition to gills.
 When a fish respires, a pressurised gulp of water flows
from the mouth into a gill chamber on each side of the
head.
 Gills themselves, located in gill clefts within the gill
chambers, consist of fleshy, sheet like filaments transected
by extensions called lamellae.
Uptake of oxygen in and carbon dioxide release from the fish
Diagram showing the structure for respiration (gas exchange) in fish.
Uptake of oxygen in and carbon dioxide release from the fish
 As water flows across the gills, the oxygen within them
diffuses into blood circulating through vessels in the
filaments and lamellae.
 Simultaneously, carbon dioxide in the fish’s bloodstream
diffuses into the water and is carried out of the body
Effects of oxygen levels on oxygen uptake by fish
 It is commonly thought that if there is not enough oxygen
in the water, then the fish will be seen gasping at the
surface but this is a last resort means to breathe.
 The first indication there may be a dissolved oxygen
problem in the water is when the fish become unusually
lethargic and stop feeding.
 As oxygen levels decrease, the fish do not have enough
energy to swim and feeding utilizes yet more oxygen.
 In terms of managing any aquatic system, it is always
advisable to increase the aeration when any fish start to
behave abnormally, before adding any form of medication
to the water.
Osmoregulation and Excretion
 An organism maintains a physiological favorable
environment by osmoregulation, regulating
solute balance and the gain and loss of water
and excretion, the removal of nitrogencontaining waste products of metabolism.
 Osmoregulation balances the uptake and loss of
water and solutes.
 Over time, the rates of water uptake and loss
must balance.
Osmoregulation and Excretion
 Water enters and leaves cells by osmosis, the movement of water
across a selectively permeable membrane.
 Osmosis occurs whenever two solutions separated by a membrane
differ in osmotic pressure, or osmolarity (moles of solute per liter of
solution).
 If two solutions separated by a selectively permeable membrane
have the same osmolarity, they are said to be isoosmotic.
 There is no net movement of water by osmosis between isoosmotic
solutions, although water molecules do cross at equal rates in both
directions.
 When two solutions differ in osmolarity, the one with the greater
concentration of solutes is referred to as hyperosmotic, and the
more dilute solution is hypoosmotic.
 Water flows by osmosis from a hypoosmotic solution to a
hyperosmotic one.
Osmoregulators expend energy to control their internal
osmolarity; osmoconformers are isoosmotic with their
surroundings.
 An osmoregulator is an animal that must control its internal
osmolarity because its body fluids are not isoosmotic with the
outside environment.
 An osmoregulator must discharge excess water if it lives in a
hypoosmotic environment or take in water to offset osmotic loss if
it inhabits a hyperosmotic environment.
 Osmoregulation enables animals to live in environments that are
uninhabitable to osmoconformers, such as freshwater and
terrestrial habitats.
 It also enables many marine animals to maintain internal
osmolarities different from that of seawater.
 Whenever animals maintain an osmolarity difference between the
body and the external environment, osmoregulation has an
energy cost.
Osmoregulation and Excretion
 Most animals, whether osmoconformers or osmoregulators,
cannot tolerate substantial changes in external osmolarity
and are said to be stenohaline.
 In contrast, euryhaline animals—which include both some
osmoregulators and osmoconformers—can survive large
fluctuations in external osmolarity.
 For example, various species of salmon migrate back and
forth between freshwater and marine environments.
 The food fish, tilapia, is an extreme example, capable of
adjusting to any salt concentration between freshwater and
2,000 mosm/L, twice that of seawater.
Osmoregulation and Excretion
 Marine vertebrates and some marine invertebrates are
osmoregulators.
 For most of these animals, the ocean is a strongly dehydrating
environment because it is much saltier than internal fluids, and
water is lost from their bodies by osmosis.
 Marine bony fishes, such as cod, are hypoosmotic to seawater
and constantly lose water by osmosis and gain salt by diffusion
and from the food they eat.
 The fishes balance water loss by drinking seawater and actively
transporting chloride ions out through their skin and gills.
Sodium ions follow passively.
 They produce very little urine.
Osmoregulation and Excretion
 Marine sharks and most other cartilaginous fishes
(chondrichthyans) use a different osmoregulatory “strategy.”
 Like bony fishes, salts diffuse into the body from seawater,
and these salts are removed by the kidneys, a special organ
called the rectal gland, or in feaces.
 Unlike bony fishes, marine sharks do not experience a
continuous osmotic loss because high concentrations of urea
and trimethylamine oxide (TMAO) in body fluids leads to an
osmolarity slightly higher than seawater.
 TMAO protects proteins from damage by urea.
 Consequently, water slowly enters the shark’s body by
osmosis and in food, and is removed in urine.
Osmoregulation and Excretion
 In contrast to marine organisms, freshwater animals are
constantly gaining water by osmosis and losing salts by
diffusion.
 This happens because the osmolarity of their internal fluids
is much higher than that of their surroundings.
 However, the body fluids of most freshwater animals have
lower solute concentrations than those of marine animals,
an adaptation to their low-salinity freshwater habitat.
 Many freshwater animals, including fish such as perch,
maintain water balance by excreting large amounts of very
dilute urine, and regaining lost salts in food and by active
uptake of salts from their surroundings.
Osmoregulation and Excretion
 Salmon and other euryhaline fishes that migrate between
seawater and freshwater undergo dramatic and rapid
changes in osmoregulatory status.
 While in the ocean, salmon osmoregulate as other marine
fishes do, by drinking seawater and excreting excess salt
from the gills.
 When they migrate to fresh water, salmon cease drinking,
begin to produce lots of dilute urine, and their gills start
taking up salt from the dilute environment—the same as
fishes that spend their entire lives in fresh water.
Excretion of Wastes
 Elimination of nitrogenous waste products from the body is a
process called excretion.
 The excretory products are formed during the amino acid
catabolism.
 These excretory products are-harmful to the body, if they are
accumulated.
 In some animals that live partly in water and partly on land, in such
forms the toxic ammonia is changed into less toxic urea in liver.
 Urea can be retained in the body for much longer period than
ammonia.
 Terrestrial animals which have scarcity of water cannot afford to
loose water from their body; In such forms nitrogenous waste is
converted into still less toxic substance called uric acid. It is excreted
in crystalline form.
Excretion of Wastes
 Animals that excrete nitrogenous wastes as ammonia
need access to lots of water.
 This is because ammonia is very soluble but can be
tolerated only at very low concentrations.
 Therefore, ammonia excretion is most common in aquatic
species.
 Many invertebrates release ammonia across the whole
body surface.
 In fishes, most of the ammonia is lost as ammonium ions
(NH4+) at the gill epithelium.
 Freshwater fishes are able to exchange NH4+ for Na+ from
the environment, which helps maintain Na+
concentrations in body fluids.
Excretion of Wastes
 Because ammonia is so toxic, it can be transported and
excreted only in large volumes of very dilute solutions.
 Instead, mammals, most adult amphibians, sharks, and
some marine bony fishes and turtles excrete mainly urea.
 Urea is synthesized in the liver by combining ammonia
with carbon dioxide and is excreted by the kidneys.
 The main advantage of urea is its low toxicity, about
100,000 times less than that of ammonia.
 Urea can be transported and stored safely at high
concentrations.
 This reduces the amount of water needed for nitrogen
excretion when releasing a concentrated solution of urea
rather than a dilute solution of ammonia.
Excretion of Wastes
 The main disadvantage of urea is that animals must
expend energy to produce it from ammonia.
 The amount of nitrogenous waste produced is coupled to
the energy budget and depends on how much and what
kind of food an animal eats.
 Because they use energy at high rates, endotherms eat
more food—and thus produce more nitrogenous
wastes—per unit volume than ectotherms.
 Carnivores (which derive much of their energy from
dietary proteins) excrete more nitrogen than animals that
obtain most of their energy from lipids or carbohydrates.
Excretion in fishes
The functions of excretion and osmoregulation are closely
related and are performed by gills and kidneys in fishes.
Excretion and Osmoregulation in Freshwater Fishes
 Because of hyperosmotic body fluid they are subjected to swelling
by movement of water into their body owing to osmotic gradient.
 Since the surrounding medium has low salt concentration, they are
faced with disappearance of their body salts by continual loss to the
environment.
 Thus, freshwater fishes must prevent net gain of water and net loss
of salts.
 Net intake of water is prevented by kidney as it produces a dilute,
more copious (i.e. plantiful hence dilute) urine
Excretion and Osmoregulation in Freshwater Fishes
Osmoregulatory inflow and outflow of salts and water in a fresh water
fish. HpU, hypotonic urine, S, salt , W, water, W+S, water and salt
Excretion and Osmoregulation in Freshwater Fishes
 The useful salts are largely retained by reabsorption into the blood
in the tubules of kidney and a dilute urine is excreted.
 Although some salts are also removed along with urine which
creates torrential loss of some biologically important salts such as
KCI, Nacl, CaCI2 and MgCI2, which are replaced in various parts.
 Freshwater fishes have remarkable capacity to excrete Na+ and Clthrough their gills from surrounding water having less than 1 mm/L
NaCl, even though the plasma concentration of the salt exceeds 100
mm/L NaCl.
 Thus NaCl actively transported in the gills against a concentration
gradient in excess of 100 times.
 In these fishes the salt loss and water uptake are reduced by the
integument considerable with low permeability or impermeability to
both water and salt also by not drinking the water.
Excretion and Osmoregulation in Fish
Exchange of water and salt in some fishes.
(a)Marine elasmobranch does not drink water and has isotonic urine.
(b)Marine teleost drinks water and has isotonic urine.
(c )Fresh water teleost drinks no water and has strongly hypotonic urine.
ASG, absorbs salt with gill; Hr NaCI(RG), hypotonic NaCI from rectal gland; SS (G),
secretes salts from gill; W, water.
Excretion of Wastes in Marine Water Fishes
 Modern bony fishes (marine teleosts) have the body fluid
hypotonic to seawater, so they have tendency to lose water
to the surroundings particularly from gill via epithelium. The
lost volume of water is replaced by drinking salt water.
 About 70-80% sea water containing NaCl and KCl enters the
blood stream by absorption across the intestinal epithelium.
However, most of the divalent cations like Ca++, MG ++ and
SO 4 which are left in the gut are finally excreted out.
 Excess salts absorbed along with sea water is ultimately
removed from the blood with the help of gills by the active
transport of Na+, Cl sometimes K+ and eliminated into the
seawater.
 However, divalent ions are secreted into the kidney.
Excretion of Wastes in Marine Water Fishes
 Thus urine is isosmotic to the blood but rich in those salts,
particullarly Mg++, Ca ++ and SO4 which are not secreted by the gills.
 Combined osmotic action of gills and kidney in marine teleosts
resulted in the net retention of water that is hypotonic both to the
ingested water and urine.
Osmotic regulation in marine boney fishes. HpU, hypotonic urine: SW, sea water,
W+S+NH3 , water, salt and ammonia, W. water