A10. Trap mounted on weir

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Figure 1: source ® Marc-Antoine Colleu

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1. Objectives

  • Assess run timing, abundance and composition of out-going or returning migratory fish stocks (including as part of ‘index river’ monitoring programmes to better understand population process over the long term).
  • Obtain data regarding the presence/absence of species).
  • Observe extent of invasive or newly arrived species
  • Screen for the incidence of fish parasites or diseases.
  • Conduct non-lethal sampling, including tagging to track individuals’ biometric measurements (e.g. changes in weight, length etc.) over their lifespan, ascertain the ratio of males to females within a population; collect scale or fin tissue samples for ageing (scales) or other purposes (e.g. genetic analysis), etc.
  • Conduct lethal sampling, i.e. autopsy, to assess pollutant accumulation within the body or causative agents in diseased fish, position in trophic web through stomach content analysis, body condition and accurate ageing through otolith (inner ear) sampling.
  • Sample genitor (parentage) for aquaculture and stocking programmes or screen fish to evaluate the efficacy of those programmes (e.g. migratory salmonids as out-going smolts or returning adults).
  • Assess recruitment of smolt and young migratory salmonids.

2. Method summary

Figure 2 ® Michel Robert, Ressource Innovation; Yann Abdallah, Scimabio Interface.

A metal trap is usually located on a fish migration route to catch adults going upstream or juveniles coming downstream.

Trapping structures may be temporary or permanent depending on the aims and duration of the study, but in all cases are likely to require permissions for deployment from the regulating authorities.

Trap design will vary depending on numerous factors including the target species, nature of the site, available resources and the duration of the study.

2.1 Upstream traps

Usually incorporate some form of inner funnel or V-shaped ‘inscale’ to guide fish into the main body of the trap where they are retained (Fig 1). Upstream traps are often located in a fishway (fish pass) associated with a weir or dam where fish will tend to concentrate as the preferred route of passage (helping to improve trap efficiency) (Fig 2). In the absence such structures, fences or guides may be used in open channels to divert fish into the trap.

2.2 Downstream traps

Downstream traps are most likely to be deployed for the interception of out-migrating juveniles, particularly salmonid smolts. Common trapping methods in this case include:

  • Inclined screen traps: usually deployed on weirs or similar features where water can fall through the screen but downstream migrants are retained and diverted to a bypass channel and holding box. (Similar grid style traps may use vertical fences to divert fish to a holding facility).
  • Rotary Screw Traps (RSTs): floating traps comprising a pontoon mounted mesh-covered conical drum. The drum mouth is directed upstream and is usually positioned where the river flow is concentrated to help maximise interception of downstream migrants (Fig 2). An internal vane within the drum causes it to rotate with the flow and move water and any captured fish to a holding box positioned at the rear of the drum. When used to trap smolts, RSTS are normally deployed as temporary structures for operation during the migration period. They are usually most effective when fished at night, and when not in use a winch mechanism allows the drum to be lifted from the water. RSTs are held in place by ropes or cables and bankside winches depending on the size of river and trap, and nature of the site (e.g. availability of trees or other possible anchor points). As floating traps, RSTs are less likely to be damaged by high flow conditions than fixed structures.

3. Considerations

Figure 3 ® Michel Robert, Ressource Innovation; Yann Abdallah, Scimabio Interface.

For any proposed trapping programme, primary consideration should be given to the welfare of the fish including:

  • the numbers of fish likely to be intercepted by the trap
  • whether the size of the trap can accommodate those numbers without crowding or undue stress
  • the frequency of sampling to avoid overcrowding or prolonged retention times
  • mechanisims for ‘opening’ the trap (i.e. allowing free passage of fish) or removing the trap when not in use
  • the interior layout of the trap e.g. avoiding or mitigating for protruding structures which may damage fish; ensuring mesh sizes or bar spacings are appropriate and will not lead to fish being ‘gilled’
  • the method by which the trap can be checked for the presence of fish e.g. approaches to reducing water flow, if required; use of hand nets, etc.
  • Protocols for sampling fish e.g. whether sampling can take place within the trap or fish will need to be transferred elsewhere; sampling procedures – including use and disposal of anaesthetics; collection of biological information (scales, size; sex, etc.); tagging; other screening, etc.
  • Protocols for release of fish e.g. facilitating recovery from sedation; release location, etc.

Alongside the above questions it is clearly important that the study objectives are carefully considered to identify likely approaches, risks and uncertainties (indeed, whether a trapping programme will provide the best sampling solution).

Where any trapping option is being examined, whether temporary or permanent, it is also important to identify the possible impact of the physical environment (e.g. flows and water borne debris) on proposed structures and, as far as possible, ensure structures are sufficiently robust to meet these demands.

Temporary structures serving as upstream or downstream traps are likely to be particularly vulnerable e.g. at times of high flow or during autumn leaf-fall, when the pressure of water, erosion, build-up of debris or direct impact can lead to damage (even in relatively small river systems). Strategies to try to mitigate for these problems will need to be thought-through and will often require regular checking of weather conditions and the facility to respond quickly to adverse changes. Health and safety requirements will need to be considered for trapping in all conditions and particularly so for the more extreme conditions.

Figure 3 shows the process of lifting a trap to collect fish; the lower portion of the trap remains submerged to retain the fish in water. Following collection, the fish are released into an open container to be netted or driven into another tank. The trap can be used at both day and night.

4. Advantages

  • Most fish passing through can be caught using this method, if it is well-designed (small fish can escape, if an incorrect mesh size is selected).
  • Allows sampling and collection of population data on different fish species.
  • Can be size selective depending on barrier and mesh size.

5. Disadvantages

  • The method is not species specific and can deter targeted fish.
  • It can delay fish migration.
  • It needs a minimum of 3 people to deploy and operate. The number of people required varies due to several factors, including fish density, transport distance and the biometric material collected.
  • Equipment is heavy.
  • Very expensive.

6. Recommendation for method application

  • Selecting the right trap design is essential since poorly constructed traps can directly impact natural fish behaviour (e.g. deter migration) and thus reduce catch rates. For example, bubbles caused by within-structure turbulence may disorient fish and discourage certain species from entering.
  • The design should not allow fish to pass under the trap while the trap is lifted from the water to collect fish caught within it.
  • It is important to have an effective process in place to take measurements and collect data to minimise fish exposure and handling.
  • To avoid deterring migratory fish downstream of the trap keep noise and presence to minimum when checking the trap.
  • Ensure team members are familiar with the trap setup, the process of taking biometric measurements and samples, tagging fish and recording data prior to the field campaign. This will ensure that tasks can be carried out efficiently and with minimal disturbance.

7. Cost

Usually expensive, ranging from £10,000 to £100,000, since the design of the trap must tailored to the site conditions. Costs will vary depending on the river width, the shape of the obstacle/weir/dam and the trap design complexity.

8. Protocol and data analysis

  • Migration dynamic.
  • Cohorts and sex-ratio.
  • All type of analysis related to biological samples and morphometry.

9. References

  1. https://canalrivertrust.org.uk/media/library/7183-cefas-report-river-tees.pdf
  2. https://www.logrami.fr/telechargement/nos-publications/rapports/CR%2520Pi%25C3%25A9geage%25202008.pdf

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