A10. Gill net

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1. Objectives

Gillnets are mostly employed by professional fishermen to catch fish to sell. Few studies use gillnets in rivers, but they can be used for:

  • Fish cohorts and length/weight ratio of fish.
  • Fish or depth distribution.

It is not known whether this technique has ever been used for assessing river restoration.

2. Method Summary

The method is mostly used in lakes, reservoirs, or marine water. A gillnet is a passive net consisting of synthetic monofilament nylon (although nets made from biodegradable materials) held at the top by a float line and at the bottom by a weight (Portt et al., 2006). Fixed points in the net are attached to one or two weights and identified at the surface by a buoy. The depth of the net can be adjusted by buoyancy setting. Fish are removed from the net on to a boat a few hours after the net has been set, or less depending on the survey. Mortality rates for this method vary depending on species and habitats, but is typically high because entangling obstructs fishes’ gills. Gillnets are highly selective because of a strong relationship between mesh size and fish size. Its effectiveness depends on its catch ability but also its ability to retain fish after capture.

3. Advantages

  • Easy to use – a small net is manageable by one person.
  • Good range in the size of fish captured (Petriki et al., 2014).
  • Range of ways to set, depending on species and habitats (Hubert, Pope and Dettmers, 2012).

4. Disadvantages

  • Hard to use for quantitative stock assessment (EIFAC, 2010) or benefits of river restoration work.
  • Catch ability related to experience of surveyor.
  • Not effective for sedentary fish.
  • Lethal or very damaging for fish, except when used for very short time periods.
  • Seasonality can make a huge difference in fish movement and density, impacting catch rate.
  • Diving birds can be caught in the net.

5. Recommendation for method application

  • Mesh size and the number of layers (1, 2 or 3, called a ‘gang’) are critical for effective fish capture. Choice of mesh size (or specific combination of gang and mesh size) is related to the targeted fish.
  • Compare the size of the targeted fish to previous studies to get information on ideal mesh size, colour, and entanglement probability.
  • Length of the net should be adapted to river size and placed to avoid strong currents, logs and branches, which can distort the net and cause damage.
  • Strong currents and perpendicular placement should be avoided.
  • Autumn is not the best season as leaves can be caught in the net, making it highly visible. Light currents with low turbidity are also undesirable conditions.
  • Adapt net characteristics to the targeted fish. Mortality varies widely depending on the mesh and net size, but also species and ambient conditions (temperature, dissolved oxygen).
  • When fish do not die, they are usually injured (scales and mucus loss, injuries on body and gills due to fighting), which increases the risk of infection and disease.
  • Some researchers associate catchability with swimming capacity of a species or individual.

6. Costs

Not expensive. Net prices range from £50 to £300. Main costs are related to the number of people, working time and boat equipment.

7. Protocol and data analysis

Sampling effort is an important data field. Gillnet effort is calculated by multiplying the length of net by the length of time it was set. The effort is quantified by catch unit per meter/hour or per 100 m/day. Depending on the area and duration of the survey, a localised fish depletion can be noted. Factors like the number of fish (equipment saturation), and caught leaves or branches can influence catchability by making nets more visible. Time of day influences activity and contributes to differences in catch rates and species richness. Dawn and dusk are known to be periods during which many fish species are more active. Normalising catches from different mesh sizes and soak times can be complex. Statistical application and standardisation of this method seems to be very tricky and uncertain. http://www.fao.org/fishery/static/eifaac/wpfmfw/DraftGuidelinesMonitoringFishFreshwaters.pdf

8. References

Note: A scarce number of articles are available to give examples of this techniques use in lotic freshwater environments. Used mostly in lakes and reservoirs, articles are mainly related to gillnet efficiency through analyses of mesh size for a dedicated species of fish.

  1. EIFAC (2010) ‘Guidelines for fish monitoring in fresh waters DRAFT report’, p. 30. Available at: ftp://ftp.fao.org/FI/DOCUMENT/eifac/wpfmfw/DraftGuidelinesMonitoringFishFreshwaters.pdf.
  2. Hubert, W. A., et al. (2012) ‘Passive Capture Techniques’, in D. L. Parrish, and T. M. S. (ed.) Fisheries techniques, 3rd edition. 3rd edn. Bethesda, Maryland: American Fisheries Society, pp. 223–265.
  3. Petriki, O. et al. (2014) ‘Gillnet selectivity for freshwater fish species in three lentic systems of Greece’, Journal of Applied Ichthyology, 30(5), pp. 1016–1027. doi: 10.1111/jai.12476.
  4. Portt, C. B. et al. (2006) ‘A review of fish sampling methods commonly used in Canadian freshwater habitats. Canadian Technical Report of Fisheries and Aquatic Sciences 2604 Fisheries and Oceans Pêches et Océans Canadian Technical Report of Fisheries and Aquatic Sciences’