THE FISHES OF THE BRACKISH WATER.
On such parts of a coast at which there is a mixture of fresh and salt
water, either in consequence of some river emptying its water into the
sea or from an accumulation of land surface water forming lagunes,
which are in uninterrupted or temporary communication with the sea,
there flourishes a peculiar brackish water fauna which is characterised
by the presence of fishes found sometimes in sea-, sometimes in pure
freshwater.
This fauna can be rather sharply defined if a limited district only
is taken into consideration; thus, the species of the brackish water
fauna of Great Britain, the Pacific coast of Central America, of
the larger East India Islands, etc., can be enumerated without much
hesitation. But difficulties arise when we attempt to generalise in the
enumeration of the forms referable to the brackish water fauna; because
the genera and families enumerated include certain species and genera
which have habituated themselves exclusively either to a freshwater or
marine existence; and, besides, because a species of fish may be at
one locality an inhabitant of brackish water, at another of the sea,
and at a third of fresh water. The circumstance that these fishes can
live in sea and fresh water has enabled them to spread readily over the
globe, a few only being limited to particular regions; therefore, for
the purposes of dividing the earth’s surface into natural zoological
regions the brackish water forms are useless. The following fishes may
be referred to this Fauna:--
1. Species of _Rajidæ_ (_Raja_, _Trygon_) prefer the mouths of rivers,
probably because the muddy or sandy bottom offers the most suitable
conditions for fishes which can feed on the bottom only; such brackish
water species belong chiefly to the Equatorial Zone, some having taken
up their abode entirely in fresh water (South American Trygons).
2. _Ambassis_, a Percoid genus, consisting of numerous small species,
inhabiting the shores of the tropical parts of the Indian Ocean and
the coasts of Tropical Australia. Many species enter, and all seek the
neighbourhood of, fresh water; hence they disappear in the islands of
the Pacific, and are scarce in the Red Sea.
3. _Therapon_, with the same distribution as the former.
4. Numerous _Sciænidæ_ of the Equatorial Zone.
5. The _Polynemidæ_, chiefly inhabitants of brackish water of the
Equatorial Zone, most developed in the Indian region, and scarce in the
Tropical Pacific.
6. Numerous species of _Caranx_ (or Horse Mackerels) of the Equatorial
Zone.
7. Nearly all species of _Gastrosteus_ enter brackish water, _G.
spinachia_ being almost exclusively confined to it: Northern Zone.
8. The most important genera of the Gobies (_Gobiina_): _Gobius_
(nearly cosmopolitan), _Sicydium_, _Boleophthalmus_, _Periophthalmus_,
_Eleotris_ (equatorial). Many of the species are entirely confined to
fresh water.
9. The _Amblyopina_, similar to the Gobies, but with more elongated
body: Tropical Indo-Pacific.
10. The _Trypauchenina_: Coasts of the Indian region.
11. Many species of _Blennius_, of which several are found far inland
in fresh waters--for instance in North Italy, in the Lake of Galilee,
in the eastern parts of Asia Minor.
12. The majority of _Atherinidæ_, and
13. The _Mugilidæ_: both families being most numerous and abundant in
brackish water, and almost cosmopolitan.
14. Many _Pleuronectidæ_ prefer the mouths of rivers for the same
reason as the Rays; some ascend rivers, as the Flounder, _Cynoglossus_,
etc.
15. Several _Siluridæ_, as especially the genera _Plotosus_,
_Cnidoglanis_, _Arius_, which attain their greatest development in
brackish water.
16. The _Cyprinodontidæ_ are frequently found in brackish water.
17. Species of _Clupea_, some of which ascend rivers, and become
acclimatized in fresh water, as _Clupea finta_, which has established
itself in the lakes of northern Italy.
18. _Chatoessus_, a genus of Clupeoid fishes of the Equatorial Zone, of
which some species have spread into the Northern Zone.
19. _Megalops_: Equatorial Zone.
20. _Anguilla._ The distribution, no less than the mode of propagation,
and the habits generally, of the so-called Freshwater-eels still
present us with many difficult problems. As far as we know at present
their birthplace seems to be the coast in the immediate neighbourhood
of the mouths of rivers. They are much more frequently found in fresh
water than in brackish water, but the distribution of some species
proves that they at times migrate by sea as well as by land and river.
Thus _Anguilla mauritiana_ is found in almost all the fresh and
brackish waters of the islands of the Tropical Indian Ocean and Western
Pacific, from the Comoros to the South Sea; _Anguilla vulgaris_ is
spread over temperate Europe (exclusive of the system of the Danube,
the Black and Caspian Seas), in the Mediterranean district (including
the Nile and rivers of Syria), and on the Atlantic coast of North
America; _Anguilla bostoniensis_, in Eastern North America, China,
and Japan; _Anguilla lati__rostris_, in Temperate Europe, the whole
Mediterranean district, the West Indies, China, and New Zealand. The
other more local species are found, in addition to localities already
mentioned, on the East Coast of Africa, South Africa, on the continent
of India, various East Indian Islands, Australia, Tasmania, Auckland
Islands; but none have ever been found in South America, the West Coast
of North America, and the West Coast of Africa: surely one of the most
striking instances of irregular geographical distribution.
21. Numerous _Syngnathidæ_ have established themselves in the Northern
Zone as well as in the Equatorial, in the vegetation which flourishes
in brackish water.
This list could be considerably increased if an enumeration of species,
especially of certain localities, were attempted; but this is more a
subject of local interest, and would carry us beyond the scope of a
general account of the distribution of Fishes.
[Illustration: Fig. 105.--_Mugil octo-radiatus._]
[Illustration: Fig. 106.--_Mugil auratus._]
[Illustration: Fig. 107.--_Mugil septentrionalis._
Heads of Grey Mullets, fishes of Brackish water.]