By William F. Royce (Auth.)
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Extra resources for Introduction to the Fishery Sciences
The result ing currents are always turbulent and depend on many factors including the shape and depth of the lake, and strength, steadiness, and fetch (length of water stressed by the wind) of the wind. Consequently they are highly irregular and largely unpredictable. What needs emphasis here is the distur bance of the deeper waters which reflects in part the disturbance of the surface and which may be significant to the fisheries. When the wind piles up the water at the downwind end of a large lake, the thermocline is also tilted downward at the downwind end.
If the river water includes organic material that uses up the oxygen, the layer of water may be a barrier to the movement of fish. 5 CIRCULATION IN ESTUARIES Estuaries are mixing basins of fresh and salt water. The circulation is much more rapid than in either the lakes or open ocean. No two estuaries are alike and the mixing is as varied as the tidal range, the river flow, the shape, size and depth of the estuary, the wind, and the effect of the coriolis force. Thus the circulation of each estuary under study needs to be individually deter mined.
All waters are relatively opaque to red light. Transparency of surface waters is commonly measured by a Secchi (pro nounced sek-key) disc 20 or 30 cm in diameter (Fig. 14). It is lowered on a graduated line until it just disappears and then raised until it just appears; the average of the two depths is called the Secchi disc transparency. This measure of transparency ranges from about 60 m in the clearest ocean water to a few centimeters in the turbid water of river mouths. It is equivalent to the level of penetration of between about 10 and 15% of solar radiation.