Grilse A salmon which natures after only one winter at sea. A fish which stays in the sea for two sea winters or more is called a salmon. Most strains of salmon give rise to some grilse and much effort has been spent through selective breeding to reduce this percentage. Farmers usually grade the fish by in mid to late Spring, when the skilled worker can tell a grilse by the beginning of the secondary sexual characteristics . These fish are harvested and sold.
Some farms satisfy niche markets for small salmon by harvesting all fish prior to grilsing.
See also smolt Gross Pathology Symptoms etc. which are apparent by viewing the body and tissues with the naked eye. Groundwater Water found beneath the surface of the earth. Generally of constant quality and temperature. Often used for hatcheries as a disease free water supply, however, many ground water supplies are from underground streams which may at times rise to the surface where they can become contaminated. Some pathogens such as viruses have been found in groundwater supplies close to fish farms, where seepage from ponds has contaminated the ground water supply.