CURRENT MOON
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Life History
Brown trout occur in a wide variety of forms, differing
substantially in appearance and in life history. Small, growth-stunted, but
highly-coloured, resident trout are common in headwater streams. Others, which
may be more silvery as juveniles, migrate downstream to better feeding
opportunities in rivers, lochs, estuaries, or the sea (see sea trout), and
eventually return to their natal areas to spawn in early winter (Oct-Dec). The
eggs (4 to 6 mm diameter) are buried in gravel in moderately flowing riffles of
small streams, usually from 1 to 5 metres wide. Spawning can also occur on clean
gravel areas in lochs, particularly where there is up-welling of
groundwater.
The brown trout is an extremely variable species in
both habit and appearance. Large migratory sea trout, for example, differ
markedly in most respects from the smaller non-migratory trout that choose to
remain in rivers – despite the fact that, in many cases, single families give
rise to both migratory types. River and loch trout are well-known for their wide
range of colourations and spotting patterns. Formerly, this led to their being
classed as many separate species, or sub-species. Now, however, all trout are
regarded as belonging to a single species, Salmo trutta, although this is not
the whole story. Modern techniques show that trout from different locations
differ genetically and that the effect is very marked. The patterns of variation
appear to reflect the routes taken by invading trout at the end of the last
glaciation, as they re-colonised Scotland's emerging freshwaters from rivers or
lakes on the edge of the ice.
Diet and Behaviour
Adult brown trout can range from 10-100 cm in length,
but typically are 20-50 cm. Smaller trout feed almost entirely on crustaceans,
worms and molluscs and on insects and their larvae. In summer, much of the diet
in highland lochs is wind-blown insects of terrestrial origin. As they grow,
larger trout feed increasingly on smaller fishes, including minnow (Phoxinus
phoxinus L.), stickleback (Gasterosteus aculeatus L.), and on other small trout
and salmon parr. A large form (known as ferox trout) found in many highland
lochs feeds largely on Arctic charr. Trout in lowland waters tend to grow
faster than those at higher altitudes. Trout in warmer waters grow more quickly,
but tend not to live as long as those in the colder, less productive highland
waters.
The original genetic patterns have been modified
considerably by later events involving exchange of fish, and genes, among
populations – and this probably continues today. On the otherhand, genetic
exchange among populations is limited by other factors such as fidelity to
particular spawning sites, differences in the timing of reproduction and the
choice of particular types of mates. The most notable example of such
segregation comes from Lough Melvin in Ireland, which contains three forms of
trout – sonaghen, gillaroo and ferox. Sonaghen are silvered plankton feeders,
gillaroo are highly coloured bottom feeders and ferox are large fish that prey
on smaller ones. All three forms spawn separately and, in spite of their
proximity during most of their lives, each form remains genetically
distinct.
Description
The European brown trout (Salmo trutta) is found throughout Scotland in both running and still waters. Our damp, equable, oceanic climate and varied geology with an abundance of gravels provide very favourable conditions for spawning, growth and survival. Brown trout are typically streamlined, highly-spotted fishes, with brown background colouration varying to yellow, and with black, orange, or red spots.
Brown trout