The school has an excellent stretch of the river Cherwell adjoining our fields and the North Oxford Angling Society, that runs our stretch of water, is very happy for us to fish there. It would not be game fishing (as there are relatively few trout on our stretch) but the earthier brand of angling known as coarse fishing. The distinction is in the species caught. In game fishing the angler is usually pursuing trout and salmon that may then be taken home and eaten. The coarse fisherman is after a wider range of species such as carp, tench, perch, chub, pike, bream, barbel and many others. These are not taken home but carefully returned to the water unharmed. The tactics used tend to be different too. The game fisherman uses artificial lures of one sort or another, whilst the coarse angler uses bait as well as lures in a range of different tactics to suit the different species and water conditions.
Our stretch of the Cherwell is noted for its bream, chub, perch, roach and tench. I have spoken to some local anglers who say that the bream are very large along the banks of what is a very clean stretch of water. By clean I mean unpolluted; the river carries a great deal of silt so the water appears very dark. This cuts out light reaching the deeper water meaning that the river is fairly free from plant life.
The Cherwell used to be packed with signal crayfish. These are an introduced species that compete with our native crayfish. Like the grey squirrel they out-competed their native cousins and pushed them towards extinction. They are officially a pest as they carry a disease that is lethal to our native species but to which they are immune. You need a license to catch them but if you do so by accident it is illegal to return them to the water! Luckily they are jolly tasty so it is not too much of a hardship to take a few home. They need to be left in some clean water for a while to flush them of the silt but then they can be cooked and eaten whilst still fresh. Since we have acquired a family of otters living on the bank opposite the school the crayfish population has plummeted. Pretty much the only evidence you see of them is discarded exoskeletons on the banks from where the otters have had a feast.
Of the vertebrate, true fish species in the river my favourite is the perch. These are thick-set and powerful animals that are aggressive hunters who lurk in ambush under any bit of cover they can find. Catching them is not easy; stealth is required if they are not to be frightened off as you approach the water’s edge. They can see you before you have any chance of seeing them because the light that enters the water refracts in their favour. I have tried to show this in the diagram below.
They have the laws of physics on their side. Although it is hard to look into the murky waters to see any fish clearly, it is relatively easy for them to detect movement on the bank from the little light that does penetrate the gloom. They have Snell’s Law to help them spot you before you see them. Snell’s Law describes the bending of light as it travels between materials of different densities – e.g. air and water. As light enters the water it refracts, so fish can see movement further back on the bank than a direct line of sight might indicate.
In a vacuum, light travels at 299,792,458 m/s (we often round this off to 300,000,000 m/s) but through different media it travels more slowly. The value by which light slows in a given material is called the index of refraction (refractive index). The table below shows these values for a number of common materials. You can see that the indices of refraction for a vacuum and for air are so close that we can ignore the difference on most occasions.
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The greater the difference in refractive index, the greater will be the degree of bend as light travels between two materials.
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You can use the index of refraction to calculate the speed of light through different materials – just divide the speed of light through a vacuum by the index of refraction for the material in which you are interested.
For example, to calculate the speed of light through water…
Some of the questions below relate to topics you may have forgotten!
Questions…
- To which taxonomic group do crayfish belong?
- What is the chief dietary nutrient that they would supply were you to eat them?
- To what trophic level do perch belong?
- What two products would you expect to form if ethanol were burned in a plentiful supply of oxygen?
- What is the approximate speed of sound in water (in m/s)?
- What is the approximate speed of light through water? (in standard form)
- What would be the approximate speed of light through diamond? (in standard form)
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