DISEASES OF FISHES
=Contagious Diseases.=--As compared with other animals the fishes
of the sea are subject to but few specific diseases. Those in fresh
waters, being more isolated, are more frequently attacked by contagious
maladies. Often these diseases are very destructive. In an "epidemic"
in Lake Mendota, near Madison, Wis., Professor Stephen A. Forbes
reports a death of 300 tons of fishes in the lake. I have seen similar
conditions among the land-locked alewife in Cayuga and Seneca Lakes,
the dead fishes being piled on the beaches so as to fill the air with
the stench of their decay.
[Illustration: FIG. 225.--Menhaden, _Brevoortia tyrannus_ (Latrobe).
Woods Hole, Mass.]
=Crustacean Parasites.=--The external parasites of fishes are of little
injury. These are mainly lernæans and other crustaceans (fish-lice)
in the sea, and in the rivers different species of leeches. These may
suck the blood of the fish, or in the case of certain crustaceans
which lie under the tongue, steal the food as it passes along, as is
done by _Cymothoa prægustator_, the "bug" of the mouth of the menhaden
(_Brevoortia tyrannus_).
[Illustration: FIG. 226.--Australian Flying-fish, _Exonautes unicolor_
(Valenciennes). Specimen from Tasman Sea, having parasitic lernæan
crustaceans, to which parasitic barnacles are attached. (After
Kellogg.)] The relation of this crustacean to its host suggested to
Latrobe, its discoverer, the relation of the "foretaster" in Roman
times to the tyrant whom he served. A similar commensation exists
in the mouth of a mullet (_Mugil hospes_) at Panama. The writer has
received, through the courtesy of Mr. A. P. Lundin, a specimen of a
flying-fish (_Exonautes unicolor_) taken off Sydney, Australia. To this
are attached three large copepod crustaceans of the genus _Penella_,
the largest over two inches long, and to the copepods in turn are
attached a number of barnacles (_Conchoderma virgatum_) so joined to
the copepods as to suggest strange flowers, like orchids, growing out
of the fish.
[Illustration: FIG. 227.--Black-nosed Dace, _Rhinichthys atronasus_
(Mitchill). East Coy Creek, W. N. Y. Showing black spots of parasitic
organisms. (From life by Mary Jordan Edwards.)]
=Myxosporidia, or Parasitic Protozoa.=--Internal parasites are
very numerous and varied. Some of them are bacteria, giving rise
to infectious diseases, especially in ponds and lakes. Others are
myxosporidia, or parasitic protozoans, which form warty appendages,
which burst, discharging the germs and leaving ulcers in their place.
In the report of the U. S. Fish Commissioner for 1892, Dr. R. R. Gurley
has brought together our knowledge of the protozoans of the subclass
_Myxosporidia_, to which these epidemics are chiefly due. These
creatures belong to the class of Sporozoa, and are regarded as animals,
their nearest relatives being the parasitic _Gregarinida_, from which
they differ in having the germinal portion of the spore consisting of a
single protoplasmic mass instead of falciform protoplasmic rods as in
the worm-like Gregarines. The _Myxosporidia_ are parasitic on fishes,
both fresh-water and marine, especially beneath the epidermis of the
gills and fins and in the gall-bladder and urinary bladder. In color
these protozoa are always cream-white. In size and form they vary
greatly. The cyst in which they lie is filled with creamy substance
made up of spores and granule matter.
Dr. Gurley enumerates as hosts of these parasites about sixty species
of fishes, marine and fresh-water, besides frogs, crustaceans,
sea-worms, and even the crocodile. In the sharks and rays the parasites
occur mainly in the gall-ducts, in the minnows within the gill cavity
and epidermis, and in the higher fishes mainly but not exclusively in
the same regions. Forty-seven species are regarded by Gurley as well
defined. The diseases produced by them are very obscurely known. These
parasites on American fishes have been extensively studied by Charles
Wardall Stiles, Edwin Linton, Henry B. Ward, and others.
According to Dr. Linton the parasitism which results from infection
with protozoan parasites will, of all kinds, be found to be the most
important. Epidemics among European fish have been repeatedly traced
to this source. The fatality which attends infection with psorosperms
appears to be due to a secondary cause, however, namely, to bacilli
which develop within the psorosperms (_Myxobolus_) tumors and give rise
to ulceration. The discharge of these ulcers then disseminates the
disease.
[Illustration: FIG. 228.--White Shiner, _Notropis hudsonius_ (Clinton),
with cysts of parasitic psorosperms. (After Gurley.)]
"Brief mention of the remedies there proposed may appropriately be
repeated here. Megnin sees no other method than to collect all the dead
and sick fishes and to destroy them by fire. Ludwig thinks that the
waters should be kept pure, and that the pollutions of the rivers by
communities or industrial establishments should be interdicted. Further
he says:
"That most dangerous contamination of the water by the _Myxosporidia_
from the ulcers cannot of course be stopped entirely, but it is evident
that it will be less if all fishermen are impressed with the importance
of destroying all diseased and dead fish instead of throwing them back
into the water. Such destruction must be so effected as to prevent the
re-entry of the germs into the water.
"Railliet says that it is expedient to collect the diseased fish and
to bury them at a certain depth and at a great distance from the
water-course. He further states that this was done on the Meuse with
success, so that at the end of some years the disease appeared to have
left no trace."
[Illustration: FIG. 229.--White Catfish, _Ameiurus catus_ (Linnæus),
from Potomac River, infested by parasitic protozoa, _Ichthyophthirus
multifilis_ Fouquet. (After C. W. Stiles.)]
=Parasitic Worms: Trematodes.=--Parasitic worms in great variety exist
in the intestinal canal or in the liver or muscular substance of fishes.
Trematode worms are most common in fresh-water fishes. These usually
are sources of little injury, especially when found in the intestines,
but they may do considerable mischief when encysted within the body
cavity or in the heart or liver. Dr. Linton describes 31 species of
these worms from 25 different species of American fishes. In 20 species
of fishes from the Great Lakes, 102 specimens, Dr. H. B. Ward found
95 specimens infected with parasites, securing 4000 trematodes, 2000
acanchocephala, 200 cestodes, and 200 nematodes. In the bowfin (_Amia
calva_), trematodes existed in enormous numbers.
=Cestodes.=--Cestode worms exist largely in marine fishes, the adults,
according to Dr. Linton, being especially common in the spiral
valve of the shark. It is said that one species of human tape-worm
(_Bothriocephalus tænia_) has been got from eating the flesh of the
European tench (_Tinca tinca_).
=The Worm of the Yellowstone.=--The most remarkable case of parasitism
of worms of this type is that given by the trout of Yellowstone Lake
(_Salmo clarki_). This is thus described by Dr. Linton:
"One of the most interesting cases of parasitism in which direct injury
results to the host, which has come to my attention, is that afforded
by the trout of Yellowstone Lake (_Salmo clarki_). It was noticed by
successive parties who visited the lake in connection with government
surveys that the trout with which the lake abounded were, to a large
extent, infested with a parasitic worm, which is most commonly in the
abdominal cavity, in cysts, but which in time escapes from the cyst and
tunnels into the flesh of its host. Fish, when thus much afflicted, are
found to be lacking in vitality, weak, and often positively emaciated.
"It was my good fortune, in the summer of 1890, to visit this
interesting region for the purpose of investigating the parasitism
of the trout of Yellowstone Lake. The results of this special
investigation were published in the Bulletin of the U. S. Fish
Commission for 1889, vol. ix., pp. 337-358, under the title 'A
Contribution to the Life-history of _Dibothrium cordiceps_, a Parasite
Infesting the Trout of Yellowstone Lake.'
"I found the same parasite in the trout of Heart Lake, just across the
great continental divide from Yellowstone Lake, but did not find any
that had tunneled into the flesh of its host, while a considerable
proportion of the trout taken in Yellowstone Lake had these worms
in the flesh. Some of these worms were as much as 30 centimeters in
length when first removed; others which had lain in water a few hours
after removal before they were measured were much longer, as much as
54 centimeters. They are rather slender and of nearly uniform size
throughout, 2.5 to 3 millimeters being an average breadth of the
largest. I found the adult stage in the intestine of the large white
pelican (_Pelecanus erythrorhynchus_), which is abundant on the lake
and was found breeding on some small islands near the southern end of
the lake.
"In the paper alluded to above I attempted to account for two things
concerning this parasitism among the trout of Yellowstone Lake: First,
the abundance of parasitized trout in the lake; second, the migration
of the parasite into the muscular tissue of its host. The argument
cannot be well summarized in as short space as the requirements of this
paper demand. It is sufficient to say that what appear to me to be
satisfactory explanations are supplied by the peculiar conditions of
distribution of fish in the lakes of this national park. Until three
or four years ago, when the U. S. Fish Commission stocked some of the
lakes and streams of the park, the conditions with relation to fish
life in the three principal lakes were as follows: Shoshone Lake, no
fish of any kind; Heart Lake, at least three species, _Salmo clarki_,
_Leuciscus lineatus_, and _Catostomus ardens_; Yellowstone Lake, one
species, _Salmo clarki_. Shoshone and Yellowstone Lakes are separated
from the river systems which drain them by falls too high for fish to
scale. Heart Lake has no such barrier. The trout of Yellowstone Lake
are confined to the lake and to eighteen miles of river above the
falls. Whatever source of parasitism exists in the lake, therefore,
must continue to affect the fish all their lives. They cannot be going
and coming from the lake as the trout of Heart Lake may freely do. If
their food should contain eggs of parasites, or if the waters in which
they swim should contain eggs or embryos of parasites, they would be
continually exposed to infection, with no chance for a vacation trip
for recuperation. To quote from my report:
"'It follows, therefore, from the peculiar conditions surrounding the
trout of Yellowstone Lake, that if there is a cause of parasitism
present in successive years the trout are more liable to become
infested than they would be in waters where they had a more varied
range. Trout would become infested earlier and in greater relative
numbers, and the life of the parasites themselves--that is, their
residence as encysted worms--must be of longer duration than would
be the rule where the natural conditions are less exceptional....
There are probably not less than one thousand pelicans on the lake
the greater part of the time throughout the summer, of which at any
time not less than 50 per cent. are infested with the adult form of
the parasite, and, since they spend the greater part of their time on
or over the water, disseminate millions of tape-worm eggs each in the
waters of the lake. It is known that eggs of other dibothria hatch out
in the water, where they swim about for some time, looking much like
ciliated infusoria. Donnadieu found in his experiments on the adult
dibothria of ducks that the eggs hatched out readily in warm water
and very slowly in cold. If warm water, at least water that is warmer
than the prevailing temperature of the lake, is needed for the proper
development of these ova, the conditions are supplied in such places
as the shore system of geysers and hot springs on the west arm of the
lake, where for a distance of nearly three miles the shore is skirted
by a hot spring and geyser formation, with numerous streams of hot
water emptying into the lake, and large springs of hot water opening in
the floor of the lake near shore.
"'Trout abound in the vicinity of these warm springs, presumably on
account of the abundance of food there. They do not love the warm
water, but usually avoid it. Several persons with whom I talked on the
subject while in the park assert that diseased fish--that is to say,
those which are thin and affected with flesh worms--are more commonly
found near the warm water; that they take the bait readily but are
logy. I frequently saw pelicans swimming near the shore in the vicinity
of the warm springs on the west arm of the lake. It would appear that
the badly infested or diseased fish, being less active and gamy than
the healthy fish, would be more easily taken by their natural enemies,
who would learn to look for them in places where they most abound. But
any circumstances which cause the pelican and the trout to occupy the
same neighborhood will multiply the chances of the parasites developing
in both the intermediate and final host. The causes that make for the
abundance of the trout parasite conspire to increase the number of
adults. The two hosts react on each other and the parasite profits by
the reaction. About the only enemies the trout had before tourists,
ambitious to catch big strings of trout and photograph them with a
kodak, began to frequent this region, were the fish-eating birds,
and chief among these in numbers and voracity was the pelican. It is
no wonder, therefore, that the trout should have become seriously
parasitized. It may be inferred from the foregoing statements that the
reason why the parasite of the trout of Yellowstone Lake migrates into
the muscular tissue of its host must be found in the fact that the life
of the parasite within the fish is much more prolonged than is the case
where the conditions of life are less exceptional.
"The case just cited is probably the most signal one of direct injury
to the host from the presence of parasites that I have seen. I shall
enumerate more briefly a few additional cases out of a great number
that I have encountered in my special investigations on the entozoa of
fishes for the U. S. Fish Commission."
Many worms of this type abound in codfishes, bluefishes, striped bass,
and other marine fishes, rendering them lean and unfit for food.
=The Heart Lake Tape-worm.=--Another very interesting case of
parasitism is that of the large tape-worm (_Ligula catostomi_)
infecting the suckers, _Catostomus ardens_, in the warm waters of Witch
Creek, near Heart Lake, in the Yellowstone Park. Of this Dr. Linton
gives the following account:
[Illustration: FIG. 230.--Sucker, _Catostomus ardens_ (Jordan &
Gilbert), from Heart Lake, Yellowstone Park, infested by a flatworm,
_Ligula catostomi_ Linton, itself probably a larva of _Dibothrium_.
(After Linton.)]
"In the autumn of 1889 Dr. David Starr Jordan found an interesting
case of parasitism in some young suckers (_Catostomus ardens_) which
he had collected in Witch Creek, a small stream which flows into Heart
Lake, in the Yellowstone National Park. Specimens of these parasites
were sent to me for identification. They proved to be a species of
ligula, probably identical with the European _Ligula simplicissima_
Rud., which is found in the abdominal cavity of the tench. On
account of its larval condition in which it possesses few distinctive
characters, I described it under the name _Ligula catostomi_. These
parasites grow to a very large size when compared with the fish which
harbors them, often filling the abdominal cavity to such a degree as
to give the fish a deceptively plump appearance. The largest specimen
in Dr. Jordan's collection measured, in alcohol, 28.5 centimeters in
length, 8 millimeters in breadth at the anterior end, 11 millimeters
at a distance of 7 millimeters from the anterior end, and 1.5
millimeters near the posterior end. The thickness throughout was about
2 millimeters. The weight of one fish was 9.1 grams, that of its three
parasites 2.5 grams, or 27-1/2 per cent. the weight of the host. If a
man weighing 180 pounds were afflicted with tape-worms to a similar
degree, he would be carrying about with him 50 pounds of parasitic
impedimenta.
"In the summer of 1890 I collected specimens from the same locality. A
specimen obtained from a fish 19 centimeters in length measured while
living 39.5 centimeters in length and 15 millimeters in breadth at
the anterior end. Another fish 15 centimeters in length harbored four
parasites, 12, 13, 13, and 20 centimeters long, respectively, or 58
centimeters aggregate. Another fish 10 centimeters long was infested
with a single parasite which was 39 centimeters in length.
"These parasites were found invariably free in the body cavity. Dr.
Jordan's collections were made in October and mine in July of the
following year. Donnadieu has found that this parasite most frequently
attains its maximum development at the end of two years. It is
probable, therefore, that Dr. Jordan and I collected from the same
generation. Since these parasites, in this stage of their existence,
develop, not by levying a toll on the food of their host, after the
manner of intestinal parasites, but directly by the absorption of
the serous fluid of their host, it is quite evident that they work a
positive and direct injury. Since, however, they lie quietly in the
body cavity of the fish and possess no hard parts to cause irritation,
they work their mischief simply by the passive abstraction of the
nutritive juices of their host, and by crowding the viscera into
confined spaces and unnatural positions. The worms, in almost every
case, had attained such a size that they far exceeded in bulk the
entire viscera of their host.
"From the fact that the examples obtained were of comparatively the
same age, it may be justly inferred that the period of infection to
which the fish are subjected must be a short one. I did not discover
the final host, but it is almost certain to be one or more of the
fish-eating species of birds which visit that region, and presumably
one of which, in its migrations, pays but a brief visit to this
particular locality. This parasite was found only in the young suckers
which inhabit a warm tributary of Witch Creek. They were not found in
the large suckers of the lake. These young _Catostomi_ were found in
a single school, associated with the young of the chub (_Leuciscus
lineatus_), in a stream whose temperature was 95° F. near where it
joined a cold mountain brook whose temperature was 46° F. We seined
several hundred of these young suckers and chubs, ranging in length
from 6 to 19 centimeters. The larger suckers were nearly all infested
with these parasites, the smaller ones not so much, and the smallest
scarcely at all. Or, to give concrete examples: Of 30 fish ranging
in length from 14 to 19 centimeters, only one or two were without
parasites; of 45 specimens averaging about 10 centimeters in length,
15 were infested and 30 were not; of 65 specimens averaging about
9 centimeters in length, 10 were infested and 55 were not; of 62
specimens less than 9 centimeters in length, 2 were infested and 60
were not. None of the chubs were infested with this parasite.
"The conditions under which these fish were found are worthy of passing
notice. The stream which they occupied flowed with rather sluggish
current into a swift mountain stream, which it met almost at right
angles. The school of young chubs and suckers showed no inclination to
enter the cold water, even to escape the seine, but would dart around
the edge of the seine, in the narrow space between it and the bank,
in preference, apparently, to taking to the colder water. When not
disturbed by the seine they would swim up near to the line which marked
the division between the cold and the warm water, and seemed to be
gazing with open mouth and eyes at the trout which occasionally darted
past in the cold stream. The trout appeared to avoid the warm water,
while the chubs and suckers appeared to avoid the cold water. It may
be that what the latter really avoided was the special preserve of the
trout, since large chubs and suckers are found in abundance in the
lake, which is quite cold, a temperature of 40° F. having been taken by
us at a depth of 124 feet.
"Since the eggs of this parasite, after the analogy of closely related
forms, in all probability are discharged into the water from the final
host and hatch out readily in warm water, where they may live for a
longer or shorter time as free-swimming planula-like forms, it will be
observed that the sluggish current and high temperature of the water in
which these parasitized fish occur give rise to conditions which are
highly favorable to infection.
"It may be of passing interest to state here what I have recorded
elsewhere, that ligulæ, probably specifically identical with _L.
catostomi_, form an article of food in Italy, where they are sold in
the markets under the name _maccaroni piatti_; also in southern France,
where they are less euphemistically but more truthfully called the _ver
blanc_. So far as my information goes, this diet of worms is strictly
European.
"It is not necessary to prove cases of direct injury resulting from
the presence of parasites in order to make out a case against them. In
the sharp competition which nature forces on fishes in the ordinary
struggle for existence, any factor which imparts an increment either of
strength or of weakness may be a very potent one, and in a long term of
years may determine the relative abundance or rarity of the individuals
of a species. In most cases the interrelations between parasite and
host have become so adjusted that the evil wrought by the parasite
on its host is small. Parasitic forms, like free forms, are simply
developing along the lines of their being, but unlike most free forms
they do not contribute a fair share to the food of other creatures."
=Thorn-head Worms.=--The thorn-head worms called _Acanthocephala_ are
found occasionally in large numbers in different kinds of fishes. They
penetrate the coats of the intestines, producing much irritation and
finally waxy degeneration of the tissues.
According to Linton, there is probably no practical way of
counteracting the bad influences of worms of this order, since their
larval state is passed, in some cases certainly, and in most cases
probably, in small crustacea, which constitute a constant and necessary
source of food for the fish. The same remark which was made in another
connection with regard to the disposal of the viscera of fish applies
here. In no case should the viscera of fish be thrown back into the
water. In this order the sexes are distinct, and the females become at
last veritable sacs for the shelter and nourishment of enormous numbers
of embryos. The importance, therefore, of arresting the development of
as many embryos as possible is at once apparent.
=Nematodes.=--The round worms or nematodes are very especially abundant
in marine fishes, and particularly in the young. The study of these
forms has a large importance to man. Dr. Linton pertinently observes:
"Where there is exhaustive knowledge of the thing itself the
application of that knowledge toward getting good out of it or
averting evil that may come from it first becomes possible. For
example, a knowledge of the life-history of _Trichina spiralis_ and
its pathological effects on its host has taught people a simple way of
securing immunity from its often deadly effects. A knowledge of the
life-histories of the various species of tæniæ which infest man and
the domestic animals, frequently to their serious hurt, has made it
possible to diminish their numbers, and may, in time, lead to their
practical extinction.
"So with the parasites of fishes. Whenever for any reason or reasons
parasitism of any sort becomes so prevalent with any species as to
amount to a disease, the remedy will be suggested, and in some cases
may be practically applied. If, for example, it were thought desirable
to counteract the influences which are at work to cause the parasitism
of the trout of Yellowstone Lake, it could be very largely accomplished
by breaking up the breeding-places of the pelican on the islands of
the lake. With regard to parasitism among the marine food-fishes,
the remedy while plainly suggested by the circumstances, might be
difficult of application. Yet something could be done even there,
if it were thought necessary to lessen the amount of parasitism. If
such precautions as the destruction of the parasites which abound
in the viscera of fish before throwing them back into the water,
and if no opportunity be lost of killing those sharks which feed on
the food-fishes, two sources of the prevalence of parasites would be
affected and the sum total of parasitism diminished. These remarks are
made not so much because such precautions are needed as to suggest
possible applications of knowledge which is already available."
=Parasitic Fungi.=--Fishes are often subject to wounds. If not too
serious these will heal in time, with or without scars. Some lost
portions may be restored, but not those including bone fin-rays or
scales. In the fresh waters, wounds are usually attacked by species of
fungus, notably _Saprolegnia ferox_, _Saprolegnia mixta_, and others,
which makes a whitish fringe over a sore and usually causes death.
This fungus is especially destructive in aquaria. This fungus is not
primarily parasitic, but it fixes itself in the slime of a fish or in
an injured place, and once established the animal is at its mercy.
Spent salmon are very often attacked by this fungus. In America the
spent salmon always dies, but in Scotland, where such is not the case,
much study has been given to this plant and the means by which it
may be exterminated. Dr. G. P. Clinton gives a useful account of the
development of _Saprolegnia_, from which we take the following:
"The minute structure and life-history of such fungous forms have been
so thoroughly made out by eminent specialists that no investigation
along this line was made, save to observe those phenomena which might
be easily seen with ordinary microscopic manipulations. The fungus
consists of branched, hyaline filaments, without septa, except as these
are found cutting off the reproductive parts of the threads. It is
made up of a root-like or rhizoid part that penetrates the fish and
a vegetative and reproductive part that radiates from the host. The
former consists of branched tapering threads which pierce the tissues
for a short distance, but are easily pulled out. The function of this
part is to obtain nourishment for the growth of the external parts.
Prostrate threads are found running through the natural slime covering
the fish, and from these are produced the erect radiating hyphæ so
plainly seen when in the water. The development of these threads
appears to be very rapid when viewed under the microscope, although
the growth made under favorable conditions in two days is only about
a third of an inch. From actual measurements of filaments of the
fungus placed in water and watched under the microscope, it was found
that certain threads made a growth of about 3000 microns in an hour.
Two others, watched for twenty minutes, gave in that time a growth of
90 and 47 microns respectively; and yet another filament, observed
during two periods of five minutes each, made a growth of 28 microns
each time. In ordinary cultures the rate of growth depends upon the
condition of the medium, host, etc."
[Illustration: FIG. 231.--Quinnat Salmon, _Oncorhynchus tschawytscha_
(Walbaum). Monterey Bay. (Photograph by C. Rutter.)]
Professor H. A. Surface thus speaks of the attacks of _Saprolegnia_ on
the lamprey:
"The attack that attends the end of more lampreys than does any other
is that of the fungus (_Saprolegnia_ sp.). This looks like a gray
slime and eats into the exterior parts of the animal, finally causing
death. It covers the skin, the fins, the eyes, the gill-pouches,
and all parts, like leprosy. It starts where the lamprey has been
scratched or injured or where its mate has held it, and develops very
rapidly when the water is warm. It is found late in the season on all
lampreys that have spawned out, and it is almost sure to prove fatal,
as we have repeatedly seen with attacked fishes or lampreys kept in
tanks or aquaria. With choice aquarium fishes a remedy, or at least a
palliative, is to be found in immersion in salt water for a few minutes
or in bathing the affected parts with listerine. Since these creatures
complete the spawning process before the fungoid attack proves serious
to the individual, it can be seen that it affects no injury to the
race, as the fertilized eggs are left to come to maturity. Also, as it
is nature's plan that the adult lampreys die after spawning once, we
are convinced that death would ensue without the attack of the fungus;
and in fact this is to be regarded as a resultant of those causes that
produce death rather than the immediate cause of it. Its only natural
remedy is to be found in the depths of the lake (450 feet) where there
is a uniform or constant temperature of about 39° Fahr., and where the
light of the noon-day sun penetrates with an intensity only about equal
to starlight on land on a clear but moonless night.
[Illustration: FIG. 232.--Young Male Quinnat Salmon, _Oncorhynchus
tschawytscha_, dying after spawning. Sacramento River. (Photograph by
Cloudsley Rutter.)]
"As light and heat are essential to the development of the fungus,
which is a plant growth and properly called a water mold, and as their
intensity is so greatly diminished in the depth of the lake, it is
probable that if creatures thus attacked should reach this depth they
might here find relief if their physical condition were otherwise
strong enough to recuperate. However, we have recently observed a
distinct tendency on the part of fungus-covered fishes to keep in the
shallower, and consequently warmer, parts of the water, and this of
course results in the more rapid growth of the sarcophytic plant, and
the death of the fishes is thus hastened.
"All kinds of fishes and fish-eggs are subject to the attacks of
such fungus, especially after having been even slightly scratched or
injured. As a consequence, the lamprey attacks on fishes cause wounds
that often become the seat of a slowly spreading but fatal fungus.
We have seen many nests of the bullhead, or horned pout (_Ameiurus
nebulosus_), with all the eggs thus destroyed, and we have found scores
of fishes of various kinds thus killed or dying. It is well known that
in many rivers this is the apparent cause of great mortality among
adult salmon. Yet we really doubt if it ever attacks uninjured fishes
that are in good strong physical condition which have not at least
had the slime rubbed from them when captured. It is contagious, not
only being conveyed from one infested fish to another, but from dead
flies to fishes." (For a further discussion of this subject see an
interesting and valuable Manual of Fish Culture, by the U. S. Fish
Commission, 1897.)
=Earthquakes.=--Occasionally an earthquake has been known to kill
sea-fishes in large numbers. The _Albatross_ obtained specimens
of _Sternoptyx diaphana_ in the Japanese Kuro Shiwo, killed by the
earthquakes of 1896, which destroyed fishing villages of the coast of
Rikuchu in northern Japan.
=Mortality of Tilefish.=--Some years ago in the Gulf Stream off
Newfoundland an immense mortality of the filefish (_Lopholatilus
chamæleonticeps_) was reported by fishermen. This handsome and large
fish, inhabiting deep waters, died by thousands. For this mortality,
which almost exterminated the species, no adequate cause has been found.
As to the destruction of fresh-water fishes by larger enemies, we may
quote from Professor H. A. Surface. He says there is no doubt that
these three species, the lake lamprey (_Petromyzon marinus unicolor_),
the garpike (_Lepidosteus osseus_), and the mud-puppy (_Necturus
maculosus_), named "in order of destructiveness, are the three most
serious enemies of fishes in the interior of this State [New York],
each of which surely destroys more fishes annually than are caught by
all the fishermen combined. The next important enemies of fishes in
order of destructiveness, according to our observations and belief, are
spawn-eating fishes, water-snakes, carnivorous or predaceous aquatic
insects (especially larvæ), and piscivorous fishes and birds." The
lamprey attaches itself to larger fishes, rasping away their flesh and
sucking their blood, as shown in the accompanying plate.
[Illustration: FIG. 233.--Catfishes, _Ameiurus nebulosus_ Le Sueur,
destroyed by lampreys (_Petromyzon marinus unicolor_ De Kay). Cayuga
Lake, N. Y. (Modified from photograph by Prof. H. A. Surface.)]