Fish Sampling 137 



ability to reproduce under current conditions. However, their absence 

 from waters containing adults does not necessarily mean that these 

 poj^ulations cannot reproduce and are "on their way out." 



(3) The production of dominant year classes of fish may be recorded first 

 through minnow seining. 



(4) Some indication of overpopulation and stunting may be gained from 

 minnow seining, although a larger seine is much more useful for this. 



Spot Poisoning. One of the more satisfactory methods of obtaining an 

 unbiased sample of the fish population of a large lake is to poison a 

 bay or lagoon connected with the larger body of water by a narrow 

 channel. If the bay is not too small or too shallow, it will probably contain 

 a population fairly comparable (in composition) to that of the larger 

 lake,^^ particularly if the fish are warm-water species and the bay is 

 treated with the chemical during the night when they are moving about 

 in the shallower parts of the lake. 



A seine or block-off net ^' deep enough to reach from the top to the 

 bottom of the bay is set across the channel connecting the bay with the 

 main lake. Fish disturbed by the chemical treatment or frightened by 

 the noises of boats are prevented from escaping by this seine. Following 

 its placement, a canvas strip approximately its same depth can be staked 

 across the channel adjacent to the seine. This canvas strip prevents the 

 circulation of rotenone-treated water from the bay into the main part 

 of the lake. The bay is then treated with derris or cube powder, 5 per 

 cent rotenone, or emulsifiable rotenone with a dosage of sufficient strength 

 to kill all of the fish trapped in the bay. These poisoned fish are picked 

 up, counted, measured, and weighed, as in a regular census. Following 

 the rotenone treatment the seine and canvas strip are left in place until 

 the rotenone has disappeared from the water, so that no fish are killed 

 outside of the bay. 



Spot poisoning may be done in open water by treating the circumfer- 

 ence of an arbitrary one-acre circle and then applying the rotenone inward 

 throughout its area. However, work in open water is not as satisfactory as 

 in an isolated bay because in the former instance the treated water may 

 move downwind out of the original circle, causing fish affected by the 

 rotenone to disperse beyond the original area of treatment. In any case, 

 it is well to pick a time when wind velocity is at a minimum to prevent, 

 as much as possible, the mixing of treated and untreated water. 



As the behavior of fishes is influenced by seasons, several spot treat- 

 ments of the same bay, made several weeks or months apart, will give 

 a better population sample than a single spot poisoning. 



Boat Shocking. In 1949, an AC row boat shocker was developed for 

 the purpose of sampling fish in lakes. ^^ This apparatus is useful for obtain- 

 ing quickly a fish sample from lakes and ponds that are sufficiently 



