Beach Seine

Contributing Author: Richard Bailey

Background and Objectives

Background

The earliest form of seining was drag netting, or beach seining, which was introduced as a commercial fishing technique in about the 1800s. Seining is a fishing technique that is traditionally done in areas where there are large schools or groups of fish. In the early years seine nets were set from large rowboats where the fishermen would use beach seines to catch salmon. They would haul the nets in by hand, either rowing their boats to the fishing grounds or hitching a tow from a steam-powered cannery tender. The typical seine net has weights on the bottom and buoys on the top to keep it afloat. These weights and floats enable the net to remain vertical when pulled through the water to entrap fish. The net is set to encircle a school of fish, and is then closed off to trap them.

Today, seines can be employed in non-wadeable systems by drift or power boats to capture both juvenile and adult salmonids in a variety of habitat types. Hayes et al. (1996) describes generic applications of beach seining for fish capture. Specific beach seine applications include capturing fish to estimate total and relative abundance, determine fish diversity and distribution (Farwell et al.1998, Allan et al (In Prep), Rawding and Hillson, 2002), broodstock capture, effectiveness monitoring, and collection of biosamples. Abundance estimates can be refined by determing net capture efficiency (Hayes et al. 1996), and the use of tagging and a mark-recapture approaches. Fish that are tagged on the first catch effort, and recaptured on the second catch effort can indicate the percentage of the total population that is captured by any single catch effort. Additional mark- recapture approaches can be used to help estimate populations of juvenile or adult salmonids within a given location and time or for a given system.

Beach seines are also used to capture juvenile fishes across a wide variety of habitats including rivers, estuarine, and nearshore lake and reservoir habitats. In these habitats, beach seines can be deployed by wading or from boats.

Rationale

Beach seining is an efficient method to capture salmonids and non-salmonid fishes in a wide variety of habitats. It permits sampling relatively large areas in short periods of time, and capture of fish without significant stress or harm. Selective capture and subsequent release are possible with beach seines. It is the preferred capture method to provide fish for mark-application for many mark-recapture based salmonid assessments. This method is also most frequently used for sampling juvenile salmonids in nearshore and estuary habitats.

Objectives

Determination of specific study objectives (the questions you want the data to answer) is critical to developing an appropriate monitoring program using beach seining. Objectives may range from sampling the fish diversity within a given habitat (low precision requirements) to being able to estimate population abundance with high accuracy and precision. The specific study question will also determine what size of fish are the target and what habitats will be sampled. These elements (fish size and sample habitat conditions) drive the selection of beach seine mesh size and characteristics , as well as the best set technique to use.

Effectiveness

Seine efficiency has been estimated by using block nets to enclose the area seined (Hayes et al. 1996). The area is then seined multiple times until no more fish are caught. Efficiency varies with the position of fish within the water column. Catches are higher for fish residing in the middle of the water column. Bottom substrate also affects efficiency. Some objectives can be met by using a single haul with a seine. Species richness, species rank, and size distribution of dominant taxa can be estimated well using one haul. Rare taxa and total abundance of all species are better assessed using multiple seine hauls. If species can outswim the net that is being set, the accuracy of the estimate is likely to be low.

  1. Sampling Design

Site selection

Seining may be carried out in a variety of habitat types, within the area of residence of the fish, depending on the population and life stage being targeted. Adult salmonids are seined from pool (holding) and nearshore lake habitats adjacent to and on the migratory routes to spawning grounds. Juvenile salmon can be fished in stream habitats, estuarine habitats, or nearshore lake and reservoir areas. Sites with irregular bottom topography, significant accumulations of debris or larger rocks may be problematic due to net snagging and abundant fish escape routes. Current velocity and depth influence site selection and choice of net design. Faster current generally requires a larger mesh size, at least in net wings, to reduce drag; while deeper waters require longer nets to reach from the surface to the substrate. Sites with firm sloping beaches are favoured.

For estuary and intertidal habitats, smaller nets can be used for sampling the shallow intertidal shoreline areas (<4 feet, 1.2m deep) with relatively homogenous water depth, velocity, vegetation, and substrate. Larger nets can be used for the intertidal-subtidal fringe with depths ranging from 6 to 15 feet (1.8 to 4.6m).

Sampling frequency

Seining for adult salmon is carried out throughout the return run, such that returning fish are captured in a manner proportional to their abundance within the river (Farwell et. al 1998,; Rawding and Hillson, 2002). These fish can be marked to establish the percent of the target population sampled using a mark-recapture approach. For some species such as sockeye, this may involve deployment of seining gear at passage sites below spawning areas. =For other adult salmon species, this may involve fishing at a variety of pool and glide locations throughout the spawning range (Allan and Bailey, in press).

To determine assemblage diversity or provide fish for biosampling one to many sets may be employed, either on one day or throughout a longer period. To establish an abundance estimate, generally more than one set is used to try to establish sampling efficiency. The area can be locked off and seined until no more fish are caught, or a mark and recapture approach can be used. Once the area is blocked off, fish caught in the first haul are marked, counted and released. On the second set, the proportion of the marked fish recaptured as compared to the total number captured provides an estimate of the total population (Chapman 1948) .These sets may occur in a single habitat or a wider range of habitats. Peck et al. (2003) provide sampling schedules for use of beach seines in habitat assessments associated with juvenile salmon.

The following assumptions from (Rawding and Hillson 2002) are made if a mark-recapture approach is used to estimate abundance:

  • Equal catchability: every animal in the population whether tagged or untagged has the same probability of being caught in the I-th sample (pi) given that it is alive and in the population when the sample is taken.
  • Handling mortality: every animal caught in the I-th sample has the same probability of being tagged and returned to the population
  • Tag loss: tagged animals do not lose their marks and all marks are recognized on recovery
  • Instantaneous sampling: all samples are instantaneous, i.e. sampling time is negligible and each release is made immediately after the sample.

Beach seining for juvenile salmonids is often conducted during the period of smolt outmigration in rivers and estuaries. Smolts will be present in the estuary as part of their sea-ward migration. Peaks in smolt migration vary by location and should be investigated as part of the planning process for a beach seining sampling effort.

For both large and small net approaches, a minimum of three sets per site is recommended (SSC 2003) Setting the net three times in a given sample location increases the accuracy of the estimate of fish density within a given area.

Data Extrapolation Within and Among Other Locations

Abundance estimates by seining, without using mark recapture, or efficiency estimates, would not be very comparable among different systems or habitats. Data could be somewhat comparable if most factors were the same, which is most likely to occur within a system being sampled by the same crews during the same sampling period. For example comparing the catch rate (e.g. number per square meter sampled) may be somewhat comparable if species, species size, habitat characteristics (e.g. river gravel bar, of medium velocity, similar water clarity and time of season), and sampling gear (seine dimensions and mesh size) and deployment methods were very similar.

III. Field / Office Methods

Setup

Several steps are needed prior to the sampling season to ensure a smooth implementation of a beach seining program.

In order to accomplish these steps, initial questions about the program have to be answered. The first question is to determine the specific objectives of the sampling effort – what are the questions to be answered by this effort. Other issues that drive the set-up process are: Where do you want to sample?; What are the habitat conditions there (e.g. lotic vs lentic, fast vs. slow currents, substrate type, beach angle, depth); What are your target species/lifestage(s)/size range(s)?

Preseason Activities for Beach Seining

  1. Inspection of seining sites, access points, and routes for boat travel where required.
  2. Choosing appropriate net designs, deployment vessels, and requirements for winches or other mechanical aids to recover nets
  3. Inspection of nets to ensure mesh, lines, floats and weights are all secure and functional.
  4. Determination of required crew size
  5. Preparation of data sheets
  6. Preparation of tags to apply (if needed)
  7. Permitting (where required)
  8. Preparation of vessels, including safety equipment

In non-wadeable lotic habitat, seines are typically set from unpowered rafts such as drift boats, or jet/propeller-driven boats. Seines may be deployed from the stern (forward travel) or bow (reverse travel) of the vessel; however, where speed is required, setting from the stern is preferred. For maximum efficiency of setting, a seine table is desirable. For powered boats when setting from the stern, a centrally mounted tow post equipped with a quick-release device is desirable, as is a cowling around outboard motors to reduce the chance of nets tangling (Fig. 1).

Typical net constructions for capture of adult salmon vary between 15 m and 100 m in length; 3 m to 12 m in depth with lead lines varying between 4lbs and 7lbs per fathom. Floats are typically installed at 50 cm intervals on the cork lines. Stretched mesh sizes vary but 5 cm (2”) mesh is commonly employed. Twines used in mesh construction range in gauge from 48 to 96; 96 being heavy and 48 being light. Twines are frequently tarred to increase durability.

For wadeable systems, small net beach seine methods are appropriate. Small nets are 80 feet (24.4 m) by 6 feet (1.8m) with 0.3cm mesh knotless nylon net (Figure 3). For non-wadeable juvenile sampling larger net (120 feet (36.6m) by 12 feet (3.7m)) is used with 0.3cm mesh knotless nylon net (Figure 4). These larger nets often have tapered depth, increasing from wings to the central bag, and may have large mesh on the wings than in the bag to reduce drag.

Figure 1. Fisheries and Oceans Canada stock assessment crew and beach seine boat, ready to make a set on the LowerShuswapRiver, October 2001. Photo credit: Richard Bailey

Figure 2. Making a set in a downstream arc. Photo credit: Richard Bailey

Figure 3. Small net beach seine methodology. A – design of net (not drawn to scale); B – setting of net on shallow beach; C – beginning to haul net (SSC 2003)

Figure 4. Large beach seine methodology. A – design of net (not drawn to scale); B – towing net; C – hauling net. (SSC 2003)

Events Sequence

During daily operations, crews arrive at the seining site, and proceed to organize and sampling equipment, prior to deploying the net. Once equipment is ready, and the crew is ready to proceed, the net is loaded onto the seine deck.

First, the towline, connecting the upper and lower bridles is attached to the quick release mechanism on the tow post. If needed, extra lengths of rope may be added to the towline to facilitate passing the line to the crew on shore, prior to release from the post. Then, the net is stacked back and forth on the seine table, corks forward and lead line to the rear, more to the side of the vessel that the line will exit. Seine sets are made by deploying the net in either a downstream or upstream arc. For downstream sets from river left, the line should exit on the port side of the vessel; for sets from river right, the line should be arranged to exit from the starboard side of the vessel. In Figure 1, the crew is preparing for a set from river left, thus the line is arranged from the port side of the stern. When the net has been fully stacked, the beach line should be tied off to a solid attachment point, preferably close to the waterline. Sets are typically made in a downstream arc, at speeds up to 20 km/hr. The boat operator should attempt to run out of net just as the boat reaches the beach, closing the set.

In slower moving, deep water, sets may also be made by stretching the net slowly, part way across the river, and securing it to rope tensioned bank to bank manually. The shoreward end of the net is secured to the beach, while the net (and boat) beyond the attachment to the tensioned line then drift downstream until the limit of the net is reached. If the goal is to collect actively migrating fish at this point, the crew maintains the boat and net in place by use of ropes to each shore. The net is left in place, and the crew observe as fish swim up into the area sectioned off by the net. Once sufficient fish have entered the net, or for a specified time period, the midpoint attachment of the net is released and the net is drawn manually to shore by crew members.

For juvenile fish sampling in wadeable systems, one end of the net is fixed as described above, and the other end is brought around by wading and then hauled in. When a boat is not used the net will be stacked on the shore where the line is attached, with lead line stacked in the direction of the arc (i.e. on the downstream side if making arc downstream). For large net approaches to juvenile sampling in estuarine areas, the free end of the net is set by boat across the current to 60 percent of the net length. The net is held open for about 4 minutes and then the free end is brought back into shore for hauling (SSC 2003). Other approaches include pulling the seine at a constant rate without holding the nest open for a set period of time. With all seine sets, lead and cork lines should be withdrawn at equivalent rates until close to shore. Once the lead line approaches the shore, it should be withdrawn more than the cork line, until a secure “pond” is formed in the bag of the net and the lead line is on the beach. Fish may then be allowed to rest within the bag until they are withdrawn for sampling, tagging, or transport to hatchery for use as broodstock.

Depending on bottom topography and the presence of debris or large rocks, nets may become trapped during retrieval. Slacking tension on lead lines and use of a gaff hook on a long pole from a boat is one method of freeing the line. If repeated sets are made around such obstacles, rebar devices can be assembled and placed over the debris to facilitate smoother passage of the net (See Figure 5).

Figure 5. Devices built out of rebar to bridge over rocks, boulders, snags, LWD etc to allow beach seining for capturing adult Chinook salmon in the Green R., Washington in 2000-2002. Welded washers allowed nailing the devices to wood, otherwise the ends were jammed into the substrate, pointing upstream. The seine would slide over such devices. Snorkeling was still necessary in some cases to free the net; obviously this is safe only in certain flows/river sizes. Snorkeling or SCUBA was required in advance to find suitable sites, and install devices (Photo credit: Peter Hahn, WDFW).

Once all fish have been withdrawn from the net, the net is cleaned of all leaf litter, sticks, rocks and other debris, checked for damage, and re-loaded onto the seine deck. Damage to seines can be repaired following instructions in Gebhards (1996 in Murphy and Willis 1996).

  1. Data Handling, Analysis and Reporting

Data collection for each beach seine set should include: