Fish are curious creatures and we have still a great deal to learn about their habits. Some like the Salmon and the Shad (Ikan tĕrubok) live in the sea and spawn in the rivers. Such fish are termed anadromous and the term is also applied to fish which make a migration from the deep sea coastwards for the purpose of spawning.
Others, like some Eels, live in the rivers and spawn in the sea. The common Eel of Europe (Anguilla vulgaris) spawns far out in the ocean, after which both males and females die, never returning to fresh-water a second time. Fishes which live in the rivers and spawn in the sea are termed catadromous.
Some fishes do not lay eggs but bring forth their young alive. Examples of viviparous fishes occur in the Shark and Ray families and also in the Blennidae, Cyprinodontidae and Scorpaenidae. Instances of functional hermaphroditism occur, and some of the Serranidae (Sea-Perches) are invariably hermaphrodite and self-fertilising.
A Sea-Bream, Chrysophrys auratus, is an example of successive hermaphroditism, the male and female sex-cells ripening alternately. As an occasional variation hermaphroditism has been recorded in such well known fishes as the Cod, the Mackerel and the Herring.[1]
The eggs of fishes may be divided into two kinds; the large (demersal ova) which are heavy and sink; and the small (pelagic ova) which are buoyant and float at or below the surface according to their density. The buoyancy of the pelagic egg depends, however, on the density of the sea and the pelagic egg becomes demersal, in position, in brackish water and in fresh water.
Demersal eggs may be either viscid and adhesive or smooth and non-adhesive.
Pelagic eggs are distinguished by their lightness, buoyancy, small size and remarkable transparency. They are always non-adhesive and free and they invariably belong to Marine Fishes. As a general rule it may be said that fresh water fish produce demersal ova and marine fish pelagic ova.
When we realise that the eggs of most Marine fishes float, it is obviously futile to speak of guarding the "spawning grounds" on our coasts. It is necessary to mention this because at one time it was thought that spawning took place on shallow banks or even close in shore but this is now known to be incorrect, except in the case of the true Herring which lays demersal eggs in comparatively shallow water, and a few less important species.
Amongst our important Marine food fishes which are known to produce pelagic eggs are members of the Herring, Mackerel, Horse-Mackerel, Sea-Perch, Mullet and Flatfish families, in fact, all our best fish.
- ↑ Camb: Nat: Hist: 1904.
