Fishing nets - Active fishing gear
Trawls - pulled nets
Trawls belong to active gear for sampling all age and size classes of fishes in whole water column.
Trawls can be used in the open water of lakes, reservoirs and rivers.
The net is usually pulled by a boat using towropes. The net moves in horizontal position in the water column.
Trawls can be divided into two groups:
1. Bongo nets - fry trawls with fixed frame
2. Adult trawls - trawls without fixed frame, the mouth is kept open by otter boards, towing by two boats or by spars
Bongo nets, fry trawls
Bongo nets and fry trawls are used for sampling the earliest life stages of fishes (eggs, larvae and fry) and also for larger water invertebrates in the open water. These trawls can be used in all depth layers of the water column with some modifications.
Bongo nets and fry trawls consist of funnel-shape netting with mesh sizes ranging from 1 to 6 mm. The mouth opening can be of square or round shape. The trawl is supported by floaters and by adjusting the length of the rope between the floaters and the trawl the desired sampling depth could be retained. Iron sledges were found to be the best weights for trawling close to the bottom because sledges also prevent the lower beam of the trawl from brushing against obstacles at the bottom.The success of trawling is derived from several basic rules:
- the speed and power of a boat used for trawling has to be relevant to both trawl and fish sizes.
- trawl size
- lengths of towropes - trace of the trawl should not follow trace of the boat
- correct position (balance) of the trawl in the water column
Sampling trace can be controlled by GPS receiver. Sampled volume is measured by flowmeter at the trawl mouth. Recommended speed of trawling is approximately 1 m.s-1.
Material
Bongo nets and fry trawls consist of fixed frame and funnel-shape nets.
The frame is round, square or rectangular.
Frames up to 1 m are made in one piece. Larger sizes of frames can be detachable in order to make transportation easy.
The net is connected to the one-piece frame by binding. In detachable frames, the net is supplemented with a double hem from tough cloth and the frame is framed up to this hem.
Trawl net closing
The lower part of the trawl net can be supplemented with a funnel. The funnel is a small funnel-shaped net stitched in the border of the trawl belly and the cod-end. When the tow is stopped or slowed down, the funnel hangs down and prevents fish from escaping from the trawl cod-end
Proven, reliable closing of the trawl net with mesh sizes 1 - 3 mm is accomplished by a metallic filter calyx, where the catch is gathered after removing the trawl from the water. Disconnection of the calyx from the net is provided by a bayonet mechanism. 
The cod ends of the trawls are made from the netting of the same mesh size as the body of the trawl or the mesh size can be smaller. The catch is removed from the cod end by unbinding a closing string.For scientific research we produce bongo nets and fry trawls in well-proven standard sizes:
| Material | Mesh size | Frame size and shape | Length of the net |
|---|---|---|---|
| Tulle - PES | 1,5x2mm | 0,9 m round | 3m |
| Netting PA 210/6 | 6mm tělo | 1,5x1,5m detachable | 4m |
| Netting PA 210/4 | 4mm sběrný válec | 2x2m | 5m |
| 6/4mm, sběrný válec | 3x3m | 6m | |
| 6/4mm, sběrný válec | 4x2 m, benthic | 6m |
Trawl with fixed frame and the funnel - detachable construction
Sběrný válec na síťovém tralu / Napojení sběrného válce / Tral s lyžinami
Research trawler used for sampling with trawls
References:
Čech, M., Kratochvíl, M., Kubečka, J., Draštík, V. & Matěna, J. (2005). Diel vertical migrations of bathypelagic perch fry. Journal of Fish Biology 66, 685-702.
Jůza, T., Kubečka, J. 2007. The efficiency of three fry trawls for sampling the freshwater pelagic fry community. Fisheries Research 85, 285-290
Adult trawls, otter board trawls
Trawls without fixed frames are kept open using two boats towing, by otter boards or by spars.
These trawls can be used in all depth layers of the water column from the surface down to the bottom.
Sampling depth is adjusted by the length of line between the trawl and floaters and by weights.
Developing adult trawls is still in progress and net sizes and shapes need to be standardized. The development is carried out with close cooperation with scientists from the Biology Centre AS CR, v.v.i, Hydrobiological Institute.
Testing trawl with otter boards
Trawling with trawl with otter boards
