A10. Hoop nets and trap nets

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1. Objectives[edit]

Hoop nets and trap nets are primarily used to study yearly changes in population density and structures. They are used for:

  • Catching fish in marshland, mapping eel migration dynamics (Baras et al., 1994), and trophic movements. However, to ensure data accuracy, they should be coupled with a tagging method to achieve this.
  • Collection of qualitative and relative abundance of fish community and describe habitat association (but not exhaustive) (Hubert et al., 2012).

2. Method summary[edit]

Hoop and trap nets are passive traps that simply catch fish that enter them. They can be placed at different locations according to the study objectives, and fish are caught due to their hiding, exploratory or foraging behaviour. The device is made by a series of rigid hoops (wood, metal or plastic) covered by web netting and composed of several interconnected net cones which funnel the fish forward but make it impossible to turn around. The ‘heart’ is the last part of the cylinder, which can be open and emptied.

These traps are fastened to stakes, allowing them to be used in currents, and are positioned facing downstream to avoid vegetation debris entering into them. They are mainly used in marshland, channel habitats, and water bodies exhibiting low flow, but can be used in medium flowing water if well secured.

The effectiveness of these devices depends not only on the level of fish activity related to environmental conditions, but also their own characteristics (hoop diameter, mesh size, entrance size) (Baisez, 2001). Environmental conditions include temperature, turbidity, water level, current, and season. These factors can have a significant impact on fish size and selectivity.

3. Advantages[edit]

  • Less invasive than active fishing (e.g. electrofishing).
  • The gear can be baited for more efficiency, depending on the species targeted (Stone, 2005), e.g. catfish.
  • The device is lightweight and can be transported, depending on its size, by one or two people.
  • Collection is fast (less than 10 minutes per hoop net).
  • Not harmful for fish, which can be released with no or few injuries.
  • Perfectly adapted to study of yellow eels and not very size-selective if a small mesh size is used.

4. Disadvantages[edit]

  • Can overestimate presence of larger individuals, which can be more mobile and are more likely to be trapped.
  • If the net is fully submerged, mammals and reptiles can be inadvertently killed. It is better to keep part of the heart above the water surface.
  • Eels or tiny fish can escape depending on the hoop net’s configuration.
  • Results can vary considerably between day and night.

5. Recommendation for method application[edit]

  • Use net mesh adapted to fish species and size targeted.
  • Orientate the hoop net parallel to the current direction and attach it securely to an anchor or stake (Portt et al., 2006).
  • Standardise the net placement and its characteristics to compare with other studies and years.
  • Estimate the number of hoop nets needed, depending on the channel length or survey area.
  • Know the arrival time of fish migrations, for example, if targeting eels.
  • Hoop nets can become overloaded when populations are large or large individuals are caught. A daily check is necessary.
  • Catch rates are often higher immediately preceding and during the spawning periods. It declines if water temperatures decrease.
  • Waterproof labels with researcher’s contact information/details are important to avoid vandalism or to be warned if a problem occurs with the net.

6. Cost[edit]

According to the size of the Hoop net, between £150 (3 hoops) and £400 (7 hoops).

7. Data analysis[edit]

  • Analysis can be carried out related to effort: number of fish/hoop net/ hours or per day.
  • Cohorts can be studied, as well as migration movements (for eels) and fish abundance.
  • Escape rate and predation by some species can have influence on data.

8. References[edit]

  1. Baras, E., et al. (1994) ‘Évaluation de l’efficacité d’une méthode d’échantillonnage par nasses des anguilles jaunes (Anguilla anguilla L.) en migration dans la Meuse’, Bulletin Français de la Pêche et de la Pisciculture, (335), pp. 7–16. doi: 10.1051/kmae:1994001.
  2. Baisez A. (2001) ‘Optimisation des suivis des indices d’abondances et de structures de taille de l’anguille européenne (Anguilla anguilla, L.), dans un marais endigué de la côte atlantique : Relation des relations « espèce-habitat ». Thèse « écologie aquatique », Cémagref Bordeaux/Université de Toulouse III – Paul Sabatier, 396p.
  3. 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.
  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’.
  5. Stone, D. M. (2005) ‘Effect of Baiting on Hoop Net Catch Rates of Endangered Humpback Chub’, North American Journal of Fisheries Management, 25(2), pp. 640–645. doi: 10.1577/m04-091.1.