HATCHERY OPERATIONS 83 



however, fish over 3 inches long can outrun the pulled seine, and are likely 

 to escape, leaving a nonrepresentative sample. This problem can be partial- 

 ly overcome by setting the net across, or pulling it into, a corner of the 

 pond instead of pulling it to a straight shore. The uniformity of fish distri- 

 bution is the most difficult aspect to determine. Many species form aggre- 

 gations for one reason or another. A seine might net such a cluster or the 

 relatively empty space between them. It helps to sample several areas of 

 the pond and to average the results, although this is time-consuming, and 

 seines rarely reach the pond center in any case. 



Fish can be concentrated for sampling if the pond is drawn down. This 

 wastes time — it can take two or three days to empty a pond of several 

 acres — and a lot of water. It also can waste a lot of natural food production 

 in the pond. Unless fish have to be concentrated for some other purpose, 

 such as for the application of disease-control chemicals, ponds should not 

 be drawn down for sampling purposes. 



In summary, pond fish should be sampled regularly, but the resulting in- 

 formation should be used for production calculations only with caution. 



Fish Grading 



Fish grading — sorting by fish length — makes possible the stocking of uni- 

 formly sized fish if this is necessary for fishery management programs. Also, 

 it reduces cannibalism in certain species of fish; some, such as striped bass 

 and northern pike, must be graded as often as every three weeks to prevent 

 cannibalism. Grading also permits more accurate sample counting and in- 

 ventory estimates by eliminating some of the variation in fish size. An ad- 

 ditional reason for grading salmon and steelhead is to separate smaller fish 

 for special treatment so that more of the fish can be raised to smolt size by 

 a specified time for management purposes (Figure 32). 



In trout culture, good feeding procedure that provides access to food by 

 less aggressive fish can minimize the need for grading. However, grading of 

 fish to increase hatchery production by allowing the smaller fish to increase 

 their growth rate is questionable. Only a few studies have demonstrated 

 that dominance hierarches suppress growth of some fish; in most cases, 

 segregation of small fish has not induced faster growth or better food utili- 

 zation. In any fish population there are fish that are small because of their 

 genetic background and they will remain smaller regardless "of opportuni- 

 ties given them to grow faster. 



In warmwater culture — and extensive culture generally — fish usually 

 cannot be graded until they are harvested. Pond-grown fish can vary great- 

 ly in size, and they should be graded into inch-groups before they are dis- 

 tributed. Products of warmwater culture often are sold in small lots to 



