Stunting of Coho Salmon Oncorhynchus kisutch Parr Rearedin Wastewater‐Seawater Ponds

Abstract

Abstract.After entering saline water coho salmon Oncorhynchus kisutch parr often cease to grow (stunt) and many stunts eventually die. Stunted coho parr occurred in an experimental wastewater‐seawater smolt production system constructed in 1971 on the northern arm of Humboldt Bay, Arcata, California, USA. In 1991 many stunted parr (1990 brood) were noted during 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‐spring rearing (January‐May). The cohort suffered a 42% mortality during summer rearing in low salinity water (7 ppt) but deaths due to stunting could not be estimated. When the summer parr were transferred for winter rearing to 15 ppt salinity water, a total mortality of 15% was nearly all related to stunting. A small sample of stunted, but otherwise normal‐appearing, parr found in smolt out‐migrant traps was retained for an additional year of rearing (February 1992—January 1993). There was 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‐smolt, and “smolt‐like” external appearances. Five groups of 1990‐brood smolts were released into Humboldt Bay. They had rates of return to their point‐of‐release (South Pond) similar to other brood years released from the Arcata ponds and other northern California hatcheries. This suggested that whatever produced stunting in the 1990 brood acted independently of endocrinological or physiological factors that produce mortalities in smolts after entering saline water.