Breathe Easy
We know oxygen is a critical water quality parameter for our
koi – a minimum of 5-6mg/l is usually recommended – but have you every wondered
how they manage to extract it quickly die without access to air, but fish
positively thrive underwater, and in this article we’ll look at how they do
this.
Oxygen in water
You or I wouldn’t give a second thought to the oxygen
content of air, however for fish it’s a different story. To begin with, water
contains much less oxygen than air 6.3cm³ per litre at 20⁰C, as opposed to
210cm³. on top of this, water is denser and move viscous than air, which means
it requires more energy to move it across the respiratory surfaces (gills in
fish, lungs in mammals).
In fact, around 10% of the oxygen absorbed by fish is
used to move water across the gills. Just to complicate things a little more,
the oxygen content of water, and the amount that fish require, depends on water
temperature. Warmer water holds less oxygen, yet fish require more of it. For
example, in one experiment it was found that at 20⁰C the resting oxygen
consumption of carp is 48mg/kg/hr, rising to 104mg/kg/hr at 30⁰C. so spells of
hot weather can place real challenges on the fish’s respiratory system to
deliver the oxygen needed for health and condition.
Gills
For most fish, including koi, the gills are the main site of
gas exchange between the blood and surrounding water. Although the skin is a
major site of gas exchange in larval fish, accounting for 85-90% of the total
oxygen uptake, in adult fish this falls to 20% or less. Each gill consists of a
branchial arch (or gill arch) that supports two comb-like gill filaments. The
gill filaments consist of primary lamellae, upon which are numerous small
secondary lamellae. It is at these secondary lamellae where oxygen is absorbed.
These are eight branchial arches in total, arranged so that all of the gill
filaments can potentially receive a steady flow of water.
The secondary lamellae provide a very thin (2-4um) barrier
between water and fish’s blood supply. This allows oxygen to diffuse from the
water into the blood, where it can then be carried to the rest of the body. The
blood flows in the opposite direction to the flow of water, in a
“counter-current’ system. This improves the diffusion of oxygen into the blood
by maintaining a better concentration gradient (i.e. the level of oxygen in
blood remains lower than in the water). Fish that are active often have
increased numbers of gill lamellae, and a greater overall gill lamellae, and a
greater overall gill are to help them obtain more oxygen from the water. For
most fish, the gill area is around 150-350mm2/g bodyweight. In very active fish
such as tuna this rises to 1500-3500mm2/g.
The gill area, coupled with the amount of water being passed
over it (respiratory volume), determines how much oxygen the fish can extract
from the water. As oxygen levels decrease, the respiratory volume increases to
compensate, and the amount of blood being pumped through the gills rises.
Unfortunately, as water flow over the gills increases it becomes harder for the
fish to extract oxygen as efficiently. Under ideal conditions, 85-90% of the
oxygen can be removed, falling to 10-20% at very high respiratory rates. If
oxygen levels are low you will see the gill covers (opercula) beating more
rapidly as the fish pumps more water.
Osmorespiratory
compromise
Because of their high permeability and surface area, as well
as being ideal for gas exchange, the gills also provide an area where ions
(changed atoms) and water can move in and out of the fish. This presents a
problem, as water will move into the fish, and important ions will move out.
The fish has to compensate for this, by producing lots of dilute urine, and
having mechanisms to take up ions into the body. For example, in a study
involving trout, a period of exercise and increased oxygen use was followed by
increased urine production. These are energy-consuming processes, and the
balance between the need for gas exchange versus the need to minimise water/ion
movement is called the ‘osmorespiratory compromise’.
Because of this dilemma, when oxygen concentrations are
high, or if the fish is inactive, only a part of the gills may be used. This
can achieved by reducing blood flow through the gills, or by adjusting the gill
filaments to reduce water flow through them. This ensures the fish receives the
oxygen it needs, whilst limiting the movement of water and ions. studies involving
species closely related to koi have also revealed that the surface area of the
gills can be reduces when oxygen demands are low.
Oxygen transport
Once in the blood, most of the absorbed oxygen is combined
with haemoglobin in red blood cells and transported to various parts of the
body. The ability of haemoglobin to carry oxygen depends on the concentration
of hydrogen ions (H+); in other words the PH level. Because tissues in the body
are constantly respiring, they are releasing carbon dioxide (CO²) into the
blood. This decreases the PH.
When the red blood cells arrive at the tissues,
the lower pH causes haemoglobin to off load its oxygen. This can then diffuse
into the tissues that need it. The blood pH remains low as it carries CO² back
to the gills. Here it diffuses back out into the water, facilitated by an
enzyme called carbonic anhydrase. The pH rises again and oxygen can once again
be picked up, and the cycle starts again.
The effect of the blood’s pH on its
ability to carry oxygen is known as the Bohr effect, and it is essential for
supplying oxygen and removing carbon dioxide.
A very low environmental pH, or lactic acid production
caused through increased activity/stress, may depress blood pH and reduce its
oxygen carrying capacity. Koi, like all fish, have mechanisms to prevent blood
pH changing, and for quickly restoring it, however under extreme conditions
these may be exhausted. Maintaining a correct pH is therefore important, as is
providing plenty of oxygen if the fish are likely to be stressed. In addition,
carbonate(KH) and general (GH) hardness should be kept at recommended levels.
For a stable pH, healthy gill function, and help the fish maintain the pH of
its blood.
Keeping koi healthy
From knowledge of how respiration works in koi, it’s clear
that for top condition we need to keep oxygen levels high. This means less of
the gills need to be used, and therefore the fish needs to use less energy for
maintaining its water/ion balance. Water quality is also important, and
mineral-poor, acidic conditions should be avoided if possible. By providing a
good environment and minimal stress, your koi will be able to obtain the oxygen
they need for top health and condition.
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