80 



FISH HATCHERY MANAGEMENT 



Figure 29. Muskellunge fry being sample-counted for inventory. (Courtesy 

 Wisconsin Department of Natural Resources.) 



(4) Several samples (at least five) should be taken. If the calculated 

 number of fish per pound (or weight per 1,000) varies considerably among 

 samples, more samples should be taken until there is some consistency in 

 the calculation. Then the sample values can be averaged and applied to the 

 total lot; all samples should be included in the average. Alternatively, the 

 counts and weights can be summed over all the samples, and an overall 

 number per pound computed. Larger samples are required for large fish. 



Even with care, the sample-count method can be as high as 15-20% inac- 

 curate. Some fishery workers feel it is necessary to weigh as much as 17% 

 of a population to gain an accuracy of 5-10%. Hewitt (l948) developed a 

 quarter-sampler that improved the accuracy of the sample count method 

 (Figure 30). 



In the total-weight method, as the name implies, all of the fish in a lot 

 are weighed, thus sampling error is avoided. Initial sample counting must 

 be conducted during the first weighing to determine the number of fish in 

 the lot, but this is done when the fish are small and more uniform in size. 

 This method involves more work in handling the fish, but is the most accu- 

 rate method of inventorying fish. 



