WF Feature
Fish are definitely mouth breathers – as this
Napoleon wrasse demonstrates.
Photo: Alex Steffe/Lochman Transparencies.
12 Western Fisheries MARCH 2007
HOW FISH
BREATHE
AND WHY THEY NEED
GOOD WATER
By Steve Ireland and Martin de Graaf
S
tating the obvious, fish - like humans
- need water to live. However, fish
need water in the same way as we need air
– in order to breathe. Whereas we get our
oxygen from the air around us, fish extract
it from the water they live in.
The problem for fish is that getting oxygen
from water is much harder than getting
it out of the air. The fact is that there is
roughly around one-third less oxygen
available in water than there is in air. As a
result, fish have evolved breathing systems
based around gills instead of lungs.
Like some humans who have a tendency
to snore, fish are mouth breathers - their
nostrils are only used for smelling.
In simple terms, when a fish opens and
closes its mouth underwater, it pumps water
through its gill system. When a fish’s mouth
opens, its gill cover – called the operculum
– is closed, sucking water into the mouth.
Similarly, when the fish’s mouth closes, the
operculum opens, drawing the water across
the gills.
Fish vary a lot in their oxygen needs and in
how efficiently the pump formed by their
mouths and the operculum work.
Those that have evolved to live in relatively
oxygen-poor water, such as shallow
estuaries or swampy waterways, like cobbler
or catfish – are relatively slow-moving in
order to survive. In contrast, fast-swimming
Western Fisheries MARCH 2007 13
fish like brown and rainbow trout need to
live in fast-flowing water that is oxygenrich in order to survive.
Those fish that have an efficient “active”
internal water pumping system do not
need to swim to breath. In contrast, those
fish whose systems are less effective or
“passive” in nature need to swim with
their mouths partly open, in order that
water is always flowing across their gills.
Like brown and rainbow trout – species
of fish that have a bony skeleton – sharks,
which have skeletons made of cartilage,
are fast-swimming fish that breathe though
their gills.
However, some shark species have
evolved a gill pump – a set of muscles
that enables them to suck in water and
push it past the gills, even when they are
stationary. On the other hand, other shark
species rely entirely on their forward
movement to draw water through their
gills – which means they have to stay in
motion virtually the whole time in order to
breathe!
Similarly, some fish that live in shallow,
oxygen-poor water have evolved additional
lung-like organs or larger gill surface
areas to help them with breathing. The
mudskipper falls into the latter category
– as we will see later in this article.
A whale shark breathing – when its gill covers
open, water is drawn from inside its mouth
across the gills. Photo: Geoff Taylor/Lochman
Transparencies
14 Western Fisheries MARCH 2007
Like lungs, gills contain lots of blood
vessels that are used to absorb tiny
particles of oxygen and send them into
the fish’s bloodstream/circulatory system.
Whereas our heart has to pump blood in
two directions, a fish’s heart only has to
pump blood in one, in a circular manner.
very small food items, then the gill rakers
will be complex and very dense, so as to
capture food. On the other hand, if the fish
eats large prey items – like other fish - the
gill rakers will be very simple and widely
spaced so as to reduce “drag” when trying
to capture another fish.
In a fish, the blood enters the heart through a
vein and exits in a similar manner, on its way
to the gills, where it flows through them and
picks up oxygen. On the return journey, the
blood leaves the gills through arteries, which
go to the fish’s body where the oxygen is
used. The blood then flows back to the heart
– and the cycle begins over again.
For this reason, in some species of fish
that hunt large prey, gill rakers may not
actually exist.
Now, we have talked briefly about what
the gills of a fish do in a breathing sense.
We must also consider the fact that they
actually perform a dual function – while
the gill filaments collect oxygen, the gill
rakers collect food (see Figure 1 opposite).
Once water has passed into a fish’s mouth,
the first part of the gills it meets is the
gill rakers, which are pieces of bone or
cartilage arranged in a comb-like structure
that act as a filter system and strain out any
material – from food particles to grains of
sand - that is floating in the water.
The structure of the gill rakers in a
particular fish tells us a lot about what
kind of food it eats. If a fish species eats
Once the water has been cleaned up of
food or other particles, the next important
part it reaches is the soft, red fleshy part
of the gill, which is made up of the “gill
filaments”. These are hundreds of very
thin membranes, which, in turn, are made
of rows of even finer membranes called
lamellae. The gill lamellae are crisscrossed with a network of capillaries – tiny
blood vessels that run between the ends of
the arteries and beginning of the veins.
As the water flows through the gills, the
oxygen in it passes into the blood flowing
through the capilliaries, while at the same
time any carbon dioxide in this blood
passes out through the capillaries into the
water, where it is carried out of the body
with the water through the gill covers/
operculum.
The gill filaments are supported by the
gill arch, which forms the vital function of
FIGURE 1: How a fish’s gills work
Gill rakers
Gill arch
Gill filaments
Water in through
mouth
Water out
through gills
Gill rakers
Gill arch
Gill filaments
Illustration:
Lauren
Poetschka.
holding the entire gill structure in place.
When a fish sucks water into its mouth,
it squeezes the gill arches so as to help
force water over the gills – which lie at the
rear of the fish’s mouth. Once the water
has flowed across its gills, the fish relaxes
the gill arches, which helps to draw more
water in through its mouth.
Jawless fish
Originally, fish were jawless and
palaeontologists have traditionally believed
that jaws evolved in fish – about 460
million years ago – because they made fish
a better hunter by being more able to take
prey into their mouths. However, there
is an interesting theory that jaws actually
developed from gill arches because the
former helped fish to breathe more easily
by forming a better water pump.
This links back to the dual purpose of
the gills mentioned earlier – which are
both a means of helping fish to filter food
from the water and to extract oxygen for
breathing.
There are some species of jawless fish
that still survive today, of which the best
known in Australia is the lamprey – the
pouched lamprey (Geotria australis)
is found between Margaret River and
Denmark in the State’s south west.
Lampreys have relatively large muscular
lips and cheeks – useful for holding onto
prey when you don’t have a jaw to do this
for you!
The eel-like lamprey can survive out of
water for a short period, as long as there
is some moisture present. In very rainy
winter weather, in order to reach a suitable
breeding habitat, pouched lampreys will
leave their freshwater home on a dark night
and use their whip-like body and suctionready mouth to climb dams and weirs.
Queensland lungfish
The lamprey is not only the only
Australian fish to date back to prehistoric
times – Queensland is home to a fish that
In a fish that hunts other fish – like coral
trout – the gill rakers are widely spaced to
reduce ‘drag’ when grabbing prey. Photo: Alex
Steffe/Lochman Transparencies.
Western Fisheries MARCH 2007 15
has been described as the missing link
between fish and the amphibious animals
from which they evolved.
The lamprey – one of the
remaining jawless fish.
Photo: Gunther Schmida/
Lochman Transparencies.
The breathing system used by Queensland
lungfish (Necerodatus forsteri)
– combining both lungs and gills - was
discovered in the late 1860s by the then
director of the Australian Museum, Gerard
Kreft, when he saw one being prepared
for dinner. At the time, the lungfish
was commonly called the “Burnett
salmon”, owing to its pink flesh, and was
regarded as something of a delicacy by
Queenslanders, who caught it in the rivers
of south-eastern Queensland, including the
Burnett River.
The Queensland lungfish has a single lung
and a set of five pairs of gills. Mostly the
lungfish uses its gills in a conventional
manner, but if the water it lives in becomes
stagnant or shallow, the fish can comes
to the surface to breathe noisily, using its
lung.
Although there are now only six species
of lungfish left in the world – one in
Australia, one in South America and four
in India – fossils of lungfish have been
found all over the world in areas where
there has been freshwater.
Adult Queensland lungfish can grow to
over one and half metres in length and
up to about 40kg in weight. Fossils of the
species have been found in New South
Wales, dating back over 100 million years
in age.
Whilst the Queensland lungfish is able to
live in water with a wide range of oxygen
levels and has survived for a very long
time, it is now listed as a “vulnerable
species” under the Commonwealth’s
Environment and Biodiversity
Conservation Act 1999. According to
The mudskipper spends much of its time out
of the water. Photo: Lochman Transparencies.
the Commonwealth’s Department of
Environment and Heritage, there is
evidence in recent years that only a small
number of young lungfish are reaching
adulthood.
Two additional key problems are said to be
the flooding of suitable spawning sites and
the amount of physical barriers in rivers,
such as dams and weirs, which stop many
of those adult lungfish that do survive
from reaching traditional breeding sites.
For successful spawning, Queensland
lungfish need shallow water with a dense
cover of water plants, such as ribbonweed.
Closer to home – the mudskipper
Closer to home but almost as strange
as the lungfish is the mudskipper
(Periophtalmus argentilineatus), which
can be found in brackish mangroves and
estuaries north of Carnarvon, along the top
end of Western Australia. Whilst still being
a fish, the mudskipper stays out of water
a lot, skittering across mud flats using its
pectoral (front) fins as a pair of legs.
In order to escape large fish such as
barramundi that hunt around the mud flats
at high tide,
mudskippers
have been
known to
actually
climb up on
mangrove
roots!
One way a
mudskipper
breathes when
it is not in the
water is by
extracting the
oxygen from
a reservoir
of water it
16 Western Fisheries MARCH 2007
stores in pouches in its gill chamber. The
oxygen contained in the water in the pouch
is enough to last the fish for a couple of
minutes, at which time it needs to refill the
pouch from a nearby pool of water or from
its water-logged burrow.
In addition to using its gill pouches, a
mudskipper has the ability to breathe
through its skin – in a similar manner
to that of true amphibians. However, its
skin must be wet in order to do this, so it
can extract the oxygen from it – which is
one reason why although a mudskipper
can live out of water for a time, it must
live in a place where the air is very humid
(i.e. damp).
Like mudskippers, crustaceans such as
crabs, lobsters, prawns and marron are
able to walk about on land for periods of
time, but breathe through gills like fish.
In a similar manner to the mudskippers,
crustaceans survive on land by keeping
their gills moist. Often the gills are well
hidden or are far from obvious – in the
case of the western rock lobster (Panulirus
cygnus) the gills are protected by being
enclosed in a chamber in their carapace.
Those crustaceans that live mainly on land
usually live near the shoreline. When the
tide is “out”, they keep their gills moist by
wetting them with body fluids – a situation
which is aided by the gills having a
tightly-sealed chamber surrounding them.
In a separate section of this article, we will
look at some examples of how the way fish
breathe is affected by their environment. g
Steve Ireland is a senior journalist in the
Department of Fisheries’ Communications
and Education Branch. Dr Martin de
Graaf is a research scientist at the
Western Australian Fisheries and Marine
Research Laboratories and specialises in
freshwater fish.
Living and breathing in a healthy environment
As we have explored in the main
part of this article, the physiology
of fish has evolved over millions
of years to provide an amazing
range of breathing methods to
help them to better cope with the
changeable environment they live
in – and to help them catch food
and/or evade capture.
However, extreme relative
sudden changes to their
environment that causes a lack
of oxygen, such as man-made
pollution or a “natural” disaster
such as algal bloom, can leave
them struggling for life.
Dr Rod Lenanton, senior finfish
scientist at the Department of
Tailor live a fast hunting life-style in
Fisheries’ Research Division, has
the surf. Photo: Eva Boogard/Lochman
been carrying out research on
Transparencies.
the fish in WA’s south-western
estuaries and coastline for close
to four decades. He says that
The lifestyle and home of another iconic
the issue of having enough oxygen in the
WA fish species, the cobbler (Cnidoglanis
water all-year-round for the estuarine fish
macrocephalus) makes an interesting
is a major one for their survival.
contrast to those of tailor. While tailor
“Different species have different abilities
to tolerate low levels of oxygen,” Dr
Lenanton said.
like the wide-open spaces of the ocean,
cobbler prefer a slower life in small
confined spaces – burrows – in estuaries.
“Rooted macrophytes and a healthy bed
of macroalgae can respire all night and
use up all the dissolved oxygen in the
water column. When the sun comes up in
the morning, photosynthesis takes place
and the dissolved levels of oxygen go
back up very quickly.
“As a result, there needs to be enough
dissolved oxygen in their burrows for the
parents and their young. If is not sufficient
– say, because the estuary they live in
has become degraded [by pollution or an
algal bloom] – this poor environment can
contribute to their downfall,” Dr Lenanton
explained.
“In a normal healthy estuarine system
where the macrophytes are growing,
the fish that live there have to cope with
the fact that dissolved oxygen levels go
down at night, particularly in shallow
areas. If you put a marine fish into this
environment, it would die, owing to the
fish being used to a constant level of
oxygen.
“In a bad system where the macros are
deoxygenated – where an intensive algal
bloom occurs and the water becomes
deoxygenated for weeks or months estuarine fish can struggle to survive. In a
normal estuary environment, the fish that
live there have to be much more tolerant
about oxygen levels than those who live
in the sea.”
Dr Lenanton said marine fish such as
tailor (Pomatomus saltatrix) were attracted
to the highly oxygenated water that
is found under waves in the surf and
seemed to thrive there. Why the fish did
so well in this environment was difficult
to work out, but he said tailor are a very
aggressive fish and possibly they needed
the high oxygen levels to sustain their
fast, hunting life-style in the surf.
Algae are a group of tiny plants that live
permanently in rivers and estuaries and
help – along with other water plants – to
oxygenate the water. However, during
WA’s long hot summer, aided by nutrients
from fertilisers that have washed into them
from agriculture and urban gardens, algae
living in WA’s major waterways, such as
the Swan-Canning and Peel-Harvey, have
grown explosively.
This has resulted in dense carpets of
algae, such as Karlodinium, Anabaena
and Microcystis, covering the surface of
the rivers. As the algae begins to break
down, this process starves the water of
oxygen, which suffocates the fish.
(See “Karlo, whitespot and other nasties,
page 8).
In September 2006, the State Government
announced they would trial the use of a
submersible oxygenation plant during
the summer of 2006/07, as part of an
experiment to try to boost dissolved
oxygen levels in the Swan River during
this crucial period. The unit, which pumps
three tonnes of oxygen into the river each
day, is located at Guildford – an area
prone to annual algal outbreaks.
Launching the unit, Mark McGowan,
the then Environment Minister, said the
Government would contribute almost $4.5
million over the next four years for the
long-term oxygenation of the Swan-River,
as low oxygen levels in the rivers and
algal blooms were occurring more often
as a result of increased nutrients entering
the waterways.
“In 2004/05, a total of 5,000 fish died and
the annual reoccurrence of fish deaths
is a serious concern, particularly for fish
stocks of high recreational value, such
as black bream. This is unacceptable
and we need to do more to prevent
this happening, which is why we have
introduced a number of measures to
address the cause of the problems, not
just the symptoms,” Mr McGowan added.
Mr McGowan also announced plans to
phase-out the use of “river-harming”
fertilisers across the coastal plain through
which the Swan River flows.
Similar but smaller scale trials have been
carried out before in the past – for four
weeks in 1997 in the Swan River and
over the 1998/99 summer in the Vasse
River, which flows through Busselton. In
the latter, it was reported that although
oxygen levels were increased in the Vasse
during the trial period, providing better
conditions for fish, there was little effect
on nutrient levels or algal blooms.
It had been hoped that the oxygenation
would limit nutrients being released from
the sediments in the bottom of the Vasse,
where they build up over winter, due to
fertiliser run-off caused by rain.
Western Fisheries MARCH 2007 17