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Report
The Danish Biological Station

to

The Board of Agriculture.

C. G. Joh. Petersen,

PrÆD:

Translated from ,,Fiskeri-Beretning for 19108.

). +
Copenhagen.
Centraltrykkeriet.
195

From

The Danish Biological Station.

1911.

Copenhagen.
Centraltrykkeriet.

TSA LE,

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FR BE VRE RE ROR

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Ul

(190 GE

Some Experiments on the Possibility of combating
the harmful Animals of the Fisheries, especially the Whelks
in the Limfjord.

In 1909 a »Committee to combat the harmful animals of the fisheries«
appointed by the Dansk Fiskeriforening, published a short report on this matter,
and the Council of the Fiskeriforening submitted a proposal to Landbrugsministeriet
that funds might be allocated for this purpose. The matter was sent to me for
explanation and this explanation, with but few changes, was as follows.

»The Committee in including 5 animals on the list of harmful animals,
namely, the sea-scorpion, stickleback, crabs, star-fish and whelks, have
specially relied upon & 40 of the Fishery Law regarding rewards for the bringing
of star-fishes to land in the Limfjord, and desire that this & shall be extended to
embrace among others the 5 animals mentioned and to apply to the whole of the
Danish sea-territory. I may remark to this, that this & is on the whole not well-
grounded; its persistence in the law is only due probably to some old regulations
for the oyster-dredgers, who were meant to land the star-fishes they fished. This
old regulation has never been carried into effect, so that we know nothing at all
as to its usefulness. We thus have no available material to build upon, and be-
fore we can obtain such, we should proceed experimentally, that is to say,
undertake such experiments on the extermination of the harmful animals as will
in the course of a few years afford information which would show,
whether they have helped to reduce the numbers of these animals or
not. It it just as important to obtain the last, negative result from the experi-
ments as the positive; as otherwise it may happen with this as with the artificial
hatching of salt-water fishes in other countries, that much money has been spent
for a long series of years, but no result has ever been noticed, whether the ex-
periments have been of use or not. I consider that it was a good thing for Den-
mark, that I at that time opposed such unripe experiments on a large scale and
thus saved the State unnecessary expenditure, and instead endeavoured to find
some other ways of increasing the quantities of fish im our small waters, namely
by the inplanting of young fish from the sea. These results could be controlled;
at some places they were an advantage, at others not.

However much I may desire with the Committee to see the numbers of
the harmful animals mentioned reduced at many places, since they cannot all be

noge
"&IGERZ

4

considered harmful everywhere, I must yet disagree with the Committee regarding
the manner in which the end has to be achieved. One thing we must remember,
namely, that the fight with these harmful animals is everlasting; the
species can never be quite exterminated. For example, howewer much the public
have helped to the extermination of the prawn as a useful animal, it has yet not
entirely disappeared from any of our small fjords. It is thus a perpetual fight,
and we must consider who has to bear the expense in the long run; the only
way this can be met is to find a profitable use of the animals in question for in-
dustrial or other purposes, as the Committee indeed also suggest; hut at present
we do not understand how to do this in Denmark. The conditions here are not
the same as in other countries, nor can we directly introduce the results from the
latter into Danish conditions; these results must first be tested and studied, then
perhaps the right man will come who can solve the problem. In my opinion,
therefore, it would not be right, as the Committee propose, to use 20,000 Kr. di-
rectly in rewards for the harmful animals mentioned indiscriminately and especially
not everywhere in Denmark, as it is quite possible, that after a number of years
have gone past we will be exactly at the same place as we are now, namely, that
we will not know whether it is of advantage to continue or not; provisionally we
should certainly keep to experiments in small waters, as the large would assuredly
show no result at present. We may remember here, how strongly the war against
the rats had to -be organised in this country, before there was any hope of achieving
anything worth mentioning. The fact is, that it is of no use to destroy a small
number of a harmful species, the remainder live and propagate just so much the
better and more rapidly; in other words, the stock remains the same every year;
only when more are exterminated than are yearly produced, can we
manage to reduce the stock, and the fight must be continuous. AÅ wood
does not disappear from cutting down the trees; to obtain this result, more must
be cut down than grow yearly, and the cutting must be continued.

The rate of propagation of the 5 animals mentioned is however very
different, and the same is the case with their biological conditions. If, from my
knowledge of these conditions, I were to mention which of them might most easily
be combated in our small waters, the whelks would be the first, next the crabs
and sea-scorpions and lastly, the sticklebacks and starfishes.

The whelks (Buccinum) are not able to swim, either as small or as adult,
and consequently cannot spread in this way, and crabs and sea-scorpions do not
propagate in several of our small, enclosed fjords; on this account their numbers
here might probably be kept down somewhat by a constant warring against them.

On the other hand, I at present consider that the fight with the sticklebacks
and starfishes is hopeless in our country.

With regard to the whelks, I have seen this year, 1909, during my in-
vestigations in the Limfjord, that it would be quite useless to give rewards for
the number of whelks the fishermen bring to land when fishing for plaice; this
number namely is too small a part of the stock. Instead of this a special whelk-
fishery might be organized here with hive-traps or such like. Such a fishery
might well be established. From a couple of motor-boats, each with 2 men and
several hundred hive-traps, some thousands of barrels of whelks could be fished

5

yearly in Thisted Bredning along, and this would probably in time reduce the
stock appreciably. The stock in Thisted Bredning may probably be estimated
at 20—40,000 barrels of whelks.

On account of the immense harm done by the whelks to the fishes entangled
in fishing-nets — both plaice and other fishes in the Limfjord and just in Thisted
Bredning — I advise that an experiment with a motor-boat should be
undertaken as soon as possible. It would also be desirable here, if the matter
could pay itself economically, but even if this were not the case, the money would
be well-spent on the experiment. If such a paying enterprise could be started in
the Limfjord, similar undertakings might perhaps be begun at. other places, but
the Limfjord is the right place for the start. We have seen how, from the study
of the biology and cultivation of the plaice, practical results were. first-obtained in
the Limfjord; perhaps the same will occur in this case. The reasons for this are
of both a biological and legislative nature, and it is not necessary to explain them
further here; but as the fjord-exploitation of the Limfjord is already much
developed, it is only consistent to endeavour to increase it by means of fjord-ad-
ministration. My advice is, consequently, that the Board of Agriculture should
this year procure a specially equipped motor-boat.

With such a boat we can investigate about the number of barrels
of whelks which can be fished in a year and thus, whether it is of any
use to go further in the matter by procuring more boats; we would see further,
if the people of Thisted Bredning will understand how to make the whelks pro-
fitable as food for pigs, fowls and ducks, for which use the whelks seem to be
well suited, and lastly, at what price the whelks can be sold.

This year, of course, the work may be restricted simply to the fishing of
the whelks and throwing them away; but it will be very unfortunate, if we cannot
at the same time make experiments on their utility.

I consider it of great importance, that the matter should be undertaken
soon and just in 1910; hivetraps, namely, can only be made in the winter, and
the Limfjord is in any case being investigated by the Biological Station this year.

Before I express an opinion as to the possibility of combating the crabs
and sea-scorpions, I should prefer to make investigations into the matter from the
Biological Station for about a year or so.«

In the winter of 1910 the Board of Agriculture allotted the funds necessary
for the Station and the experiment of fishing up the whelks in Thisted Bredning
was begun on the 4th of April 1910.

The whole matter of the destruction of plaice in the nets by the whelks
had been naturally under my observation in several previous years. It was thus
in accordance with my wishes that Dr. Th. Mortensen, the assistant at the
Biological Station, in 1895 carried out the enquiry mentioned on p. 46 of the
Committee's communication, in order to ascertain how many of the plaice caught
in the nets are destroyed in one to two nights; but I was not at all clear, as to
how we should organize an attack on the whelks. It was not until 1909 that I
began again to examine into the matter, more particularly, in connection with other

6

investigations regarding the inplanting of plaice, which has now been set on foot
on a large scale.

Some few figures will explain better than many words how the matter
really stands.

Over large areas of the Limfjord, especially in the central parts, two species
of predatory whelks occur in great quantities; one a large form, the common dog-
whelk or the »Konkylier« of the Limfjord (Buccinum undatum), and a smaller, Nassa
reticulata.. They are both shown in fig. 1. They have long snouts furnished with
sharp teeth, by means of which they can tear up all the flesh they can get hold
of. Their mode of attacking the plaice is to gnaw a hole in the skin and put

Fig. 1.

their snout through this hole; they then devour all the muscular tissue, leaving
only the skin and bones. The slow-moving whelks however cannot seize the plaice
when swimming freely, but when the plaice are caught in the gill-nets (see fig. 2),
which always reach to the bottom, they are often so entangled or so enfeebled,
that they sink quite down to the bottom and lie quietly; they thus become an
easy prey for the whelks. In figures 3 and 4, several whelks are seen attached to
the plaice just brought up in the nets; the latter are dead in the first place, but
they are also partially devoured by the whelks and of no use as human food.
In fig. 3 at least 3 Buccimum are seen and 2 in fig. 4 together with a starfish
(Asterias).. The small whelks, Nassa, attack the plaice in a similar way and, on
account of their greater numbers, are perhaps still more harmful than the large.
On the other hand, the plaice are not so much in danger of the starfishes
(Asterias), as the latter ordinarily only get time to suck at them outside on the
skin, but they also might kill the fish. It can readily be understood, that
the whelks do an immense amount of harm in waters where the fishery

cQ

and prices are based on the capture of living plaice, especially when the
fish are caught in nets which stand for many hours (not in seines), in accordance
with the bye-laws of the Limfjord for the greater part of the year. In the case
of the trawls, which are quickly drawn up, the whelks do not get time to attack
the plaice; but the trawl has other drawbacks, which, however, need not be further
discussed on this occasion.

The whelks find the plaice and their food on the whole by means of their
finely developed sense of smell. They also attack all other fishes fixed in the nets;
in a few hours 10—20 whelks will often fix themselves on to a plaice or other
dead fish on the bottom of the sea. The whelks are so numerous, that some

Fig. 2.

hundreds, both large and small, can be fished in 24 hours in a hive-trap with
a piece of split cod as hait. We may imagine, that in the centre of the Limfjord
several whelks occur on an area no larger than the floor of a room; as they are
able to crawl rather rapidly, we can understand how, in the course of a few hours,
they might easily find a dead fish lying on the bottom and devour the muscles.
On the other hand, they will not touch rotten fish; experiments with soiled bait
in the hive-traps have clearly proved this. Sometimes dead fishes can be found
in numbers on the bottom of the Limfjord; this was the case in 1909 in the spring,
when a great number of eels had been frozen dead in the winter; they were rotten
before the whelks had any desire to feed, but I have never seen anything similar
in the summer; everything eatable which sinks to the bottom is evidently quickly
devoured at that season.

It is very difficult to find out, how great is the number of net-entangled
plaice which are devoured by the whelks throughout the year, as-this depends on
so many circumstances. For example, it is especially in the warmest summer

Fig. 3.

months, that the worst attacks of the whelks take place; but the amount of the
damage done daily by the whelks depends greatly on the way fishermen
deal with their nets. If they let the nets stand out for 24 hours, most of the
fishes attached in it are often devoured, and if a storm rises which prevents the
nets being taken up just one single day, so that they remain out 48 hours, practi-
cally everything is destroyed. If, on the other hand, the fishermen attend to the

Fig. 4.

9

nets both morning and evening, as is now often the case, for instance at Thisted
in the summer, not nearly so much harm is done, and if they use the method of
beating the water in the daytime and thus have the nets out for only a short
time, almost no harm is done. But this mode of fishing requires the fishermen
to remain uninterruptedly by their nets — for which tolerably fine weather is ne-
cessary, so that it is not easily carried out with sufficient profit. We may reckon,
however, that all in all the attacks of whelks lead to a loss of "/,rd of the year's
catch in the central parts of the Limfjord; consequently, the damage done is
very considerable, and on account of this I have made great efforts to find

Fieæ5:

a suitable use for these whelks, which might perhaps make the fishing of them
pay itself and thus be carried on without special support, for example, from the State.

The whelks, the large (Buccinum), are acknowledged to be an excellent
bait for eels in summer and for cod in winter, and during my investigations this
year a part also has been sold to fishermen in Thisted, for use in the eel-fishery;
but the local use is too small, when many whelks are fished. Experiments have
also been made with the salting of the whelks, taken out of their shells, but
on the west coast they were refused owing to the salt making them too hard. I
have sent whelks slightly salted to Norway, where salted mussels (Mytilus modiolus)
and cuttle-fish are known to be much used for bait; but this experiment did not
succeed either; it appears that the whelks require so much salt to keep, that it
makes them too hard. Next I tried solutions of formaline, and though experi-
mental fishing for cod gave much fewer fishes per 100 hooks with such bait in
the Limfjord than with fresh whelks, it is possible, that these prepared in forma-
line might be used on the west coast, where the fishermen are usually obliged to

use salted, not fresh bait. If numbers of whelks could be fished during the
2

10

cold season — which however was proved impossible by the experiments with
hive-traps — they could be used in great quantities on the west coast. There the
fishermen use immense quantities of living mussels, which are fished in the Ise-
fjord or in the eastern part of the Limfjord and can be taken there in an almost
unlimited number.

Thus I have not been able to get the whelks used in sufficient quantities
in the summer. To investigate whether they might possibly be used for other
purposes, for example, as food
for man, pigs, fowls etc., I have
had them tested by Professor K.
Rørdam, and he reports that
the whelks dried at 100? contain
about 79% shell and ca. 21,
soft parts —thus, only "/sth. Ofthis
fifth, however, %/,,ths consist of
albuminous matters or related
nitrogenous compounds. Ås the
weight of the living whelks is
reduced on drying to somewhat
under the half, it is seen that
the predominant ingredients are
lime and water.

The large whelks fished in
England are eaten in great quan-.
tities, for example in London, but
I consider it impossible to get
them used in this way in our
country. On this account I endea-
voured to get them prepared as
food for fowls or dogs at one of
the Copenhagen factories, where
such food is made; hut as the
whelks became rotten so
quickly in the warm season
that the steamers refused
to take them on board, this experiment also failed.

This rapid decomposition of the whelks in the warm season prevents any
use of them except locally, and I considered it inexpedient and too expensive at
the present stage of the matter, to have a drying-faetory built in Thisted. Nor
have I tried on a large scale to get them cooked and used as pigs' food, as ex-
perience has shown that the removal of the shells must be performed with
great care to prevent the stomachs of the pigs being wounded by the sharp
pieces of shells, especially by the central spiral inside the shell; this is difficult to
remove except by using the fingers to every cooked and crushed whelk. The
result is, that all these experiments, which have taken up much of my time, have
not succeeded, at least for the time being. Apart from the local use of the

Fig. 6.

del

whelks as food for fowls and ducks and as bait, I can recommend
nothing at present with certainty. But in Thisted the ducks and fowls which
live by the harbour, like to eat the whelks brought to land by the fishermen; I
have bought and eaten one of the ducks fed in this way; the taste was not in the
least disagreeable, like ducks fed on fishes; the reason being, that the whelks
contain but a small proportion of fat.

I may now turn to a discussion of the experimental fishing undertaken
in 1910, to give information regarding the number of barrels of whelks, which
might be fished in Thisted Bredning from a motor-boat with 2 men.

In 1909, for the purpose of obtaining whelks for salting and for other

(Fies7:

investigations, I had procured ca. 50 whelk hive-traps, bait-traps as they
were called in the Sound, where in earlier years (in the eighties) they were used
for obtaining hait. This fishery has now almost entirely ceased there, it is said
for want of whelks; nor do I know of other places in the country where the
method is still used. These whelk hive-traps are represented in fig. 7, one hanging
outside the boat ready to be sunk down to the bottom. They are made of willows
and have a funnel-shaped opening et each end; one of the ends is loose, to be
taken off when new hait has to be placed inside and the entrapped whelks taken
out. The traps must have a weight to make them sink and lie steady, or the
whelks will not enter. They are furnished with a rope, which at the surface of
the water ends at a wooden buoy, so that the traps can be found again. It was
at once found, that it was difficult in the Limfjord to buy bait for these traps.
Fresh fish, dried fish, salted fish were all too expensive. I then ordered second-
class, but fresh, split cod through an Icelandic firm; it cost but some few øre per
pound and turned out to be good. Tainted split cod, however, which I once got

in the Limfjord, was of no use;
it smelt badly and the Buceinum
would not touch it; Nassa how-
ever entered the traps but
only in small number. Fresh
but unsalted cod seems to be
the best bait of all, but as a
matter of fact it is dear in these
fjords.. To economize with the
bait, the split cod, which is
otherwise rapidly devoured by
the whelks so that the traps
catch no more, the pieces of
split cod were enveloped in a
strong steel wire through which
the snouts of the whelks could
not penetrate. The bait could
then last for some days in spring
and the trap continue to fish;
as soon as the water gets warm,
however, the bait must be chan-
ged almost daily or it will rot.
During the latter period the
steel wire was given up; but,
to keep the bait from the whelks
Bi & as long as possible, the pieces
of split cod were hung by a
wire under the roof of the trap, thus away from the bottom. In this way we
managed to keep the bait from being eaten too rapidly. In 1909 by means of
these traps we were sometimes able to procure sufficient quantities of whelks, often
some barrels full; but as we could not look after this whelk fishery constantly, we
could not draw any conclusions from our fishing in 1909 as to how many barrels
a boat, only occupied in this fishery, can fish in a year. Sufficient information
however was obtained to serve as a basis for the start of such an experimental
fishery in 1910.

Å suitable motor-boat, such as is shown in figs. 5—7, was procured; it
was furnished with a small davit and with a bollard on the motor, as shown in
fig. 7, for hauling up the traps; two fishermen from the Limfjord were engaged,
partly at fixed wages, partly paid, first 10 later 20 øre for each harrel of whelks
of ca. 190 Pd. The fish-exporter Ottesen in Thisted was the corresponding man-
ager of the boat, and on the 4th of April 1910 the fishery could begin.

The boat had ca. 240 whelk-traps at its disposal; they were all put out,
but they were not all attended to every day; storms often broke off the fishery.

On p. 13 a list is given of the number of barrels of whelks, daily
fished from the åth of April to the 8th of November, on which day I stopped
the fishery.

13

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"OIGI 'AON 'g 0] ER 'G WOJJ Sulupag pajsiyL ur <3oysy 494 MA

14

Ås shown by the table of the daily and monthly hauls of whelks, the
fishery was small in the month of April, increasing in May and most abundant
in June, in which month 1045 half-bushels (»Skæpper«) of whelks were fished. It

Im

jun

Z"

==

==

=4 =E

ER "==

EN ET

SEN —
EN <=

mme?

)

Thisted Bredning. Number of Buccinum. 34 stations with tbe 1,2 m? bottom-sampler.
The area fished with hive-traps is indicated at A.

looked as if July would be a good month, but from the 19th to the 27th of July
the fishery was very bad; a phenomenon which occurred simultaneously with a
great mortality among the plaice caught in the nets; not that these were all attacked
by the whelks, but many, though untouched by the whelks, were nevertheless dead
in the nets. I imagine that both phenomena were connected with the specially

il)

intense warmth of this past summer. In August and to the middle of November
the fishery decreased, and in October it gave but 296 half-bushels.

To gain an idea of the quantities of whelks caught in the Bredning during
this fishery, reference may be made to fig. 8, which shows a large heap of whelks
thrown up on the beach east of Thisted harbour. This heap only represents
however the fishery of ca. 8 days im the summer. The whelks were deposited
here to die; they decomposed very quickly and spread an intolerable stench in
the neighbourhood. As mentioned, a part of the whelks fished was also used as
bait and as food for fowls; but only a small part of the quantities fished could
be used in this manner in the summer time.

It soon became clear, that all the 240 traps could not be attended to
every day; this took too much time; consequently, we let them remain out on an
average for 2—3 days before taking them up; in this way more were caught per
trap each time than otherwise would have been the case. As expected, it was
necessary each time to move the traps ca. 10—20 fathoms or they fished
less; evidently a sign that the stock of whelks was reduced when the traps
remained at one place for a couple of days. After some search in the Bredning
we found that the majority of the whelks occurred in the eastern part of Thisted
Bredning. All the traps were later put out here in 2 long rows at distances of
ca. 15—20 fathoms between each; as these rows were constantly mowed each time
they were attended to, a not inconsiderable area of the Bredning was fished over
in this manner, probably "/,th of that part of the Bredning which is outside the
6 meter line and in which the majority of the whelks live (see Chart p. 16). To
investigate how much the stock was reduced in the summer at the places fished
over, some traps were sunk here; it then proved, that the catch in these was only
1/rd of what was fished immediately. We cannot conclude from this, however,
that 2/.rds of the whole stock there was really fished away; we know, namely,
that not all the whelks go equally readily ilnto the traps. The experiments
were made during the warm period, in July, however, when the whelks were taken
in greatest quantity; but we cannot know for certain, whether they will all try to
enter the traps at this period.

So much can therefore be said with certainty, that we have not fished
away all the whelks on "/;th of the area of the Bredning.

To form any judgment of the utility of the whelk fishery gener-
ally, however, it is of the greatest importance to know the approximate
quantities of whelks found in the whole Bredning; if, namely, but a
small part of the number is fished away yearly, the stock will probably
with but little difficulty be able to replace it, and thus nothing lasting
is attained.

Already in 1909, before it was quite decided to undertake this whelk
fishery, I had arranged several experiments to test this question.

In our experiments made for other purposes to determine the actual number
of animals per m? ([] meter) in Thisted Bredning, a bottom-sampler was used
with a spread of "/,, m?. In 1909 100 stations were investigated with this bottom-
sampler, and the result obtained was 1,3 Buccinum per-m?. With regard to Buc-
cinum, however, the numbers obtained by means of this bottom-sampler are so

16

small, that we cannot trust them. Another bottom-sampler, with a fishing-capacity
of 1,2 m? was then constructed and used in the investigation of 34 stations (see
Chart, p. 14) scattered over the whole area of Thisted Bredning outside the 6 meter
line. The catch with it was 53 Buccinum, somewhat evenly distributed over the
whole area, but in the eastern part the number was distinctly greater, just where
in 1910 the most whelks were also found. This series of stations also gave 1,3
Buccinum per m?; but I must consider it a chance, that the numbers for both the
large and the small bottom-sampler show just the same result, 1,3; the reason is,
that the small one also takes quite small Bueccrnum.

I would not trust this apparatus alone either and arranged for a diver to
go down to the bottom at different places in Thisted Bredning, to investigate how
many Buccinum he was able to find on a definite area; a method I had earlier
used in the oyster investigations. The area investigated by the diver was deter-
mined from the anchored boat by the length of the air-tube, but also in another
manner, by measuring an area of the bottom with a line. Buecinum however was
not so easy to take as the oysters; they are more huried in the soft bottom and
only appear when the diver has disturbed the bottom with his feet; they collect
specially in the foot-prints.

At one place the diver investigated ca. 106 m? and took 100 Buecinum

a second » ” » ” 36 » » » 128 »
- third » » » 05065 » y 70 y
-fourth 2 » y sb 5 97 K

At the last two places the diver was of the opinion that he had only
caught "/,rd of the Buccinum present there. If we therefore multiply the last two
numbers with 3, he observed altogether 729 Buecinum on 354 m?, or ca. 2 Bucci-
num are found per m?.

Owing to the difficulties of the diving operations on soft bottom, we kept
him to the less soft, where we know that most Buccinum are found; owing to
this probably we got a number somewhat greater than that given by the bottom-
sampler, namely 2 instead of 1,8 per m?. Whichever number we take as basis for
a calculation of the number of Buccinum living in Thisted Bredning, we find great
quantities of whelks there.

Outside the 6 meter line the area of Thisted Bredning is, namely, ca. 65
million m?; consequently, we find either 84,5 millions or 130 millions of
these whelks on the whole area, in addition to the smaller whelks
(Nassa) which are ca. 5 times more numerous.

If we reckon in round numbers with 100 millions of Buccinum, this gives
ca. 19,131 barrels =— 26,754 hectol., as 5,200 Buccinum on an average make one
barrel. In comparison with such numbers, the quantity fished by us in
1910, 480 barrels, is obviously quite insignificant; the stock is easily
able to replace the number of whelks fished, if that is to say the con-
ditions for their growth are present.

Professor W. Johannsen has kindly undertaken a numerical revision of
the numbers found by means of the large bottom-sampler (1,2 m?) and has given
me the following information about the result. The number of Buccinum, 1,8 per m?
or ca. 85 millions, found in Thisted Bredning outside the 6 meter line in October

17

1909, has an average error of 16 millions. When twice the average error is taken
as range, this gives as outer limits respectively 53 and 117 millions. It is but
little probable that the size of the stock lies outside these limits. The number of
Buccinum, 480 barrels or ca. 2,5 millions, taken by the fishery in 1910, only re-
presents consequently "/,,th of the calculated quantity of the stock. And even in
the most favourable case with regard to the importance of the catch, that the real
size of the stock is at the lowest of the limits above-mentioned, the catch is only
1/4; but this case is not more probable than the opposite, that the real size of
the stock is at the upper limit, and in this case the catch is but "/,,th of the stock.

The result of these calculations thus gives us, that the number of Buccimum
fished in 1010 is but a small part of the stock of whelks, and it may be said,
further, that great quantities of Buccinum live in near the shores, where it has
not been possible to obtain samples with the bottom sampler; these quantities have
not at all been included in any of these calculations.

There is reason to believe, however, that the quantity of whelks was not
quite as great in 1910 as in 1909; on the one hand, the 0,1 m? bottom-sampler
used at 200 stations did not take as many Buceinum in 1910 as in 1909, and on
the other, the catch in the whelk-traps was not so great in 1910 as I had reason
to expect from 1909. In 1909 34 traps were taken up 7 times from the 3.—16.
October, and had consequently been out as a rule but 24 hours each time; 69
half-bushels in all were fished. This makes 0,29 half-bushels per trap per day. In
the beginning of October 1910 from the 3.—17., however, the traps which were
now out for 2 to 3 days gave but 0,19 half-bushels per trap each time; thus not
nearly so many; during this period no less than 1175 traps were examined. The
two grounds fished on lie near together; but it must be admitted, that it is difficult
to draw any definite conclusions from this material, as even a short distance can
affect the result, and in addition to this the nature of the bait may also be of
importance; both series of data however indicate the same, namely, smaller quan-
tities of whelks in 1910 than in 1909. The statements of the fishermen are also
in accordance with these results. The whelks have not been so bad in 1910 as
in the year before; they have especially not damaged the fishes in the nets so
much in 1910 as in 1909.

In 1908 an unusual number of small and large cod immigrated into
Thisted Bredning and probably into the whole Limfjord, it seems also, into the
whole of Denmark; people could be seen trailing for cod in and outside Thisted
harbour in 1909 during the whole of the summer, something not known there
usually, and in 1910 the cod were still there. As these cod grew, larger they
could devour the Buccinum; and the contents of their stomachs distinctly show
that they do so on a large scale. In a cod of ”/,—"/, a kg. the soft parts with
operculum of as many as 6-—7 Buccinum were often found. It is remarkable that
the shells themselves of the whelks are seldom seen in the cod. I have only
seen this outside the Limfjord; a half digested and almost transparent shell was
all that was left. As we know how firmly the whelk is attached to its shell, we
cannot doubt that the cod must swallow the shell to get the soft parts;
the latter are not digested until the gastric juices have dissolved the shell. It is
possible that the shells are so seldom found in the somachs because I have only

3

18

investigated the latter in the daytime; probably the cod in the Limfjord feed in
the nighttime. However the case may be, many Buccinum have been devoured
by the cod in 1909 and 1910; and I consider it probable, that we have here the
reason of the smaller quantity of the whelks in 1910. To judge from the results
of the small bottom-sampler, the numbers of Nassa on the other hand have not
decreased; I have never seen them more numerous than in the spring of 1910;
but so far as known Nassa is not sought after by cod or by other animals in
Thisted Bredning, so far as I know. Its shell is stronger than that of Buccinum.

It is quite possible, however, that the supposed reduction in the numbers
of Buccinum is also due to other causes than the attacks of the cod. Buccinum
belongs to the molluses whose young do not float about in the water; they remain
at the bottom when they have crept out of the egg-capsule; but in spite of eager
search in these years we have only succeeded in finding extremely few,
fully developed young in the otherwise very numerous egg-masses,
which Buccinum every spring deposits on stones and shells in the Bredning. Nor
have we been able to find, with but few exceptions, the small, fully developed,
free-living young on the bottom, where however there must be many per m?, even
if only one comes out of the many egg-capsules. Later in the summer,
however, many eggs are found dead and not developed in the capsules
and many seem to be attacked by fungi or bacteria; it is probably not
unreasonable to suppose that epidemics may arise, where so many Buccinum live
so close together. In any case the small number of quite small Buccinum in the
Bredning is remarkable. The whelk seems to take several years before it reaches
its full size, thus to grow slowly; if there is not a sufficient augmentation from
the side of the young, the stock will be appreciably reduced in the course of
some years.

It is said, that these whelks have spread very much in the Limfjord during
the last 10—20 years; that they first appeared in the western parts of the
fjord, where they are now less numerous, whilst the central parts of the
fjord are now teeming with them. If this is really the case, that they have been
numerous in the western parts, e. g. in Kaas Bredning, we may indeed hope,
that they will also to some extent disappear in the central parts; in the western
parts, at any rate, they are not nearly so troublesome as elsewhere, and their
number is obviously much fewer per m? there. It is chiefly to the west and north
of Mors and in Livø Bredning, that they are now so numerous.

I may also mention here one thing more regarding the biology of Buccinum.
As I can scarcely believe, that the dead fishes and on the whole the dead animals
which fall to the bottom, can provide sufficient food for these whelks, I have tried
to find out what they live on from the Buccinum we caught; but I have always
found but small quantities in their stomachs and always only some indeterminable
slime; what they feed on normally in the Limfjord, therefore, I do not
know; but everything thrown out, dead fish, dead oysters etc. are attacked by
them at once, so long as it is fresh. They know probably how to find similar
food not only among dead animals; they probably also know how to attack
the living animals, such as small mussels etc. How the large whelks can
open mussels is in fact well-known; I may refer here to a paper by H. S. Colton

19

(Proceedings of the Acad. of Nat. Sc. of Philadelphia 1908. Jan. pp. 3—10 with
5 Plates), which explains, how the larger whelks can open large mussels and eat them.

That it is the month of June especially, which shows the greatest numbers
of whelks caught, is certainly connected with the fact, that at this time the ma-
jority of the whelks have just deposited their eggs; and it is a well-known pheno-
menon, that for many species of animals the feeding time as a rule succeeds the
spawning time. —

If now we are not disposed to believe, that the whelks of themselves
will decrease in numbers, then the above investigations indicate clearly, that
June and a part of July are especially the months when they might
be fished for with the greatest success by means of the hive-traps. It
is possible indeed that we might be able to find a still better method of fishing
them; but so long as we do not know of such a method, we cannot take it into
consideration. As we must remember, that the quantities of the whelks in Thisted
Bredning amount to at least ca. 20,000 barrels, it will probably not be of any
use to fish less than about the half or third during the year, and it must
also be remembered, that the fewer the whelks become, it will be so much the
more difficult to get many of them. In other words, about 74 or 50 times more
whelks must be fished in the month of June than the motor-boat has taken this
past year in June; or if we also include July in the fishing period, 25—40 boats
must work just as hard in two months as this one boat has worked. If we
imagined, that the fishermen in Thisted would spend 1 to 2 months in fishing
whelks with a subvention from the State of 2 Kr. per hbarrel of whelks fished,
the expense would amount to 10 to 20,000 Kroner per year, thus much more
than the inplanting of plaice in Thisted Bredning costs. It would thus be much
better to inplant more plaice into the Bredning, if this can support
more than the 60 per hectare it now gets. It is so difficult to combat the
whelks even in Thisted Bredning alone, so long as we have not found any other
method of fishing whioh can perhaps give other results, or until we have found
some profitable use for the whelks themselves, so that the fishery would at
least for a great part pay itself. And yet we must consider these whelks
(Buccinum) as much easier to combat than other animals, which might in
masses suddenly come wandering into the fjord or as young drift in with the
current (star-fishes), or as adult swimming fish (sticklebacks). I am inclined
to believe, that the fight against the harmful animals of the fisheries is much more
difficult than is usually imagined; but 6ne thing is certain, the matter must be
investigated separately for each single species and regard taken of the
methods of capture available at the time and the possible methods of
making use of the products. These investigations show to the full, how useless
is the system of rewards for the whelks brought on land on a small scale. It
would be much more profitable to think out some way of avoiding the
damage the whelks do, by giving the fishermen the right to make
certain regulations with regard to the gill-net fishing especially in the
warm months, as for example, that the interval between the times the
nets are taken up should not be too long; I have often heard wishes put
forward by the fishermen on this matter; but I need not enter further into this

20

side of the question; when the fishery laws come eventually to be revised however,
this question should not be forgotten.

It was my intention, as mentioned, to follow a plan in these investigations,
which would give a positive answer in the course of time, as to whether it would
be of use to do this or that, or whether it would be useless, instead of blindly
beginning a series of undertakings, which however long they may be continued,
would give no answer (premiums). I believe that I have carried through
the plan as well as the circumstances for the present permit. If new
moments of importance should appear in the future, the question could always be
taken up again; we now know a good deal more regarding the whelk question in
the Limfjord than before these investigations were begun; thus, we know, that
it might well be within the power of the State to exterminate almost
all the Buccinum in Thisted Bredning in the course of some years, but
we also know, that the money would be badly spent, we should rather
use a portion of it on more extensive inplanting of plaice in the Limfjord. Ås is
known, the State has experimentally given out enormous sums for the collection
and distruction of the May bugs, with the result that the May bugs have now
almost disappeared; but as Professor I. E. V. Boas writes in his book on the May
bug in 1904, their disappearance is not due to their collection, but to the
attacks of their natural enemies (bacteria). I believe that we have reason
to hope, that the same will happen with the whelks (Konkylierne) in the Limfjord.

I have very often pointed out, that man can do much to reduce the
numbers of a species in the sea, and can also often manage to make their numbers
very few, e. g. the prawn in Denmark. But what has this cost, both in work
and money? — In the good years for the prawn in the Limfjord in 1895 and 96,
the value of the traps and purse-nets amounted to ca. 50,000 Kr.; the
value of the boats and the work is not included; but hundreds of fishermen were
occupied in the fishery. When so much energy is being used, much can be done;
it succeeded; in 2—3 years the prawn became more scarce, and is so still; it
paid the expense with 70—80,000 Kroner per annum, and between 100 and 200
thousand pounds were fished yearly of these animals in the Limfjord; and the
fight against the few remaining still continues whenever the opportunity presents
itself. I certainly believe that it would be much easier to reduce the numbers
of the whelks in the Limfjord; but I must consider the sums necessary for this
purpose as badly used, — at any rate, so long as no new, unknown moments
appear, which might considerably facilitate the fishing and reduce the expense.

C. G. Joh. Petersen: Om de skalbærende Molluskers Udbredningsforhold i de danske
"… Have indenfor Skagen 1888. . 89. 1162 pg. 2 Kort.

Det videnskabelige Udbytte af Kanonbaaden »Hauch«s Togter i de danske Have
indenfor Skagen. 1893. 49, 464 pg. Atlas i folio.

Indhold: Echinodermata (Petersen), Diatomeer (Cleve), Undersøgelse af noglf Bundprøver (Rør-
dam), Mollusca (Petersen), Cephalopoda (Posselt), Crustacea' malacostraca (Meinert),
Hydrografi (Rørdam), Polyzoa (Levinsen), Ascidiæ simplices (Traustedt), Annulata,
Hydroidæ, Anthozoa, Porifera (Levinsen), Nogle alm. Resultater (General Results.
Engl.) (Petersen).

C. F. Drechsel: Oversigt over vore Saltvandsfiskerier med Kort og Planer. 1890. 49,
100 pg. Med et Tillæg: »Om Naturforholdene indenfor Skagen« af C. G. Joh.
Petersen. Med Kort. 49. 46 pg.

Fiskeri-Beretning for 1888—89, 89—90, 90—91, 91—92, 92—93, 93—94, 94—95.
95—96, 96—97, 97—98, 98—99, 99—1900, 1900—01, 1901—02, 1902—03,
1903—04, 1904—05, 1905—06, 1906—07, 1907—08. 1908. 1909.

Beretninger fra den danske biologiske Station findes paa dansk publicerede i de til-
svarende Fiskeri-Beretninger.

I. Fiskenes biologiske Forhold i Holbæk Fjord. 1890—91. 63 pg. Med et Kort.
II. Om vore Kutlingers (Gobius) Æg og Ynglemaade. 1892. 9 pg. Med 2 Tavler.
II. On the Eggs and Breeding of our Gobiidæ. 1892. 9 pg. Two Plates.
III. Det pelagiske Liv i Fænø Sund etc. 1893. 38 pg. Tabeller.
III. The Pelagic Life in Fænø Sound etc. 1895. 38 pg. Tables.
IV. Om vore Flynderfiskes Biologi og om vore Flynderfiskeriers Aftagen. 1594. 146 pg.
2 Tavler. 1 Kort og mange Tabeller.
IV. On the Biolegy of our Flat-fishes and on the decrease of our Flat fish Fisheries. 1894,
146 pg. 2 Plates. 1 Chart. Many Tables.
V. Den alm. Aal (Anguilla vulgaris T.) anlægger før sin Vandring til Havet en særlig For-
plantningsdragt. 1896. 35 pg. Med 2 Tavler.
Etc. 64 pg. SAR
V. The common Eel (Anguilla vulgaris T.) gets a Particular Breeding-dress before its Emi-
gration to the Sea. 1896. 35 pg. With 2 Plates.
Etc. 64 pg.
VI. Om Rødspætteyngelens aarlige Indvandring i Limfjorden etc. 1887. 49 pz. 1 Kort. 2 Tabeller,
VI. The Yearly Immigration of young Plaice into the Limfjord etc. 1897. 48 pg. 1 Chart.
2 Tables.
VII. Plankton-Studier i Limfjorden. 1897. 23 pg. 1 Kort. 4 Tabeller.
VII. Plankton Studies in the Limfjord. 1897. 23 pg. 1 Map 4 Tables.
VII. Om et Skovl-Vaad til Undersøgelse af dybere Farvande. 1898. 24 pg. 10 Figurer.
VII. An Otter-Seine for the Exploration, of Deeper Seas. 1898. 24 pg. With 10 Figures.
IX. Travlinger i Skagerak og det nordlige Kattegat i 1897 og 98. 1899. 56 pg:.1: Kort.
IX. Trawlings in the Skager Rack and the Northern Cattegat in 1897 and 98. 1899. 56 pg.
With One Map.

Fortegnelse over Aalerusestader i Danmark etc. — Mindre Meddelelser. 1899 og 1900. 36 pg.

Et farvetrykt Kort.
List of The fAalerusestader” in Denmark, etc. — Smaller Communications. 1899 and

1900. 37 pg.

SE

XIL

XV.
XVI.
XVI.
XVII.

XVII.
XVIII:

XVII.

X1xX.

. I. Torskens Biologi i de danske Farvande. II. Om andre Torskefisk i vore Farvande

III. Nogle almindelige Betragtninger om Fredning, Lovgivning etc. IV. Ændringer og
Forbedringer af Skoyl-Vaad til zoologisk Brug. 1900 og 1901. 44 pg. .

. I. The Biology of the Cod in the Danish Seas. II.-On other Codfishes in our Seas.

HI. Some General Observations on Protection, Legislation etc. IV. Alterations and
Improvements on Otier-seines for Zoological Purposes. 1900 and: 1901. 44 pg.

I. Hvor og under hvilke Forhold kunne Rødspættens Æg udvikle sig til Unger indenfor
Skagen? 1 Kort. IM. Smaahvarrernes (Zeugopterus-Slægtens) Unger. 1 Tavle: III. Kunne
vi optage Konkurrencen med Udlandets Da DERS i vore Farvande udenfor det danske
Søterritorium? 1902 og 1903. 36. pg.

I. Where, and. under. what Conditions, can the Free of Plaice be developed into Young
Fish within the Skaw? . II. On the Young Stages of the. genus Zeugopterus. (With one.
Plate.) III. Can we enter into Competition with the. Foreign Steam-trawlers in our Seas
outside the Danish Territorial. Limit. 1902 and 1903. 33 pg.

. Fiskeæg og Fiskeyngel i de danske Farvande. (Undersøgelser i 1904 og tidligere Aar.

1903 og 1904. 81 pg.

. Eggs and Young of Fishes in the Danish Waters. (Investigations during 1904 and earlier

years.) 1903 and 1904. 85 pg.

. I. Om Lysets Indflydelse paa Aalens Vandringer. II. Om Aalens Alder og Vækst. 1906.

!

39 pg.

. I. The. Influence of Light on the Migrations of the Eel. II. Age and Rate of -Growth of

the Eel. 1906. 359 pg.

. Studier over Østersfiskeriet og Østersen i Limfjorden. Med et Kort, Temperaturkurver,

3 Tabeller og 2 Tekstfigurer. 1907. 70 pg.

I. First Report on the Oysters and Oyster Fisheries in the Lim Fjord. (70 pg.)
Aalegræssets (Zostera marina's) Vækstforhold og Udbredelse i vore Farvande. 1908. 61 pg.
On the Ecology and Distribution of the Grass-Wrack (Zostera marina) in Danish Waters.
(With -9 figures.) 1908. 62. pg.

Studier over Østersfiskeriet og Østersen i Limfjorden. 1908, 24 pg.

II. Second Report on the Oysters and Oyster Fisheries in the Lim Fjord. . 1908... (24 pg.)
I. Om Udbyttet af Limfjordens Fiskerier i de senere Aar samt om Rødspætteindplant-
ningen i 1908, Med 6 Tabeller og 1 Kort, Il. Biologiske Undersøgelser over Limfjordens
Rødspættebestand. Med 3 Tabeller. III. Om. Rejefiskeriets Udbytte fra 1885—1907 og
om dets Forbedring «ved Fredning. Med 3 Tabeller. 25 pg.

I. The Yield of the Limfjord fisheries in recent years and the transplantation of plaice
in 1908. With 6 tables and 1 chart. II. Biological investigations on the stock of plaice
in the Limfjord. With 3 tables. III, The Yield of the Prawn fishery from 1885—1907
and its improvement by means of protection. With 3 tables. 25 pg.

Nogle Undersøgelser over Muligheden af at bekæmpe Fiskeriets Skadedyr, særlig Sneglene
i Limfjorden. 1911, >20 pg.

;Forfattede af.C. G. Joh. Petersen, undtagen:
Report XIII. .. 4. Otterstrøm.
00" XIV, IL. K.' J. Gemzøe.
Es RIV: C. H. Ostenfeld.

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