Natural reproduction of coho salmon (Oncorhynchus kisutch) in two Wisconsin tributaries of Lake Michigan (Little Scarboro Creek, Kewaunee Co., and Fischer Creek, Manitowoc Co.), is limited by an unusually high mortality of eggs and preemergent embryos. Of approximately 1800 coho salmon eggs planted in six study redds (spawning beds) within Fischer Creek (November 1972), none survived to hatching. The 1500 eggs planted in five study redds within Little Scarboro Creek produced 21 sac fry.

The bottom materials of both streams are comprised of at least 15% (by volume) particles smaller than 0.84 mm in diam. Streambed gravels containing this quantity of sand, silt and clay do not allow for intragravel water of sufficient dissolved oxygen concentration or velocity to meet the oxygen requirements of developing salmon eggs. Water surrounding eggs planted in Fischer Creek and Little Scarboro Creek had mean dissolved oxygen concentrations of 3.96 and 6.22 mg/liter, respectively.

The extremely low mean (2.83 C and 2.67 C) water temperatures of Fischer Creek and Little Scarboro Creek were an added cause of egg loss. In response to low temperature, planted eggs hatched after a prolonged development period of approximately 145 days. Gravel shifts caused by variable stream flow in Fischer Creek may also be responsible for killing naturally deposited eggs there