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Annual report of the Fishery Board for Scotland for the year ended .. online

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knowledge between the larval and adalt stage of common fishes, which
only in a few cases is partly bridged over. But it is also of interest for
practical purposes in connection with the hatching and rearing of food
fishes, and will, no doubt, in the future become much more important. It
became desirable in 1885 at the Marine Hatchery at Floderig to under-
take an experiment to clearly show the vitality of artificially produced cod
fry. A large tank was constructed on shore, into which cod fry from the
hatchery was placed in the spring, and most of the young fishes survived
the following winter. Some of them at that time had a size of about 10
to 12 inches; others were smaller. This experiment, which was carried
out by Capt. G. M. Dannevig, the director of that Hatchery, is described
in the annual report of that institution.* It is perhaps, the only one of
its kind on a large scale, where the young fry could find suitable condi-
tions for growth and development, and the only case, as pointed out by
Mr. Cunningham,! in which larvae artificially hatched from marine pelagic
fish eggs have been successfully reared in captivity ; the success he attri-
buted to the fact that the young fry were turned into a large reservoir of
clean sea water.

Later, Cunningham described rearing experiments that he undertook in
the spring of 1894, J with whiting, flounder, and plaice. The fry were kept
in a tank in the laboratory ; it would seem that the two former species
soon disappeared, while the yolk was absorbed in the plaice larvae after
the elapse of five days, when the fry began to feed. They soon died, how-
ever, without having undergone any important change in development
In another experiment he succeeded so far that two larvae were found
alive, thirty-seven days after the yolk had been absorbed ; but it appears
that the advance in development, also in this case, was very small In
the first experiment the yolk was absorbed in so short a time, that it is
clear the temperature must have been high, probably at least about 12** C.
or thereabout, and there is little reason to think the water became
remarkably colder soon after. But then those two specimens that were
left at an age of thirty-two days, ought, according to the results of my
experiment this year, to have been far advanced in development, and
signs of the transformation should have been visibla When such was
not the case, the explanation can only be that one thing or another in
connection with Cunningham's arrangements were unfavourable to the fry.
The majority had died previously, and the two specimens left must have
been in an abnormal condition. I have taken notice of this here, as the
results of the above experiments partly correspond with what I experi-
enced in the same season (1894). (Twelfth Annual Report of Fishery
Board for 1895, part iii. p. 216.)

Experimentfug at the same time, we both followed much the same
system, trying filtered and untiltered water in succession, and always
maintaining a constant current, and we both sought for the food chiefly
in tow-net collections. Our success was also much about the same, a
few individuals being kept alive for a considerable length of time ; but
the advance in development that these reached was very small. Although
these experiments were advances, and in the right direction, it was evi-
dent that something of importance was lacking with the arrangements,
that accounted for the sickly and abnormal state of the fry.

* Beretning for femaaret, 1883-88. Arendal, 1889. hth Annucd Report Fishery
Board, Part III. 1886.
t Jour, Marine Biological Ass,^ New Series, vol. ii., No. 3, May. 1892,
t Jour. Marine Biological Ass., New Series, vol. iii. No. 8.



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of the Fishery Board for Scotlai\d, 177

Below I shall mention what should principally be considered in rear-
ing experiments with larvsB from pek^c fish eggs. It seems strange
that a fish in the post-larval stc^e should be so dif&cult to keep in con-
finement^ while the preceding and following stages offer no difficulty. It
has been said, but without good foundation, that the difficulty lies in
the supply of food alone and sufficient currents of water ; but it will be
shown kiter that the question of water is rather secondary, and that the
food can be procured without the least trouble. In fact, when the water
for the rearing-jar or tank is taken direct from the sea, it is difficult to
avoid having forms of suitable food present. But the quantity of food
would have to be increased artificially according to the number of larva
present. The principal points are — (1) The water arrangements ; (2) The
question of light ; (3) The temperature ; and (4) The food.

An important point is the question whether or not a constant current of
water should be employed. A stop was put to my most successful experi-
ment in 1894 by an overflow carrying away the fry, which were all lost.
This time, therefore, I thought that the constant current of water through
the jar might be avoided. It is doubtful whether the passing of perhaps
ten or twenty times more water through the jar than is necessary for the
fry is of any use-— a small number of larval fishes, such as are yet nourished
by the yolk, will thrive in a small quantity of water for days, providing the
air is not artificially forced out by an increased temperature. And the
same is the case with young flat-fishes after they have passed the transfor-
mation. So it is difficult to see why similar circumstances should not
suit this intermediate post-larval stage equally well. The constant current
is clearly not necessary for the supply of air. In a hot season a con-
stant run of cold, perhaps artificially cooled water, would be a means of
keeping the temperature down as desired ; but this may be done equally
well by less injurious precautions.

There is now left the third point, that may be put forward in support
of the necessity of the constant water supply, namely, thai tlie regular
current has always been found tolincrease the activity of tlie fry. This is
undoubtedly so ; the fry are always found more * vivid ' in appearance when
the water is in movement, than when quiet. In the hatching-boxes, a
certain amount of current is always found satisfactory and necessary for
the welldoing of the fry. But it is to be remembered that a great
number (perhaps a quarter of a million or more) of newly-hatched larvae
in this case are crushed into the narrow capacity of about one cubic foot
of water. And as these fry, at this early stage, are nursed from the yolk
and therefore take no active part in feeding, no attention can be derived
from the latter in the perpetual activity on the part of the fry when
striving against the constant currents. These, again, are of great advan-
tage in preventing the fry from accumulating in great crowds, which
naturally prove fatal to a great number of them ; they are simply suflb-
cated in the crush. And the perfect condition of the fry when turned
out from the hatchery shows, that this exposure to the currents in the
apparatus has done no harm. In a rearing experiment such crowding
can be avoided, and matters are diflerent as the young larva has then
got to look for and catch its own food. When a constant circulation is
maintained in a small jar, the small organisms which the larvse require
for nourishment are sure to be pressed against the outlet side, and thus
be drawn away from the attention of the larvae.

But I have found that a slight movement in the water, whether con-
stant or not, is desirable, as it will always increase the activity amongst
food organisms that otherwise remain quiet. Such a movement may
easily be arranged artificially.
AC



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178 Part III. — FifteerUh Annual Report

During my experiment last summer, I obtained iBMt satisfactory
movements in the water through the eSkci of variations in the tempera-
ture of the surrounding air. ^ee description of the arrangements.]

Another point in connection with the water supply is the filtration,
which, if thoroughly applied, will remove, perhaps, all organic and inor-
ganic matters, leaving the water as a pure and simple solution of salts.
There may be no harm in this when food is put in artificially, and it cer-
tainly offers conveniences in several ways. But for rearing purposes 1
find that unfiltered water that is relatively free from decomposing organic
substances is all that is wanted.

The answer to the question whether the presence of abundant light or
occasional exposure to the direct rays of the sun is of importance to the de-
velopment of larval and post-larval fish produced from pelagic eggs is
given by nature herself. In the sea the eggs of a plaice, for instance, are
developed near the surface in full exposure to the daylight. The larval
stage (during the absorption of the yolk) is also passed much in the same
level, and though the post-larval fish may occasionally be found at a
greater depth, I think I am safe in saying that this stage should also be
called a pelagic one. And since this early development is accomplished by
nature in the full light of da)*, it would be a simple waste of energy even
to attempt a rearing experiment in a dull or much shadowed place in a
laboratory.

In accordance with this, it is a long recognized fact in hatcheries that
such apparatus give by far the best result that are most abundantly sup-
plied with light. The presence of abundant light is of importance for the
well-doing of the larvae directly, and it also has a marked effect upon the
minute organisms that compose their food; but experiments are still
required to show to what extent a dull light would be effective.

It is well known that a high temperature increases the rate of growth
of the embryonic and larval fish, and it is therefore only reasonable to
suppose that the chances of success in a rearing experiment with the post-
larval stage would be increased relatively as the time was shortened. And
we know that eggs and young larvae are also able to resist considerable
changes in the temperature. But this, of course, is limited ; and it must
be remembered that in a rearing experiment the circumstances have also
to be made suitable for the living food. The various species of food fishes
will here appear more or less different, so it will be necessary to experi-
ment with each of them separately in order to be able to give even
approximate figures for the most suitable temperatures. With the plaice
I found it risky to allow the temperature to rise above WO when dealing
with the post-larval stage, while it is known that the same fish, after the
transformation has taken place, will thrive well in much hotter water.

What has now been said respecting the water supply, the influence of
light and a high temperature on the success of rearing experiments on a
small scale, is also proved by the success of my ex[)eriments in 1896, of which
I shall now give an account The question of food can then be more con-
veniently dealt with.

The apparatus and arrangements employed for the I'earing experiment
were very simple. As a rearing-jar, a large carboy-shaped glass vessel was
used, witii a diameter of 20 inches ; it was a little flattened at the bottom,
and 18 inches deep ; the opening of the neck was 9 inches. This vessel
or jar was placed in a shallow tub, filled with sand, so that its bottom was
evenly supported all round to a height of about three or four inches, and
it was placed at a height of about two feet above the floor, near a comer,
having a window on each side four feet distant from the jar ; one window
faced to north and the other to east. Light was supplied from the back



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of the Fishery Board for ScotUmi, 179

throagh more distant windows in the hatchery. The supply of light was
therefore good, and any part could in the daytime be closely examined
without interfering with the fry.

The water supply was taken direct from the harbour, and was in no
case filtered. Heavy sediment, bits of sea-weed, etc, were easily separ-
ated out. The jar was filled to a height of about ten inches, or a little
above the middle, and would contain about ten gallons. The water was
generally renewed once a day, or twice, if the weather was hot, in order
to prevent the temperature rising too high, which was ii^'urious to the
fry and the living food.

There were therefore no currents similar to those produced by a constant
run, but all the same the water in the jar was perhaps never at rest
During most part of the day the temperature of the water was kept below
that of the surrounding air. The consequence was, that the glass sides
got heated first, and transferred this heat to the nearest layer of water.
This would expand as its temperature increased, and consequently rise to
the top, being always replaced with water from the lower and inner parts
of the jar, where the temperature was lower. This led to a constant
movement upwards of the water nearest the glass sides in the daytime,
and it was particularly vivid when the sun-rays were allowed to strike
the jar.

On the 6tii of May the first experiment was begun when plaice larvae,
eight days old, which had just absorbed the yolk, were placed in the
rearing jar. Natural food was provided by tow-net collections from the
harbour, containing small organisms that were likely to suit Some of the
larvas soon began to feed. When they were sixteen days old it was necessary
for me to leave the station, and on my return I found the larvsB all dead ;
food had been properly supplied during my absence, and I am incUned
to think that an excessively high temperature was the cause of death.
During the sixteen days, a few of the larvie had attained a considerable size
and advance in development, as will be seen from PL IV. fig. 2.

Though the larvae died, this first attempt was encouraging, and gave
good prospect of success, and another hatch was started soon after witii the
same arrangements. The plaice eggs had been artificially fertilized on
April 28th, and hatched on the 9th to the 11th May. Most of the larvsB
had the yolk absorbed on the 18th, when they were placed in the rearing
jar. Tow-net collections from the harbour were supplied twice a day, and
the water was at first changed at the same time. About 1200 larvse were
put into the jai*, and they soon divided themselves into three different
classes: — (1) the majority remained persistently swimming near the surface,
with the head touching the water line and the body forming an angle with the
latter of from 30' to 46'.

These larvas paid no attention to the food, — if they touched against
anything they would swim frightened away, and showed more activity than
ever. Though originally richly pigmented (which I consider a sign of
well-doing), they gradually became pale, and, giving up this first-men-
tioned peculiarity, they soon joined the second class, (2) pale and thin
individuals, that occasionally made an attempt to catch small crusta-
ceans that appeared near them. They generally remained very quiet,
and were mostly near to, or on, the bottom, resting on their bead ; in all
cases their head was inclined to drop downwards as soon as the body was
at rest*

* This, I think, is an old experience amongst investigators on this subject, but
has been explained differently. Guided by an experiment, I suj^cated in 1894 [F,
B, \Wh An. Hep,, Part III,, page 216], that this sickly state of the fry with the
above-mentioned tendencies, was due to want of currents ; and later Prof. Mcintosh



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180 Part IIL — Fifteenth Annual Report

Larvse of this second class are in reality in an unhealthy condition, and I
have never found any food in their stomachs.

I am not yet certain, but there is reason to think that these two classes
of larvae in reality had been starving while in the hatching apparatus,
where they perhaps were retained rather long.

(3) The third class of fry in the rearing jar formed the minority ; they
had a healthy appearance, and were richly pigmented. They were seen
at all levels, as a rule swimming slowly, and were always looking for
food. This they did so intently that hardly any object in the water was
passed unobserved.

Tenth to Fifteenth day after Hatcldng,

Though appearing at all parts of the jar, these feeding larvae were chiefly
found in places where the living food most accumulated, viz. : at the side
nearest the w ndow, where the light was strongest. The food, as men-
tioned, consisted of tow-net collections from the surface of the harbour,
taken at all states of the tide. These collections varied in richness, ac-
cording to the weather, but I never failed to get multitudes of diatoms,
larval and adult crustaceans (chiefly copepoda), and larval mollusks, etc.

Although the feeding fry pay attention to perhaps anything suspended
in the water, living or dead, only few forms were eaten at the time. The
first food found in their stomachs was as follows :— (1) Diatoms {Guin-
ardia flaccida)^ a commonly occurring form ; (2) A small larval mollusk
(Bttccinuml), measuring 0*16 mm. in length and 0'112 mm. in breadth
(PL I. fig. 11). The mollusk was found three or four times more frequently
than the diatoms, and it is noteworthy that I in no case found both forms
in the same specimen at the same time. However well filled the stomach
was, its contents in this earliest post-larval stage were almost without excep-
tion one thing only. It seemed as if the young fish had become familiar
with one particular kind of food, and took that alone for some time. But
as different individuals fix upon different forms, there is no reason to
conclude that one kind of food alone was of vital importance.

The larval stage of the plaice (from the time of hatching till the yolk
is absorbed)^ has been described by several authors; M'Intosh and
Prince, t Cunningham, ^ Fullarton,§ Ehrenbaum,|| and others.

There is some difference between the various authors in respect to the
size of the plaice at the end of the larval stage, due no doubt to the varia-
tion in the size of the egg, and my observations are in near correspond-
ance with those of Ebenbaum ; he gives the average size at the end of
the larval stage as 7*5 mm., or slightly more ; while I have found the
average size of this stage to be 7*2 mm., while specimens are frequently
found that measure 7*5 mm.

expressed the same view with reference to larval turbot [F, B l^th An. Rep.f Part
IILf page 229]. This appears to be the case during the first few days of the post-
larval stage, to which the experiments were confined ; but I now doubt whether
this somewhat artificially produced activity will ever prove beneficial to the
rearing fry.

* While Cunningham, and lately Ehrenbaum, appljr the term 'larvce' to the
young fish, fi*om the moment of hatching till it has acquired the form and habit of
the adult, M'Intosh has divided this period into two stages— the larval, fix>m hatch-
ing till the absorption of the yolk is completed, and post- larval, fi^m this time till
the adult form appears. The latter is at least the most definite, and has been
followed in this paper.

+ Trajis. Hoy, Soc, Edin,, voL xxxv. part iii., 1SS7-8S, p. 840.

t Trans. Roy, Soc. Edin., vol. xxxiii. part L, 1887, p. 92.

S Ninth Annual Report of Fishery Boards part iii. 1890, p. 311.

II Eier und Larven von Fisehen der dexUschen Bucht, Keil and Leipzig, 1897.



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of the Fishery Boa/rd f<yr Scotland. 181

The general characters at the end of the larval stage of the plaice may
be summed up as follows : — ^The larvae is large, compared with others
originating from pelagic eggs ; the eyes prominent, with a golden lustre
(in reflected light) ; embryonic fin broad, with slight black pigment spots
on the ventral side ; pectorals large ; and the gut, which at the time of
hatching, forms a single lobe, is now divided into two parts, of which the
foremost or middle gut is forming a single sling. This is more complete
and distinct a few days later. The general pigmentation of the larva is
bright yellow, with a few black spots, particularly on the ventral side.
Cimningham mentions three rows of pigment along the body ; these are
more prominent later. These characters remain nearly unchanged during
the first seven days of the post-larval stage ; the chief difference being a
rapid growth of the pectorals, and an increase of black pigment, which
hsa now become stellated, particularly in the lateral line of the body j and
the pigmentation in general. has spread more to the embryonic fin.

Sixteenth to Twentieth dan ^fl^ Hatching,

The active larvae that first appeared at the surface, had in this period
begun to quieten, got paler in pigmentation, more inclined to rest with
the head downwards, and some of them died.

Those, again, that had been feeding from the beginning continued to do
so ; they were always actively in search for food, which they captured with
great voracity. Having discovered a prey, the larvae would approach it
slowly to within 1 or 1*5 mm., when it would contract [its body laterally,
forming part of an S, and then suddenly throw itself forward, catching
the object instantly. Another form of larval mollusk was now commonly
seen in the stomachs, a little larger than the one first mentioned (fig. 12).
The stomach of fig. 1 shows how greedily this specimen had been feeding
on this particular form. The young larvae were often seen to catch various
forms of larval crustaceans; these, however, were not found in the
stomachs at this period, so they had not been swallowed. Often I had an
opportunity of observing how well armed forms were captured and then
again rejected with violence.

At the end of this period most of those larvae had died, that from the
beginning showed the mentioned peculiar symptoms and never took food ;
while the mortality amongst the feeding ones was very low.

On the 17th day the average length of three specimens was 7*40 mm.,
and the average breadth of same 1*44 mm. On the nineteenth day I found
one specimen 8 00 mm. long and 152 mm. broad (measured just behind
the gut). The difference from the previous period consists chiefly in a
further increase in the black stellate pigment, which has now spread mort
evenly over the body, partly covering the original yellow pigmentation j
the appearance of the larvae now is therefore a faint yellow-greyish colour.
Only a few black pigment spots are seen on the dorsal marginal fln, while
the ventral part of the body and fin is reachly covered with them ; very
prominent is a row of black stellate pigment, which extends ventrally
from near the snout backwards, along the abdomen and the edge of the
marginal fin ; it extends over four-fifths of the length of the whole fish.
Anterior to the posterior end of this pigment row is noticed an even
development in breadth and thickness of the body, which now appears
denser than before ; the posterior portion has remained thin, and a dis-
tinct narrow or tapering is therefore now formed on the body at this
place.

The tail has remained unchanged (homocerc), and there is no appear-
ance of fin rays.



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182 Part HI.— Fifteenth Anmud Report

The eyes are radiant and with a bright silvery hue, which makes them
very prominent Most of the vertebra are now distinct and the clavi-
cular bones prominent.

Twenty-first to Twenty fifth day.

When changing, the old water in the jar was removed by means of a
syphon ; to the end of this was fixed a large glass filler, across the open-
ing of which was tied a piece of fine cloth, so that food or fry should not
be carried away. The latter remained in the jar constantly, and some
water would therefore always be left each time. And while the upper
part of the glass sides could be properly cleaned daily, the bottom had
gradually become covered with accumulating sediment from the tow-
nettings. In this sediment was soon gathered a great number of crusta-
ceans and larval molluscs, and during this period the fry were frequently
found at or near the bottom, though they were never seen resting on the
latter.

Now I also found that the larvce took a greater variety of food, the addi-
tion chiefly consisting in minute larval forms of crustaceans. It was diffi-
cult to come across well-preserved specimens without killing larger numbers
of larvsB than was advisable at the time, but I have figured some forms as
they appeared in the stomach of the living fish (figs. 1 3 and 1 4). I mentioned
that during the previous stage the larvse often caught hold of well-armed



Online LibraryFishery Board for ScotlandAnnual report of the Fishery Board for Scotland for the year ended .. → online text (page 25 of 52)