RETENTION OF ADULT SALMON 



The objectives in holding pond design 

 during the retention period are: first, to pre- 

 vent the fish from incurring further self-inflicted 

 injuries during the holding period; second, to 

 induce tissue repair in previous injuries; and 

 third, to reduce the incidence of disease. 



Holding pond design 



The recommended design for holding 

 ponds is shown in figure 1. Ignoring the trap- 

 ping facilities, it consists of a rec tangular pond 

 150 feet long, 50 feet wide, with a maximum 

 depth of 5 feet, sloping sidewalls, an undershot 

 diffusion for water inflow, and a picket "V" at 

 the outflow. Several of these features are de- 

 signed to prevent self-inflicted injury. 



Prevention of self-inflicted injury: -- 

 The undershot diffusion panel introduces the in- 

 flowing water at the bottom of the pond 20 feet 

 from any obstruction . Salmon rarely jump at 

 such an inflow and then only if the quantity and 

 velocity of the inflow is such as to produce a 

 marked upwelling at the water surface . Under 

 such circumstances the fish may jump toward 

 the upwelling, but they would have to jump a- 

 cross the 10 feet square diffusion and an addition- 

 al 20 feet before striking an obstruction . No 

 fish has ever been observed to make such a jump 

 in this type of holding pond. In fact, with 5 feet 

 of water over the diffusion grating and 35 cfs of 

 inflow there is so little surface agitation that the 

 fish do not jump in this area . The tendency is 

 for the fish to sound and nose the diffusion grate 

 but never to the extent that visible injury is in- 

 curred. The undershot diffusion, properly 

 placed, eliminates all the dangers of self-in- 

 flicted injury normally encountered in vertical 

 picket inflow weirs . 



In addition to fighting the inflow, salmon 

 in confinement will, on occasion, make a series 

 of leaps . If the water at the edges of the pond is 

 deep the fish may jump out on the bank and either 

 injure itself in its struggle to return to the pond 

 or be trapped and die . Sloping the banks so that 

 there is a 1-foot rise in 3 feet appears to provide 

 sufficient warning so that the fish diverts from 

 the shallow water . Salmon also have a tendency 

 to jump at straight side walls; sloping banks 



avoid this tendency. In this type of holding 

 pond, the diffused water inflow and sloping 

 banks have successfully eliminated the problem 

 of self-inflicted injury. 



Factors influencing tissue repair and 

 disease inhibition: - -The problems of tissue re- 

 pair and disease inhibition are related. In fish 

 with open wounds it is a race between healing 

 and the invasion of fungus ( Saprolegnia parasitica) . 

 Fish and Rucker (1953) demonstrated that the in- 

 cidence of columnaris disease ( Chondrococcus 

 columnaris ) was significantly higher in injured 

 fish. Both these diseases become more virulent 

 at water temperatures over 60° F . . Low water 

 temperatures, then, become a significant factor 

 in the successful retention of adult salmon be- 

 cause of the retarding effect on disease develop- 

 ment. At high water temperatures, however, 

 some other factor must be employed to reduce 

 the probability of infection . 



Salmon seek the deepest water available 

 to them when in confinement. Such water in a 

 holding pond has the slowest current and the low- 

 est rate of interchange . With low water inflows 

 the water tends to stratify and the flow becomes 

 laminar with the deepest areas the most affected. 

 These deep areas, then, without an adequate inflow, 

 become essentially dead and provide excellent 

 locations for the concentration of waterborne 

 disease organisms. The situation is further ag- 

 gravated by the collection of diseased and injured 

 fish in these areas . At low temperatures when 

 columnaris and fungus are of low virulence the 

 resistance of the fish is usually adequate to com- 

 bat the infection but at water temperatures of 

 65° F. or higher the situation can become explosive 

 in nature with the injured fish succumbing first 

 but losses not necessarily confined to this group. 



Effect of current velocity on disease in - 

 hibition -- A water inflow of 35 cfs is recommend- 

 ed for the holding pond shown in figure 1 if water 

 temperatures above 60° F. are to be expected dur- 

 ing the holding period. This pond is 175 square 

 feet in cross section and 35 cfs of inflow provides 

 an average current velocity of 0.2 fps. Actually, 

 of course, such a current does not exist immed- 

 iately adjacent to the bottom but it has been 

 demonstrated that this average velocity is suf- 

 ficient to provide the pond with excellent disease 

 inhibition qualities. 



