North American Journal of Management 11:167-176. 1991 © Copyright by the American Fisheries Society 1991

Stepped-Oblique Midwater as an Assessment Technique for Rainbow Smelt RICHARD A. KIRN* AND GEORGE W. LABAR School of Natural Resources, University of Vermont Burlington, Vermont 05405, USA

Abstract.—Discrete-depth midwater trawling for rainbow smelt Osmerus mordax at night in Lake Champlain revealed a delayed vertical migration of smaller fish and size partitioning vertically within the . These behavioral observations led to the development and evaluation of a nighttime stepped-oblique midwater trawling technique. This latter method proved effective in obtaining large catches of rainbow smelt (mean catch per unit of effort [CPUE], 215-655 fish/ 55-min trawl) and provided estimates of relative abundance with reasonable precision (CPUE ± 13-50% from June through October), as well as estimates of length and age distributions unbiased by vertical migration behavior. Rainbow smelt Osmerus mordax serve as the vide information on the sexually mature portion principal forage species for stocked salmonids in of the population, limiting the value of such sur- Lake Champlain (Kirn 1986) and many other New veys for monitoring year-class dynamics. In ad- England waters (Kircheis and Stanley 1981). The dition, this approach is not feasible for waters with desirability of rainbow smelt as a food and forage significant lake-spawning populations of rainbow fish, and the relative ease with which they are smelt, such as are found in Lake Champlain (Plo- transferred to other waters, is resulting in a con- sila 1984). In recent years, hydroacoustic sampling tinually expanding distribution of this species has proven to be a reliable technique for moni- (Maydenetal. 1987). toring the abundance of stocks (Argyle Knowledge of forage-fish population dynamics 1982; Heist and Swenson 1983), but requires ex- is an important component of recent freshwater pensive equipment and must be integrated with salmonid introduction and restoration programs, other sampling gear to determine species com- especially in situations such as in Lake Champlain, position and obtain biological samples. where recruitment of salmonids relies almost ex- Daytime bottom and midwater trawling in Lake clusively upon hatchery production and is there- Huron indicated that discrete portions of the rain- fore independent of forage abundance. The ability bow smelt populations inhabited both the bottom to recognize fluctuations in rainbow smelt abun- and midwater during the day, and were therefore dance before predator growth is affected is partic- unavailable to a single trawling method (Argyle ularly critical given the potential for extreme vari- 1982). Observations of size partitioning among ability in the abundance of rainbow smelt, which daytime rainbow smelt distributions have been apparently is unrelated to stock size, predation, associated with bottom depth and possibly water intensity, or disease (Smith 1972; Havey temperature (MacCallum and Regier 1970; Argyle 1973; Kircheis and Stanley 1981; Selgeby 1985). 1982; Plosila 1982). Length or age compositions Sampling rainbow smelt populations has been estimated under these conditions become a func- complicated by size- or age-specific seasonal dis- tion of trawling location and effort rather than a tributions (MacCallum and Regier 1970; Argyle random sample of the population at large (Argyle 1982; Plosila 1982) and diel vertical migrations 1982). (Ferguson 1965; Gray 1979; Heist and Swenson Midwater trawling at night, when diel vertical 1983). Population dynamics of Great Lakes rain- migration of rainbow smelt provides a concen- bow smelt have been assessed from spring spawn- trated, pelagic distribution, has shown promise as ing runs, during which concentrations of rainbow an effective sampling strategy. Rainbow smelt con- smelt in tributaries are vulnerable to a variety of stituted 90-99% of the nighttime pelagic fish pop- sampling gears (Luey and Adelman 1984; Frie and ulation sampled with midwater trawls in the upper Spangler 1985). These surveys, however, only pro- 50 m of Lake Superior (Heist and Swenson 1983). Nighttime midwater-trawl sampling for cisco Coregonus artedii in Lake Champlain has also 1 Present address: Vermont Fish and Wildlife De- produced large catches of rainbow smelt (G.W.L., partment, Roxbury, Vermont 05669, USA. unpublished data). 167 168 KIRN AND LABAR

Considering the influences of rainbow smelt be- graduated and ranged from 20.3 cm at the head- havior on past sampling strategies, the specific ob- rope to 1.3 cm at the -end liner. Although this jectives of this study were to develop and evaluate net was capable of retaining some young-of-the- a nighttime midwater-trawling technique for sam- year rainbow smelt, many escaped. For this rea- pling rainbow smelt populations. Results from son, young-of-the-year fish, distinguished by size preliminary discrete-depth trawls, also presented, and transparency, were not included in data anal- led to the development of a stepped-oblique mid- yses. water-trawling technique. This technique was The net was towed at 1.1 m/s by the Vermont evaluated for consistency in estimating length and Fish and Wildlife Department's 9.4-m research age distributions and relative abundance of rain- vessel Dore. Temperature and net depth were bow smelt. The influence of the temporal and spa- monitored with an ultrasonic temperature-pres- tial movements of rainbow smelt were also eval- sure transducer attached to the port bridle near uated and used to develop an overall sampling the headrope. strategy for monitoring the population. Discrete-depth trawls.—Discrete-depth trawls were performed during June, August, and October Study Area 1984, and during June and August 1985. Prelim- Lake Champlain is a typically dimictic lake lo- inary trawls in June 1984 sampled sites in Malletts cated within a north-south, glaciated fault be- Bay and the Northeast Arm (Figure 1). Subsequent tween the Adirondack Mountains of New York trawls were confined to the Main Lake. and the Green Mountains of Vermont. Meyer and Discrete-depth trawls consisted of a series of 20- Gruendling (1979) provided a detailed account of min trawls at each of three predetermined depths: the limnology of Lake Champlain. Lake dimen- midway between the surface and the top of the sions are length, 172 km; maximum width, 19 km; metalimnion, at the top of the metalimnion, and maximum depth, 129 m; mean depth, 22 m; sur- at the bottom of the metalimnion. Because of a face area, 1,140 km2. Plosila and Anderson (1985) paucity of rainbow smelt (4% of the total catch in estimated 45% of the lake to possess deep, cold June), trawls midway between the top of the meta- water with sufficient dissolved oxygen to support limnion and the surface were replaced by trawls salmonids. 10 m below the metalimnion after June 1984. In Potash et al. (1969) described five limnologically the absence of thermal stratification, depths of 15, distinct water masses in Lake Champlain: South 25, and 35 m were sampled. All three depths were Lake, Main Lake, Malletts Bay, Northeast Arm, sampled before a second series of trawls was re- and Missisquoi Bay (Figure 1). New York and peated in sequence. Trawls conducted in June 1984 Vermont state investigators have further were begun within 10 min after sunset. Trawls partitioned the lake into 15 management zones. conducted after June 1984 were generally initiated Sampling during this study concentrated on zones within 90 min after sunset. Two series of discrete- 3 A and 3B of the Main Lake (Figure 1), where the depth trawls were conducted per night except in highest densities of lake trout have been reported August 1984, when three series were conducted at (Plosila and Anderson 1985). The Main Lake ac- two of the four sites sampled, and in October 1984, counts for 82% of the entire volume and 60% of when three series were conducted at 25 and 35m. the total surface area of Lake Champlain (Potash Stepped-oblique trawls.— Stepped-oblique trawls et al. 1969). Maximum surface temperatures are were deployed at least 10 m above the lake bottom, reported to reach about 23°C, while summer hy- or to a maximum depth of 35 m. Once set, the polimnetic temperatures average 6°C (Meyer and net was fished for 5 min at each 3.3-m depth stra- Gruendling 1979). Summer stratification of the tum, in stairstep fashion, until the net reached a Main Lake is reported to generally begin in June depth of about 7 m, when it was completely re- (Meyer and Gruendling 1979). Thermal stratifi- trieved. During preliminary discrete-depth trawls cation was only observed in this study during Au- in this study and earlier work (G.W.L., unpub- gust trawl sampling, when the metalimnion had lished data) in Lake Champlain, few rainbow smelt an upper boundary at 10-12 m and a lower bound- were captured in the upper 7 m of the water col- ary ranging from 17 to 25 m. umn. Although the actual net dimensions during trawling were unknown, it was assumed that the Methods 3.3-m depth strata were sufficient to ensure that Rainbow smelt were sampled with a 5-m x 5-m the net sampled the entire water column. A rep- midwater trawl. Mesh size (stretched measure) was licate trawl was then fished along the same course MIDWATER TRAWLING FOR RAINBOW SMELT 169

FIGURE 1.—Map of Lake Champlain (left) showing water masses and fishery management zones 3 A and 3B. In the large-scale map of zone 3 A and 3B (right), the lines connecting triangles, squares, and circles indicate shallow (<45-m), intermediate (45-75-m), and deep (>75-m) trawl sites, respectively. in the opposite direction. Based on results from sured to the nearest millimeter (total length). Ad- the discrete-depth trawls, stepped-oblique trawl- ditional fish were counted. ing was begun at least 1 h after sunset to allow Sites in the 45-75-m and below-75-m depth rainbow smelt to complete vertical migration. ranges resulted in stepped-oblique trawling times Replicate stepped-oblique trawls were used to of 55 min (i.e., 11 "steps"). Due to the more shal- evaluate spatial and seasonal differences in rain- low deployment of the net at sites above 45 m bow smelt abundance and length and age distri- deep, trawl times were limited to 35 min (i.e., butions. Sites were chosen within management seven "steps"). Therefore, to obtain a standard- zones 3 A and 3B (Figure 1) in each of three depth ized total rainbow smelt catch per trawl (CPUE) ranges: above 45 m, 45-75 m, and below 75 m. as an index of relative abundance, smelt catches Replicate stepped-oblique trawls at these six sites from the shallow sites were proportionately ad- were scheduled for August and October 1984, and justed to represent a 55-min trawl (i.e., [catch/35] for April, June, August, and October 1985. Smelt x 55). The time of deployment and retrieval of catches from these sites and an additional Main the net was not included in CPUE calculations. Lake site were also used to evaluate differences in The two-sample Kolmogorov-Smirnov (K-S) abundance and length distributions between rep- statistical procedure (SPSSX 1983) was used to licate trawls. evaluate differences between length distributions and statistical analysis. —When of replicate stepped-oblique trawls and replicate available, a random sample of 100 rainbow smelt discrete-depth trawls, and between different depth from each discrete-depth trawl and 200 rainbow strata sampled by discrete-depth trawls. Before smelt from each stepped-oblique trawl were mea- analysis, rainbow smelt lengths were truncated to 170 KIRN AND LxBAR the nearest 5-mm increment. Paired /-tests were used to determine systematic differences in total rainbow smelt catch between the first and second replicate stepped-oblique trawls from all sample sites and dates. Variations in the relative abundance of rainbow smelt due to management zones, sampling period, and water depths were examined by analysis of variance based on a hierarchical classification of the 1985 stepped-oblique trawl CPUE (Snedecor and Cochran 1980). To evaluate the precision of abundance estimates, for rainbow smelt from stepped-oblique trawling, 95% confidence inter- vals were calculated for mean CPUE for each man- agement zone when all three sites within the zones were sampled. Similarly, precision estimates for mean CPUE of each' sampling period were cal- culated when all six sites were sampled (two man- agement zones, three sites per zone). Length (mm) Age structure of rainbow smelt captured in FIGURE 2.—Length-frequency distributions of rain- stepped-oblique trawls was used to evaluate the bow smelt captured in replicate discrete-depth trawls catch efficiency of this technique (Ricker 1975). fished at 15 m on June 22, 1984, in Inland Sea, Lake Otoliths were used for age determinations, as de- Champlain. scribed by Kirn (1986). Complete annulus for- mation and thus recruitment of rainbow smelt to suggesting a delayed upward vertical migration of the next age-class was considered to begin in June, smaller fish. In June 1984, length distributions the time of the first trawl sampling following rain- were significantly different (K-S, P < 0.01) for all bow smelt spawning. four comparisons between replicate trawls (i.e., Age composition of combined replicate trawl same depth). Total catches of rainbow smelt were samples, determined from stratified sampling also influenced by the timing of the trawl sample. (Ketchen 1950), was examined statistically for dif- In June 1984, rainbow smelt captured in the sec- ferences due to location and water depth with ond series of trawls accounted for 87% of the total Pearson's chi-square analysis (Dixon et al. 1983). rainbow smelt catch, which also reflected a delayed For each sampling period, comparisons were made vertical migration of smaller fish into the water among sites; among depths (i.e., <45 m versus column. 45-75 m versus >75 m); within specific depths, To reduce the effects of the delayed vertical mi- between zones {e.g., <45 m, 3A versus 3B); and gration of small fish on estimates of rainbow smelt within zones, among depths. Length distributions length distributions, discrete-depth trawls con- (10-mm increments) were similarly tested. ducted in August 1984 were started at least 60 min after sundown. Length distributions differed sig- Results nificantly (P < 0.05) between replicate trawls (i.e., same depth) in only 2 of 19 comparisons, indi- Discrete-Depth Trawls cating that the vertical migration of rainbow smelt In all, 77 discrete-depth trawls were conducted into the water column had generally been com- at nine sampling sites throughout Lake Champlain pleted by 1 h after sunset. during 1984-1985. Rainbow smelt dominated the Subsequent discrete-depth trawls conducted in catch and accounted for 98% of the total; cisco October 1984 and in June and August 1985 did accounted for 1% and were the next largest com- not reveal statistical differences (K-S, P > 0.05) ponent. between replicate trawls in 12 comparisons, al- Preliminary discrete-depth trawls in June 1984 though trawling was begun within 60 min after revealed a difference in the timing of vertical mi- sunset. gration for distinct size-classes of rainbow smelt Size partitioning of rainbow smelt, such that (Figure 2). Smaller fish (<120 mm) were appar- yearling smelt migrated higher into the water col- ently less available for capture in earlier trawls, umn (Figure 3), was observed at all four sites sam- MIDWATER TRAWLING FOR RAINBOW SMELT 171

120 140 160 180 85 105 125 145 165 185 Length (mm) Length (mm) FIGURE 3.—Length-frequency distributions of rain- bow smelt captured in replicate discrete-depth trawls fished at 18 and 28 m on August 27, 1984 in Main Lake, Lake Champlain. Arrow indicates mean length of year- ling rainbow smelt. pled in August 1984 and was again apparent in October 1984 (one site) and June 1985 (one site). Highly significant differences (K-S, P < 0.01) were observed for length distributions among depth strata in these six cases. Trawls in August 1985 (one site) did not reveal this pattern (K-S, P > 0.25). During periods of thermal stratification (i.e., August in 1984 and 1985), greatest catches (82- 98%) of rainbow smelt were observed from trawls fishing the lower limit of the metalimnion and in the hypolimnion. Stepped-Oblique Trawls 85 105 125 145 165 185 Altogether, 67 stepped-oblique midwater trawls Length (mm) were fished in the Main Lake during 1984-1985. FIGURE 4.—Length-frequency distributions of rain- Rainbow smelt again dominated the catch and bow smelt captured in replicate stepped-oblique trawls accounted for 97% of the total catch, followed by during June 1985 in zones 3A and 3B in water deeper cisco, which accounted for 3%. Replicate stepped- than 75 m (upper graph) and in water 45-75 m deep oblique trawls provided reasonably consistent (lower graph) in Main Lake, Lake Champlain. length distributions of rainbow smelt. Only 6 of 31 comparisons (19%) of length distributions from replicate trawls were significantly different (K-S, Total catches of rainbow smelt were also gen- P < 0.05; Table 1). However, statistical differences erally consistent between replicate trawls, except did not necessarily imply practical biological dif- in April 1985 when rainbow smelt were concen- ferences, particularly at significance levels of 0.01 trated for spawning (Table 2). Total rainbow smelt < P < 0.05 (Figure 4). Only 2 of 31 comparisons catch was significantly different between replicate (6%) were found to be significantly different when trawls (paired /-test, P < 0.05; Table 3) only when tested at P < 0.01 (Table 1). April 1985 data were included. 172 KIRN AND LABAR

TABLE 1.—Probability values3 resulting from Kolmogorov-Smirnov comparisons of rainbow smelt length dis- tributions captured in replicate stepped-oblique midwater trawls in zones 3A and 3B of Lake Champlain, 1984- 1985. Numbers in parentheses are the combined sample sizes of replicate trawls.

Zone3A Zone3B Date <45 m 45-75 m >75 m <45m 45-75 m >75m X" Aug 1984 0.528 0.327 0.000** 1.000 (404) (400) (388) (394) Oct 1984 0.797 0.388 0.955 0.050 0.021* 0.414 (440) (398) (330) (396) (402) (459) Apr 1985 0.096 0.890 0.994 0.061 (329) (313) (328) (201) Jun 1985 0.865 , 0.393 0.022* 0.387 0.013* 0.322 (365) (400) (398) (295) (398) (315) Aug 1985 0.003** 0.810 0.010* 0.490 0.975 0.236 (394) (400) (397) (399) (401) (292) Oct 1985 0.144 0.911 0.984 0.584 0.964 (311) (401) (405) (400) (400) a Asterisks indicate significance at P < 0.05* or P < 0.01**. b One site, designated "X," did not conform to any of the three depth categories.

Trawl catches of rainbow smelt, examined with catches of rainbow smelt observed in August and a hierarchical analysis of variance, did not reveal October were associated with recruitment of year- differences between management zones (P > 0.25) ling fish to the catch (Table 5). However, due to or among sampling periods within zones (0.10 < the association of CPUE with water depth, month- P < 0.25). A significant difference (P < 0.05) in ly mean CPUE may not be accurate for sampling total catch was observed between depths within periods in which all sites (and therefore depths) sampling periods. This statistical procedure does were not sampled (August 1984, October 1984 and not provide further information on the nature of 1985). these differences (i.e., which depths differed). Catches in April 1985 were the most variable Trends in mean CPUE were associated with wa- and resulted in 95% confidence intervals (CI) for ter depth and sampling period. Greatest catches CPUE estimates of ±99% and ± 117% for zones of rainbow smelt were generally associated with 3A and 3B, respectively, and ±74% for the com- shallow sites (<45 m) throughout the study (Table bined estimate (Table 4). The most consistent 2). Monthly mean CPUE ranged from 215 rain- catches were in June 1985, when the 95% CI for bow smelt in April to 655 and 502 smelt in August the mean CPUE estimate was ±13% and ±27% of 1984 and 1985, respectively (Table 4). Larger for zones 3A and 3B, respectively, and ±15% for

TABLE 2.—Mean catch per unit of effort (CPUE)3 and coefficients of variationb (in parentheses) of rainbow smelt from replicate stepped-oblique midwater trawls at various sites in the Main Lake of Lake Champlain during 1984- 1985.

Zone 3 A ZoneSB Date <45 mc 45-75 ffl >75m <45mc 45-75 m >75m Aug 1984 1,008 312 407 (24.6) (10.2) (18.8) Oct 1984 648 729 264 531 470 (26.8) (15.6) (42.5) (22.4) (35.1) Apr 1985 30 401 154 569 117 15 (37.4) (95.6) (15.6) (84.0) (13.7) (61.3) Jun 1985 317 268 284 232 318 158 (26.2) (12.6) (5.7) (33.9) (1.5) (17.0) Aug 1985 825 631 392 638 383 147 (34.2) (14.7) (12.6) (30.5) (26.9) (4.4) Oct 1985 146 329 680 335 366 (16.9) (6.9) (16.2) (14.6) (21.3) * CPUE = rainbow smelt catch per 55-min trawl. Catch does not include young of the year. b Coefficient of variation » 100 x SD/mean. c Trawls lasted for 35 min. Catch expanded to represent a 55-min trawl; CPUE = (catch/35) x 55. MIDWATER TRAWLING FOR RAINBOW SMELT 173

TABLE 3.—Paired /-test comparisons of total rainbow TABLE 5.—Mean catch per unit of effort (CPUE)* and smelt catches from replicate stepped-oblique midwater percent composition (in parentheses) of rainbow smelt trawls in Lake Champlain, 1984-1985. age-classes derived from stratified sampling of stepped- oblique midwater trawl catches in Lake Champlain, 1984- Two-tailed 1985. Comparison N t > b All trawls 31 2.14 0.041 Age All trawls except Date 1 2 3 4 >5 Apr 1985 25 1.50 0.150 Aug 1984 352 210 83 10 (54) (32) (13) (2) (0) the combined estimate. Trawling in August 1985 Oct 1984 171 225 106 17 9 (32) (43) (20) (3) (2) provided the only additional complete sample, Apr 1985 88 74 37 15 which resulted in a 95% CI for the mean combined (41) (35) (17) (7) (0) CPUE estimate of ±29%. October samples were Jun 1985 11 107 106 34 5 (4) (41) (40) (13) (2) complete only for management zone 3B in 1984 Aug 1985 161 188 120 29 4 and 1985, but revealed more precise estimates of (32) (38) (24) (6) (1) mean CPUE (±31% and ± 32%, respectively) than Oct 1985 99 110 122 36 4 August catches within the same year and zone (27) (30) (33) (10) (1) (±50%, both years). a CPUE = rainbow smelt catch per 55-min trawl. Catch does not include young of the year. b 3 Recruitment to next age-class was assumed to occur in June TABLE 4.—Mean catch per unit of effort (CPUE) for samples. management zones 3 A and 3B in Lake Champlain from stepped-oblique midwater trawls, 1984-1985. Ninety- five-percent confidence intervals (95% CI) are presented In an attempt to address short-term temporal for each management zone, individually, and combined variability in CPUE at a specific site, additional when complete samples (i.e., two trawls per site, three sites per zone) were obtained. stepped-oblique midwater trawls were fished at the shallow (<45-m) and deep (>75-m) sites in man- Date and agement zone 3B on four and five consecutive 5 statistic Zone3A Zone3B Combined nights, respectively, in August 1987. Daily mean Aug 1984 CPUE estimates were substantially more variable Mean 1,129 575.5 654.6 at the shallow site (Figure 5), possibly due to the SD 357.3 387.7 N 1 6 7 greater influence of physical structure on the dis- 95% CI ±50% tributions of rainbow smelt. In this particular case, Oct 1984 the shallow site transects the mouth and a portion Mean 688.3 421.3 528.1 SD 130.0 162.7 198.2 of a bay, whereas the deepwater site transects a AT 4 6 10 much more uniform habitat of open water greater 95% CI ±31% than 75 m in depth (Figure 1). Apr 1985 Mean 195.0 233.3 214.5 These data were also used to estimate the num- SD 240.9 339.8 281.6 ber of trawls necessary to obtain certain precision N 6 6 12 estimates for mean CPUE at individual sampling 95% CI ±99% ±117% ±74% sites (Snedecor and Cochran 1980). To obtain a Jun 1985 Mean 289.5 235.8 262.7 precision of ±25% of the mean CPUE at the 95% SD 46.7 80.6 68.7 CI, 3 and 11 trawls would be required at the deep N 6 6 12 and shallow sites, respectively. Mean CPUE pre- 95% CI ±13% ±27% ±15% Aug 1985 cision estimates of ±50% would only require one Mean 615.8 388.8 502.3 and three trawls, respectively. SD 236.3 240.7 256.3 Rainbow smelt age composition and length dis- N 6 6 12 tribution varied significantly among sites and 95% CI ±31% ±50% ±29% Oct 1985 among depths throughout this study (Table 6). Mean 237.0 460.0 370.8 However, there was no systematic association of SD 110.1 182.2 189.1 age or length distributions with depth or location. AT 4 6 10 95% CI ±32% Consequently, random variation among sites was assumed, and age structure was summarized in a CPUE = rainbow smelt catch per 55-min trawl. Catch does not include young of the year. each sampling period for all sites (Table 5). Ages b N = number of trawls. 1-3 accounted for about 90% of the total catch in 174 KIRN AND LABAR

Discussion 270- < 45 m Koeller et al. (1986) stressed the need to address 240 species-specific behavioral characteristics (e.g., diel 210 3 or ontogenetic vertical migration) when a survey design was developed for monitoring relative LLJ 180 4 abundance of pelagic fishes. Discrete-depth mid- aD. o 150 water trawling in this study provided important ? 1 observations of rainbow smelt nighttime vertical 120; ' t distributions, although these observations pre- 90 clude the use of this technique for population as- ' sessment. Delayed vertical migration of smaller en size-classes of rainbow smelt was observed in June 1984 trawl sampling. Size or age partitioning, ob- served vertically within the water column at night 270 in this study, has been previously reported for hor- 240 > 75 m izontal (MacCallum and Regier 1970; Argyle 1982; 210 Plosila 1982) and daytime vertical distributions of rainbow smelt (Ferguson 1965). LLJ 180 These behavioral observations must be consid- CL 150 4 ered when a quantitative sampling technique is 0 2 120 3 4 developed for rainbow smelt. Sampling errors in- 90 duced by delayed vertical migration of smaller fish ' * + - + can be easily avoided by sampling after the upward 1234 5 vertical migration has been completed. Size or age Night number partitioning of rainbow smelt in the water column, FIGURE 5.—Mean catch per unit of effort (CPUE; catch however, requires uniform sampling of each depth per 55 min) of rainbow smelt captured in stepped-oblique stratum to obtain unbiased estimates of length and midwater trawls on consecutive nights in management age distributions. Stepped-oblique midwater zone 3B of Lake Champlain in August 1987. Vertical trawling, as described in this study, avoided po- line represents ±SE; the number above the bar represents tential sampling errors introduced by the vertical the number of trawls on a particular night. migration behavior of rainbow smelt and provided consistent catches and length distributions of rain- all sampling periods. Although yearling fish con- bow smelt, as well as sufficient samples for bio- stituted a substantial proportion of the total catch, logical information. rainbow smelt generally were not fully recruited Spatial and seasonal variability must also be to the midwater-trawl gear until age 2. considered when an overall sampling strategy for

TABLE 6.—Chi-square analysis of rainbow smelt age compositions and 10-mm length distributions from stepped- oblique midwater trawls in Lake Champlain, 1984-1985. Significance levels are presented from comparisons of age compositions and length distributions (in parentheses) among sites; among depths; within depths, between zones; between zones; and within zones, among depths.

Within zones, Within depth, between zones among depths Among Among Between Date sites depths <45m 45-75 m >75m zones 3A 3B Aug 1984 <0.001 <0.001 <0.001 <0.001 <0.001 (<0.001) (<0.001) (<0.001) (<0.001) (<0.00l) Oci 1984 <0.001 <0.001 <0.001 0.222 0.001 <0.001 <0.001 (<0.001) (<0.001) (0.397) (0.160) (0.093) (<0.001) (0.002) Apr 1985 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 (0.008) (0.001) (0.135) (0.010) (0.028) (<0.001) Jun 1985 <0.001 <0.001 <0.001 <0.001 0.371 <0.001 <0.001 <0.001 (<0.001) (0.003) (<0.001) (0.012) (0.031) (0.003) (<0.001) (<0.001) Aug 1985 <0.001 <0.001 0.179 <0.001 0.014 <0.001 0.001 <0.001 (<0.001) (0.001) (<0.001) (0.020) (<0.001) (<0.001) (0.183) (<0.001) Oct 1985 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 (0.023) (0.006) (0.091) (0.460) (0.002) (0.135) (0.359) MIDWATER TRAWLING FOR RAINBOW SMELT 175 monitoring rainbow smelt populations is devel- smelt (>90 mm) were most likely the result of oped. Sampling several depth ranges in an area, immigration rather than the fish attaining a gear- as in this study, should help to avoid potential selective size threshold. sampling errors that may result from environ- Gear restrictions limited trawl sampling to the mentally induced spatial or temporal distribu- upper 35 m of the water column during this study. tions. Length and age distributions were inconsis- To obtain representative samples of the rainbow tent among sites within sampling periods, although smelt population with stepped-oblique trawls, we no systematic association with location or water must assume that the majority of rainbow smelt depth was observed. were confined to this stratum. If a substantial por- Although substantial catches of rainbow smelt tion of the rainbow smelt population was not sam- were observed at all sites, the largest catches of pled in our sites deeper than 45 m, we would ex- smelt were generally associated with shallow sites pect larger rainbow smelt to be systematically (<45 m) during this study. These findings are con- underrepresented in these samples when com- sistent with observations on Great Lakes rainbow pared with samples from shallow (<45-m) sites smelt populations (Ferguson 1965; Gray 1979; (where the entire water column was sampled), due Crowder 1980; Heist and Swenson 1983). Heist to stratification of larger rainbow smelt in deeper and Swenson (1983) reported highest densities and waters. This pattern was not observed. abundances of rainbow smelt in shallow waters The results of this study suggest that stepped- (<50 m) of Lake Superior, with a significant de- oblique midwater trawling at night is an effective cline in densities with increasing bottom depth. sampling technique for obtaining estimates of rel- They suggested that nearshore distributions of ative abundance with acceptable precision levels rainbow smelt were associated with warmer tem- and unbiased estimates of length and age distri- peratures and daily vertical movements. The noc- butions of rainbow smelt. Replicate stepped- turnal spawning habits of rainbow smelt (Scott and oblique trawls provided CPUE estimates for in- Crossman 1973) probably resulted in the highly dividual management zones with precisions of variable catches among sites and between replicate ± 13-50% for samples in June through October. trawls in April. The effect of shoreward spawning The addition of a third oblique trawl per site would movements on abundance measurements also led reduce these ranges an additional 5-9% per man- Argyle (1982) to recommend fall surveys due to agement zone if similar variation between trawls the more predictable locations of rainbow smelt is assumed. This trawling procedure is mechani- at that time. cally "clean," unlike , and was es- Although June sampling provided the most con- sentially species specific for rainbow smelt, which sistent rainbow smelt CPUE among sites (95% CI constituted 97% of the total catch in Lake Cham- = ± 15%), sampling in August and October was plain. Although a large vessel and net were used more advantageous, because in those months, in this study, the procedure and equipment could trawls provided the greatest rainbow smelt catches, be easily scaled down for use with small boats to including large catches of yearling smelt. The sea- provide an inexpensive and effective monitoring sonal peaks of rainbow smelt CPUE in August technique for state agencies with limited resources. observed in this study were also reported for Lake Acknowledgments Erie (Ferguson 1965). Increasing water tempera- tures and thermal stratification were cited as the We thank Richard Furbush, Captain of the RV major factors initiating the return of rainbow smelt Dore, who provided valuable assistance through- to deeper waters and increasing their tendency to out the study. We also thank Kathy Newbrough form schools. During the summer months in Lake for her help in the laboratory, field, and office. Erie, both yearling and adult rainbow smelt were Gary Badger and Michael Constanza provided ad- observed moving into the thermocline and becom- vice on statistical techniques, and Barry Johnson ing more vulnerable to trawling gear (MacCallum critically reviewed the manuscript. The Vermont and Regier 1970). Thermal stratification of Lake Fish and Wildlife Department provided the use of Champlain was not observed during trawl sam- their research vessel. The project was funded by pling in 1985 until August, when increased catches the U.S. Fish and Wildlife Service, Project AFS- reflected recruitment of yearling rainbow smelt to 6-R-2. the trawl. Considering that young-of-the-year smelt References as small as 40 mm were often captured in large Argyle, R. L. 1982. Alewives and rainbow smelt in numbers, increased catches of yearling rainbow Lake Huron: midwater and bottom aggregations and 176 KIRN AND LABAR

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