proximately 14.5 km before entering the Salmon 

 River, 17 km from Lake Ontario (Figure 1). About 

 6(y7c of Orwell Brook is considered adequate for 

 successful salmonid reproduction with suitable 

 substrate generally consisting of gravel ( 1-2 cm in 

 diameter) and pebbles (3-6 cm in diameter). The 

 maximum summer water temperature recorded 

 during 1977 and 1978 was 21° C. Mean monthly 

 stream discharge from June to October 1978 was 

 0.26 m^/s. Salmonids, cyprinids, and catostomids, 

 in order of abundance are the principal compo- 

 nents of the Orwell Brook fish fauna. 



A single 100 m station was established on Or- 

 well Brook 3 km above the Salmon River. This 

 section was generally characteristic of the lower 

 portion of Orwell Brook. Sections of the stream 

 were visually examined weekly from early May to 

 mid-June in 1978 in order to estimate the approx- 

 imate time of peak emergence of salmon fry. Peak 



emergence, as used in this study, occurred when 

 the densities of coho and chinook salmon and 

 steelhead were highest in Orwell Brook. Collec- 

 tions of juvenile salmonids were made monthly 

 from May to October with a 3 m minnow seine. 

 Supplemental observations on salmon emergence 

 were also made in May 1979. Monthly population 

 estimates derived using the Chapman mark- 

 recapture index (Ricker 1975) and average 

 monthly weights of juvenile salmonids were plot- 

 ted with the area beneath the curve providing an 

 estimate of total production (Chapman 1968). A 

 logarithmic plot assuming an exponential decline 

 in monthly densities was used to determine the 

 population size at peak emergence for coho and 

 chinook salmon. This method, based on the as- 

 sumption that natural mortality is greatest just 

 after emergence and then gradually diminishes, 

 has previously been employed to estimate the 



Figure l . — The study area near the south- 

 eastern shore of Lake Ontario in central 

 New York. 



551 



