Trout Biology:
Trout spawn during winter by shedding their eggs into excavations in gravel. The eggs are then covered over with gravel to form a completed nest known as a "redd". Water then percolates through the redd and brings dissolved oxygen to the eggs. There must be little silt amongst the gravel as silt can clog the pore spaces . The classic spawning site for an upland stream is on the upstream face of a gravel bar where the current is gathering speed. At such places water is pulled by the flow down through the gravel to the eggs bringing dissolved oxygen.
Once emerged during the spring the trout try to seek cover in shallow slower flowing water compared to the nest sites. The trout try to seek cover typically under stones, aquatic plants and overhanging vegetation. This habitat is typical of upland streams, for this reason trout tend to thrive in tributaries and are less abundant in larger lowland rivers.
As trout grow the fish progressively use deeper water, moving out from the edges of the streams and often dropping downstream from the smaller nursery streams, this is known as the 'swim up', this usually occurs twenty to fifty days from hatching.
Adult trout lie in areas of slack current, but never far away from a strong flow which carries down a steady supply of food. Trout feed upon aquatic invertebrates, particularly crustaceans insects on or near the stream bed. An important component of their diet is terrestrial insects which have fallen or been blown from trees, bankside plants, or simply out of the air . However cover still plays an essential part in the trouts life cycle, typical cover which is preferred includes undercut banks with draping grasses, branches, tree roots and submerged boulders.