A10. Seine net

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Seine netting in a lake. © Marc-Antoine Colleu

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1. Objectives

Seine nets can be used for several purposes:

  • Qualitative assessment of fish community.
  • Relative fish abundance (summary across species) in a river reach or specific habitats.
  • Sampling fish population for tagging.
  • Netting for fish fry.

2. Method summary

Figure 1. Wrap around (also known as drag-down) seine netting begins on a lowland drain. Four staff members on the bank are pulling the heavily leaded driving net (blue) via the head-line, whilst two staff in the margins gently draw along the lead-line. Source: Environment Agency.

Seine netting is an active sampling method that is carried out to catch fish. A range of freshwater bodies can be inventoried with seine nets but the technique is most useful and effective on large rivers and lakes. Small, shallow or fast flowing water bodies are better sampled using alternative techniques such as electric fishing.

A Seine net is principally constructed from a rectangular panel of fine but strong mesh netting and may be between 5 meters and over 150 meters long. The depth of net is similarly varied, but a net at least 1.5 times the depth of the water to be sampled is generally considered optimal. Various mesh sizes are also available, the Environment Agency frequently use a 10mm knotless mesh, which is soft and helps minimize damage to fish. A series of corks attached along the upper edge or ‘head-rope’ hold the net vertically in the water column, whilst the bottom edge, the ‘lead-line’, is weighted to maintain contact with the channel substrate. Various weighting options are available with lead loading required to maintain contact increasing commensurate with depth, flow and method used.

Various seine netting techniques exist, but all essentially involve surrounding the target area with a wall of net, extending from surface to substrate which can be withdrawn to concentrate fish within into an ever decreasing area, eventually forming a pouch known as the ‘bag’ which will contain the catch. Although seine netting appears, and is, a simple technique, the skill needed by a team to ‘work’ the seine net and use the method efficiently is considerable, a snag, tangled net or lifted lead-line could allow fish to escape and the whole operation end in failure.

The retrieval process begins by bringing the two ends of the net together at the point where recovery will occur, ideally this area should have a gently sloping bank is present and adequate space for the operation to occur. The net is withdrawn by hauling on the head-line whilst a team member in the margins also gently draws in the lead-line and this may involve ‘hand over hand’ pulling or, if particularly heavy, by gripping the headline and walking a few steps backwards; either way the net should be carefully stacked as it is withdrawn with headline and corks on the outside and lead-line in the middle. Once the net and cork-line is almost ashore, focus should turn fully to the lead-line and it is crucial at this point to keep it from lifting from the bed. The remaining lead-line is then drawn in, alternating between either sides of the net until fully ashore, the net will now be sealed and fish will be forced into the back of the seine creating a ‘bag’. The net is held in the water and fish are removed with hand nets and placed in aerated containers or floating keep cages, the latter being a better choice as they reduce the risk of thermal shock and water quality deteriorating within an enclosed container. If an unexpectedly large catch is made then it is sometimes possible to stake out the seine net in the margins and use it as a large keep cage, processing fish straight from the catching net itself.

Careful management of the net during retrieval is vital for the efficiency of a netting operation and the net should always be neatly stacked, leads central with corks on the outside, as it is hauled in and as it leaves the water. Although this may seem obvious, it is surprising how quickly a net may become tangled, twisted or stacked on top of itself, wasting time and effort whilst these issues are addressed before the net can be retrieved or redeployed.

If large numbers of fish or macrophyte is present within the seine ‘bag’ then it will be useful to have team members hold the cork line up to stop the weight of that contained within the net from drawing them under allowing fish to escape and if a large amount of silt is present then is important not to draw the net in too far following the lead line being ‘sealed off’ as this may concentrate fish and silt together in the bag and potentially lead to mortality as their gills become clogged with fine sediment. It should also be remembered that the seine netting may not be an appropriate technique during particularly hot weather, on slower flowing channels water quality may already be sub optimal and could deteriorate further if sediment is agitated by dragging seine net leads through a soft substrate. Lakes are often better sampled during the cooler months when water quality and fish stocks are more resilient. It is suggested that it would be wise to sample water quality, particularly temperature and dissolved oxygen, prior to any sampling via seine net occurring.

3. Seine netting techniques

Figure 2. ‘Wrap around’ seine netting on a lowland drain showing layout of the stop nets (red), driving net (blue) and the catching net (yellow) which is laid across the channel and parallel along the bankside. Source: Environment Agency

Seine net techniques include (EIFAC, 2010):

  • Wrap-around
  • Stop-net netting
  • Isolated area netting
  • Netting for fish fry
  • Sampling marginal communities

3.1 Wrap-around

The Wrap-Around or ‘drag-down’ technique uses four seine nets, two are located at upstream and downstream end of the sampling area as ‘stop-nets’ to limit fish movement. A heavily leaded ‘driving-net’ is also laid across the river at the upstream limit and a catching-net, generally three times the width of the river, is laid across the river at the lower limit with the excess net lain out parallel along the bankside margin Figure 1.

Members of the survey team are positioned on each bank to draw the driving net along the channel using long ropes attached to the headline and two people walking in the margins, if operator safety and water depth does not preclude this, gently drawing the lead line along, again using ropes, to stop the lead line lifting as pressure on the net builds Figure 2. The net will form a U shape behind those pulling the net and this should lie roughly central in the channel and ideally with two meters of cork-line will be running parallel to the bank and tight in the margins to ensure a good seal and stop fish escapement. The lead line must maintain contact with the substrate to stop fish passing underneath, whilst the head-rope should stay above the surface to avoid fish escaping over the top. Carp and pike will regularly jump the headline of even a well handled seine and this unfortunately cannot be avoided.

When the driving net almost reaches the catching net it should be wrapped around the back of the driving net and sealed off. The driving net can then be removed and stacked leaving the fish contained within the catching net, which can then be retrieved in the usual manner. The captured fish are then weighed, measured and counted. At this point the EA will retain the fish for processing, but some may choose to utilise a mark and recapture technique. The catching net and driving net are then reset and the process is repeated. The number of fish captured (or re-captured dependant on methodology used) in the second trawl gives an indication of netting efficiency. This technique should only be used in waterbodies with minimal flow velocity else the driving net be pushed into the wrap or stop net, tangling the two and making removal of fish and retrieval of the nets difficult.

3.2 Stop-net netting

Figure 3. Setting a seine net within stop nets on a large lowland drain. Source: Environment Agency

Stop-Net Seine Netting utilises a single long seine net that is set between two stop-nets placed at the upper and lower extent of the studied river reach. This technique is commonly used by the EA on our larger lowland watercourses (30-100 meters wide) where the width of the river channel precludes the use of wrap around netting. Stop nets are hand hauled across the river channel on long ropes whilst the longer ‘catching’ seine net is set by boat to encircle the survey area. The catching net is best deployed over the bow of a punt that is reversed around the sample area using an outboard motor. Care should be taken to select a stable boat with sufficient freeboard to safely carry the weight of a wet seine net plus crew and a long-shaft outboard should also be used, weight from the net and staff in the bows can lift the stern and the sometimes the prop of a short shaft engine out of the water.

Whilst the momentum of the boat will help to feed the net out, two team members in the bows also assist by pitching the net over the bow and making sure it does not become snagged. It is imperative that the boat has no protruding bolts, rivets etc. or the seine will invariably catch upon these and making the setting process extremely difficult and those team members feeding the net out should also be careful to avoid themselves becoming snagged on the net and being pulled overboard by the momentum of the boat. Excellent communication between the boats skipper and the team, as well as life jackets, are essential for this job. Once set the catching seine is hauled in by hand and the fish are removed. The seine net is then reset and the operation repeated. Figures 3. Figure 4 shows stop-net seining utilised at the limit of what is practical and achievable on the Relief Channel in Norfolk. These surveys require 100m stop nets, a 200 meter seine net and specialist equipment to transport and deploy it. Seine netting on this scale needs a team of at least six people and the effort involved should not be underestimated, a team can haul almost 3km of heavy seine during a single survey.

3.3 Isolated area netting

The Isolated Area method is used on rivers greater than 90 m wide, where the stop-net seine technique becomes increasingly impractical. This method cannot be employed in anything but minimal water velocity. The survey area is encircled with a pair of 100m+ seine nets that are laid out from a boat. The inner catching net is drawn in normally and the captured fish removed and processed. The catching net is then reset and the procedure repeated. The outer encircling net can then be drawn in at the end of the survey and any additional captured fish removed.

3.4 Netting for fish fry

The sampling location for the netting of fry is very important. Fry are normally found in channel margins and different species often inhabit specific microhabitats. A seine with a fine knotless mesh of 3 – 5 mm is set in a semicircle from the bank and drawn in. Electrofishing will be more suitable in weed covered locations, as submerged vegetation may lift the net and allow fish to escape or obstruct the fine meshed of the seine making retrieval and extraction of the fish a matter of some difficulty and risk to the fish captured. Particular care should be taken of these young life stages with are extremely fragile and susceptible to damage and scale loss.

4. Snagging

It is worth spending a little time on a major issue associated with this survey technique as no matter how diligent the prior inspection of the survey area, and it is always advantageous to thoroughly assess the area prior to putting a net in the water, seine nets will occasionally become stuck fast. This may be due to the apparatus snagging on a submerged structure or perhaps the net has ‘dug in’ to soft sediment or the top of a gravel bar etc. The EA now use acoustic survey techniques before netting new venues and this has saved deployment of nets around objects as diverse as sunken trees, quarrying apparatus and, more recently, a submerged Land Rover.

There are various methods that maybe attempted to free the net, each of which have advantages and disadvantages and the following text is not a definitive method of retrieving a snagged seine, merely details the initial steps that EA survey staff will attempt upon becoming hung up on an obstruction. It goes without saying that a thorough risk assessment should be made prior to any recovery effort.

The first method attempted will generally be a straight pull with all members of the netting team standing in-line and hauling the corks only, this is hard work but will sometimes pull the net free; however care must be taken as too much effort will risk tearing the net and losing the fish within. If a straight pull is ineffective, then the large and stable punt used to set the net may be utilised to free the snagged apparatus. The net must be first tensioned via hand-hauling on the head-line from the bank and this will generally cause the corks to dip underwater in the general vicinity of the snagged area. The net can frequently then be observed pulling taught down towards the obstruction and if grasped the boat can be manoeuvred until the net is pulling vertical in the water column, i.e. the boat is directly above the snag. This allows the team to lift the lead-line and mesh up and away from the snag, often without causing any significant damage to the seine. Any hauling and lifting will be conducted over the bow rather than the beam as this will reduce the risk of the boat tipping over whilst pulling against the snag and lifting the heavy leads and netting upwards. Once freed the net may be hauled from the bank, whilst still partially lifted by the boat, allowing the seine to pass over the obstruction. When thought to be clear, the net is dropped and normal retrieval is resumed.

5. Advantages

Figure 4. Seine netting on a 100m wide channel. Source: Environment Agency
  • Simple method for sampling a large area in a relatively short time.
  • More efficient than ‘passive’ equipment, which relies on fish movement.
  • Small nets are inexpensive, lightweight, easily transportable, storable, and reparable, larger nets increasingly expensive, difficult to store and deploy etc. commensurate with size.
  • Not restricted by water turbidity, i.e. fish can be caught in nets without having to see them.
  • If used well and under appropriate conditions there will be minimal effects and harm on fish population, i.e. fish are stressed but few should be harmed.
  • Efficient method to catch large amount of fish if abundant population.
  • Most effective and requires less effort in slow flowing water.

6. Disadvantages

  • Requires previous experience using equipment and knowledge of fish habitats and behaviour.
  • Seine nets can be hard to retrieve when large nets are used or when a silt substrate or hitherto unseen sunken structure is encountered.
  • Longer nets are very bulky and heavy requiring specialist apparatus to transport to site and deploy.
  • Inefficient in more complex habitats (e.g. streams with in-channel trees, rocks, macrophytes etc.), when used in rivers with certain substrates (fish more likely to escape under nets when substrate is uneven/irregular, e.g. cobbles) and fast-currents (Crane and Kapuscinski, 2018).
  • Hard to catch benthic species, especially small individuals as they escape under the lead.
  • May not provide a representative sample of the fish community (eels and benthic species are hard to catch), so it needs to be carried out in combination with another method (usually electrofishing).
  • Fragile species and juvenile fish are more likely to suffer mortality.
  • Method is not selective; all species in survey area captured and recorded.
  • Sample effort and results for seining are more variable than electrofishing.
  • Data comparison between sites may be more difficult since survey effort varies, due to differences in operator efficiency, mesh size, net length and river depth etc.

7. Recommendation for method application

Figure 5. Seine netting a lake. Source: Environment Agency
  • Verify that net characteristics (mesh size, length, depth) are suitable for the survey objectives, water features, and targeted species (Environmental Protection (Water) Policy, 2018).
  • Be careful with the weight and strength of the net; it can greatly influence its durability.
  • Avoid raising the lead from the riverbed unnecessarily, otherwise fish will escape – net avoidance is one of the main sources of bias.
  • Keep constant survey conditions as fish location and behaviour can be influenced by season, temperature, and time of the day (Ríha et al., 2008).
  • Lower fish stress, air exposure, and handling by using rubber nets and oxygenated containers, or better still keep cages.
  • A team of at least three people is recommended. Longer and heavier nets will obviously require larger survey teams and EA surveys generally require between four and six members of staff.
  • Leave the seine bag in the water as much as possible to prevent undue stress to the fish.
  • Particular care should be taken when sorting through the seine ‘bag’ due to the risk of sharp items present, this could range from broken glass and rusted cans to fisherman’s hooks and hypodermic syringes. Stout gloves designed to protect against needle stick injuries are therefore a wise precaution.
  • Cleaning the equipment is vitally important to prevent the transfer of pathogens and invasive species. Check, clean, dry guidance is available from DEFRA https://secure.fera.defra.gov.uk/nonnativespecies/checkcleandry/documents/check-clean-dry-england.pdf.
  • When damaged the net must be repaired to avoid statistical bias due to fish escaping through holes. This is simple but can be laborious work if large tears are present.
  • A seine net should be examined for damage prior to survey, and this is particularly important if stored outside where rodent damage may occur. The net should also be inspected after survey, and between hauls if possible, especially if it has become snagged at any point.

If conducting several surveys ensure that the same method is followed, the same equipment is used and that surveys are conducted under similar conditions (i.e. depth or temperature). This will allow data comparison between sites.

8. Costs

Cost is very much dependant on the number of nets and people needed to undertake the process. Net price varies by length, mesh type and lead loading required. A recent quote (07/2020) for a 100m x 4m seine net was £2600. Further costs are related to working time, transport requirements such as trailers for large nets and boat / outboard equipment for deployment.

9. Data analysis

Sampling effort is quantified by catch per haul, if all hauls have similar characteristics. Catch per area sampled is also used for assessing density (catch per m²). Data must be corrected for catch efficiency related to species and size classes (Crane and Kapuscinski, 2018), as well as net specification and sampling conditions (Taylor et al., 2011).

10. References

  1. Crane, D. P. and Kapuscinski, K. L. (2018) ‘Capture efficiency of a fine mesh seine in a large river: Implications for abundance, richness, and diversity analyses’, Fisheries Research, 205(September 2017), pp. 149–157. doi: 10.1016/j.fishres.2018.04.018.
  2. 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.
  3. Environmental Protection (Water) Policy (2018) Monitoring and Sampling Manual Biological assessment Sampling fish communities using seine nets 4 Permits and approvals See Appendix 1 for example equipment checklist. Available at: https://www.ehp.qld.gov.au/water/monitoring/sampling-manual/pdf/biological-assessment-sampling-fish-communities-using-seine-nets.pdf.
  4. Ríha, M. et al. (2008) ‘Dependance of beach seine net efficiency on the net length and diel period’, Aquatic Living Resources, 21, pp. 411–418. doi: 10.1051/alr.
  5. Taylor, P. et al. (2011) ‘Transactions of the American Fisheries Society The Efficiency of a Seine Net’, 8659 (January 2014), pp. 37–41. doi: 10.1577/1548-8659(2000)129<0901.