John L. Forney
Department of Natural Resources, Cornell University
Ithaca, New York
High-speed Miller samplers have been used in Oneida Lake to estimate abundance of larval walleyes during the pelagic stage since 1966. Similarity between estimates from successive surveys in the same year and the existence of consistently higher larval populations in years when larvae were stocked demonstrated the reliability of the sampling scheme.
Des échantillonneurs ultra-rapides Miller sont utilisés depuis 1966 au lac d'Oneida pour évaluer l'abondance des larves de Stizostedion vitreum au stade pélagique. Les analogies entre les estimations obtenues à partir de nouvelles prospections réalisée au cours de la méme année et l'existence de populations de larves de plus en plus nombreuses chaque fois que les larves ont été stockées, ont démontré que ce système d'échantillonnage était sûr.
2. DESCRIPTION OF GEAR
3. SAMPLING DESIGN
4. RESULTS AND DISCUSSION
4.1 Successive Population Estimates
4.2 Contribution of Stocked Larvae
4.3 Efficiency of Stratification
Larvae of many freshwater fish become pelagic following hatching and occupy the limnetic zone (Faber, 1967). Few quantitative estimates of larval abundance have been reported although fluctuations in adult stocks are frequently attributed to changes in early survival. Failure to develop precise methods of measuring abundance of larval fish in lakes is somewhat surprising considering progress made in assessing stocks of larvae in the open sea where sampling is far more demanding. In lakes, population boundaries are clearly defined, dispersal of larvae is restricted and larvae of some species hatch over a short time so the entire cohort is vulnerable to capture.
Techniques for estimating abundance of larval walleyes (Stizostedion vitreum) were developed at Cornell University in cooperation with the New York State Department of Environmental Conservation and studies were supported by Federal Aid in Fish and Wildlife Restoration funds. Investigations were carried on in Oneida Lake located in central New York State. Oneida is a shallow, eutrophic lake with a surface area of 207 km2. It has a mean width of 6.1 km, a length of 33.6 km from east to west and has a mean depth of 6.8 m and maximum depth of 16.8 m. The lake is ice covered from January to late March or early April. Temperatures reach 10°C in early May and rise to about 27°C in summer.
The walleye is the most abundant predator in the lake. Adult walleyes, which number between 0.5 to 1.0 million (Forney, 1967), spawn in April over rock or rubble along the shoreline and in streams. The largest spawning run enters Fish Creek at the east end of the lake. Eggs hatch in late April or early May and the larvae appear in the limnetic zone soon after hatching.
Larval walleyes were collected in fibreglass samplers (Miller, 1961) with a 14 -cm diameter body and 10-cm orifice. Opaque, dark green samplers were used in 1966–68 and translucent, unpigmented samplers in subsequent years. Samplers were equipped with nets constructed of 333 μ Nitex. Two samplers were attached to each of two cables with 7-kg depressors and towed from a boat propelled with twin 40-hp motors.
A Clark-Bumpus flowmeter was mounted on the cable between two samplers to determine the volume filtered. The flowmeter was calibrated by towing over a known distance and it was assumed that all water encountered was filtered. Filtration efficiency of the samplers approaches 100 percent, even when partially clogged (Miller, 1961).
The programme for estimating abundance evolved from observation of vertical and spatial distribution of larvae (Houde and Forney, 1970). Newly-hatched larvae 7.0–7.5 mm long were most abundant near bottom but older larvae concentrated in the upper 4 m. Larvae drifted passively until the yolk-sac was absorbed and the spatial distribution was governed in part by the wind regime following hatching. After the yolk was absorbed at a length of 8.5–9.0 mm there was a progressive inshore movement and highest densities were found in bays.
Abundance was estimated from the catch at randomly selected stations within three strata when walleyes average 9–10 mm. These strata encompassed the volume of water between the 2 and 4 m contours (65 × 106 m3), between the 4 and 6 m contours (148 × 106 m3), and within the 6 m contour to a depth of 6 m (729 × 106 m3). In most years 20 stations were selected in each strata. Since the volume of the two inshore strata was less than the volume of the off- shore stratum sampling efforts was concentrated where highest densities of larvae were anticipated.
At each station four samplers were towed during the day parallel to a depth contour and the catch pooled. Depth of samplers was selected at random with two above and two below the median depth of water or to a maximum depth of 6 m. Samplers were towed for five minutes at a speed of 3.6 m/s with the exception that tows in the strata over 6 m deep lasted 7½ minutes in 1968–73. Four samplers filtered about 40 m3 in five minutes.
The estimate of the larval population followed the method of complete enumeration of selected subsamples outlined by Regier and Robson (1967). Population of larvae in each strata was the product of the mean catch and number of sampling spaces. Number of subspaces was determined by dividing the volume of a strata by the mean volume strained by four samplers. Strata estimates were summed to determine the total population. Variance was calculated for the population in each strata and strata variances summed to estimate variance for the total population.
Larval walleyes were also present in catches used to estimate abundance of larval yellow perch (Perca flavescens). The perch survey began three to eleven days after the walleye survey and employed the same gear, but stratification and sampling effort were different (Noble, 1968). In the perch survey the lake was split along both axes with further vertical stratification and sampling for perch was concentrated in open water where density of perch was high and walleyes relatively scarce. The perch survey was modified by adding stations to the inshore strata in 1968 and in 1970 through 1973, and a second estimate of the larval walleye population was derived from the catch and volume filtered.
Population estimates derived from successive surveys conducted five to eleven days apart were is close agreement (Table I). Number of larvae was high in 1968, 1970 and 1972 and low in 1971 and 1973. Sampling in both surveys apparently encompassed the population and was adequate to detect annual fluctuations in larval abundance. Although samples were not taken between shore and the 2 m contour or below 6 m in either survey, low density of larvae below 6 m was verified by the capture of only one walleye in 80 samples taken at 7 to 12 m in 1968–71. Risk of damage prevented the use of high-speed samplers in water less than 2 m deep but walleye larvae were seldom captured in meter nets towed between shore and the 2 m contour.
Agreement between successive surveys could be fortuitous if recruitment of larvae to the pelagio mode was balanced by increased avoidance of samplers by larger larvae. Although some recruitment may have occurred between surveys, length distributions of larvae were nearly normal indicating recruitment was not substantial (Table II).
Noble (1970) examined the relation between avoidance and length of larvae by modifying the Miller sampler and sampling technique. Night sampling, less conspicuous samplers and an electrical field proceeding the gear resulted in larger catches indicating walleye larvae could avoid samplers towed at 3.6 m/s during the day. Most tests were conducted when wall- eyes averaged 12 to 16 mm so the size at which larvae began avoiding samplers was not established. However, the ratio of day to night catches increased steadily from 1.2 when larvae averaged 12 mm to 9.3 when larvae were 16 mm.
The assumption that population estimates were not seriously biased by avoidance seems justified since abundance was measured when walleyes averaged less than 12 mm except in 1972 (Table I). In 1972 walleyes grew from 10.0 to 13.8 mm between the first and second survey but catches did not indicate significant avoidance. Contrary to expectations the second estimate of abundance slightly exceeded the first.
Recently hatched larvae, one to three days old, were stocked in 1966–68 and in 1970 and 1972 at about the time eggs spawned in the lake and tributary streams were hatching (Table III). These larvae were reared from eggs collected from Oneida Lake fish and incubated in water from Soriba Creek. Larvae were released in open water near the mouth of Soriba Creek on the north- west shore one to two weeks before the larval walleye survey. Larval density was high in years when walleyes were stocked and substantially lower in years when natural reproduction was not augmented.
Contribution of larvae from natural spawning varied from 6.6 to 10.2 and averaged 8.3 million in 1969, 1971 and 1973. If spawning success was similar in other years, then the contribution of stocked larvae to the population at 9–10 mm can be obtained by subtracting 8.3 from each estimate. A correlation of .78 between the number of larvae stocked in five years and their apparent contribution to the population supports the assumption that larval catches reflect real differences in density. Failure to show a higher correlation is attributed to the limited precision of estimates and possibly to variability in survival of larvae.
Stratification was not as effective in controlling sampling error as anticipated. Fry density and the variance of catches was higher in the inshore strata where sampling effort was concentrated but the distribution of larvae was different each year (Table IV). In 1972 the density of larvae in the inshore strata was about 40 times the density in open water while in 1971 the difference was less than two-fold. Consequently the efficiency of stratification relative to a completely random sampling design jumped from 67 in 1971 to 398 percent in 1972.
Stocking affected the distribution of larvae and consequently the effectiveness of stratification. Larvae which were stocked along the northwest shore were transported into the shallow western end of the lake and into bays along the south shore. Most larvae originating from natural spawning probably hatched from eggs spawned in the east section of the lake or in Fish Creek since larvae density was higher in the east than in the west half of the lake in 1969, 1971 and 1973 (Table V). These larvae apparently were not transported inshore as rapidly as stocked walleyes as shown by the high proportion of larvae in open water in 1971 and 1973 (Table IV). Stratification could be adjusted for changes in stocking policy but the distribution of larvae is also affected by the wind regime, growth of larvae and other factors which are less predictable (Houde, 1968).
Evaluation of walleye population estimates by repeated sampling and by introducing known numbers of larvae into the population indicates that the techniques for estimating abundance are reliable. Standard errors of the estimates ranged from 11 to 29 and averaged 20 percent. This level of precision is adequate for preliminary studies of population dynamics in an area where current information is sparse. Using similar techniques and about the same sampling effort, numbers of larval yellow perch in Oneida Lake have been estimated with a standard error of 15 to 25 percent (Personal communication R.L. Noble). Although the two species are genetically related, larvae exhibit marked differences in distribution, behaviour and abundance which suggests sampling procedures developed for these species may be applicable to other larval fish.
|(× 106)||(× 106)|
|(mm)||11–13 May||23–24 May||23–24 May||29–30 May|
|Year||Number of Larvae (× 106)|
|Weighted mean catch/haul||1.196||0.792||1.444||.442||2.643||.391||1.828||.496|
|Relative efficiency %b||497||128||118||235||142||67||398||83|
|Mean total length||9.1||9.1||9.0||9.2||9.0||9.6||10.0||9.0|
a Boundaries of the strata described in text
b Relative efficiency =
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