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Soil & Water Conservation Society of Metro Halifax (SWCSMH)

January 04, 2016                               Fisheries- Recreational           



Over half of all vertebrates are fishes. The most diverse and successful vertebrate group, they provided the evolutionary base for invasion of land by amphibians. In many ways amphibians, the first terrestrial vertebrates, can be viewed as transitional-fish out of water.

The story of vertebrate evolution started in the ancient seas of the Cambrian Period (590-505 million years ago), when the first backboned animals appeared. 1

Almost all the fishes that live today (21, 000 species) are bony fishes.

Figure 1: The major fish groups. 1


Characteristics: 1

From whale sharks that are 18 metres long to tiny cichlids no larger than one's fingernail, fishes vary considerably in size, shape, colour, and appearance. Some live in freezing Arctic seas, others in warm freshwater lakes, and still others spend a lot of time out of water entirely. However varied, all fishes have important characteristics in common.

  1. Gills: Fish are water-dwelling creatures and must extract oxygen for their metabolism from oxygen gas dissolved in the water around them. They do this by swallowing a great deal of water. The water passes over fine filaments of tissue rich in blood vessels, called gills, in the back of the mouth, and then back out of the body through slits in the side of the throat. The blood moves opposite the flow of water.
  2. Backbone: All fishes have an internal skeleton with a backbone surrounding the spinal cord, although it may not necessarily be made of bone. The brain is fully encased within a protective box, the skull or cranium, made of bone or cartilage.
  3. Single-loop blood circulation: Blood is pumped from the heart to the gills. From the gills the oxygenated blood passes to the rest of the body, then returns to the heart. The heart is a muscular tube-pump made of four chambers that contract in sequence.
  4. Nutritional deficiencies: Fishes are unable to synthesize the aromatic amino acids and must consume then in their diet. This inability has been inherited by all their vertebrate descendants.

Eutrophication: an enrichment problem 2

Normal lakes that have minimal levels of nutrients are said to be enriched, or oligotrophic. An oligotrophic lake has clear water and supports small populations of aquatic organisms. Eutrophication is the enrichment of water by nutrients; a lake that is enriched is said to be eutrophic. The water in a eutrophic lake is cloudy and usually resembles pea soup because of the presence of of vast numbers of algae and cyanobacteria that are supported by the nutrients.

Although eutrophic lakes contain large populations of aquatic animals, different kinds of organisms predominate there than in oligotrophic lakes (cf. Figure 2 below).

In an unenriched lake there is a higher concentration of dissolved oxygen. In eutrophic lakes, on the other hand, the deeper, colder levels of water are depleted of dissolved oxygen because of the high BOD caused by decomposition on the lake floor. Therefore, the natural fish population may be replaced by warm-water fishes, such as catfish and carp, that can tolerate lesser amounts of dissolved oxygen (cf. Figure 2 below).

Over vast periods of time, oligotrophic lakes and slow-moving streams and rivers become eutrophic naturally. As natural eutrophication occurs, these bodies of water are slowly enriched and grow shallower from the immense number of dead organisms that have settled in the sediments over a long period. Gradually, plants such as water lilies and cattails take root in the nutrient-rich sediments and begin to fill the shallow waters, forming a marsh.

Eutrophication can be markedly accelerated by human activities, and it results from the enrichment of water by inorganic plant and algal nutrients- most commonly in sewage and fertilizer runoff.

Figure 2: A comparison of the features of an oligotrophic lake and a eutrophic lake 2


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