CHARACTERISTICS AND HABITS OF FISHES
Fishes are the most primitive vertebrate, i.e. backboned, creatures
known. All reptiles, birds, and animals have gradually evolved from
fish-like ancestors by a series of age-long processes, the stages of
which are recorded in fossilized remains that are found in various
rock strata throughout the world. A fish lives exclusively in water.
It has no lungs, but extracts oxygen from the water as it passes over
the surface of its gills. Instead of limbs, it has fins, with which
it balances itself and propels itself through the water. Its skin is
either bare, e.g. the cat fish, or is covered with scales, e.g. the
herring, or with bony plates, e.g. the sturgeon. The skin of certain
sharks is studded with minute teeth and produces, when cured, the
well-known shagreen leather. In nearly all cases the skin of fishes
is liberally supplied with small glands which constantly produce a
lubricating mucus. This mucus greatly reduces friction between the fish
and the water through which it moves.
The body of a fish is adapted to move swiftly and smoothly through
the water; it is shaped more or less like a torpedo, but this form is
greatly modified in different species. Certain species of fish living
at the bottom of the sea, for example skates and rays, have become
flattened, as though by a pressure applied vertically downwards.
Others, for example plaice, flounder, sole, appear to have been
flattened sideways. In the various members of the eel family, the body
is greatly elongated.
[Illustration: FIG. 1]
The body of a fish is generally coloured and marked in such a way
that it becomes practically invisible when seen from above or below,
the under-surface being silvery white, and the upper surface generally
olive or blackish-green. Sometimes, as in the mackerel, the upper
surface is mottled, resembling rippled water.
Most small fish in ponds and streams reflect their surroundings so
well, and are coloured and marked in such a way, that they are almost
invisible to the large fish, for example pike, that prey upon them.
Generally, they reveal their presence by the flash of light reflected
from above by their scales, as they turn suddenly to snap at a morsel
of food. In the same way, many predatory fish, e.g. the angler fish,
resemble their surroundings so closely that the fish for which they are
lying in wait swim within easy reach of them without perceiving their
danger. Many fishes, particularly in tropical waters, are remarkable
for their bright and gorgeous colouring. It is impossible to preserve
these colours in their natural brightness after the fish have been
taken from the water, but amongst the brightly coloured corals, and
anemones and seaweeds, in the crystal clear water of their natural
environment, they flit like gorgeous tropical birds in a tropical
forest.
=Distribution.= Fishes are found in practically every ocean, lake and
river in the world, with a few notable exceptions, such as the Dead
Sea, in which the concentration of salt is too high. They appear to
exist at all depths of water, and have been found in the sea as deep
down as 2,720 fathoms. Fish living at this depth generally possess
enormous mouths, long, attenuated, soft bodies, and are equipped with
highly developed phosphorescent organs.
The distribution of a particular species appears to depend upon the
salinity of the water, the temperature of the water, the kind and
quantity of food available and the prevailing intensity of sunlight.
It is possible to divide fish into four well-defined groups, according
to the salinity of the water in which they are found: (1) Marine fish:
those that live always in the sea, for example herring, haddock,
shark. (2) Fresh-water fish: those that live always in fresh water,
for example carp, trout, pike. (3) Many fish live in brackish water,
and appear to be able to accommodate themselves easily to considerable
changes in salinity, e.g. sticklebacks, gobies, grey mullets and
blennies. Such species naturally are widely distributed; thus, a
particular kind of grey mullet (_Mugil capito_) is found without any
appreciable difference in form on nearly every coast of the Atlantic
Ocean. (4) The fourth group of fish are migratory. Some species, for
example salmon and shad, live and develop in salt water, but ascend
rivers to spawn, i.e. to lay their eggs, in fresh water. Others, such
as eels and certain pleuronectids, for example the flounder, live and
develop in fresh water, and descend rivers to the sea to spawn. Many
fresh water fish, e.g. trout, forsake the large streams in the spring
and ascend small brooks, where the young can be reared in greater
safety.
Of these different groups or species, the marine fishes are
industrially by far the most important, for at least two-thirds of
all the fish in the world live in the sea, and the capture of these
sea-fish in enormous quantities constitutes the fishing industry, with
which we are concerned.
The different species of marine fishes can be divided into three
well-marked groups, according to their habits and habitats.
[Illustration: FIG. 2
COD (_Gadus morrhua_)
Length up to 5 ft.; usually caught at about 3 ft.
_Food._—Small crustaceans, molluscs, and young fish.
_Range._—North of Norway and Iceland to the Bay of Biscay, and
from Greenland to New York.]
(1) There are the true deep-sea fishes that live at the bottom of
the sea, for example cod, haddock, plaice, sole. These are called
“demersal” fish. Fish, like birds, inhabit a medium that is
continuous throughout the world. A glance at the map of the world will
show that the three great oceans—Atlantic, Indian and Pacific—are
united in the southern hemisphere. In Tertiary times, it is practically
certain that the Pacific and the Atlantic oceans were also united
at Darien, and that the Mediterranean was united with the Red Sea.
Apart, therefore, from differences in local conditions, for example
of temperature and food supply, there is practically no obstacle to
the world-wide distribution of any particular species of fish. At
the bottom of the sea, the temperature, the food supply, and the
general conditions of life are singularly uniform all over the world,
consequently there are no barriers at all to the dispersion of demersal
fish, and we find various species widely distributed in all seas.
Demersal fish, on the whole, are more primitive in type than those that
live nearer the surface. They have well-developed senses of touch and
smell by means of which they hunt for their food. They differ markedly
in structure and shape from surface or shallow-water fish, their bodies
being designed to resist the greater pressure of deep water. The
body is generally lean and is enclosed by a wall of muscular fibre.
Shallow-water fish, if introduced into deep water, would be crushed
inward by the pressure. Similarly, the deep-living, demersal fish are
unable to accommodate themselves to shallow water and, if placed in it,
soon become unhealthy. A cod floats helplessly on its side when placed
in shallow water, owing to the dilatation of its swimming bladder. If
the bladder is pricked it collapses, and the fish is able to regain an
upright position. This is done when cod and other similar demersal fish
are kept alive in sea-water tanks on board ship, to be delivered to
the markets alive. In Denmark, fish are delivered alive to the shops.
When fishes from great depths are brought to the surface, their bodies
break into pieces owing to the reduced external pressure, the scales
start from their skin and the eyes from their sockets.
There are two distinct types of demersal fish: the “round” and
the “flat.” The body of a round fish is more or less circular in
cross-section, for example cod, while that of a flat fish is flattened,
for example sole, ray.
The most important edible demersal fish can be classified as follows—
(_a_) The _Gadidae_—related to the cod.
Cod—inhabits northern waters, notably the North of Britain, Iceland
and Newfoundland.
Ling—inhabits northern waters: West of Scotland and Ireland, and
North towards Iceland and Newfoundland.
Haddock—inhabits northern waters. Nearly half the total catch is
obtained in the North Sea, from the White Sea to the Bay of Biscay.
Whiting—found in great numbers in the North Sea. It is more coastal
than the cod or haddock.
Hake—found from Norway to the Mediterranean. The greater part of the
catch is obtained off the south-west of Ireland. Hake is also caught
off Morocco and in the Bay of Biscay.
(_b_) The _Pleuronectidae_—related to the plaice and sole.
Sole—a shallow-water fish, common in the Irish Sea, and particularly
abundant in southern waters down to Morocco.
Plaice—inhabits northern waters—all round Britain and Iceland.
Flounder—inhabits estuaries, for example, of the North Sea and the
Baltic.
Halibut—inhabits northern waters. It attains a large size, six feet
or more.
[Illustration: FIG. 3
LEMON SOLE (_Pleuronectes microcephalus_)
Length up to 16 ins.
_Food._—Small crustaceans and worms.
_Range._—From North of Europe to the Bay of Biscay.]
[Illustration: FIG. 4
SKATE (_Raia batis_)
Length up to 7 ft.
_Food._—Crustaceans and molluscs, and fish.
_Range._—Round the British Isles and along the coast of Western
Europe.]
Turbot—not very abundant. It inhabits the deeper parts of the North
Sea.
Brill—inhabits southern waters, and is fairly abundant.
(_c_) The _Raüdac_.
Skates and rays—found all round Britain, more particularly the
Western area of the English channel.
(2) The various species of fish that inhabit the surface waters of the
sea are called “pelagic.” They include the herring, mackerel, tunny,
flying fish, sword fish, and many sharks, also various marine mammals,
such as whales, grampuses, porpoises, dolphins. Amongst pelagic fish
are included some of the smallest (plankton) as well as some of the
largest (whales) of all living creatures. Pelagic fish pass their whole
life swimming at or near the surface. They enter the shallow water
offshore only for prey or, in some cases, periodically to spawn. The
majority spawn in the open sea, far from land. Unlike demersal fishes,
the distribution of the different species of pelagic fishes depends
very much upon local conditions of light, water temperature, and the
character and quantity of food available. They do not hunt their food
individually to the same extent as demersal fishes, but generally
filter it from the water as it passes through their gill-openings.
Although not so widely dispersed as demersal fish, they are, in
favourable circumstances, dispersed over large areas by swimming and by
ocean currents.
All pelagic fish are “round.” With the exception of the mackerel, the
important edible pelagic fishes belong to the herring family, and are
known as the Clupeidae. They include—
Herring—found from the White Sea to the Bay of Biscay. It is the
most abundant of all food fishes.
Sprat—found from the North of Europe to the Mediterranean.
Pilchard—ranges from the English Channel to Madeira and the
Mediterranean. Skipper “sardines” are young herring, pilchard, and
brisling.
There is also—
Mackerel—found from the North Sea to Madeira and the Mediterranean.
(3) The shallow-water of the seashore is inhabited by certain animals
(shellfish) not found elsewhere, including various mollusca, e.g.
mussel, cockle, oyster and periwinkle, and crustacea, e.g. lobster,
crab, prawn, shrimp. In addition to these, there are various species of
immature offshore fish, e.g. plaice and dabs. The inhabitants of this
shallow, coastal water are called “littoral” fish. The distribution of
such littoral fish depends not only upon the water temperature and the
amount of light, but also upon the character of the shore—whether it
is rocky, or soft and sandy—and more especially upon the animal and
vegetable products of the adjacent land, e.g. plants, seaweed, worms.
Littoral fish do not swim very far, but become scattered inadvertently
over considerable distances by currents and other mechanical means.
[Illustration: FIG. 5
HERRING (_Clupea harengus_)
Length slightly above 12 ins.
_Food._—Plankton (_copepoda_).
_Range._—From the White Sea to the Bay of Biscay.]
Certain kinds of shellfish, for example oysters, mussels, cockles,
live in the sand or attached to the stones or seaweed on the seashore,
generally between high and low watermarks. They obtain their food
from the water as it streams over their gills. They require adequate
room for growth and development, and constant irrigation by water
containing sufficient floating food. When mussel beds or oyster beds
become overcrowded, the fish are ill-nourished, their health is
impaired and their growth is arrested. It has been shown that, if they
are transferred to new beds, their condition rapidly improves and
ultimately they increase considerably in size. All edible shellfish
need systematic care and attention. Their cultivation by man affords
the simplest instance of an attempt at a systematic aquiculture.
=Food.= The surface water of the sea abounds in minute forms of
vegetable and animal life. This vast floating population of microscopic
organisms is called the “plankton.” Just as man and all land animals
depend ultimately for their food supply upon grass and other
green-leaved plants which, under the influence of sunlight, are able
to transform the inorganic constituents of the atmosphere and the soil
into organic foodstuffs—albumen, fat, carbohydrates—so the minute
unicellular marine plants of the plankton are able, under the influence
of sunlight, to convert the inorganic constituents of their environment
into fat, albumen and carbohydrate. Upon these minute organisms,
therefore, directly or indirectly, all marine life depends.
In addition to these minute plants, the plankton contains nearly all
forms of marine life at some stage or other of their life history. Fish
are only found in it as eggs, or larvae. Crustacea of all kinds are
present, and form one of its most important constituents. Crabs and
lobsters spend their larval, free-swimming career among the plankton,
until they reach the adult stage and settle down to the bottom. Various
minute crustacea, known as “Copepoda” (lit., oar-footed) spend the
whole of their lives drifting about in the surface water. They occur
in incredibly large numbers, and are the most abundant of all forms of
marine life. These copepoda form the main source of the food of pelagic
fish, such as the herring, mackerel and sprat.
The larvae of the edible molluscs, oyster, mussel, cockle, develop in
the warm surface water until they settle to the bottom and begin their
adult life.
There are also many larval forms of marine worms and jellyfish, and
many kinds of microscopic, unicellular organisms, some of which are
vegetable and others are clearly animal. The chief animal forms belong
either to the Infusoria, the Foraminifera or the Radiolaria. The shells
of the two latter forms accumulate at the bottom of the sea, producing
the deposits known as the Globigerina and Radiolarian oozes. In this
way, chalk deposits were formed in primitive times.
The most important vegetable planktonic organisms are the Diatoms.
Their accumulated shells form important deep-sea deposits.
The numerous varieties of planktonic life can thus be divided into two
groups: those minute animal and vegetable organisms that pass the whole
of their existence at the surface of the sea—the true constituents of
plankton all the year round—and the eggs and larvae of many species
of fish that are found among the plankton only at certain times of the
year—notably in spring and summer.
The quantity of organic food substances such as albumen, fat and
carbohydrate, that is contained in the plankton produced annually by
a given area of the sea, has been compared with the quantity of such
substances produced by a similar area of land in crops such as pasture,
hay, lupine and peas. In this way, it has been estimated that the
productivity of the sea is about 20 per cent less than that of average
land.
[Illustration: PLANKTON: LARVAE
1. Crab zoea; 2. Fish egg; 3. Sea Urchin pluteus; 4. Barnacle nauplius;
5. Fish larva; 6. Mussel larva; 7. Copepod nauplius; 8. Worm larva.]
[Illustration: PLANKTON: UNICELLULAR ORGANISMS
1, 2, 3, 7, 10, 11, 12, 13, 16, 17, 18. Diatoms; 4, 5, 7, 9,
Peridinians 8. An Algal spore; 14. Noctiluca; 15. A Radiolarian.
FIG. 6]
Unlike that of the land, the productivity of the sea is greater in
colder latitudes than in the tropics. This somewhat unexpected fact is
attributable to the action of denitrifying bacteria which, flourishing
more readily in warm, tropical waters, effectively reduce the amount
of available nitrogen compounds in the water. In colder waters,
denitrifying bacteria are less active, and nitrates and nitrites are
available in larger quantities for the nourishment of the plankton.
All the great fisheries of the world are prosecuted in cold or
temperate seas; as examples of this we have the Banks of Newfoundland,
the cod fisheries of Norway, and the great trawling grounds of the
North Sea and the North Atlantic.
All fish, during the larval stage of their development, feed first upon
the contents of the yolk sac which, when they are hatched, is attached
to their ventral surface. When the yolk is absorbed, the larvae feed
upon the microscopic plankton that abound in the water on every side.
The surface water, with its warm temperature, high plankton content
and sunlight, forms an ideal nursery for the very young fish of all
species. Demersal fish, as they complete the larval stage of their
development and descend into deeper water, have to rely for their food
either upon the various species of young shellfish and crustacea that
drop from the surface water as they develop, or hunt for their food
amongst the small fish, mollusca, crustacea, worms and seaweeds of the
sea-bottom. Plaice feed chiefly upon cockles and other mollusca, which
in their turn feed upon diatoms. The cod is almost omnivorous, greedily
devouring small fish, crustacea, worms or mollusca; its favourite food,
however, is shrimps and prawns. These, in their turn, feed upon smaller
invertebrates, for example small jellyfish and larval molluscs, and
these upon microscopic plankton.
Pelagic fish, herrings and mackerel, feed almost entirely upon the
larger plankton, mainly copepoda (small, shrimp-like crustacea). These
may be present in the surface water in enormous quantities at certain
times. In many cases, shoals of herring or mackerel probably follow
special swarms of copepoda. Mackerel also feed upon young fish, hermit
crabs, and prawns.
With a few notable exceptions, the various species of demersal fish
feed upon smaller fish. Thus—
The hake, normally a deep-water fish, ventures inshore in pursuit of
herrings, pilchards, mackerel.
The ling, turbot, brill, dog fish live entirely upon small fish. The
dog fish swarms on certain fishing grounds and is often a serious pest
to the drift-net fishermen, destroying their nets as well as the fish
that are attached to them.
The whiting, like the cod, feeds upon small fish, and upon crustacea
and mollusca.
The food of the haddock consists of mollusca, crustacea and marine
worms, etc.
The sole lives on small crustacea, for example shrimps, and marine
worms.
Skates and rays feed upon mollusca and crustacea.
Most shellfish live in shallow water and feed upon the plankton.
The methods by which fish obtain their food differ greatly according
to the species of the fish. Pelagic fish, e.g. herring and mackerel,
sprat and pilchard, obtain their food almost automatically as they
swim open-mouthed through the water in which it abounds. These direct
plankton-feeders possess comb-like structures—the gill-rakers—attached
behind the gill openings, and as the food-bearing water streams through
the mouth and gill openings of the fish, these structures strain the
food from it. The fish licks the plankton from its gill-rakers with its
tongue and swallows it.
Many pelagic fish, e.g. carp, trout, salmon, look for their food while
swimming through the well-lighted surface water.
Demersal fish—flat fish, cod, haddock, etc.—seek their food by scent
and touch. The cod possesses a barbel attached to its chin, by means of
which it feels for its food.
The Angler or Devil fish is a curious creature, from three to four
feet long, and appearing to consist almost entirely of head. It has a
large mouth, and teeth that are hinged so as to admit food, but prevent
it from escaping. The devil fish has a long feeler on the top of its
head, terminating in a tassel which, moved by the water, attracts the
attention of small fish and lures them to their fate. This tassel is
a sensory organ and, when it is touched by the small fish, the angler
fish snaps upwards with unerring aim at a point immediately in advance
of the tassel.
The dog fish seeks its food exclusively by scent. If its sense of smell
be destroyed, it ceases to feed spontaneously.
The sole also seeks its food by smell. It is quite unable to recognize
a worm by sight or touch, even when hung just above its head, but feels
aimlessly over the ground seeking it by smell.
=Reproduction.= Fish are male and female and, with few exceptions,
reproduce their kind by laying eggs. The number of eggs laid by an
individual female fish during a single spawning varies greatly,
according to the species. The average number of eggs spawned by a
single female fish in the course of one season, is—
Ling 18,500,000
Turbot 8,600,000
Cod 4,500,000
Flounder 1,000,000
Sole 570,000
Haddock 450,000
Plaice 300,000
Herring 32,000
Shark } {A few—not more
Dog fish} {than a dozen.
Skate }
[Illustration: FIG. 7
HERRING EGGS—×5]
[Illustration: FIG. 8
PLANKTON CONTAINING FISH EGGS—×3
The large egg is that of a plaice: the smaller ones are cod and whiting.
The copepod is a calanus.]
The eggs of the cod, whiting, haddock, fluke, plaice, etc., are
relatively small, varying from 1/6 of an inch in the case of a halibut,
to 1/25 of an inch in a flounder. The eggs are discharged into the
water by the female. This process takes place gradually, and generally
occupies many weeks. A few of the eggs come to maturity at a time, and
are extruded. They are fertilized in the water by the spermatazoa of
the male, which are discharged into the water at the same time as the
eggs. The fish, both male and female, are closely crowded together on
the spawning grounds, so that the fertilization of the eggs is fairly
complete. With few exceptions, the eggs of most species are buoyant
and float to the surface, where they drift in the warm surface water
until, happily, they hatch. Unhappily, however, a very large proportion
of them never reach maturity, for, either as eggs, embryos or larvae,
or post larval young fishes, they soon fall a prey to marauding fish.
It is estimated that, of the thirty-two thousand eggs laid annually by
each female herring, not more than two reach maturity.
The spawning grounds of the herring are not definitely known. Research
is being carried out at present with a view to solving this question.
Haddock are to be caught in various likely parts of the sea, marked
with the place of capture, and their interiors examined for herring
spawn.
Certain demersal fish, notably shark, dog fish and skate, deposit a
few large, demersal eggs—about a dozen in the year—in a carefully
selected spot. The incubation period of these eggs is unusually long,
being from six months to over a year, according to the species and the
temperature of the water.
Parental care is exhibited by very few fishes in this part of the
world, although many foreign fish build nests and care for their
young, often carrying them in their mouths. Certain kinds of dog fish
and angel fish keep their young inside their oviducts until they
are completely formed. The only notable example of a fish common to
British waters that exercises parental care is the stickleback. Spawn
is deposited by a number of different females in a nest constructed
of stones and weed, and is guarded by a male until all the eggs are
hatched.
The eggs of the crustacea, for example the lobster, are found attached
in large numbers to the swimmerets—feathery processes that are
situated underneath the tail. When in this condition, the lobster is
known as “berried,” and, if captured, should be returned to the sea.
The eggs are sticky and are laid while the lobster lies on her back,
and so become attached to the hairs of these feathery processes.
Berried crabs, prawns and shrimps may also be observed on the seashore
in the spring and early summer.
The mollusca, e.g. mussels, periwinkles, oysters, deposit their eggs
in the sea-water. The eggs float to the surface, hatch out, and drift
about with the other constituents of the plankton. The fully developed
larvae fall to the sea bottom and become attached to seaweed and stones.
The period of incubation of fish eggs varies according to the species
of fish, and for the same species is prolonged by a low temperature.
Plaice eggs, fertilized in January, hatched in eighteen days; others,
fertilized in April, were hatched in nine days.
All fish, on emerging from the egg, enter upon a larval stage in which
they resemble each other very closely (_see_ Fig. 1). (Thus, the larvae
of plaice are quite symmetrical, like those of the cod or other round
fish.) The newly hatched larvae drift helpless in the water for two or
three weeks, during which time they subsist upon the contents of the
yolk sac, which they carry attached to their ventral surface. When this
is exhausted, they feed upon the microscopic plankton which abound in
the surrounding water.
The characteristic forms of the different species of flat fish are
gradually assumed by the young fish during the period of their larval
development. The appearance of a newly-hatched young plaice exhibits
little change during the first week or so, other than that due to
the gradual disappearance of the yolk sac. The young fish grows very
slowly, and, twenty-one days after hatching, is only 3/8 of an inch
in length. For thirty days the development of the young fish is
entirely symmetrical. During the succeeding fifteen days, the shape
and appearance of the fish become profoundly modified. The left eye
gradually moves upwards and forwards, until it attains its final
position above and in front of the right eye. At the same time, the
fish gradually acquires a new swimming position, finally swimming on
what is really its left side. This left side becomes colourless. With
these changes in form and habit, there proceeds a transformation in the
diet of the fish. At twenty-one days it feeds upon the young stages of
various crustacea. Gradually it acquires a taste for copepoda and the
larvae of mollusca and crustacea. After its metamorphosis is complete,
it feeds upon various worms, small shrimps and small, bottom-living
crustacea. The adult plaice feeds upon mollusca of the cockle and
mussel families.
=The Migration of Fishes.= Fishes, like birds, migrate over great
distances at certain seasons of the year. In most cases, this migration
occurs just before spawning, and is evidently connected directly with
the spawning instinct. True marine fishes, such as the herring,
haddock, plaice, cod, associate in vast numbers at spawning time,
choosing a locality in which the temperature and food supply will be
favourable to the development of the young larvae. Generally, the
spawning ground is in deep water. The eggs are buoyant, and drift up to
the warm surface water and hatch out amongst the plankton. The herring
differs from most other pelagic fish in laying its eggs in relatively
shallow water, over a rocky bottom covered with seaweed. The eggs are
denser than sea-water and are covered with an adhesive substance, so
that they sink to the bottom and become attached to the stones and
seaweed.
It is at the time of this annual migration to the spawning grounds that
the fish are most profitably caught, for not only are they gathered
together in large numbers, but, just before spawning, the fat content
and general condition of the fish, and therefore its food value, reach
a maximum. After spawning, the food value of the fish is at a minimum,
and remains comparatively low until a few months before the next
spawning.
The plaice migrates in the autumn from the feeding grounds in various
parts of the North Sea to the spawning grounds near the Straits of
Dover. Spawning takes place between December and March. In the spring
and summer it returns northwards to the feeding grounds in the centre
of the North Sea.
In the Irish Sea, there are two distinct annual migrations of plaice.
The first occurs in summer (from June to September), the larger plaice
moving from the warmer, shallow water inshore to the deeper, cooler
waters offshore. In winter and spring, (from October to May), the
mature plaice migrate from Morecambe and Liverpool bays to the spawning
ground in deep water to the North-East of Douglas (Isle of Man).
In winter, also (from November to January), a large number of plaice
gather in Red Wharf Bay, off the north coast of Anglesey, probably
because it is sheltered from the prevailing south-east winds. In
February they commence their spawning migration round the coast of
Anglesey to Cardigan Bay.
Certain species of fish, instead of migrating from one part of the sea
to another, migrate from the sea to rivers (anadromous), or from rivers
to the sea (katadromous).
Thus, in the spring or autumn, according to species, the anadromous
salmon and shad ascend rivers to spawn. The eggs are deposited on clean
gravel in clean water, where they are likely to remain undisturbed. The
salmon does not feed when in the river, and after spawning, becomes
very thin and in poor condition.
The Alaskan salmon, from which the bulk of American canned salmon
comes, exists in five species. It has a similar spawning habit to the
British salmon, except that the same species always tends to use the
same rivers. Once having spawned, the fish dies, so that the parents
never see their offspring. The young larvae hatch out in the fresh
water and make their way to the sea, where they pass the whole of their
lives until they are mature, some years later, and then, in their turn,
ascend the rivers to spawn.
Eels are normally fresh-water fish. After living for six or seven
years in rivers and ponds and streams, they become mature and migrate
to the sea to spawn. This spawning always takes place in deep water
(over five hundred fathoms), the particular region chosen depending
upon the species. Eels from the British Isles and North-West Europe
spawn in deep Atlantic, some hundreds of miles west of Ireland. In
the autumn, the mature eels move down the rivers to the sea. When
approaching maturity, the yellowish coat of the eel changes to silver.
These “silver” eels pass into the sea and are never seen again. It
is probable that the eel only spawns once in its life and then dies.
The spawn floats to the surface and hatches out into curious little
transparent, leaf-shaped larvae. These larvae develop rapidly into
elvers and commence the return journey to the shores and rivers. In the
spring, the young eels ascend the rivers in enormous swarms. Many of
them leave the rivers and travel over damp ground and grass to isolated
pools and lakes. It is probable that the eels that are found in the
Thames travelled overland from the Severn.
The Baltic flounder migrates in winter from rivers and estuaries to
the open sea, and spawns in spring in deep water. It returns in the
summer when the spawning is over. By observing the movements of marked
fish, it has been shown that the fish move at an average rate of from
three to four miles per day. During its seaward migration, the flounder
takes no food, but uses the material stored up in its tissues for the
development of its reproductive organs.
In addition to these spawning migrations, there are migrations that are
prompted by a search for food, or for warmer or colder water.
In northern and temperate seas, the surface water grows warmer with the
spring. This warming influence spreads northwards from the equator,
producing what is known as the annual wave of sea temperature. A direct
result of the rise of temperature and the increased sunshine is a
rapid increase in the amount and quality of the plankton. It is not
surprising, therefore, that fish migrate in the wake of this annual
wave of sea temperature, attracted by the increased food supply, and
possibly, also, by the warmer water.
The mackerel is a southern fish, and prefers the warm water of the
Mediterranean and West African coast. In spring, as the wave of rising
sea temperature travels northwards, it migrates to the English Channel
and the North Sea. This migration is often directly associated with the
presence, in large quantities at that season, of a particular kind of
copepod in the surface water of the English Channel.
=Phosphorescence.= Many marine creatures, ranging from deep-sea fish
living in the dark abysses of the ocean to various species of the
minute plankton drifting in the surface water, possess phosphorescent
organs, which emit light of low intensity similar to that of a
glow-worm and firefly. In many cases the light appears to possess some
important function, and highly specialized organs are developed. In
such cases the light is only emitted in response to some stimulus—thus,
the phosphorescence of the surface water of the sea, when disturbed by
the blade of an oar, is due to the disturbance of myriads of minute
planktonic organisms, equipped with phosphorescent organs, either
protozoa or protophyta; many pelagic copepods are phosphorescent. In
other cases, phosphorescence appears to be a more or less accidental
by-product of some other process, and of little or no significance. The
substance which produces the glow is contained in the slimy secretion
produced by the epidermal glands of the fish, and, as phosphorescence
can only occur in the presence of oxygen, it is evident that the light
is produced by the slow oxidation of this substance. The colour of the
light emitted by marine organisms is generally blue or light green, but
red and lilac also have been observed. The distribution and colour of
the light or lights produced by individual fish vary with the different
species. In many cases it would appear that these points of light
provide the means by which fish recognize each other in the dark depths
of the ocean. Some fishes possess highly developed phosphorescent
organs known as photophores, consisting essentially of a group of
gland cells that secrete the phosphorescent fluid. These organs are
generally distributed in rows along the sides and ventral surface of
the fish. Some fishes possess more complex and highly developed organs
containing, in addition to the gland cells, a system of blood vessels
and nerves, a transparent, protecting membrane and reflector, an
iris-like diaphragm and a lens. These more complex organs are generally
larger and less numerous than the simpler ones. Possibly they are used
to search for, or to attract, prey.
The phosphorescence of decaying fish and meat is due to the presence
on the fish or meat of certain bacteria of putrefaction, which are
themselves phosphorescent. When seen under the microscope, the
individual bacteria appear as shining points of light.