Life History

Redd A shallow trench cut in the riverbed by the hen trout, in which the eggs are laid and fertilized. The term is sometimes also used to describe the banks of gravel where the trout spawn.

Milt A milky fluid produced by the testes of the cock trout that contains the sperm.

Ova The hen trout's eggs, which are laid in a redd and then fertilized by the milt. The same term is used for embryo trout while they are developing within the egg membrane. Thus ova can be described as ‘un-eyed', where the embryo has not developed eyes, or 'eyed' where the embryo eyes are clearly visible beneath the egg membrane.

Alevin The young trout after it has left the egg. It retains the large yolk sac from which it obtains its food supplies, and remains hidden in the gravel surrounding the redd. In North America alevins are sometimes called 'sac-fry'.

Fry The young trout after it has exhausted the yolk supplies and begun to seek food. Sometimes the fry stage is split into two: unfed fry where the fry has not begun to feed, but has lost the alevin yolk sac; fed fry where the fry is feeding actively.

Parr The older fry stage, where ovoid "parr markings' are well developed along the side of the fish.

Smolt The post-parr form in which the young of sea-going trout migrate from freshwater to the sea.

Post-smolt A term sometimes used to describe the period of the life of the sea trout between its arrival in the sea and its return to freshwater as a sexually mature fish. This term might also include those steelhead, cutthroat and Atlantic sea trout that return to freshwater after a few months at sea but which do not breed (herling); these later return to the sea before returning as adult sea trout.

Kelt The adult trout after spawning; the rigours of the breeding season results in a massive wastage in body tissues so that the conditions of kelts is usually poor.
Recovering kelts are sometimes called ‘mended’ or ‘well-mended kelts’. Those that recover completely can be difficult to separate from ‘maiden’ trout (those that have yet to spawn) other than by a scale reading.

Scale reading A technique whereby the life history of an individual trout can be deduced from the microscopic examination of its scales. As the trout grows it lays down rings, analogous to the annual rings laid down by trees, on each scale. From these can be ascertained the age of the fish, in sea trout the number of years of river and sea life, and whether the fish has bred.


Genetics and Evolution

Genes Chemical molecules, held within the nucleus of the cells that make up the body of the trout, which determine growth, form and structure, coloration, physiology and behaviour of the trout. An individual obtains half of its genes from each parent. Variation in genetic structure can result in one individual or one population of trout being quite distinct from other individuals or populations. The effect of genes within an individual or population can be greatly modified, by environmental factors, so there may be argument as to whether a physical feature in the trout really is a consequence of genes.

Gel electrophoresis A modem chemical technique (also known as 'genetic fingerprinting') that can determine precisely the genetic composition of a population of trout. Gel elecrophoresis has proved that some trout populations and varieties are genetically discrete, and not the result of environmental factors.

Evolution The process whereby the genetic structure of a trout population is moulded by the environment. The pressures of natural selection will tend to remove some genes in favour of other advantageous genes that render the trout population better adapted to its environment. Thus two lakes or rivers, colonised by trout from one population, may give rise to two genetically different trout populations.

Taxonomic status The differences between populations of trout may be such that the populations can be named as separate species, or subspecies within one species, or varieties within one subspecies of a species. Which of these three categories a population of trout is placed in depends largely on the arbitrary opinion of taxonomic biologists, as the rules that define the terms species, subspecies and variety have many exceptions.


Aquatic Environment

Oligotrophic The purest of waters that produce little trout food and where trout populations are small and growth rate slow.

Eutrophic Lakes and rivers that produce large amounts of animal and plant life. Such waters may have large populations of fast-growing trout.

Mesotrophic Waters that lie between the extremes of oligotrophic and eutrophic conditions. Because eutrophic waters may verge on the state of over-eutrophication, mesotrophic waters are perhaps the ideal trout habitats.

Over-cutrophication Waters that have artificial fertilisers or organic pollutants added may frequently suffer from oxygen shortage, especially in spells of hot dry weather. This usually results in a death of trout.

Pollution The addition of material to water that results in an alteration to the plant and animal communities of that water. Pollutants may be e.g. organic (raw sewage, wood pulp) or inorganic (chemical fertilizers, disolved gases from 'acid rain', lead from mining, cyanide from industrial outfalls).