5. SAMPLING WITH TRAPS

John F. Craig
Freshwater Biological Association
Windermere Laboratory
Ambleside, Cumbria, England

5.1 INTRODUCTION

A trap is a passive fishing gear which is static and relies on the movement of fish. Usually it is fitted with a device that deters the fish from leaving the trap once they have entered. Brandt (1964) uses this last criterion for distinguishing between artificial shelters [such as bundles of brushwood for catching eels (Anguilla anguilla)] and true traps.

Traps, made of wood, metal, netting or plastic, can be fixed or portable. They can be used to catch adults and juveniles of migrating or resident species. Often the traps are associated with barriers and leaders which guide the fish into them. Many different kinds of traps have been designed and used for commercial freshwater fisheries (for example eels, salmonids, lake whitefish, percids). Traps, used as sampling gears for stock assessment and biological studies, have originated from these designs in most cases.

5.2 PERMANENT AND SEMI-PERMANENT TRAPS USED FOR CATCHING MIGRATING FISH

To catch migrating species of fish, traps can be built across rivers and streams. The basic idea is to filter out fish descending and deflect fish ascending into traps. These traps are normally positioned in conjunction with a barrier (such as a weir or a screen). One main problem with such barriers is that they also filter out debris, “even up to the size of oak trees” (Allan, 1966). Traps have usually been constructed at the mouth of a river or where a stream enters a lake so that they attempt to catch all migrants from one water body to another. Some traps have been installed in large dams across rivers so that anadromous fish may be counted and given passage through the installation (Trefethen, 1968). However, the following discussion will be devoted to barriers built specifically for catching migrating fish for stock assessment and biological purposes.

5.2.1 Anadromous fish (see also Chapter 10)

Blair (1956) describes a simple trap system constructed in a river mouth to catch ascending adult Atlantic salmon (Salmo salar) and descending salmon smolts (Figure 5.1a). Blair states that the system is easily dismantled and transported. Sections of netting clogged with debris during flooding can be readily replaced. Ruggles (1975) used a modified commercial trap to sample Atlantic salmon runs (Figure 5.1b). The size of the gear can be varied dependent on river width, depth and volume of flow. To catch ascending adult salmon, a 15 cm mesh (knot to knot) leader and a 9 cm mesh pound netting is used. To catch smolts, a 6 cm mesh leader and a 3 cm mesh pound netting is substituted. Ruggles (1975) collected data on year-to-year changes in timing of migrations and composition and relative abundance of individual river populations of Atlantic salmon. Fish captured were in good condition and mark-recapture experiments could be carried out. The pound size is important in the determination of sampling mortality, as there appears to be heavy mortality with over-crowding. Krema and Farr (1974) have used floating fish traps to capture homing adult Coho salmon (Oncorhynchus kisutch). Traps used for catching salmon smolts are usually only operational during low river discharges (McGrath, 1975). Menzies (in Allan, 1966) stated that he

Figure 5.1 Examples of traps to catch anadromous fish:

1. Plan drawing of a two-way trap system to catch migrating salmon in rivers. The arms of the fence are made from sections of netting suspended from steel cables which are supported by a number of piers (redrawn from Blair, 1956).

2. Plan drawing of a pound trap set for use in estuaries to catch salmon (from Ruggles, 1975).

thought traps for adult fish were satisfactory in any size of river but smolt traps were best confined to small rivers. A great deal of manpower is required in keeping traps clean of rubbish, especially during time of flood. Craddock (1958) found that downstream traps tended to overflow in very high water. Some attempts have been made to overcome this by incorporating modifications to the screens (Hunter, 1954) and by building self-cleaning screens (John, 1954). Other traps built to catch downstream migrating salmonids and overcome the problem of heavy loads of debris include the scoop trap (Meecham, 1964) and the migrant dipper (Mason, 1966).

5.2.2 Catadromous fish

A number of different types of trap have been constructed to catch migrating eels, mainly for commercial purposes. Permanent eel traps can be built in streams and small rivers where there is a regular water flow, current and volume of water are not too great, floating debris is of manageable quantities and the trap does not block a navigation route. Frost (1950) describes the large eel weirs on the River Bann in Northern Ireland. A simpler eel trap has been used in Canada (Day, 1948). Two wings guide the fish into a long, small-meshed, tapering bag inside of which is a cone-shaped funnel (Figure 5.2a). The wings, made from boards or brushwood, are permanent while the wooden frame with the funnelled bag is removed at the end of each fishing season. Eels may also be caught by a horizontal filter (Figure 5.2b). There is usually a roof over the catching chamber and the system is used for trapping fish migrating at night. Lowe (1952) found that eels migrating at night could be deflected by lights and guided into traps.

Figure 5.2

5.2.3 Non-seagoing fish

Shetter (1938), Carbine and Shetter (1943), Wolf (1951), Whalls, Proshek and Shetter (1955), and Campbell (1967) describe a number of fish weirs and traps to catch fish of varying species moving between tributaries and the main stream and between river and lake. These all work on the same principle but various modifications are made to filter out debris and catch the fish unharmed. Craig (unpublished) has used a two-way trap to catch lake trout (Salmo trutta) moving up a small afferent stream to spawn (Figure 5.2c). The rails can be replaced by screens and used to catch juveniles moving downstream to enter the lake. The angle of the rails makes it easy to clean them of debris.

5.2.4 Some general conclusions

Care needs to be taken when entire runs of fish are intercepted by barriers across streams and rivers. There may be a delay in the adult spawning run and thus the reproductive process. This may adversely affect the strength of the next year class. Foerster (1968) discusses the effectiveness of catch: escapement ratios in the run of sockeye salmon (Oncorhynchus nerka) in relation to the commercial exploitation of these fish. He states that it is important that a suitable division between commercial catch and escapement applies throughout the season, and in the case where there is a river system with many separate tributaries, each individual run should be protected. Handling should be kept to a minimum to avoid stressing the whole population. Thus it is important to devise a good sampling technique. Ruggles (1975) suggests that many of the disadvantages in terms of interfering with the migrations of large populations of fish can be overcome by sampling with a constant fishing effort. How representative the sample is of the entire population depends on the method of catch. Several units of gear can be employed to obtain estimates of variance.

Hellawell (1973) believes that there are serious disadvantages in using traps in terms of cost and manpower. He recommends the installation of effective automatic fish-counters for collecting data; however, there are still many problems with fish-counters (for example, they count objects other than fish) and information obtained from them is limited. Obviously, the type of system used depends on the information required and the physical make-up of the site chosen (see Chapter 10).

5.3 PORTABLE TRAPS MAINLY USED FOR RESIDENT SPECIES

5.3.1 Trap nets and pound nets

Nomura et al. (1968 and 1968a,b,c,d,e,f,g) have made a study of trap net design especially in relation to current. Although these studies have been undertaken in the sea, many of the findings are applicable to fresh waters. A scaled-down version of the trap net is described by Crowe (1950). A small portable unit of this kind (Figure 5.3a) weighs about 45 kg and can be set from a large rowboat in depths down to about 12 m. Latta (1959) found that these nets were selective for different species and depending on the mesh size, did not adequately sample the smaller fish. He also found the traps were selective among the larger fish and he suggests that population estimates should be based on each size group of a species. Mortality rates, based on the decrease in numbers of fish in successive age groups, must be adjusted for net selectivity.

A pound net suitable for sampling has already been described (Figure 5.3b). Leopold et al. (1975) summarized data on the commercial use of a number of pound nets of various sizes adapted to conditions of the lake in which they were used and to the behaviour of different fish species. Pound nets were found to be seasonally selective with the highest catch occurring during the spring spawning period.

(a)	Drawing of a trap net which is shown anchored to the bottom. Fish are extracted through the lace holes after the end pot has been lifted into a boat. (Redrawn from Crowe, 1950).
(b)	Sketch of a typical hoop net with two funnels.
(c)	An eel basket made from wickerwork. (Redrawn from Tesch, 1977).
(d)	A ‘Windermere perch’ trap. The trap can be located by attaching a rope and buoy. In Windermere, to prevent interference, the traps are laid in series on a submerged line which can be found with a plant grab. Also shown in the figure is a sketch of the slit entrance of the ‘Swedish’ perch trap.

Figure 5.3 Examples of portable traps

5.3.2 Hoop and fyke nets

Hoop nets are conical fish traps made of twine hung round frames usually made of willow (Salix spp. and having two funnel-shaped throats in series. Hoop nets can vary greatly in size but typically there are usually about six or seven hoops the diameters of which range in decreasing size from 1.6 m at the front to 0.6 m at the end (Figure 5.3b). In running water, the entrance is directed downstream and the cod-end is staked upstream. In quiet and still waters, wings and a leader can be added to the hoop net and it is then termed a fyke net. Brandt (1971) states that Danish fishermen employ fyke nets for eel fishing with over 10 hoops. Moriarty (1975) has successfully used a series of fyke nets in estuaries, lakes and large rivers to catch immature eels. Eight to ten small fyke nets of this type can be operated by one man. Numbers caught vary seasonally and the gear is very specific in the species it catches. When the nets were fished in 12 lakes, 3237 eels were caught but only 139 perch (Perca fluviatilis), 34 pike (Esox lucius), 7 rudd (Scardinius erythrophthalmus), and 3 gudgeon (Gobio gobio) entered the nets (Moriarty,1975). Tesch (1977) gives an account of how fyke nets may be set for eels in estuaries and rivers by using anchors. However, in lakes, fyke nets are usually set inshore or in shallow mid-lake regions where they are supported by stakes driven into the bottom. To set the nets, submerged and emergent vegetation needs to be cut. Leopold et al. (1975) describe a modified type of fyke net used for commercial catches to which a semi-circular hoop is added; the cord (which is weighted) of the semi-circle lies on the bottom. This net does not need to be supported by poles and thus can be set in deeper parts of the lake. According to Leopold et al. (1975), it can be laid on any type of bottom and vegetation need not be cut, as the weighted cord presses it down. Bernhardt (1960), with certain reservations, concluded that fyke nets set parallel to the bottom contours in lakes caught more fish than those set perpendicularly. Lagler and Ricker (1942) found that fyke nets were selective for species and caught more sport fish than coarse fish. This they checked with seine net catches.

Fyke nets used in commercial lake fisheries in Poland catch mainly pike, tench (Tinca tinca) and roach (Rutilus rutilus) (83 percent of total catch). In catches with modified fyke nets (provided with a semi-circular hoop in front) set usually in deeper parts of lakes than the ordinary fyke nets, bream (Abramis brama) are also caught (Leopold et al.., 1975). Oliva and HolĎik (1965) used hoop nets with two wings (fyke nets) to estimate fish population numbers in the KliĎava reservoir. They found that the fyke nets selected perch, tench, pike and rudd, but did not catch roach, chub (Leuciscus cephalus), dace (Leuciscus leuciscus), carp (Cyprinus carpio), Crucian carp Carassius carassius), common gudgeon, brown trout and Danubian salmon Hucho hucho). The nets that they used allowed fish below 10 cm to escape. The catches from the traps were compared to those caught in a series of gillnets.

5.3.3 Other small entrapment devices

A number of small portable traps have been developed. The simplest is probably the minnow trap (Yoder, 1948) which has been used mainly for catching bait fish for anglers. The trap can be made out of a glass jar fitted with a funnel and it is baited with bread or rolled oats. It can be used to sample minnow (Phoxinus phoxinus) populations in the littoral regions of lakes, in ponds and in streams. Paulson and Espinosa (1975) have used a modified minnow trap in the limnetic areas of reservoirs to sample young-of-the-year threadfin shad (Dorosoma petenense). Relying on the fact that threadfin shad are positively phototactic, a light source was included inside each trap to attract the shad. The traps can be suspended to desired depths by ropes secured to moored buoys. Burgner (1962) used floating lake traps to measure abundance of young red (sockeye) salmon. He found seasonal changes in the vulnerability to the fishing gear of red salmon fry and three-spine sticklebacks (Gasterosteus aculeatus) and considered this a serious problem in indexing their abundance.

Transparent traps made out of plastic have been used to catch juvenile fish (Breder, 1960; Casselman and Harvey, 1973) (see also Chapter 3). Tesch (1977) discussed various baskets for eel fishing (Figure 5.3c) including those made from wicker-work and the modern form made from plastic.

Wire-mesh traps for catching adult fish have been used extensively (Ricker, 1942; Worthington, 1950; Allan, Herbert and Alabaster, 1958; Alabaster, 1959; Thorpe, 1973; Craig, 1974; Jensen, 1976). In the United States these are often referred to as “Carlander” traps and in the United Kingdom as “Windermere perch” traps (Figure 5.3d). They can be used in rivers, streams or lakes and in areas of submerged and emergent vegetation. In steeply shelving lakes, the bottoms of the traps are flattened to stop them rolling (Worthington, 1950), but in shallow lakes the traps can be cylindrical (Craig, 1974). Traps of this type can be collapsible for easy transport (Anon., 1954) or collapsible and made with nylon netting to avoid corrosion, distortion in the shape of the trap, injury to hands and injury to the fish (as is often the case with hexagonal wire netting) (Houser, 1960). Plastic netting can also replace wire netting (Craig, 1974).

5.3.4 Efficiency of traps

Traps are highly selective for species and size of fish. The mesh size determines the lower length limit of fish trapped. Craig (1974) found wire-mesh traps with 0.64 cm mesh trapped perch from 5 cm and those of 1.28 cm mesh caught fish from 9.5 cm. The upper limit depends on the size of the funnel entrance. The funnel entrance can be round (“Windermere perch” trap) or a vertical slit (“Carlander” trap, “Swedish perch” trap) (Figure 5.3d). Within a species the traps can also be selective for sex (Worthington, 1950; Alm, 1952; Svärdson, 1952; Stott, 1970; Craig, 1974). An important aim is to try and judge the selectivity of a fishing gear at the outset of any investigation and if necessary use other fishing gears to achieve this. Scott (1950) sampled two sections of a river with hoop nets, wire baskets, rotenone and a permanent trap installation. Each of the methods failed to reveal the presence of one or more important species. However, the methods were to an appreciable extent complementary and Scott found it possible to obtain significant information on nearly all major species. Carter (1954) compared the efficiency of nine types of fishing gear and found variation between the gears in the species selected and in the quantity of fish caught measured as catch per unit effort. For example, heart-lead nets caught the greatest weight of fish per net day than any of the other types of gear but caught 84.94 percent game fish. Wood baskets produced the lowest average return per unit of effort but harvested 79.50 percent commercial fish.

Trap catches are influenced by the season (Craig, 1975; Leopold et al., 1975). Stott (1970) claims that they can be correlated with water temperature, although Craig (1974) disputes this finding. Bagenal (1972) suggested that, due to the seasonal changes in catching power of the traps, comparisons of two populations could only be made if the fish were trapped at the same time.

Efficiency of traps can be modified. Wings and leaders did not improve the catch of “Carlander” traps, but covering them with brushwood reduced their effectiveness (Beall and Wahl, 1959). Craig (1974) found that catches were reduced if wire-mesh traps were covered with black polythene and deduced that the sight of fish already in the trap acted as a bait to lure other fish in. Most of the traps described are unbaited. Presence and type of bait influenced numbers of eel-caught in eel-baskets (Tesch, 1970). Beall and Wahl (1959) used cottage cheese to bait traps and found that it increased trapping efficiency for bluegill sunfish (Lepomis macrochirus). Cobb (1954) caught 52 percent by weight of the total catch of bluegill sunfish using soybean cake as bait, compared to 8 percent in unbaited traps.

Stott (1970) found that roach catches increased 11-fold on application of 0.1 to 0.5 ppm copper sulphate. Similar effect is described by Tompkins and Bridges (1958) with regard to fyke net catches. They make the dangerous suggestion that such an addition might be valuable in making population estimates. It is far more likely that it would add one more unknown dimension to selectivity. Sears (1953) placed traps in a pond near an inflow pipe. Large-mouth black bass (Micropterus salmoides) fry were attracted by the splash of the falling water and the current which resulted.

These few examples show that attention should be paid to any apparently minor changes in construction, in place or time of setting or in baits, if traps are used as sampling gear.

Few people have critically analysed the efficiency of the gears that they have used, but still have attempted to make quantitative estimates of population statistics. There are exceptions to this (Latta, 1959; Bagenal, 1972). Bagenal derived confidence limits for the mean catch of perch in relation to the number of traps used during the spawning season (Figure 5.4). He suggests that not less than 10 traps are needed to obtain consistent results; but for different kinds of traps for different species and water bodies, a different number of gears may be more appropriate.

Figure 5.4 The variability in the mean catch of perch in relation to the number of traps used during the spawning season. The 95% (¤) and 99% (¤) confidence limits are expressed as the multiplier and divisor or the geometric mean. For example, the 99% limits of the geometric mean with eight traps are 4× and 1/4 the geometric mean. (Redrawn from Bagenal, 1972)

5.3.5 Some general conclusions

The main characteristics of the various types of traps used for sampling fish populations are summarized in Table 5.1. For all their shortcomings, traps have many advantages. They can be used in a wide range of habitats (streams, rivers, estuaries, ponds and lakes) and in deep and shallow water. In most cases labour of operation is economical and, with appropriate precautions, fish can be caught unharmed. Information concerning density (both spatial and temporal) and migrations can be collected as the fishing effort remains constant from day to day. Le Cren, Kipling and McCormack (1977) have used the catch per unit effort of successive year classes to estimate population numbers of perch in Windermere. The unit of effort was one trap fished throughout six weeks during the spawning season (Figure 5.5)

Other examples of the use of wire-mesh traps for population estimates are mark-recapture experiments (Ricker, 1942; Thorpe, 1973; Craig, 1974; Jensen, 1976). However, population estimates based on traps must be treated with extreme caution (Craig, 1974, 1975a).

Figure 5.5 An example of long-term data collected by using traps. The graph shows the catch per unit effort (c.p.u.e.) against time (1948–1977) of perch caught in the north basin (*) and south basin (° ε ¤) of Windermere at standard sampling sites using ‘Windermere perch’ traps.

c.p.u.e. = one trap for a whole season (see Le Cren et al., 1977)

Table 5.1 Summary of characteristics of different types of traps that can be used as sampling gears

Other examples of the use of wire-mesh traps for population estimates are mark-recapture experiments (Ricker,1942; Thorpe, 1973; Craig, 1974; Jensen, 1976). However, population estimates based on traps must be treated with extreme caution (Craig, 1974, 1975a).

5.4 REFERENCES

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