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Chapter 6: Coldwater Fish in Small Standing Waters

Nigel P. Lester, Paul E. Bailey, and Wayne A. Hubert

6.1 INTRODUCTION

6.1.1 Definition of Water Body

This chapter describes standard techniques for sampling coldwater fishes in small stand-ing waters. Within the context of this book, coldwater fish species are those that prefer water temperatures less than 158C, and small standing waters are lakes and reservoirs where surface area is less than 200 ha. Chapter 7 of this book describes sampling coldwa-ter fishes in large standing waters (i.e., surface area > 200 ha). The criterion that separates small and large waters is arbitrary and does not imply that different methods are required depending on, for example, whether a lake is 199 or 201 ha. Two chapters are dedicated to sampling coldwater fishes in standing waters because lake size varies by several orders of magnitude and some differences in sampling methods are needed to achieve efficient sampling at both ends of the lake-size continuum. Although it is clear that some differ-ences in methods are necessary to accommodate extremes in lake sizes, it is not clear when the transition from methods for small lakes to methods for large lakes should apply. To bridge this gap, we describe methods of sampling coldwater fish in small lakes and reser-voirs that are compatible with a subset of the methods proposed for coldwater fish in large lakes and reservoirs (see Chapter 7).

The method proposed for sampling coldwater fish in small standing waters is depth-stratified summer gill netting. We acknowledge that coldwater species often inhabit the same lakes and reservoirs as warmwater fish. Thermal stratification during summer influ-ences the depth distribution of coldwater and warmwater species, and a depth-stratified survey can sample both temperature guilds. For this reason, gill-netting methods for sam-pling coldwater fishes have been chosen so that they are compatible with gill-netting methods proposed for sampling warmwater fishes (i.e., Chapters 2 and 3).

6.1.2 Fish Assemblages Targeted

Most small lakes in central and southern North America cannot support coldwater spe-cies because water temperatures often exceed 208C during summer. Suitable summer habitat for coldwater species typically exists in deep lakes, where thermal stratification

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occurs during the summer to supply a coldwater hypolimnetic refuge, or in shallow lakes at high elevations in mountainous areas or high latitudes in Canadian provinces or Alas-ka. Such lakes are common on the boreal shield and mountain zones of North America and somewhat common at high-elevations on the western Great Plains.

Coldwater fish assemblages in small lakes and reservoirs can include several important sport fishes, most of which belong to the salmonid family. In boreal lakes, lake trout and brook trout are the primary target species for sport fisheries management, whereas lake whitefish, ciscoes, and burbot receive relatively little attention from anglers. Other target species in the boreal zone include Arctic char (in the north) and Arctic grayling (in the north and west). Rainbow trout and cutthroat trout are commonly the focus in mountain lakes. Other trouts and salmons are found much less frequently in mountain lakes, and nongame species are rarely found in this environment. In plains lakes, rainbow trout are the most commonly stocked species, with lake trout, brook trout, Arctic grayling, cut-throat trout, and brown trout also occurring. Nongame species, primarily suckers, com-mon carp, and minnows, are also common in plains lakes.

Because fisheries management is often driven by sport fish issues, sampling methods focus on sport species and the length ranges of fishes typically harvested by anglers. We describe methods to obtain indices of abundance, length structure, and body condition of these species. Measurements of other life history traits (e.g., age, maturation, or fe-cundity) are not covered, although fish to provide these data can be collected using the standard sampling methods. Although the methods focus on sampling over length ranges of fish exploited by anglers, information about smaller or larger members of the fish as-semblage is often desired. For this reason, we describe optional sampling methods for assessing other length-classes of fish. We also identify environmental data that are needed when designing a survey or monitoring program and interpreting results.

6.1.3 Standard Sampling Methods

6.1.3.1 Logic of gear selection

Standard methods for sampling coldwater species in small standing waters are based on several caveats. A reliable index of abundance may be obtained by sampling when the spa-tial distribution of fish is relatively even and stable. Therefore, the sampling time should avoid periods when fish are heavily congregated due to spawning behavior or when spatial distribution is in flux due to changing temperature. The best sampling time for small coldwater lakes is usually during summer when lakes are thermally stable and spawning behaviors by target species are not occurring. Thermal stratification of lakes during sum-mer implies that optimal temperatures for coldwater species vary with depth. Therefore a depth-stratified sampling design is needed where stratification occurs. Because target species vary in terms of their association with benthic and pelagic zones, a gear that can sample both habitats is needed.

Given these caveats, we identified gill netting during summer as the most appropriate method to sample coldwater fishes in small lakes and reservoirs. Gill nets can be set on the

87coldwater fish in small standing waters

bottom to assess fish abundance in the benthic zone or they can be suspended at various depths to assess abundance in the pelagic zone. The latter method of deployment is less common and more difficult, but it is a means to supplement benthic gill netting when indexing fish abundance (Appelberg 2000).

6.1.3.2 Limitations of selected gear

Use of gill nets to sample fish populations may be controversial because fish captured in gill nets frequently die. When gill nets are set overnight, many captured fish may not survive. Consequently, sampling can impose mortality on fish populations and raise con-cerns that the population may be impacted by sampling. In most instances, the number of fish killed during gill-net sampling is very small relative to the population size and sampling mortality is insignificant relative to natural and fishing mortality rates.

A method that reduces sampling mortality involves setting gill nets for short periods of time (e.g., 2 h) during the day. Because fish are held captive for a shorter period, they are more likely to survive during and after capture. This technique has been tested in On-tario and survival of coldwater species sampled during the summer averaged 70% with 2-h sets, whereas survival was typically less than 35% with overnight sets. Short-duration sets are more laborious than overnight sets, but they may be needed in situations where lethal sampling cannot be tolerated.

Gill netting is a passive capture technique (Hubert 1996), and capture efficiency depends on the movements of fish, as well as factors influencing the ability of fish to avoid or escape nets. Differences in the catchability of daytime and overnight gill netting occur due to diurnal differences in movement patterns of various species and life stages. Catchability differences may also be due to differences in net avoidance because nets are more visible during the day, although this effect varies depending on water clarity. Net-ting studies on lake trout have shown that water clarity has no effect on overnight netting success but can have a large effect on daytime netting success. The effect may be small at low and medium levels of water clarity (e.g., Secchi depth < 8 m), but increases as water clarity increases beyond these levels.

Gill nets are size-selective because captured fish are usually gilled (held by mesh slip-ping behind the operculum) or wedged (held by mesh around the body; Hamley 1975; Hubert 1996). The efficiency of a mesh in retaining a fish depends on fish girth relative to mesh perimeter. When girth is less than mesh perimeter, a fish can swim freely through the mesh. When girth is slightly greater than mesh perimeter, a fish can enter a mesh but cannot pass through the net. When girth is much larger than mesh size, a fish cannot enter a net because its head is too large. The modal girth size of fish captured in a mesh is typically about 1.25 times mesh perimeter. For most trouts and salmons, girth is ap-proximately one-half of total length. Thus, given that mesh perimeter is four times bar mesh size, the modal length of fish caught in a mesh is approximately 10 times the bar mesh size. This rule of thumb works well for most trouts and salmons when mesh size is less than 50 mm (fish length < 500 mm total length). Although it slightly overestimates

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the modal length of fish captured in larger mesh, it is a useful approximation of the length range of fish captured by a given mesh size.

Identifying the range of mesh sizes needed to sample a desired length range of fish is relatively easy, but determining the length selectivity of the catch is not easy. Length selectivity describes how the length composition of the catch is related to the length com-position of the fish population. This relationship depends on length-related differences in the probability of encountering a net, contacting the net after encounter, and being retained after contact (Anderson 1998). Indirect methods of estimating length selectivity, based on mesh size and body dimensions, are effective in describing length-related differ-ences in the capture efficiency of fish that encounter a net (Hamley 1975; Kirkwood and Walker 1986; Hansen et al. 1997; Millar and Fryer 1999; Grant et al. 2004), but these methods do not account for length-related differences in encounter rate. Because larger fish swim faster and are more likely to encounter a net, gill netting selects for fish that are larger than the length predicted by net characteristics alone (Rudstam et al. 1984). Consequently, indices of abundance and length composition based on gill netting are generally biased toward larger fish.

6.2 BENTHIC GILL NETTING

6.2.1 Gear Specifications

The standard net for benthic gill netting is described in Table A.3. It contains eight meshes, ranging in bar mesh size from 19 to 64 mm (Figure 6.1) This size range of meshes is effective for sampling trouts and salmons in the length range of approximately 190–650 mm total length. In this book, this net is referred to as “core mesh” because it is the standard adopted for all systems where benthic gill netting is used (see Chapters 2, 3, 6, and 7). Core mesh data can be used for reporting standardized catch per unit effort (CPUE) and length com-position of species for comparison among lakes or within a lake through time.

If data on smaller or larger fish are needed, additional sizes of mesh can be strung on the end of core mesh nets or the additional mesh can be fished independently as separate

38 mm 25 mm 57 mm 19 mm 44 mm 32 mm 64 mm 51 mm

1.50 in 2.25 in 1.00 in 1.75 in 0.75 in 2.50 in 1.25 in 2.00 in

1.8 m

24.8 m

Figure 6.1 North American standardized core benthic gill net, with bar mesh sizes and order diagramed. Pelagic net has same mesh sizes and order but depth of 6 m. Add-on meshes possible to sample additional fish sizes and longer net lengths possible by combining nets.

89coldwater fish in small standing waters

nets. The latter approach is recommended because it avoids the possibility that presence of additional panels of mesh affect catch in the core net due to leading or baiting effects. Attaching additional panels with different sizes of mesh to the core net may be more con-venient, but this approach should not be adopted without testing whether it affects the catch in the core net. If additional panels of mesh are attached to the core net, they should be separated by a space to reduce possible leading effects.

Table A.3 specifies the diameter of the monofilament for each mesh size and panel or-der in the net. Ideally, the monofilament should be sufficiently thin to reduce visibility of the net and increase its catchability. However, the monofilament must also have enough strength to hold fish (i.e., not break) when the net is lifted out of the water. For this rea-son, monofilament diameter increases with mesh size. The value chosen for each mesh exceeds the minimum required to hold the weight of a fish that is expected to be caught in that mesh. Although thinner monofilament would suffice, thicker twine was adopted to produce a more durable net.

Panel order in the core net refers to the sequence of mesh sizes. The standard order of panels of different mesh sizes (mm) is 38, 57, 25, 44, 19, 64, 32, and 51. This order is a compromise between graduated and random. It was chosen to minimize the effect of leading fish along the net, a phenomenon that makes large fish more catchable.

Other attributes of the standard gill net are described in Table A.3. Each panel is 1.8 m in height and 3.1 m in length. Given eight panels of mesh, the standard net length is 24.8 m.

6.2.2 Operation and Deployment

Sampling may be conducted using either overnight sets or short-duration sets. When overnight sets are used, nets are set late in the day and lifted the next morning so that set duration spans two crepuscular periods. The target duration is 17–19 h (e.g., set between 1400 and 1800 hours and retrieved between 0800 and 1200 hours). When short-duration sets are used, nets are set and lifted during daylight hours and the target duration is 2 h.

The standard orientation of the gill net relative to depth contours is perpendicular or oblique but not parallel. The end of net (i.e., panel one with 19 mm bar mesh or panel eight with 51 mm bar mesh) that is set closest to shore is randomly assigned.

Deployment and retrieval of a net should follow the procedure described by Hubert (1996): “The net, anchored at both ends, is set as an upright fence of netting along the bottom. Prior to setting, rig the net with appropriate anchors, lines, and buoys. When setting the net, drop the anchor over the bow, and back the boat as the net is played out by handling the float line and shaking out tangles in the mesh.” When the net anchors reach the bottom, continue backing up, gradually playing out the rope attached to the floating buoy until the net hits the bottom and is taut (Figure 6.2). When retrieving the net, start at the downwind end and pull the net over the side of the boat. To avoid tangles and reduce the total work load, stack the gill net into a tub and remove fish as they come out of the water.

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Figure 6.2 Deploying a gill net to sample coldwater fish in a lake.

6.2.3 Sampling Season

Sampling is to be done during the summer when thermal conditions are relatively stable and fish are not congregated due to spawning behavior. Operationally, we define summer for most lakes as the period after thermal stratification and when surface temperature ex-ceeds 158C. In many alpine or Arctic lakes, water temperatures do not exceed 158C and summer is defined as the 4–6-week period of maximum water temperatures.

6.2.4 Sampling Site Selection

Creating depth strata in the sampling design is necessary in many lakes and reservoirs because lentic systems are generally thermally stratified when sampling takes place. Rec-ommended depth strata are 2–6, 6–12, 12–20, and 20–35 m, but this may vary among waters depending on the location of the thermocline, the bottom contours, and the pro-portion of the water surface area of the lake or reservoir within each stratum. If the maximum depth of the lake exceeds 35 m, then the deep stratum may also exceed 35 m, but that is an unlikely circumstance in small lakes and reservoirs. The zone less than 2 m deep is excluded because the standard gill net does not sample effectively in water depths less than its height. Strict adherence to this classification of depth strata is not manda-tory because processing of data can accommodate differences in strata definitions when a standard CPUE is calculated (see below). Other depth strata boundaries are acceptable provided they span the entire range of depths of the lake.

Additional habitat strata (e.g., landscape features or substrates) may be used, provided that they can be identified and mapped. Quantification of the projected surface area of

91coldwater fish in small standing waters

individual habitat types is needed to produce stratified (i.e., area-weighted) estimates of CPUE.

Sampling sites are selected randomly within each stratum. If fish populations in a lake are resampled at a future point in time, a new set of random sites is picked. The minimum number of sites sampled in each stratum is three because estimates of catch variance within each stratum are needed to calculate the variance of the stratified CPUE (see below). If the planned number of sites is larger than three times the number of strata, then the allocation of sampling among strata should be designed to minimize the vari-ance of the stratified CPUE estimate (i.e., increase precision). The optimum allocation (Cochran 1977) is to sample each stratum in proportion to its size (i.e., surface area) and the variability of its catch. Because variability of the catch increases with abundance, a useful rule of thumb is to sample each stratum in proportion to the product of stratum area times expected CPUE.

If prior information about the relative abundance of fish in different strata is lacking, the number of sites sampled in each stratum should be proportional to stratum area. In thermally stratified lakes, this strategy will not be very efficient for estimating CPUE of coldwater species because a large proportion of the sampling will be allocated to the large shallow stratum (i.e., epilimnion) where coldwater species are not likely to be found. Therefore, for stratified lakes, we recommend minimum sampling (i.e., three sites) in epilimnetic strata and sampling proportional to stratum area for deeper strata. If previ-ous surveys indicate that CPUE differs among strata, this information should be used to optimize the sampling design, as outlined above. Relatively more sampling would be allocated to high-density strata.

6.2.5 Computation of CPUE and Size Structure

Catch data should be recorded for the core mesh. The data reported for each set of a net include set time, lift time, depth at each end of the net, and number and lengths of each species caught for each mesh size. In addition, a sample of fish weights should be recorded so that the weight–length relationship can be described. The set and lift times are not needed to calculate CPUE, but they are useful for checking that the data are consistent with stated standards. The depths over which the net was set are used to allocate a set to specific depth stratum.

When overnight sets are used, one unit of effort is an overnight set. When short-du-ration sets are used, one unit of effort is a 2-h set. The CPUE indexes the catch of one net set. Overnight CPUE and 2-h CPUE are different indices of abundance and should not be converted to a common unit simply by adjusting for the difference in set duration.Because data are collected using a depth-stratified design, calculation of mean CPUE must also be stratified. Mean CPUE is first calculated for each stratum, and then a weighted mean CPUE is calculated, where weights are assigned based on the projected surface area of each stratum (see Chapter 11 and references therein). This stratified estimate corresponds to the estimate of mean CPUE that would be expected if sampling was not

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stratified (i.e., sample sites were randomly selected from the entire lake). The advantage of stratification is that the variance of this estimate is likely to be less than the variance re-sulting from random sampling without stratification. The variance of the stratified mean CPUE is calculated from variances of catches within each stratum (see Chapter 11 and references therein).

Stratified estimation methods are also used when calculating length structure of the population. Because the length composition of fish sampled in different strata is likely to vary (e.g., Rhea et al. 2005), an overall estimate of length composition (and mean length) must be weighted by estimates of relative abundance in each stratum. See Chapter 11 and references therein for details.

When reporting CPUE and length statistics, the duration of sets (overnight or short-duration) must be identified and the mean duration of sets should be noted. If additional panels of meshes are used in the survey, results should be reported separately for the core net and the additional panels of meshes. Also, the additional mesh sizes and their method of deployment (attached to core mesh or separate net) should be documented.

6.2.6 Amount of Effort

The amount of sampling needed depends on survey objectives and the required precision of CPUE estimates. If the objective is to detect temporal changes in abundance within one lake, then precise estimates of CPUE are usually desired because precision dictates how much of a change can be detected. Alternatively, if the objective is to test whether CPUE differs from a specified level, a precise estimate may not be necessary. For example, if the true deviation was very large, then a relatively imprecise estimate of CPUE would suffice.

Calculation of the sample sizes needed to meet these various objectives requires a base of data that describes within-lake variation in the catches of targeted species (Peter-man 1990; Lester et al. 1996). General sample size equations for stratified and random sampling are described in Chapter 11 and references therein; however, there are sampling size considerations specific to small coldwater lake sampling. We recommend sample size guidelines based on lake area. For overnight sampling, follow the recommendation of Appelberg (2000): the minimum effort is eight sites for lakes of 50–200 ha and four sites for lakes less than 50 ha. For short set sampling, the recommended minimum effort is four times the values given for overnight sets: 32 sites for lakes of 50–200 ha and 16 sites for lakes less than 50 ha. These guidelines assume sampling at least three sites within each stratum. However, sampling effort may be reduced for lakes where sensitive species exist in order to reduce impacts on these species.

The sampling guidelines do not address sample size issues concerning estimation of fish length and weight statistics. The number of fish sampled will vary depending on CPUE. If CPUE is low, more net sets may be needed to obtain a desired sample of fish. Recommended sample sizes and methods of estimating these statistics are described in Chapter 11.

93coldwater fish in small standing waters

6.2.7 Ancillary Data Needs

Ancillary data needs include a bathymetric map as well as measurements of several envi-ronmental variables that can be obtained during the survey. A bathymetric map is needed to design the survey and to obtain a depth-stratified estimate of CPUE. Environmental variables are depth at which a Secchi disk is no longer visible, a water temperature pro-file, and a dissolved oxygen profile. Although these variables are not needed to calculate CPUE, they are valuable when interpreting results.

6.3 PELAGIC GILL NETTING

Pelagic gill netting, which samples fish suspended in the water column, is much less com-mon than benthic gill netting, primarily because setting suspended gill nets is difficult. We are not aware of any emerging standards in North America, but there is a proposed standard gill-netting approach using two types of nets, one for benthic sampling and one for pelagic sampling, in Europe (Appelberg 2000). The nets are constructed with the same mesh sizes, but the pelagic net is much taller (i.e., 6 m) than the benthic net (1.8 m). The same approach for sampling coldwater species is to be applied in small standing waters of North America.

6.3.1 Specifications

The net used for pelagic gill netting is the same as for benthic gill netting, except that the net height is 6 m (20 ft). Other attributes are as described in Table A.3.

6.3.2 Operation and Deployment

Pelagic gill netting may be conducted with either overnight sets or short-duration (2 h) day-time sets, but given the increased labor involved in setting this gear, short-duration sets may not be practical. For overnight sampling, nets are set late in the day and lifted the next morn-ing so that set duration spans two crepuscular periods. The target duration is 18–20 h.

Orientation of the gear relative to contours is random. The procedure for setting a net is the same as for benthic gill netting, except that a series of floats and dropper lines are used to suspend the net at a prescribed depth. The same technique is used to sample coldwater fish in large standing waters (see Figure 7.3 and Box 7.2).

6.3.3 Timing of Sampling

Like benthic gill netting, pelagic gill netting is conducted during the summer when lakes are thermally stratified.

6.3.4 Selection of Sampling Sites

Nets are set over the deepest part of the lake or in each deep basin. A depth profile is taken by sampling at depths of 0, 6, 12, 20, and 30 m, the maximum depth being no deeper than 6 m above the lake bottom.

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6.3.5 Computation of Effort and CPUE

Computation of CPUE is the same as for benthic gill netting, except that a depth-stratified estimate is not calculated. Instead, mean CPUE is reported for each depth stratum.

6.3.6 Recommendations for Amount of Effort

Ideally, at least three sets at each depth interval should be sampled, as this will allow calculation of variance in CPUE. In practice, the time available for pelagic netting (i.e., number of nights) may be dictated by the sample size chosen for benthic gill netting.

6.3.7 Ancillary Data Needs

Ancillary data needs are the same as for benthic gill netting.

6.4 FINAL CONSIDERATIONS

Standard sampling protocols for coldwater species in small standing waters involve the use of two types of gill netting, benthic and pelagic. Both methods are needed to conduct whole-lake surveys, but only one may be necessary to monitor temporal trends in targeted fish populations.

Bar mesh size (mm)

Mon

ofila

men

t dia

met

er (m

m)

0 10 20 30 40 50 60 700.0

0.1

0.2

0.3

0.4

0.5

European netNorth American net

Figure 6.3 Comparison of the North American and European standard net for benthic gill netting.

95coldwater fish in small standing waters

The benthic gill-netting method is a compromise seeking common ground among a variety of standard methods used by state and provincial agencies in North America. The core net concept provides an opportunity for agencies to adhere to an international standard without compromising local needs. The range of mesh sizes in a core net (i.e., the standard benthic net) implies fish length selectivity of the gear that focuses on trout in the approximate length range of 190–650 mm total length. The CPUE reported for core nets can be used to develop indices of abundance for fish in this length range. If data are required for a wider length range, additional panels of different mesh sizes may be used. Provided that data are reported separately for the “core net” and “additional panels” of meshes, a standard for comparing data among lakes and reservoirs is maintained.

The pelagic gill-netting standard was not based on a current protocol because gill nets have not been widely used in this way in North America. In defining a pelagic fish sam-pling standard, we followed the European approach where the same mesh sizes are used to sample both habitats, but taller nets are used in the pelagic zone (Appelberg 2000).

Although our approach to developing a gill-netting standard has followed the Eu-ropean example, the standard net (which is labeled the North American net) is not the same as the European net (Figure 6.3). The North American net has a range of mesh sizes of 19–64 mm, whereas the European net has a range of mesh sizes of 6–55 mm. A different set of meshes was selected because management agencies in North American generally have objectives that target larger fish (i.e., the length of fish typically harvested by anglers). The North American net is effective for sampling trout and salmon in the length range of 190–650 mm, whereas the European net samples a different length range (63–550 mm). The nets also differ in monofilament diameter where overlapping mesh sizes occur, with a thicker diameter used in the North American net compared to the Eu-ropean net. This difference makes the North American net more durable, but it probably increases its visibility and reduces its catchability to some extent.

To address concerns about lethal sampling with gill nets, an alternative that attempts to use gill nets as a live capture method has been identified as a standard protocol. Gill netting can be done using overnight or 2-h set durations. Although the 2-h method is less lethal than overnight sampling, it is also less efficient and more vulnerable to variation in catchability due to water clarity.

6.5 REFERENCESAppelberg, M. 2000. Swedish standard methods for sampling freshwater fish with multi-mesh gill

nets. Fiskeriverket Information 2000:1, Goteborg, Sweden.Anderson, C. S. 1998. Partitioning total selectivity of gill nets for walleye (Stizostedion vitre-

um) into encounter, contact, and retention components. Canadian Journal of Fisheries and Aquatic Sciences 55:1854–1863.

Cochran, W. G. 1977. Sampling techniques. Wiley, New York.Grant, G. C., P. Radomski, and C. S. Anderson. 2004. Using underwater video to directly esti-

mate gear selectivity: the retention probability for walleye (Sander vitreus) in gill nets. Cana-dian Journal of Fisheries and Aquatic Sciences 61:168–174.

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Hamley, J. M. 1975. Review of gillnet selectivity. Journal of the Fisheries Research Board of Canada 32:1943–1969.

Hansen, M. J., C. P. Madenjian, J. H. Selgeby, and T. E. Helser. 1997. Gillnet selectivity for lake trout (Salvelinus namaycush) in Lake Superior. Canadian Journal of Fisheries and Aquatic Sciences 54:2483–2490.

Hubert, W. A. 1996. Passive capture techniques. Pages 157–192 in B. R. Murphy and D. W. Willis, editors. Fisheries techniques, 2nd edition. American Fisheries Society, Bethesda, Maryland.

Kirkwood, G. P., and T. I. Walker. 1986. Gill net mesh selectivities for gummy shark, Mustelus antarcticus Gunther, taken in eastern Australian waters. Australian Journal of Marine and Freshwater Research 37:689–697.

Lester, N. P., W. I. Dunlop and C. C. Willox. 1996. Detecting changes in the nearshore fish com-munity. Canadian Journal of Fisheries and Aquatic Sciences 53(Supplement 1):391–402.

Millar, R. B., and R. J. Fryer. 1999. Estimating the size-selection curves of towed gears, traps, nets and hooks. Reviews in Fish Biology and Fisheries 9:89–116.

Peterman, R. M. 1990. Statistical power can improve fisheries research and management. Cana-dian Journal of Fisheries and Aquatic Sciences 47:2–15.

Rhea, D. T., W. A. Hubert, R. S. Gangl, and R. A. Whaley. 2005. Temporal and spatial variation in relative abundance and length structure of salmonids in reservoirs. North American Jour-nal of Fisheries Management 25:1301–1309.

Rudstam, L. G., J. J. Magnuson, and W. M. Tonn. 1984. Size selectivity of passive fishing gear: a correction for encounter probability applied to gill nets. Canadian Journal of Fisheries and Aquatic Sciences 41:1252–1255.