| dc.description.abstract | During the last 10 years we have been experimenting in the laboratory
at the Flødevigen Sea-Fish Hatchery with the rearing of several
species of salt-water fishes. The intention has been to bring about
better methods for experimental purposes and to get some information
as to what factors may be of importance for the survival of the eggs
and larvae in nature. In this paper we are dealing with herring, cod,
and plaice, especially the results from the two last seasons.
In order to attain success when rearing fish-larvae, especially those
species having an air-bladder, it is important to control the gas content
of the sea-water. It is essential that when larvae are reared in small
aquaria at a pressure of but one atmosphere the gas content of the seawater
must be kept relatively low. It is assumed that the gas pressure
and not the content of gas is the chief factor. Larvae living in the sea
at a water pressure of e.g. 10 metres will probably sustain a gas content
in the sea-water which will be detrimental near the surface. In this
connection it is worth mentioning that the young cod-larvae react at
once to changes in water pressure. When the water pressure is about
5 metres the larvae in a closed aquarium will keep to the surface. When
the water pressure is shut off the larvae will straightaway sink apathetic
to the bottom, but swim up again when the water pressure once more is
put on. Experiments indicate that cod-larvae with a big air-bladder may
recover when they are transferred to an aquarium covered with silk
net and lowered to a depth of 5 to 10 metres.
The eggs of the pelagic spring spawners are not affected by reasonable
variations in the gas content.
If the gas content is not reduced in the aquaria the larvae will get
gas-bubbles in the intestines. This does not occur till the yolk-sack is
resorbed - and the intestines and gills are functioning. It is assumed
that the gas is liberated when the sea-water passes the gills. Small gasbubbles
are formed inside and swallowed.
In the case of herring the gas-bubbles are fatal. In the plaice the
gas-bubbles will pass through the anus - and are not likely to harm
the young. In the case of the cod we have not observed the gas to be
within the intestines. In early stages - about a week after the resorbtion
of the yolk-sack - the gas occurs free in the cavity. The intestines
may be ruptured-or the gas may originate from the early air-bladder.
In older stages the air-bladder will expand, the young will turn belly
upwards and die, or the air-bladder may be ruptured. In older stages
of the plaice - up to a length of some centimetres at least - fine gasbubbles
rnay accumulate in the veins, and the fish will succumb.
In the laboratory the gas content may be reduced by filtering the
sea-water slowly through fine sand. Great quantities of gas will accumulate
in the sand. The composition of this gas O²/N² is somewhat
the same as in the atmosphere, the content of oxygen being slightly
higher. Some of the gas in the sea-water in the laboratory may originate
from the atmosphere - by leakage in the pump bearings. Studies of
the gas of the air-bladder of cod (by Miss Eva Henly) indicate that
oxygen is the operative factor.
The results of the rearing experiments have improved to a very high
degree after the gas problem was taken into consideration. The larvae
of herring, cod, and plaice (and other flat-fishes) will thrive for weeks
in the small aquaria used, containing but 35 litres. The flat-fishes are
ordinarily reared without any mortality to the bottom stage in numbers
from 1 to 3 thousands in the same aquarium. In this way the critical
period - the moment when feeding begins - has been overcome.
Experiments have shown that the cod-larvae may live in filtered
water - without food - for several days after the yolk-sack is resorbed,
and still thrive when fed.
If we want to rear the fish further we encounter other problems.
Will the feeding with the nauplii of Artemia be sufficient in the long run?
Do they contain enough vitamins? Experiments have been made to
feed the nauplii with flagellate cultures for some days before giving them
to the young fish. Those experiments are to be continued.
Experiments to furnish the young herring, about 2 months of age,
with freshly caught plankton were negative. The herring did not take
the copepods or Daphnia, and the plankton introduced seemed to kill
the Artemia. We are now using fine chopped Mytilus edulis - even for
the herring.
The volume must be augmented - larger aquaria, more water, and
larger filters. Experiments to rear cod and plaice in our large rearing
boxes for lobster using but slightly filtered water were entirely negative.
The results were the same when transferring several species of flatfishes
after they had passed the stage of transformation.
At present we have a good sample of herring 6 months of age,
plaice ½ & 1½ year, and soles 5 years of age - all brought up from
eggs in the laboratory.
The last season we had some mortality of the young when 2-3
months of age. The symptoms in the plaice indicated an attack of
Vibrio anguillarum. In the herring the symptoms indicated Lentospora
cerebralis. The cod died before we were aware of the cause of the mortality
and could take steps to reduce it. The attack on the plaice was
stopped by adding some dissolved salt to the sea-water used. The
herring reared in a larger aquarium in the laboratory survived, but in
an ordinary aquarium (35 l) they succumbed.
The infection this year came probably from our water reservoir.
This was partly emptied in early winter, the algae and animals living
along the walls died - and we were not able to have it cleaned out
because of high snow and frost.
Several experiments indicate that the young cod especially is very
sensitive to products of metabolism. In one instance the water was drawn
from a container where few cod of 1-2 years of age were living. The
fry died. But when the older fish were removed, and the container
cleaned a new batch thrived. It is impossible to say wether the mortality
in the first experiment is to be ascribed to the chemical effect of the
products of metabolism or to bacteria.
Our experiences from last season give a warning that bacteria and
products of metabolism may be a problem of higher order. Our earlier
investigations in the inner Oslo-fjord, highly contaminated by sewage,
have shown that the survival of the eggs and larvae of the spring spawners
here is very poor. And in some places, where contaminated water
occurs, even the grown up cod may suffer a great mortality, evidently
caused by Vibrio anguillarum.
The problem of rearing for experimental purposes has been our
main object. Experiments on a large-scale rearing have to be postponed
till we have aquired a relatively full knowledge as to the factors of main
importance.
Besides this work we have made some observations on the behaviour
of the eggs. Many of the observations have to be confirmed by special
experiments. In order to bring those points under consideration we will,
however, mention some of them.
The hatching and rearing has ordinarily been undertaken at an
ordinary temperature of the sea-water during the season. In some
experiments the water has been cooled or heated artificially. As we have
no thermostats available for that purposes it has not been possible to
keep the temperature quite constant.
At a temperature of approximately 2° C the cod-eggs hatch normally.
herring-eggs will also hatch at that temperature, but the larvae
will have difficulties in escaping the eggs, and a heavy mortality occurs.
Temperature-variations between 2° and 10° give high mortality to cod-eggs.
In some rearing experiments the mortality among the young
may be somewhat higher when the temperature is raising. The question
of the relative gas content here comes into the problem.
Fungus may be detrimental to the eggs. If they are heavily attacked,
the eggs will sink. This occurs especially in foggy weather, and when
the water has a relatively low salinity. The attack may be reduced by
transferring the eggs for half an hour to sea-water to which has been
added some salt. In some cases the cod-eggs have been infected by a
flagellate living close inside the shell. Both these pests have been observed
in nature - the last mentioned even in the middle of Skagerak.
Cod-eggs hatched in circulating - but not renewed - water (constant
salinity), gave a poor result. Bacteria may here be involved -
or the products of metabolism.
The influence of light has been studied more fully. The experiences
from the hatching boxes indicate that the hatching of the cod-eggs gives
the best results near the window. It is, however, difficult to say anything
conclusive of the influence of the light in this case. In the boxes with
less light there will also be a higher growth of fungus.
Experiments with plaice-eggs indicate that normal daylight near
a window - but shadowed for direct sun - gave poor results. And in
experiments in the open air the eggs have died. On the other hand the
plaice-eggs will hatch well in total darkness. It appears, however, that
the plaice-eggs hatched in total darkness will give a higher percentage
of larvae with abnormal pigmentation. The dorsal side is quite white,
only the fins are pigmentated. When those plaice in the bottom stage,
or later, are transferred to our salt-water pond under normal daylight
and normal food for several months, no alteration takes place.
Experiments also indicate that the plaice-eggs react with light.
They may sink in the middle of the day and come to the surface again
during the night. | en |
