Morphology and Basic Structures of Fisheries

Water is the environment in which fish is found carrying out its normal activities throughout its life time. It is the habitat in which fish survives. Water is of different types and this determines what species of fish that can be found in it. In the same vein fishes do migrate from one water body to another especially at certain period of the season or stage in their life time.

This movement is however characterised by specific requirements of the salt content of the water body needed at a time and this can be movement from fresh water to salt water or vice versa.

However, while taxonomy is the science of classification, the use of migratory pattern of fish and the nature of water in which it is found is used for grouping.

1. Grouping of Fish

The habitat in which fish is found varies and these are marine, brackish and fresh water habitat. The salt content of these water bodies makes the distinction between one and the other. The salt content (salinity) is expressed in part per thousand (ppt).

Water body with salinity greater than or equal to 35ppt is marine water while brackish water has salinity ranging from 1-34 ppt. Any water-body with salinity less than 1ppt is said to be a fresh water habitat such as lakes, rivers, streams and springs.

Fish species are of diverse water quality requirement either at certain stages of their life or throughout their life time for specific activity and therefore have to sojourn through different water bodies of significantly varying salt content. Based on this pattern of migration for which ever reason, fish can be said to be anadromous, catadromous or diadromous.

(i) Anadromous: Fish are said to be anadromous when they are found migrating from a salt water body like brackish water into a fresh water body e.g. Salmon.

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(ii) Catadromous: They are said to be catadromous when they are found migrating from a fresh water body into a salt water body e.g. Eel.

(iii) Diadromous: Diadromous fish spend their life both in fresh water and salt water bodies simultaneously without any barrier to period of the season or metabolic activity e.g. Chrysichthys nigrodigitatus.

2. Classification of Fish

Just like in plant classification or other higher animals, fish classification also follows the same pattern which starts from Phylum to species i.e.

Phylum—- Sub-phylum—- Super class—- Class—- Sub-class—-
Order—- Sub-order—- Family—- Genus—- Species

Species of fish comprise a group of fish whose member have similarity in structure or and appearance and are capable of breeding among themselves. Species is the last and forms the basis for taxonomic grouping of plants or animal thereby becomes more to be reckoned with for various purposes including research.

Stream size plays a critical role in the distribution of freshwater fishes. Some species have specific habitats, such as headwater streams with significant groundwater influences.

Others are less particular, inhabiting a wide array of stream sizes through an extensive geographic range. So when a fish is to be identified from within a genera (that group of fish with similar structure or appearance), it is the species name that will be indicated and which does the separation.

Therefore, classification to species level has to be done correctly since not all members of the same genera possess the relevant characteristics for culture purposes.

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