STUNTING OF COHO SALMON ONCORHYNCHUS-KISUTCH PARR REARED IN WASTE-WATER-SEAWATER PONDS

After entering saline water coho salmon Oncorhynchus kisutch parr ofte n cease to grow (stunt) and many stunts eventually die. Stunted coho p arr occurred in an experimental wastewater-seawater smelt production s ystem constructed in 1971 on the northern arm of Humboldt Bay, Arcata, California, USA. In 1991 many stunted parr (1990 brood) were noted du ring transfer of juveniles from a 0.02-ha shaded pond used for summer rearing (July-December) to a 0.15-ha unshaded pond used in winter-spri ng rearing (January-May). The cohort suffered a 42% mortality during s ummer rearing in low salinity water (7 ppt) but deaths due to stunting could not be estimated. When the summer parr were transferred for win ter rearing to 15 ppt salinity water, a total mortality of 15% was nea rly all related to stunting. A small sample of stunted, but otherwise normal-appearing, parr found in smelt out-migrant traps was retained f or an additional year of rearing (February 1992-January 1993). There w as a 29% survival in these stunts, with survivors attaining an average fork length of 16 cm (range 12-21 cm). The survivors exhibited parr, pre-smelt, and ''smelt-like'' external appearances. Five groups of 199 0-brood smelts were released into Humboldt Bay. They had rates of retu rn to their point-of-release (South Pond) similar to other brood years released from the Arcata ponds and other northern California hatcheri es. This suggested that whatever produced stunting in the 1990 brood a cted independently of endocrinological or physiological factors that p roduce mortalities in smelts after entering saline water.