Fishery biodiversity, from individual populations to whole species and even complex communities, has been declining in response to the reduced health of aquatic ecosystems and invasions by nonnative species.

Pollution is a major health hazard to achieving fresh water fish diversity conservation which may arise through inappropriate agricultural practices, the destruction of watersheds, opening up of river banks leading to silting of river beds and loss of water courses.

Marine fishes are also threatened, primarily by over fishing and pollution, as well as habitat loss. Overall, the status of aquatic species appears to be deteriorating faster than that of mammals, birds, reptiles and other terrestrial animal groups.

1. Continental Shelf Ecosystem

The continental shelf area is characterised by a variety of physical and chemical forcing. Physical forcing is represented by tide, major current system, wind mixing, wind-induced upwelling, and sea-level change.

Chemical forcing includes salinity and other materials that flow in from rivers and that are deposited from the atmosphere by wind and rain. It is characterised by large spatial and temporal variability.

Productivity on the continental shelf area is much higher than that of the open ocean. The shelf comprises only 7% of the ocean but is responsible for at least 25% of oceanic primary production and interactions between the pelagic (planktonic) community and the benthic community is stronger.

However, most of the continental shelf is deep enough so that light cannot reach the bottom, resulting in less primary production there. Accordingly, the benthic community depends mostly on the production in the upper water column.

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2. Seasonal Changes

Primary production of the pelagic community often depends on the light energy and nutrient supply, and these factors change with season. Solar radiation increases during the dry season and the water column becomes stratified, whereupon the phytoplankton biomass increases using nutrients in the surface water.

Nutrients and biogenic materials in the upper water used by phytoplankton eventually drop out of the euphotic zone. Nutrients transported as organic particles recycle in deeper layers or on the sediments.

When the water is stratified, nutrient depletion is severe in the upper water column because the nutrients transported do not return and as well the increased by zooplankton reduce the biomass of the phytoplankton bloom.

This situation of low phytoplankton community remains at the water surface until onset of rainfall when the stratification weakens out and nutrients are supplied from the deeper water.

3. Physical and Chemical Forcing

The seasonal changes just discussed are part of the basic cycle of plankton communities in the ocean. On the continental shelf, additional physical and chemical forcing is important for the enhancement of production and for variability of ecosystem structures. There mechanisms are discussed below:

(i) Estuarine and River Plumes

River and estuarine plumes often extend over the continental shelf or even reach to the open ocean. The fresh and brackish waters of estuarine and river plumes are important sources of nutrient, organic material, and other chemical materials for the continental shelf.

Discharges of suspended materials from rivers and bays decrease light penetration in the upper water column and sometimes result in lower primary production near the mouth of the rivers and bays.

However, phytoplankton, typically euryhaline diatoms, increased their biomass by using abundant nutrient in the plumes after these sediments settled down and their density peaks at some distance from the river mouth.

(ii) Tide

The tide is one of the important hydrodynamic forcings in the continental shelf ecosystem. The relationship between tidal current and bottom topography induces the mixing of the water column.

A combination of strong tidal energy and a shallow bottom induces thorough vertical mixing and sometimes draws back into the system, too strong a mixing may decrease the light intensity that is required for phytoplankton production. This distribution of tidal mixing also corresponds well to the distribution of fish larva and benthic organisms.

It is important to also note that tidal motion is the residual current which is induced by oscillatory motion of the tide and which transports materials on the continental shelf.

(iii) Internal Wave

When tidal motion or current contacts the bottom topography and the water is stratified, a wave is generated at the boundary between the stratified water. It is thus called internal wave because the wave motion is below the surface.

Circulation around the internal waves also creates small-scale divergence and convergence and positive buoyancy materials often converge at the water surface.

Floating materials as well as zooplankton that swim upward can be aggregated at the convergent area. This may be an important transport mechanism for zooplanktons on the continental shelf.

(iv) Coastal Upwelling

Wind-induced coastal upwelling is a well-known hydrodynamic factor on the continental shelf which is common in Oregon, California, North Africa and Angola. Wind action induces an offshore surface current and upward movement of water from the subsurface layer along the coast and this result in the zones being highly productive and excellent fishing grounds.

However, the intensity of upwelling varies spatially because of differences in wind fields and topography and temporally because of the seasonal changes in wind velocity.

This variability is important for the biological community, as phytoplankton and fish behaviour are often evolutionary adapted to the variability of these short time scale. Smaller-scale wind-induced upwelling is frequently observed at the various continental shelves and is also important for ecosystem productivity.

4. Diversity Pattern of Fish

In recent years, a decline in artisanal fishery is an indication of environmental degradation and possible changes in water quality of the lagoon mainly and with biological consequences for the biota in the environment.

This situations shall however be addressed in order to safeguard our fishery from total collapse as well as the economic implications and the scientific undertone relent to it.

Worldwide, there are approximately 25,000 species of fishes. Nearly half of these occur in fresh waters, which are surprisingly large considering the small amount of fresh water compared to salt water, and this indicates the amount of isolation and speciation that has occurred in our rivers, lakes, and springs.

Within fresh water, riverine habitats typically harbour greater fish diversity than do reservoirs, lakes and springs. Nigeria inshore water for instance provide habitat for approximately 71 families of about 157 species of which the following are prominent; Pseudotolithus senegalensis, P. typus, P. Elongates, Brachydeuterus auritus, Aurius spp. Ilisha africana, Ephippus spp., Lutjanus spp. Ethmalosa fimbricata, Mugil spp. Gymnarchus niloticus etc.. while the offshore waters, tuna and tuna-like fishes are most important.

They include the skipjack, the yellow tin tuna, the big eye tuna, Coryphaena hippurus, Euthynnus alletteratus, Caranx crysos and Elagatis bipinnalata.

Total biodiversity, however, includes more than species. Genes, population, subspecies, and communities as well as species are species are primary components of diversity.

Maintenance of diversity below the species level often is overlooked for those fishes organized more or less into isolated populations, such as anadromous fishes or desert fishes.