7029 - Laserfiche WebLink

Home Browse Search

-3- after that described by von Geldern (1972). It measured 3.05 m (10 ft) square at the mouth, 15.24 m (50 ft) long with bar mesh net tapering fra. 20.4 an (8 in) in the throat to 0.32 an (1/8 in) at the cod end. A pair of depressors and hydrofoils, attached at the corners of the trawl mouth, functioned to hold the net open while fishing. Galvanized wire rope cables (1/8 in) running fr(n each winch were used in deploying and retrieving the trawl. A standard tow was developed and used to permit consistent sampling and replication. During each standard tow the boat was operated at 1100 rgn (1.6 m/sec or 3.6 mph) while 45.72 m (200 ft) of cable was played out and innedi_ately retrieved. Average volume of water sampled was 8,178.44 m3, and maximum depth fished was about 10.7 m (35 ft). The oblique tow allowed equal sampling of the water column from the surface to maximum depth, rather than sampling shad from any one depth. For consistency, sampling was done at night when shad were distributed in a dense and uniform pattern in the epi- limnion, rather than grouped in schools as found during the day (Houser and Dunn 1967, Netsch et al. 1971). Sample nights were selected during the period between new moon and first quarter to ensure dark nights and eliminate vari- ability caused by moonlit nights. Three trawling transects were selected to sample lake areas near the dam, midway up lake, and near the Colorado River inlet (Figure 1). Wahweap Bay was sampled two nights per month, with Bullfrog and Hite sampled one night per month. Wahweap, Bullfrog, and Hite were sampled on consecutive nights to allow comparison under approximately similar times and conditions. Four standard tows were made per sample night. Lighted bouys, permanently fixed in position, were used to mark trawl transects.