7603 - Laserfiche WebLink

Home Browse Search

5 Due to varying flow level throughout the year, sampling techniques had to be varied as well. During pre-runoff and runoff river flows from April to June, sampling was conducted primarily using a flat-bottomed john boat powered by an outboard jet unit for radio tracking and sampling gear transport. Another similar john boat outfitted with electrofishing capability was also used when flow conditions warranted. A "block and shock" technique was developed in conjunction with CSU Larval Fish Lab personnel familiar with radio tracking work on the Yampa River. The essence of the technique was to place either trammel or gill nets in river habitats that appeared suitable for the target fish species and subsequently create some disturbance in these habitats to. initiate fish movement, thereby increasing the capture potential of the passive net. In practice, this meant setting nets in low flow habitats and using electrofishing to either capture target fish species outright or drive fish into the nets. This technique appeared particularly effective in the spring when rising river flows created side channel and tributary backwaters that could be completely blocked from the main channel with nets. Using the jet boat, powered manually by oars if river flow permitted, a net was extended across the mouth of the backwater. Prior to complete blocking of the backwater, the electrofishing boat would be passed into the backwater channel with the backwater mouth blocked immediately afterwards with the net. This was all performed manually if possible to eliminate engine noise from disturbing fish prematurely. Northern pike, channel catfish, and Colorado squawfish appeared quite susceptible to this "block and shock" approach judging from sample sizes compared to continuous shoreline shocking. Nets used in this process included 30.5 m trammel nets, l.8 or 2.4 m deep, with 150 mm mesh outside walls and 25 mm inner wall; or a 38 m Monofilament gillnet, 1.8 m deep with five experimental mesh panels ranging from 12.7-44 mm mesh sizes. The electrofishing boat was equipped with a Coffelt VVP-15, 5000 watt generator, and 25 cm diameter stainless steel sphere anodes suspended from shielded dropper cable made of 9.5 mm steel cable. Output power was set initially at the highest level practical for stunning fish (200-400 volts, 2-10 amps) at the head end of a given backwater, usually some distance from the blocked mouth. Output power was decreased near the blocking net such that voltage read below 100 v and amperage was less than 0.5 amps. During decreasing run-off flows and post run-off flow levels, use of the electro fishing boat throughout the river reach was progressively limited, and ultimately impossible. Backwater habitat was eliminated and suitable habitat for net use was restricted to select pools and large eddies. Net use was also limited by the presence of drifting masses of Cladophora in the river, which invariably clogged the net after 30 min or less. Access to a given river reach was practical during the late summer and fall seasons only by canoe. Nets would be set in large pools or eddies under suitable conditions and while the nets were set, crews would attempt to capture fish via angling from the shore or canoe. Excessive noise would be made using the canoe hull while floating in the pool/eddy habitat in an attempt to move fish into the nets. All gamefish species captured by these means were enumerated by species, measured for total length and weighed. Northern pike collected at Craig or upstream were tagged by inserting a Floy tag near the base of the dorsal fin.