 |
Diseases of Fish
|
A General Overview
This column, which I hope to be a regular fixture, in this new
and most exciting of mediums, will try and concentrate on the
practical considerations relating to the health of your fish,
in the real world of aquarium keeping.
Although one may think otherwise, the problems that we face as
aquarists, are not exactly the same, as similar fish will face
in their natural habitat, although the actual parasites,
bacteria, etc. will in so many cases be those, that do
sometimes create problems in the wild.
Similarly, in the ever growing field of Aquaculture , there are
differences, between this method of keeping fish and
maintaining them in the average aquarium. Only by being aware
of these differences, and the distinct possibilities that each
form can manifest in possible problems , can we begin to both
understand the potential difficulties, and take sensible steps
to minimize the kinds of diseases or problems that may arise,
or hopefully avoid them altogether.
Fish disease in the
Natural state.
The first and major difference between fish in Aquaculture, and
the same fish living in Nature, is the sheer volume of water
that each fish has access to. This means that although many
fish can asymptomatically harbour a parasite, for extended
periods e.g. subepithelially , when the parasite is stimulated
into a reproductive mode, which typically results in it
multiplying itself by hundreds or even thousands of times, the
chances of each new spore or Trophont etc, finding a new host,
in the wild, is many hundreds of times less , than in an
Aquarium or an Aquaculture situation.
|
|
Even
when we allow that typically about half the
species of fish shoal , the bodies of water are
so large, & other forms of predation so
prevalent, that only a minute percentage of the
parasites, new spores etc, will have the chance
to re-infect another fish.
In large part the
same logic applies to bacterial & viral
diseases in Nature.
However when man
or natural events interfere with the Lakes or
Rivers, or the Reef environment, which is
unfortunately in contemporary times an everyday
event, we hear concomitant reports of fish
kills with alarming frequency. The corelation
to ecological damage cannot br
refuted.
Pathogenic
bacteria are everywhere. When we are healthy,
your natural body resistance keeps these
pathogens at bay.
|
However if for any reason we become weakened, for example by ,
exposure to excessive cold, or damp sleeping conditions, these
same pathogens often flare up, causing us to become mildly or
even seriously ill.
Similar consequences can happen to fish in their home of
rivers, lakes, and oceans.
In Nature when any of the normal background parameters, e.g.
temperature of the water, pH, alkalinity, purity ( freedom from
pollutants), oxygen values, & more are suddenly changed,
from the preferred habitat in which the fish has evolved over
eons of time, then the same background pathogens, or parasites,
will become very active as the fishes immune system becomes
weakened, & unable to provide the amount of suppression to
keep these undesirable forms in check. This weakened condition
and lowered resistance to disease results from environmental
stress.
The single greatest cause of
fish disease.
Thus a result of
environmental stress large numbers of fish are lost
as fish kills in rivers, lakes, and other natural
bodies of water , & most of you will have read
about same in your local press, or even seen it at
first hand. Today this happens with monotonous
regularity.
In Nature when the
prevailing conditions are ideal , only the
occasional fish, usually an older one, will become
ill. Natural selection usually takes care of such
fish, as they are less able to avoid becoming food,
for the predators that are omnipresent. This is
part of Nature’s checks & balances. These
weakest had they survived, may have passed on
infections to their peer group, but by this process
of Natural elimination, this ecobalance keeps down
the pool of infection potential, to manageable
proportions. |
|
For this
survival of the fittest rule applies to the entire natural
world, and has the practical effect to ensure that only healthy
or relatively robust fish survive to reproduce their kind, or
sometimes be captured for the benefit of man, either as food,
or as Aquarium specimens. Later in these articles I will
compare the drastic changes that take place, both before we
ever see the fish, and after it reaches it’s new home.
All scuba divers who have the interest and understanding of
ecology, report it is rare indeed, to find fish in Nature
showing disease signs, such as we often observe in the
Aquarium.
Although essentially fish in the wild have a built in awareness
of possible dangers, they are also relatively calm, most of the
time because they have either many of their own kind around
them, as well as a normal habitat . These factors prevent the
kind of stress related adrenalin surge , which is the precursor
to a weakening of their immune system , in the Aquarium. All to
often they have either none, or only one or two of their own
kind. Alternatively sometimes in the case of fish that are
natural loners, we place 2 of them in an Aquarium, and they
display aggression or worse to each other. Either of these
unnatural conditions can cause tremendous stress.
The ability to recreate a environmental conditions which
minimize stress is of vital importance in maintaining good fish
health.
Furthermore, in Nature fish have typically available to them ,
a large range of organisms, to serve as food, or at the least
such organisms as they have evolved to find suitable, as a
complete food for their growth, reproduction and good
health.
Compare this to the typical diet we give in an Aquarium, which
has on average some 10 or more different species of fish, each
of which in nature will predate upon different organisms to
grow and become healthy, & we feed them one size fits all
the same packet of food, day in & day out, with little
regard to their individual requirements.
Is it any wonder that the combination of these factors result
in high levels of stress ? This, given other factors to be
discussed, can & does result in the fatal outbreaks of
disease that are the cause of so many Aquarists eventually
giving up the art of successful Aquarium keeping.
|
|
Natural Normality , in
Nature.
Although this
may appear to be a redundant play on words, it
is worth emphasizing that in the natural , the
photoperiod is natural. When the sun comes up
it gets light. In Spring, when most fish start
to sexually mature it is in response to the
natural increase in photoperiod, & with the
accompanying rise in temperature in many parts
of the world. When dawn arrives it is a slow
process, & the same at dusk.
The salient point
is that any subtle changes in the quality of
the water, is usually caused by other natural
changes, e.g. such as the rush of melting
snows, which can bring about a normal and
useful stimuli to the fish
|
Compare this to what
is typical in the Aquarium.
We configure the lighting to come
on at times often to suit our preferred observation times. It
is most typically the same all year round, and bears no
relationship in many cases, to what is the natural day for the
fish. The light comes on suddenly usually within a few seconds,
the same when it goes off.
Temperature is controlled by
clever thermostats, which keep the water, at what our books
& specialists advise us, the same number of degrees from
Jan-Dec. No diurnal change as happens however modestly even in
the Tropics.
In Nature in Rivers, in most
Lakes, and on the Oceans, moving water, renewed constantly,
changed & buffeted by wind, rain, tide, & run off from
the mountains, keeps a constant purification going. This helps
to overcome the natural pollution which would otherwise occur.
Furthermore the myriad of natural organisms, utilize so much of
the biota that is created along with the excretory products of
the Fauna, so as to ecologically prevent in most cases, any
excess becoming a cause of fatality to the world surrounding
any one species.
Compare this to the typical
Aquarium. Some of us do change a percentage of the water in the
Aquarium, usually about 10% (if we do) weekly. Many do not even
do this.
Instead of a natural ecosystem to
manage the various subtle changes that are bound to occur, when
we keep our specimens in an artificial habitat, we rely upon
filters, often changed infrequently, UV’s to kill off as much
as can, Ozone to purify, chemicals to supplement what we think
needs to be replaced, ( the evidence for such replacement is
too often questionable).
It’s rather like asking us to
feel good in the rather aseptic environment of a hospital ward.
Probably alright, (some today even question that), but it
hardly leaves the average person feeling that they would like
to go there as they felt so good in that atmosphere.
In both freshwater & marine
environments there are many forms of natural plant life . Many
of these act as part of the food chain either directly for the
fish, or indirectly by contributing to other forms of life
which eventually are eaten by the fish.
I know of no “plastic” forms of
plant that exist in nature, but far too many of our Aquarists
seem to think that this artificial media, looks nice and is
useful. I personally would seek to disagree.
Furthermore in many of revered
Public Aquariums, I have seen the same plastic rubbish replace
the real thing, as evidently the professionals who are employed
to run these Mecca’s of entertainment are not provided with the
means by their money masters, to achieve what should be for so
many good reasons an essential educational tool.
If one talks “off the record” to
many of the Curators of too many Public Aquariums, you may be
horrified to hear just how often they have to replace their
fish etc. In some cases the record is as bad as Aquarists who
have little or no experience. So much for conservation of
Nature.
Again the reason is in many
cases, that to create a stress free environment is one that
requires a great deal of expertize, planning & the funds
& personnel to make it work, on a long time basis. When
this is absent, whether it be in a Public Aquarium or one in
our home, then stress is a typical consequence, &
fatalities are to be expected, as disease manifest
itself.
Once again the major reason is
the stress factor put upon the fish, which is not in accord
with their experience in Nature.
Finally in Nature, because as I
have said above, natural selection sorts out the weak leaving
the best to reproduce, one tends to have stronger
specimens.
In many Fish Farms for the Hobby,
(primarily now speaking of freshwater species), the selection
of a limited number of parents, with a restricted diverse gene
pool, can sometimes create specimens that can be more
susceptible to a particular disease or problem. This occurs
even though they are in many cases better adapted to the
environment they will ultimately arrive to, i.e. an Aquarium.
This aspect of breeding from selected stock, whilst an
imperative in Agriculture, & becoming so in Aquaculture,
has the effect of reducing to a dangerous degree the various
heritable factors that in Nature are so diverse, & that
have given the natural world an ability to overcome all kinds
of Natural disasters, & disease. Much more has to be done
to both understand, & provide realistic answers to the
kinds of problems here outlined, & we will return to this
subject later on in the series, in some more detail.
The problems that we face as
aquarists, are not exactly the same, as fish will face in their
natural habitat, although the actual parasites, bacteria. will
in so many cases be those, that create problems in the
wild.
--------------------------------------------------------------------------------
by Dr John Prescott -
April 2008
Back to
Top
Dr John Prescott is a qualified vet with over thirty years
experience of research into fish keeping. He has written a
series of articles on fish diseases, their causes and
treatments.
Source:
http://www.aquarium-gardening.com
|
