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Corvalis Research Lab
Description of Trapping Methods
SAMPLING PROTOCOLS FOR DOWNSTREAM MIGRANT FISH TRAPS

SALMONID LIFE-CYCLE MONITORING PROJECT

OREGON DEPARTMENT OF FISH AND WILDLIFE

____________________________________________________________________________

 

Table of Contents

A. Estimating Abundance of Downstream Migrant Salmonids

  1. Trap Design
  2. Site Selection
  3. Sampling Duration
  4. Trap Efficiency Estimates
  5. Fish Handling
  6. Data Analysis

B. Trapping Adult Salmonids

C. Estimating Adult Abundance

D. References

____________________________________________________________

 

Estimating Abundance of Downstream Migrant Salmonids

Trap Design

    Downstream-migrating juvenile salmonids are trapped each spring using either a rotary screw trap or an inclined plane trap. The inclined plane trap is identical to that described by McLemore et al. (1989). This trap consists of a revolving screen suspended between two pontoons. A 12-volt DC motor turns the screen. Downstream migrants are swept onto the screen by the stream current and transported to the back end of the trap with the aid of perforated L-shaped cups that are attached to the screen at two-foot intervals. Fish reaching the back of the trap are dropped into a live box where they can later be enumerated. The revolving screen traps currently in use have either 2 ft or 3 ft wide screens.

    In larger streams a floating trap that relies on an Archimedes screw built into a screen covered cone that is suspended between two pontoons is used. The large end of the cone, (either 5 ft or 8 ft in diameter, depending on the size of the stream) is placed upstream into the stream current. With the lower half of the cone in the water, the water pressure forces the cone to turn on a central shaft, much like a turbine. Downstream migrating fish that enter the cone are trapped by the rotating screw and forced into a holding box at the end of the trap.

    Neither of these two trap designs is appropriate for all streams or flow conditions. The type and size of trap used is both a function of the size and flow characteristics of the stream being sampled, and the size and species of the fish that are targeted for trapping. In general, the screw trap is more effective in larger streams because it requires adequate water depth to accommodate the fixed fishing position of the screw as well as adequate water velocity to turn the screw. Because of their smaller size and adjustable screen depth, the revolving screen traps are best suited to smaller streams. The passive capture design of the screw trap generally means that it is more effective in catching larger steelhead and cutthroat trout than the revolving trap because these larger fish are sometimes strong enough to swim off the revolving screen before being deposited in the trap live box.

    Because these traps must operate during high streamflows, there is a risk that the traps will become jammed with debris and thus not fish the entire time between trap checks. For the revolving screen traps, a 12-volt clock is connected to a fuse inline with the trap motor. If the trap is jammed with debris, the fuse is blown, stopping the clock. By recording the starting and ending time on the clock, the length of time the trap was fishing before becoming jammed can be determined. The fuse has the additional benefit of preventing motor burn out by cutting off power to the motor if the trap becomes jammed.

    For the screw traps, a hubodometer, identical to those used by the trucking industry is used. The hubodometer is placed on the front spindle of the screw trap. The hubodometer records the distance that the screw turns between trap checks. By monitoring the number of revolutions per minute, and knowing the distance that the screw turned, the length of time the trap fished before jamming can be estimated.

    Predation by larger fish on smaller fish can be a problem in the trap live box. To minimize this, fir boughs or other hiding cover are often placed in the trap live box to give smaller fish a place to hide. Fish mortality can also occur in the screw trap live box during high streamflows. The resulting high water velocities can push smaller fish to the back of the live box, eventually exhausting them and leading to mortality. To reduce water velocities in the screw trap live box, a v-shaped plywood deflector that creates a pocket of calm water for small fish is sometimes placed in the live box.

     

     

  1. Site Selection

    Regardless of the type of trap used, the selection of the site to place the trap is critical for successful trapping. The traps should generally be fished near the head of a pool, just below a section of fast flowing water. Streamflow should be moving in a straight line as it enters the trap. Pools with sharp changes in direction that result in large back eddy currents should generally be avoided. In smaller streams, small boulders, sandbags, or screened weir panels can be used to improve the hydrodynamics of the site.

    Both trap designs require streamflows that with a minimum water velocity to allow traps to function properly. For revolving screen traps, McLemore et al. (1989) suggests a minimum water velocity of 0.9 meters/second, with most efficient operation at 1.5-2.0 meters/second. We have found an optimum water velocity of 0.8-2.0 meters/second for the screw traps operated in Oregon coastal streams.

      Other considerations important in the site selection process are the availability of large trees or other suitable anchor sites, and anticipation of how well the trap will function at different streamflows and how easy it will be to monitor at high streamflows. Special attention should be given to insuring that the trap has a safe refuge during extreme high streamflow events.

     

  2. Sampling Duration

    Sampling begins by the first week in March and continues until the catch decreases to low levels, usually by the first of June. Traps generally are operated 24 hours per day 7 days per week and are monitored daily.

     

  3. Trap Efficiency Estimates

    Because neither trap samples 100 percent of the water column, only a portion of the total number of downstream migrants are captured. A variety of factors, including changing streamflow, changing fish size, behavior, and species composition can influence the proportion of the total migrant population that is captured by the trap (trap efficiency). To accurately estimate the number of downstream migrating juvenile fish, trap efficiency must be measured on a regular basis. To accomplish this, up to 25 fish of each age class/species (see data sheet, appendix A) are marked and released each day. To insure that the fish used for trap efficiency estimates are migrating fish, they are selected from those fish captured in the trap the previous night. Because frequently not enough fish are caught in certain species/size classes to enable an accurate trap efficiency estimate on a daily basis, a weekly estimate is calculated instead using the following formula:

     

    Ni = (ni) / (mrecapi/mreli)

     

    Where Ni = total number of migrants passing trapping location in week I,

    ni = number of unmarked fish caught in trap in week i (Monday-Sunday trap catches),

    mrecapi = number of marked fish recaptured in trap on week i (Monday-Sunday trap catches)

    mreli = number of marked fish released above the trap in week i (Sunday- Saturday)

     

    The total number of fish migrating past the trap site for the season is then estimate by:

     

    Ntot = å Ni

     

    The importance of expanding trap counts by appropriate measures of trap efficiency is illustrated in tables 1 and 2. In the examples shown, misinterpretation of the data would have resulted if trap efficiency estimates based on species and size class characteristics had not be used to adjust the estimate of downstream migrants.

     

    Table 1. Comparison of unadjusted trap catches and adjusted estimates of total migrants for three species of salmonids leaving Tenmile Creek, spring 1993.

Species

Unadjusted Trap Catch

Seasonal Trap Efficiency

Adjusted Estimate of Migrants

Coho (Age 1+)

2,429

48%

5,050

Steelhead (>120mm)

1,298

17%

7,591

Chinook (Fry)

242

63%

387

 

Table 2. Comparison of unadjusted trap catches and adjusted estimates of total migrants for three size groups of juvenile steelhead in Tenmile Creek, spring 1993.

Size Group

Unadjusted Trap Catch

Seasonal Trap Efficiency

Adjusted Estimate of Migrants

60-89 mm

952

26%

3,719

90-119 mm

546

22%

2,516

120-200 mm

637

13%

4,977

    To obtain the most accurate estimate of trap efficiency, marked fish are released at dusk. This is done for two reasons: 1) most downstream migrating juvenile salmonids on the Oregon coast migrate at or a few hours after dusk, and; 2) daytime releases of marked fish at the same spot every day can lead to increased predation by resident cutthroat trout that establish feeding stations at the release point. Dusk releases appear to reduce this predation by reducing the exposure time of marked fish to predators.

    Fish marked for efficiency estimates are given adequate time to recover from handling prior to release. Because the smolt traps are typically checked in the morning, a timer-activated, self-releasing live box is used that enables marked fish to recover all day before being released at dusk. This device consists of three dark-colored five-gallon buckets that are suspended between two small floating pontoons. A spring wound timer is connected to a 12-volt automobile door lock actuator. At the appropriate time, the timer energizes the door lock actuator, which pulls a pin releasing the buckets. The buckets pivot on a pipe inserted through holes in their base, turn upside down, and release the fish. Each bucket has wire mesh panels along their sides to allow transport of oxygenated water into them. To avoid predation problems in the holding buckets, fish are separated by size. Typically fry are placed in one bucket, coho smolt sized fish in another, and larger trout in a third bucket. Periodically the fish are examined just prior to release to make sure that there is no mortality and that the buckets dump at the appropriate time.

    The release location for marked fish for trap efficiency estimates is located far enough upstream so the fish can evenly mix with unmarked fish moving downstream, yet not be so far upstream as to cause an extracted period of migration of marked fish over multiple days. Marked fish are typically released at least two pool/riffle units, but no more than 300 meters, above the trap. This is based on our experience and is consistent with the recommendations of Roper (1995).

     

  1. Fish Handling

    Generally all fish handled for marking or length measurements are anesthetized with MS222. When anesthetizing fish, it is important to remember that water temperature, anesthetic concentration, and fish density and size can all increase the stress load on the fish. Care is taken that no more fish are anesthetized at one time than can be safely processed. During warm water periods or when large numbers of fish are being processed, the anesthetic water is regularly changed to keep it cool and well oxygenated.

    Several methods of marking fish for trap efficiency estimates are used. Usually, a small scissors or a razor blade is used to remove a portion of a fin. When using fin clips, typically the tip of the upper or lower caudal fin is removed. This technique is the only reliably safe way to mark small fry. For larger fish, a Panjet needleless injector is typically used to inject a small amount of colored acrylic paint under the skin.

     All newly marked fish are placed in the timer actuated release device, and a count is made of unmarked and recaptured marked fish. Each week, up to 25 length measurements are made for each species/size class (see data sheet instructions). Once all recaptured marked fish and unmarked fish are processed, the unmarked fish are released far enough downstream to minimize the potential of them swimming back upstream and reentering the trap.

     

  2. Data Analysis
During the early or late portion of the migration period, only a few fish may be caught during the week, and no marks may be recovered, making it impossible to calculate a weekly trap efficiency (note that this is the case for the first two week and the last week of trapping in Table 3.). To expand trap catches for these weeks a trap efficiency estimate based on the entire season is used. While this may not be extremely accurate, the total catches for these weeks are generally small, and their expanded estimates of migrants represent a small portion to the total population.

 

Table 3. Estimate of the number of age 1+ coho salmon migrating past the trapping location in East Fork Lobster Creek (Alsea River), spring 1994. Weekly catches were expanded based on weekly trap efficiency estimates. A total estimate of migrants was made by summing weekly estimates of migrants.

 

Week

Number captured in trap (a)

Number of fish marked (b)

Number of marked fish recaptured

Estimate of trap efficiency

Estimated number of migrants

Feb. 1-6

3

2

0

0.485

6 (c)

Feb. 7-13

0

1

0

0.485

0 (c)

Feb. 14-20

21

13

4

0.308

68

Feb. 21-27

54

50

6

0.120

450

Feb. 28-Mar. 6

77

74

29

0.392

196

Mar. 7-13

95

94

52

0.553

172

Mar. 14-20

123

90

45

0.500

246

Mar. 21-28

119

109

42

0.385

309

Mar. 29-Apr. 3

178

160

96

0.600

297

Apr. 4-10

106

115

65

0.565

188

Apr. 11-17

115

102

62

0.608

189

Apr. 18-24

179

158

78

0.494

362

Apr. 25-May 1

151

141

54

0.383

394

May 2-8

243

173

88

0.509

477

May 9-15

59

76

42

0.553

107

May 16-22

58

61

28

0.459

126

May 23-29

19

22

8

0.364

52

May 30-Jun. 5

1

0

0

0.485

2 (c)

Jun. 6-12

---

---

---

---

---

TOTAL

1601

1441

699

---

3,641

Seasonal trap efficiency (total recaps/marks released) .............................0.485

(a) Numbers in "CAUGHT" column represent fish caught in the trap Monday through Sunday.

(b) Numbers in the "MARKED" column represent fish marked and released upstream of the trap for trap efficiency tests Sunday through Saturday.

(c) No marked fish recaptured, seasonal trap efficiency used to expand the week’s trap catch to estimate of total migrants for the week.

  For some streams, the number of migrants of a particular species that are caught in the trap are not sufficient to obtain a weekly trap efficiency estimate. This may result from a low number of migrants, a low trap efficiency for this particular species, or a combination of both. For these fish, a trap efficiency for the entire season is calculated based on the total number of marks released and recaptured while the trap was in operation (Table 4). This seasonal trap efficiency estimate is used to expand the number of fish caught in the trap during the season to obtain an estimate of total migrants.

 

Table 4. Estimate of the number of age 1+ steelhead trout (>90mm) migrating past the trapping location on Moon Creek (Nestucca River), spring 1994. An estimate of the total number of migrants was made by dividing the seasonal trap catch by the seasonal trap efficiency.

 

Week

Number captured in trap (a)

Number of fish marked (b)

Number of marked fish recaptured

Estimate of trap efficiency

Estimated number of migrants

Mar. 2-6

0

0

0

0.182

0 (c)

Mar. 7-13

1

1

0

0.182

5 (c)

Mar. 14-20

2

2

0

0.182

11 (c)

Mar. 21-28

1

1

0

0.182

5 (c)

Mar. 29-Apr. 3

0

0

0

0.182

0 (c)

Apr. 4-10

2

2

0

0.182

11 (c)

Apr. 11-17

1

0

0

0.182

5 (c)

Apr. 18-24

2

3

0

0.182

11 (c)

Apr. 25-May 1

1

1

0

0.182

5 (c)

May 2-8

0

0

0

0.182

0 (c)

May 9-15

2

1

1

1

2

May 16-22

2

2

0

0.182

11 (c)

May 23-29

2

3

2

0.667

3

May 30-Jun. 5

3

3

0

0.182

16 (c)

Jun. 6-12

3

3

1

0.333

9

---

---

---

---

TOTAL

22

22

4

---

94

Seasonal trap efficiency (total recaps/marks released) .............................0.182

Estimated migrants based on seasonal trap efficiency.................................121

(a) Numbers in "CAUGHT" column represent fish caught in the trap Monday through Sunday.

(b) Numbers in the "MARKED" column represent fish marked and released upstream of the trap for trap efficiency tests Sunday through Saturday.

(c) No marked fish recaptured, seasonal trap efficiency used to expand the week’s trap catch to estimate of total migrants for the week.

 

This approach may lead to a less accurate estimate, because most marked fish may be released under different flows than when the majority of marks are recovered. More importantly, if the total number of marks recovered is low (say less than 5), the expanded estimate of migrants may change significantly if the number of marks recovered is changed by even one fish. In some streams, trap efficiencies and total number of fish caught can be improved by adding weir panels to force more of the stream flow in front of the trap.

 

Calculating the 95% confidence interval for the population estimate

    Weekly population size (N*) and variance is estimated using bootstrap methodology (Thedinga et al. 1994). The bootstrap estimate of the variance is used because it can be easily extended to include other sources of variation, such as mortality of marked fish and tag loss, in the estimated variance. Recaptured marked fish (r) and total captured fish (c) are assumed to be binomial random variables (r~bin(m,e), c~bin(N*,e) where m is the number of marked fish in the population, e is the trap efficiency, and N* is the estimated population size. Repeated samples are drawn from these distributions and a population size is estimated as m*c/r. The variance is estimated from the distribution of these simulated population estimates. We use 1000 iterations to estimate the bootstrap variance. Variances from each sampling period are summed and the 95% confidence interval estimated as +/- 1.96 * sqrt (variance). Sampling periods (weeks) can be combined if a Chi Square test reveals no significant differences between the trap efficiency estimates for the two time periods.

 

Trapping Adults

Adult salmon will be captured in traps installed in fish ladders except at West Fork Smith River where a floating weir with trap box will be operated. Fish capture in the traps will be treated in the following manner:

  • 1. Fish will be dip-netted from the trap and handled with wool gloves soaked in Pro-poly-aqua.

  • 2. Fish will not be anesthetized. They will be placed in a hooded measuring cradle.

  • 3. Fork length will be recorded.

  • 4. Two numbered Floy tags will be inserted below the dorsal fin, one on each side.

  • 5. Marked hatchery fish will not be passed above the traps. Most sites should see few, if any, hatchery fish. At these sites the occasional hatchery fish will be opercle-punched and released downstream.

  • 6. Hatchery fish will occur in significant numbers at the North Fork Nehalem Trap.

    • a. Hatchery coho will be returned to Nehalem Hatchery for disposal. These fish will be killed at the trap unless the hatchery is in need of broodstock.
    • b. All unmarked coho will be examined for CWTs with a detection wand and any with tags will be returned to the hatchery.

    • c. Hatchery steelhead will be returned downstream to a designated site and returned to the river to recycle them through the fishery.

  • 7. Scale samples will be collected from all unmarked coho passed upstream at the North Fork Nehalem trap in 1998 because ~10% of the hatchery coho returning will be unmarked.

  • 8. Wild fish passed above the traps will be released in protected areas either in the ladders or nearby. A special area was designed for this purpose in the West Fork Smith River trap.

 

Estimating Adult Abundance

Because the sites may not be total blockages to fish passage, a simple mark-recapture method will be used to estimate the total number of each species passing the trap site. Tagged and untagged fish will either be counted during spawning surveys in the basins above the traps or recaptured in traps located upstream from the primary trap (e.g. Little Nestucca River).

 

REFERENCES

McLemore, C.E., F.H. Everest, W. R. Humphreys, and M.F. Solazzi. 1989. A Floating Trap for sampling downstream migrant fishes. Research note PNW-RN-490, Pacific Northwest Research Station, United States Forest Service, Corvallis, Oregon.

Roper, B.B. 1995. Ecology of anadromous salmonids within the Upper South Umpqua Basin. Doctoral Dissertation. University of Idaho, Moscow, Idaho.

Thedinga, J.F., M.L. Murphy, S.W. Johnson, J.M. Lorenz, and K.V. Koski. 1994. Determination of Salmonid Smolt Yeild with Rotary-Screw Traps in the Situk River, Alaska, to Predict Effects of Glacial Flooding. NAJFM 14:837-851.

 


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