Ontario trapnet, Trap net

ONTARIO TRAPNET

End of spring trap netting is a live release trap netting program designed to estimate the size of a fish stock. Includes biological measures to assess the status of walleye populations.

End of Spring Trap Netting (ESTN) is a standard live release trap netting program designed to estimate the relative abundance of a fish stock, and provide other biological measures to assess the status of walleye populations in Ontario.

Fall Walleye Index Netting (FWIN) (Morgan, 2002) is the preferred standard netting procedure for monitoring Ontario walleye populations. FWIN should be used where mortality is not an issue as it samples a greater diversity of habitat (i.e. not just shoreline/littoral zone) and provides better information on the forage fish community, especially yellow perch. FWIN also allows for better age determination through the use of otoliths and can provide information on fecundity and maturity. ESTN provides a standard procedure to accommodate managers of fisheries where walleye mortality associated with gill netting is not acceptable. Fish attribute data such as age structure, growth, condition and recruitment can also be collected through ESTN, but since it is a live release method, data on maturity and fecundity are not collected.

ESTN is an adaptation of the Nearshore Community Index Netting (NSCIN) program (Stirling, 1999) that was designed to provide trend through time information on nearshore fish communities. The ESTN method was proposed because the population density of walleye is better reflected when trap netting is completed in late spring and early summer, rather than during the NSCIN period of August and early fall (Sampling Standards and Diagnostics Working Group, Percid Synthesis, 1998). A properly conducted ESTN should, at a minimum, provide fisheries managers with abundance and biological attribute data that can be used to make relative comparisons to provincial benchmark values from other lakes in Ontario. Preliminary analysis of data from lakes studied by Quetico-Mille Lacs Fisheries Assessment Unit and Kawartha Lakes Fisheries Assessment Unit indicate that there is a strong relationship between ESTN catches and relative abundance of walleye greater than 200 mm. Benchmarks are currently being refined and will be documented in a future Fisheries Assessment Unit report.

A successful index netting program requires stringent standardization of gear and methods in order to reduce sampling variability. This manual describes the standardized methods and provides the technical information necessary for project leaders and field crews to conduct the ESTN field program. It provides detailed sampling standards with regard to gear specifications, deployment of gear, selection of sampling locations, determination of sample size requirements, procedures for processing fish and procedures for data entry. Standard ESTN data collection techniques have been designed to be compatible with the software packages FISHNET2 and FISHNET3 provided by the Ontario Ministry of Natural Resources (OMNR). A standard ESTN data entry template is available in FISHNET3.

ESTN may not be suitable for monitoring walleye in all lakes. Lake size, littoral zone slope, and the amount of aquatic vegetation are just three factors that may limit the number of acceptable netting sites for the ESTN standard trap nets. Anyone considering this standard netting program should familiarize themselves with this manual’s Gear Description (section 3.0) and Site Selection (sections 4.1 and 5.1) sections to determine if the ESTN program is suited to the lake they wish to study. Examination of a lake contour map, or an actual visit to the lake, may be required to determine whether a particular lake is suitable for assessing with this program.

 Survey design and methods

The ESTN method utilizes a random sampling design in which the individual sampling units are selected without replacement. Sampling sites should be selected in a spatially random fashion to avoid any biases in the site selection process (e.g., to avoid selecting sites that are closest to the lake’s access point or sites that represent preferred habitat for a particular fish species).

Sites must not be rejected without a valid reason (e.g. water too shallow or too deep, areas with high shoreline development, impeding boat and seaplane traffic, unsuitable topography, etc.) The selection of sample sites is essentially a map exercise that is completed in the office using a random number table to select specific sites.

 Sampling methods

The basic sampling methods for ESTN TRAPNET are summarized in Table 1. The field survey occurs in late spring when surface waters reach 12 °C and may continue until the surface temperature reaches 18°C. This temperature window allows the post-spawning redistribution of walleye from spawning sites and ensures that sampling is completed before fish leave the nearshore zone for deeper water. In most parts of Ontario, the sampling window begins in mid-May and extends to mid-June (a 4 week sampling period).

A standardized six foot spring-haul trap net (see section 3.2) is set and left to fish overnight. The duration of the set is 24 hours, acknowledging that this is a target only and some reasonable variance is acceptable. The number of sites which can be sampled in a day will depend on catch size, daily travel time to the waterbody, waterbody size, crew experience and fish sampling protocols. For a typical lake, a crew of two people should be able to lift, process the fish, and reset at least two spring-haul trap nets per day. At that rate, at least 16 random sites could be sampled in 9 consecutive working days or in 2 standard 5-day work weeks. On small lakes with low to moderate catch rates, crews could likely handle up to 4 spring-haul trap nets per day.

Table 1: Summary of methods for ESTN

Site, set, lift, fish catch and fish sampling information are recorded for each netting effort on standard ESTN data forms (Appendix B). The acceptable ranges of the net setting criteria (in round brackets) are also included on the forms to provide a quick reference for field crews. The required data collection for each netting effort has been selected and designed to minimize observational error and subjectivity, without requiring an inordinate amount of field collection time.

Sample size

Sample size decisions may be influenced by a variety of factors. Practical considerations such as cost and availability of manpower or equipment, but more importantly, decisions on sample size should be linked to the survey objectives. If the objective is to test a hypothesis and statistically detect differences in abundance between years or lakes then the sample size should be chosen so that the test is performed at a reasonable level of power. Power is the probability of detecting a difference if it exists. If the objective of the survey is to obtain estimates of fish abundance at a point in time (i.e., a “snap shot”), then sample size should be dictated by a desired level of precision in estimating the mean catch. For instance, a fisheries manager may want to know if a lake fits into a “good” or “bad” category for a certain fish species, compared to other lakes in the province. Relative comparisons based on such categories usually require less precision and therefore fewer net sets than most hypothesis testing analyses. When the number of samples per lake increases, the precision level increases. Therefore, the appropriate number of samples to collect (i.e., nets to set) depends on the purpose for doing the ESTN program.

A minimum sample size of 16 sets per lake is recommended for making relative comparisons of fish abundance (i.e., comparing results to provincial benchmark categories). More sampling should be considered for relatively large lakes and spatial stratification may also be advantageous to reduce the logistical problems associated with long travel distances between sites. Based on within-year variability in catch from previous ESTN projects, 16 sets per lake will usually produce (on average) a 95% confidence interval whose total width is approximately 100% of the mean. Increasing the sample size to 20 sets per lake should produce 95% confidence intervals that are approximately 88% of the mean.

A minimum sample size of 30 sets per lake is recommended where higher precision is required (e.g., FAUs statistically testing for differences between years or lake characteristics). For high precision targets, increasing the sample size to 30 sets per lake should produce 95% confidence intervals that are approximately 70% of the mean.

In addition to the number of sets per lake, it is recommended that ESTN be conducted on each study lake for at least 2 consecutive years. Using the mean of two years greatly increases the chances of obtaining a mean catch value that more accurately reflects the actual population abundance, rather than a mean catch value influenced by unique weather conditions or other phenomena that occurred during only one sampling period.

Gear description

A trap net is a passive fishing device that entraps fish as a result of their own movement and subsequent reactions to the net. The net consists of two main parts, a leader, and a head. The leader, constructed of mesh netting, is designed to impede the normal movements of fish and subsequently lead fish out to the head. The head, also constructed of mesh netting, is designed to capture fish by funneling them into a box-shaped crib. The most common type of trap net used to conduct the ESTN program is a spring-haul trap net. Spring-haul refers to the method of setting the net without the use of pulleys (down-hauling), lifting the net by bringing it in over the boat, and the location of the zipper near the house end of the crib. Down-haul trap nets may also be used, but are not recommended due to the extra rigging and net handling time required. For this reason, the gear requirements and net handling methodology presented in this manual are based on the spring-haul trap net (see Figure 1).

How fish are captured

Fish moving in the littoral zone often travel parallel to shore and lake bottom contours where they encounter the trap net leader that is set perpendicular to shore. Very small fish may pass through the mesh leader, while larger fish swim around or away from the obstacle. Other fish may actually be attracted to the net as it offers a source of cover, and some fish may be attracted to fish already trapped in the net. Eventually, some of these larger fish will encounter the heart or wing of the trap net. To follow the heart, wing or leader away from the net requires that the fish swims back towards shore. Alternatively there is a gap between the leader and heart. Fish that pass through the gap enter into the heart and house of the net where they may continue to move about. Due to the shape and location of the gap opening created by the trap net hearts, fish rarely encounter the gap opening at the appropriate angle to exit the net’s enclosure. The funnel-shaped tunnel at the back of the house gradually leads fish into the narrow restricted end of the tunnel where there is another opening. Fish dart through the final tunnel opening and into the crib. Once trapped in the box-shaped crib, the only available escape route is the tunnel opening suspended in the middle of the crib. Due to the shape and location of the tunnel, fish following the crib walls rarely encounter the tunnel opening and rarely escape.

The entire net must be set properly for fish to successfully make the journey along the leader, through the gap, through the tunnel, and into the crib. A proper set requires that the leader is relatively straight, tight, and not twisted. The gap opening must be held open by the heart and wings that are relatively tight, at proper angles, and not twisted. The sides of the heart and house should be taut. The tunnel must be clear of obstructions and suspended in the crib. There must not be any holes in the mesh netting that will allow fish to escape from any area of the net. The net should be well secured with anchors to prevent wind and wave action from loosening the net and thus reducing its efficiency.

The standard ESTN trap net

Any trap net (e.g., spring-haul or down-haul) used to conduct the ESTN program should conform to the standard set of specifications. Standard mesh sizes, colour, measurements, etc., are provided on the following two Standard Trap Net Description Forms (see Figures 2 and 3). Mesh size and colour should be identical to the ESTN standards. Net dimensions should be within at least 10 percent of the standards. If trap nets do not conform to these standards, then fishing results will not be valid for comparative purposes.

The spring-haul ESTN trap net can be constructed using either heavyweight or lightweight materials. The lightweight net is less bulky and about 18 percent lighter and may be preferred for lakes where access is more difficult (e.g., fly-in or portage access lakes) or when smaller boats are being used. The heavyweight net is likely more durable and is recommended for more severe conditions when strong winds and hard lake bottoms can result in increased net wear.

The standard ESTN trap net description – form 1

Construction Specifications (To ensure a well built and durable spring-haul trap net):

1. Double selvage lines of black polypropylene twine must be attached around the entire perimeter of the mesh netting before attachment to the black 95 cm (⅜ in.) polypropylene lines.
2. Double selvage lines are to be attached to the black 95 cm (⅜ in.) polypropylene lines with a minimum of 3 hitches per mesh using no less than #15 black treated nylon twine or equivalent.
3. Black 0.95 cm (⅜ in.) polypropylene chafing lines are to be attached across the entire bottom of the leader and The chafing lines are to be attached to the 0.95 cm (⅜ in.) polypropylene lines, that the mesh was attached to in #2 above, with a minimum of 3 half hitches at every third mesh, using no less than #15 black treated nylon twine or equivalent. Note: The chafing line may be omitted on the leader of the lightweight net at the purchaser’s discretion.
4. Wing and leader bridles, brails, brail lines, lift line and marker, toggle ring (brail ring), and anchor lines are to be attached, and a zipper installed at the house end of the crib, such that the net is “ready to fish.”

Net storage

If the trap nets are clean at the end of the fieldwork, they can be dried by being blocked up in a warm storage room. Make sure that each net is exposed to the air all the way around the bundle, and that any water running off will drain away from the net. Ensure that your storage room is sealed to prevent mice or squirrels from getting to the nets. These animals can damage the nets by chewing on the netting materials, and will often build nests and over-winter in the net bundles.

If a net has weeds or algae clinging to it, the net should be unpacked and cleaned by pressure spraying and/or hanging on a drying rack. If quickly dried, algae will break down and fall off the net. If drying outside, do not expose the net to the sunlight for a long period of time, as the sun will deteriorate the netting materials. Do not leave a dirty net tightly bundled. The plants and algae will hold moisture in the bundle and cause the net to rot. Polypropylene nets are more resistant to rotting than other traditional materials and will last much longer if properly maintained and stored in a building where they are protected from sunlight, moisture, and nuisance animals.

The trap nets must be carefully inspected during the off season for rips and tears. This inspection should be completed at the beginning of the storage period to provide ample time to conduct, or budget for, the appropriate repairs. Watch for areas where the mesh has separated from the lead line (bottom of net) as a result of damaged lashing. When feasible, major repairs should be done by the manufacturer to ensure that the correct techniques and materials are used.