A10. Trawl net

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1. Application and Purpose

Though more associated with marine environments, trawl nets can be used in estuaries or large freshwater habitats for:

  • Catching fish of a variety of life-stages.
  • Estimating fish abundance.
  • As part of mark recapture programmes.
  • Monitoring of long river reaches.

2. Method Summary

The trawl net consists of one or more large conical net(s) towed by a boat. There are two basic types of trawl net. The beam trawl has a rigid frame around its mouth with a weighted, shaped base to enable the net to be towed along the river bed and is used exclusively for benthic species. The otter trawl has a weighted footrope and buoyant headrope around the mouth and is used in conjunction with otter boards which use the force of the water against them to keep the mouth of the net open as it is being towed. The fishing depth can be adjusted by the boat speed (between 1.5 and 4.5 knots, depending on environmental conditions), length of warp (towing ropes) and size and weight of otter boards. Otter trawls can even be used to catch surface-living species; exact specification for a trawl net for any given situation will depend on species and life-stage sought. While trawl nets are usually associated with commercial fishing, the method may be suitable for assessing freshwater and migratory fish in large rivers and estuaries.

3. Advantages

  • Time efficient; can sample very large areas in few hours.
  • Can be conducted alongside commercial trawl catches (in estuaries) to monitor fish stocks.
  • Can provide defined sample area (mouth of net area x distance covered)
  • Can provide a larger sample than any other method due to the area able to be fished in a relatively short time.

4. Disadvantages

  • Requires a boat of size and power suitable for size of net and conditions.
  • Can only provide minimum estimates of fish abundance per unit area or per unit effort.
  • Larger trawls require a winch to haul in.

5. Recommended method

  • Inform choice of survey timing and location through consultation with locals, e.g. fishermen.
  • Keep materials (e.g. boat, net) and parameters (e.g. speed, fishing depth) consistent for each run.
  • Split the area into several zones and plot transects using a GPS.
  • Record environmental conditions (e.g. temperature, turbidity, tide, salinity, etc.) and survey parameters (e.g. depth of trawl, transect line locations etc.).

Keep fishing effort constant in relation to distance not time, since weather conditions (e.g. tide, waves etc.) can cause distance fished per unit time to vary.

6. Costs

Can have high costs associated with boat hire; prices will vary depending on the survey effort. Trawl nets vary in expense but for freshwater river applications, small beam and otter trawls can be purchased for a few hundred pounds.

7. Data analysis

Sampling effort can be counted by estimating fish density related to catch per unit effort – time to complete the transects – (Fischer & Quist, 2014) distances, or surface covered by the net.

8. Protocols

See (Fischer & Quist, 2014)

9. References

  1. Brosse, L. (2003) Caractérisation des habitats des juvéniles d ’ esturgeon européen, Acipenser sturio, dans l ’ estuaire de la Gironde : Relations trophiques, hiérarchisation et vulnérabilité des habitats. Université Paul sabatié, CEMAGREF.
  2. Butcher, A. et al. (2005) ‘A comparison of the relative efficiency of ring, fyke, fence nets and beam trawling for estimating key estuarine fishery populations’, Fisheries Research, 73(3), pp. 311–321. doi: 10.1016/j.fishres.2005.01.014.
  3. Carr-Harris, C. et al. (2015) ‘Juvenile Salmon usage of the Skeena River estuary’, PLoS ONE, 10(3), pp. 1–21. doi: 10.1371/journal.pone.0118988.
  4. Fischer, J. R. and Quist, M. C. (2014) ‘Gear and seasonal bias associated with abundance and size structure estimates for lentic freshwater fishes’, Journal of Fish and Wildlife Management, 5(2), pp. 394–412. doi: 10.3996/082013-JFWM-054.
  5. Jůza, T. et al. (2010) ‘The influence of the trawl mouth opening size and net colour on catch efficiency during sampling of early fish stages’, Fisheries Research, 105(3), pp. 125–133. doi: 10.1016/j.fishres.2010.03.010.
  6. Lauronce, V. and Rochard, E. (2019) ‘La coopération internationale pour la sauvegarde de l ’ esturgeon européen, Acipenser sturio Contexte’, in.