Fish Sampling 131 



qiiently, to determine the numbers and species present, he must resort 

 to Hve-trapping techniques. It is seldom necessary to kill these specimens, 

 whether or not their number is large enough to have any significance in 

 relation to the remaining population. 



Reasons for Sampling Fish Populations 



There are several justifiable reasons for sampling fish populations. An 

 adequate sample allows an appraisal of the components of a population, 

 and exposes those segments of it, having sizes and numbers satisfactory 

 for angling. As described in another chapter, the main causes of poor 

 fishing are (1) overpopulation and stunting of desirable species, and (2) 

 an overabundance of undesirable species with a concurrent shortage of 

 acceptable ones. Hence, once the causes of poor fishing have been ex- 

 posed, it is possible to plan a method of improving the population. For 

 example, an excessive number of stunted crappies can be thinned out by 

 partial poisoning; however, if there are overabundant and stunted bull- 

 heads as well, complete elimination and restocking may be necessary. 

 Obviously, a sampling method that will expose only the crappies does not 

 provide a satisfactory diagnosis. 



It is frequently necessary to demonstrate to fishermen and owners that 

 lakes are not "fished out." This is commonly called for in waters close to 

 urban centers where species exposed to heavy fishing pressures may 

 know a frog from a frog "popper" and a worm on a hook from a free one. 

 These "fished-out" lakes are often filled with "wise" fish and fishermen 

 will keep trying to catch them (at the same time receiving health and 

 aesthetic benefits) if the fishery biologist can demonstrate that desirable 

 species are present. 



Regular annual sampling should be done on important impoundments 

 not only to record changes in the relative abundance of species, but also 

 in their length-frequency distribution and their condition from year to 

 year. Fish taken with various sampling techniques should be measured 

 and weighed individually, and scale samples obtained where there is an 

 indication of stunting or of exceptionally rapid growth. These data allow 

 an annual appraisal of the status of all important species. When this 

 information is integrated for several successive years, it shows unmis- 

 takable trends that may call for certain management practices. 



Table 6.1 shows a hypothetical length-frequency distribution for blue- 

 gills in an imaginary lake. In 1955, there were adequate numbers of large 

 bluegills belonging to the 1953 year class ( determined from scales ) . These 

 fish showed an average Index of Condition (C) of 8.0 or higher which 

 demonstrated that the fish were relatively fat. In collections of 1955 and 

 1956 no excessively large year class more recent than 1954 was present 

 although this year class was fairly well represented. However, the collec- 



