1962). It appears paradoxical but true that the 

 conditions of a natural environment do not add 

 up to the ideal situation for the life of a bottom 

 inhabiting mollusk such as the oyster. Location 

 on rocks and undenvater structures above the 

 mud line appears to offer a more favorable environ- 

 ment. 



Determination of the amount of silt setthng 

 on the bottom can be made with a mud trap of 

 the type shown in fig. 372. The trap consists 

 of a metal funnel riveted to a metal frame and a 

 quart size container (fruit jar) screwed to the 

 funnel. The trap is set on the bottom for the 

 desired length of time, then carefully lifted, and 

 the amount of sediment settled over the area of 

 the funnel and inside the jar is measured. 



Data obtained with a mud trap of the type 

 shown above measure the rate of settling of sedi- 

 ments on the bottom. One should bear in mind, 

 however, that a certain portion of the sediment 

 deposited during slack tide may be washed away 

 as the current velocity increases with tidal 

 changes. Furthermore, mud already settled on 

 the bottom may be stirred by wave action and 

 resettle on an adjacent area. The actual accumu- 

 lation of sediment can be measured with a mud 

 board of the type shown in fig. 373. A wooden 

 stake 36 inches high supports a flat board 18 

 inches long and 6 inches wide, mounted hori- 

 zontally, and the stake is forced into mud so 

 that the board is level with the bottom. This 

 type of trap can be used conveniently on tidal 

 fiats. 



A trap to be used below low water can be made 

 of a flat board ji square yard in area. To prevent 



CONE FUNNEL 



'2 STRAP IRONS FORMING 4 LEGS 



Figure 372. — Mud trap for collecting sediments settling 

 from water. 



10" 



36" 



-18"- 



<-Z"-^ 



3/4" 



Figure 373. — Mud board for measuring the accumulation 

 of sediment on tidal flats. 



the loss of sediment the board has slanted borders 

 about 4 inches wide, projected above its upper 

 surface. The trap is set on four short legs; its 

 surface is ruled in squares to facilitate the measure- 

 ments of the area covered with sedunent. Tiie 

 thickness of the sediment is measured with a 

 ruler. Since the board cannot be lifted without 

 disturbing and losing the accumulated sediment, 

 observations must be made by a diver. 



An indirect estimate of the amount of material 

 in suspension can be made by using a Secchi 

 disc and recording the depth of extinction of white 

 color. The results are affected by the visual 

 acuity of the observer, illumination, and the 

 condition of the sea surface. 



The amount of suspended material in a sample 

 of water can be determined with a simple tur- 



414 



FISH AND WILDLIFE SERVICE 



