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Distribution of Rainbow Smelt and Alewife Larvae Along the North Shore of Lake Ontario
J. Great Lakes Res. 10(3):273-279 InternaL Assoc. Great Lakes Res., 1984
DISTRIBUTION OF RAINBOW SMELT AND ALEWIFE LARVAE ALONG THE NORTH SHORE OF LAKE ONTARIO
Thomas G. Dunstall Ontario Hydro Research Division 800 Kipling Ave. Toronto, Ontario M8Z 584
ABSTRACT. Distribution and abundance of pelagic larval fish were determined for 12 sites on the north shore of Lake Ontario between Pickering and Wellington. Alewife (Alosa pseudoharengus) and rainbow smelt (Osmerus mordax) larvae represented over 99.9% of the total catch in the nearshore zone between the 3- and 13-m depth contours. Yolk-sac alewife and rainbow smelt larvae were widespread, while most post-yolk-sac alewife larvae occurred in sheltered waters east of Colborne; developing alewife larvae moved away from shore in upper water strata. Rainbow smelt larvae were concentrated in surface waters shortly after hatching, with a subsequent rapid decline in abundance as larvae moved away from shore to deeper strata. ADDITIONAL INDEX WORDS: Fish populations, nearshore processes, power plants.
INTRODUCTION Fish entrainment and impingement at electrical generating stations on the lower Great Lakes have been greatest among schooling pelagic fish, particularly rainbow smelt (Osmerus mordax), alewife (Alosa pseudoharengus), and gizzard shad (Dorosoma cepedianum) (Kelso and Milburn 1979). Cooling water intake location is likely to influence the extent of fish entrapment, and knowledge of the distribution, abundance, and behavior of nearshore fish may provide a basis for selecting the most appropriate location for water withdrawal. Rainbow smelt and alewife spawn in nearshore waters along the north shore of Lake Ontario. Distribution of larvae of these species following hatching, and prior to adopting adult behavioral patterns is, however, not well known. Previous studies have described larval fish distribution in portions of Lakes Michigan (Jude et al. 1979, 1980; Tin and Jude 1983; Wells 1973), Huron (Loftus 1979a, b; O'Gorman 1983), and Erie (Mizera et al. 1981, Cooper et al. 1981). In Lake Huron, nursery grounds for rainbow smelt occurred where topographical features of the shoreline provided calm water, while alewife larvae were uniformily distributed (O'Gorman 1983). The purpose of the present study was to determine the nearshore (bottom depth 3 to 13 m) distribution and abundance
of rainbow smelt and alewife larvae along a 170km portion of Lake Ontario and to determine if the occurrence of larvae bore any relationship to physical features of the environment. STUDY AREA Twelve 2-km-square sites between Pickering, just east of Toronto, and Wellington, Prince Edward County (Ontario, Canada) were studied (Fig. 1). The lack of prominent headlands in the region encompassing the four western sites (A to D) exposes the inshore zone to wave action and longshore currents generated by winds from the west, south, and east. Gentle to moderate increases in depth, without sharp drops, are common. Bottom gradient averages about 10 m per km. Substrate in this region, determined by closed-circuit television adapted for underwater use and examination of ponar grab samples, consists of a heterogeneous mix of glacial sediment ranging from the base material clay and sand to boulders. At remaining sites, east of Colborne, variation in topography provides a degree of protection from wind-generated waves and currents. Included in this area are two large bays, Presqu'ile and Wellers, a headland, Presqu'ile, and three islands. Five shoals are found within the 20-m depth contour. Bottom gradient is more variable than in the west-
273
T. G. DUNSTALL
274
43 o ' - -
US_A
--l
PRESQU'ILE
BAY .../ PORT HOPE
I
~
LAKE
ONTARIO
5-m DEPTH CONTOUR
20-m DEPTH CON TOUR
o
km
25 I
!
TORONTO •
TOWNS
FIG. 1. Location of larval fish collection sites (boxed letters) on the north shore of Lake Ontario (see stippled area in inset), 1978-1979.
ern portion of the study area, ranging from 5 m per km near Wellers Bay to 15 m per km east of Nicholson Island. Substrate at sites E and F consists of glacial sediment while farther east, sand (sites G, I, K, L) and bedrock (sites H, J) predominate, particularly offshore at depths greater than 4 to 7 m. METHODS Larval fish were collected during five sampling cycles between 23 May 1978 and 1 June 1979 (Table 1). At sites B, D, H, and I, samples were taken from the surface stratum (0.5-m depth) along the 3-, 7-, and 13-m bottom contours, approximately 1 m off bottom along the 7- and 13m contours, and at mid-depth (6 m) over the 13-m contour. Collections at remaining sites consisted of surface tows at the 3- and 13-m contours and bottom tows at the 13-m contour. In the first three cycles some sites and collections at the 6- and 12-m strata were omitted. Day samples only were taken and each site was not visited more than once per cycle. A series of three tows was taken parallel to shore at evenly spaced intervals over a distance of about 2.4 km at each depth contour and stratum sampled. Two 0.5-m diameter conical nets with a length-to-diameter ratio of 3: 1 and mesh size of 363 /Lm were used during each tow, resulting in six replicate collections for each bottom contour and
stratum sampled. Towing time was 5 min at a boat speed, determined with an Ott current meter, of approximately 1.2 m . s -1. Upon retrieval of nets, the catch was preserved in formaldehyde (5070 by volume). Surface lake water temperatures were recorded for each collection. Nets used for surface water collections (0.5-m depth) were mounted flush to a metal support frame suspended from the bow on the port and starboard sides of the boat, thereby sampling ahead of the wake created by the boat. A second frame holding two nets, separated by a distance of 1 m, was used to sample at depths of 6 and 12 m. Guy wires attached to the outside corners of the upper support frame, two 22.7-kg depressors mounted on the lower side of the frame, and the flush mount of nets within a double-ring swivel assembly ensured unobstructed presentation of nets at a right angle to the tow direction. Nets were open when lowered to depth, which took about 15 sec, but were pulled closed from the boat with auxiliary ropes prior to retrieval. Volume of water filtered per tow, as determined using General Oceanics current meters positioned in the center of the nets, averaged 60 m 3• This value was used in calculations of larval fish densities, expressed as numbers per 1,000 m 3 of lake water, assuming 100070 filtering efficiency of the nets. Species identifications, made with the aid of a
DISTRIBUTION OF RAINBOW SMELT AND ALEWIFE
275
TABLE 1. A summary oftimes, sites, species composition, and catches of larval fish collected in Lake Ontario during 1978-1979. See Figure 1 for identification of sites. Sample Cycle
2
Approximate Duration (Weeks)
Taxa
23 May-15 June 1978
3
Rainbow Smelt
882
All
O.5-m depth only
14 July-29 August 1978
6.5
Alewife Rainbow Smelt Threespine Stickleback
15,856 7
All I
All, except sites E, F, K, and L (O.5-m depth only)
Dates
3
31 August-2 October 1978
4 5
23 April-30 April 1979 28 May-l June 1979
4.5
Total Number Caught
Sites Where Species Caught
Stations Sampled
4
F, G, H
Alewife
1,666
A,B,G H, I, J
All at sites A, B, C, D, G, H, I, J. (Sites E, F, K, and L, omitted).
Coregonid Rainbow Smelt
1 5,542
J All
All All
stereomicroscope, were based primarily on Lippson and Moran (1974) and Cooper (1978). Total length of larvae was measured to the nearest 0.5 mm for complete samples which had fewer than 30 fish or for subsamples of 30 fish in samples with greater abundance. RESULTS Species Composition Alewife and rainbow smelt comprised 730,10 and 27%, respectively, of the 23,958 larvae collected (Table 1). Four threespine sticklebacks (Gasterosteus acu!eatus) and a single coregonid (species not determined) were also collected. Rainbow Smelt Rainbow smelt larvae were taken in late May to mid-June collections at all sites in both years of study. Continual recruitment and rapid dispersal of larvae during this period were indicated since there was no overall increase in total length with time. Mean total lengths of larvae for the individual sites sampled over the 2 years of study varied between 5.3 and 6.3 mm. The coefficient of variation for fish larvae length data averaged 8.4% for individual sites. Avoidance of nets by larger larvae was likely although seven fish with a mean total length of 9.8 mm (range 6.0 to 23 mm) were taken at site I on 17 July 1978. No rainbow smelt larvae were taken after mid-July, although three other sites were sampled in the last half of that month. At remaining sites, larger rainbow smelt larvae may have been missed since no samples were taken
during the mid-June to mid-July period. Abundance of rainbow smelt larvae was not related to lake water temperatures at time of sampling, although highest densities were found at temperatures ranging from 6 to 14°C. Sampling at sites between Darlington and Wesleyville (B to D) was conducted from 29 to 31 May in both years (Table 2) and may have preceded the period of greatest larval rainbow smelt abundance. At Proctor Point (H) and Presqu'ile (I), abundance of larvae was low in 1978 and exceptionally high in 1979. The difference between years may have reflected time of sampling relative to peak hatching period. Temporal variability associated with
TABLE 2. Mean density or range in mean density (number per 1,000 m 3) of rainbow smelt larvae in the 0.5-m stratum over the 3- and 13-m contours for Lake Ontario sites (see Fig. 1) sampled during 23 May-15 June 1978, and 28 May-l June 1979. Site
Density or Density Range (No/1,000 m 3)
I,H,D,B,C
23-31 May 1978 13-83
A,E,F,G K,J,L
7-15 June 1978 160-250 19-63
A,B,C,D,K,L,G J,E,F,
28 May-1 June 1979 10-81 180-210 2,900
H I
1,800
276
T. G. DUNSTALL
sequential sampling of the sites obscured differences in densities among sites during 1978 when the sampling cycle exceeded 3 weeks. Extreme variability in spatial distribution was evident in 1979, when all sites were visited within a 5-day period. During 1979 larvae were most abundant (> 1,000 fish per 1,000 m 3) in the Proctor Point (H) - Presqu'ile (I) region with moderately high abundance of larvae at sites immediately east (J) and west (E, F) of this area. Densities were lowest « 100 fish per 1,000 m3) along relatively exposed portions of shoreline (A to D) and in the Wellington (K)-Sandbanks (L) area. No apparent relationship existed between larval rainbow smelt density and substrate type for the 12 sites sampled. Densities of rainbow smelt larvae at a particular site were generally greatest in surface waters over the 3-m depth contour (Table 3). In 1979, decreased density of larvae with increasing bottom depth and, to a lesser extent, from surface to bottom, was characteristic of larval fish distribution at the Proctor Point-Presqu'ile sites (H, I), which accounted for 84% of all larvae caught, and most other sites. The decline in larval fish density with bottom depth was not as pronounced in 1978, possibly the result of differences between years in time of sampling relative to peak hatching. Densities of rainbow smelt larvae over the 7- to 13-m contours exceeded densities over the 3-m contour at Darlington (B) and Wesleyville (D) in both years, at Robinson Point (1) in 1978, and at the Sandbanks (L) in 1979.
gest that after hatching, rainbow smelt larvae dispersed offshore to deeper strata. Alewife Alewife larvae, collected in two sampling cycles between 14 July and 2 October 1978, showed a progressive increase in mean length (Fig. 2). Small larvae ( < 7.0 mm) comprised the bulk of the catch up to 18 August and were present in samples collected up to 31 August. Water temperatures at which alewife larvae were collected ranged from 13 to 21°C, although densities of larvae were generally highest in August when temperatures were approximately 18°C. 35,-----------------------,
665 152
30
25 58 331
E E J: f<.:J
51
84
20 508
Z W
...J ...J
« fa
15
f-
10 H
36 131
i'l + t J
I
H
C
K E L
B
G
AJ
F
G
TABLE 3. Distribution of rainbow smelt larvae (percent) at all stations (stratum-depth contour) sampled during 23 May-I5 June 1978, and 28 May-I June 1979. Values averaged for intensively sampled sites (B, D, H, I) and all sites (see Fig. 1). Sites B, D, H, I
All Sites
Strata Sampled (m)
3m
7m
13 m
3m
13m
1978
0.5
49.7
36.7
13.7
76.8
23.2
1979
0.5 6 12
73.4
16.6 2.0
5.6 1.8 0.6
89.9
8.9
Year
1.2
Mean total length of larvae for each contour and strata sampled was generally 5.6 mm. In 1979 larvae in mid-depth and bottom samples along the 13m contour averaged 5.8 and 6.1 mm, respectively. Although no statistically significant differences in size of fish among locations existed, the data sug-
20 JULY
10
20 AUGUST
10 SEPTEMBER
FIG. 2. Total length (mm) of alewife larvae collected during 1978 with means, standard deviations (vertical box), and ranges (vertical lines) indicated. Associated letters and numbers represent sites sampled (see Fig. 1) and number of larvae measured, respectively.
To evaluate movement of alewife larvae following hatching and identify potential nursery areas, larvae were divided into three size categories and catch data were compared for all surface collections from the 3- and 13-m depth contours. Small larvae ( < 7.0 mm), which included all yolk-sac larvae, comprised 36.70/0 of this total catch, while intermediate-sized larvae (7.0 to 12.5 mm) and large larvae (> 12.5 mm) represented 36.2 and 27.1 % of the catch, respectively. Small alewife larvae were present at all sites with
DISTRIBUTION OF RAINBOW SMELT AND ALEWIFE
277
TABLE 4. Mean densities (number per 1,000 m 3) and size composition (percent) of alewife larvae in the 0.5-m stratum over the 3- and 13-m contours for Lake Ontario sites (see Fig. 1) samples during 14 July-2 October 1978). Size Composition (Percent)
Density Site J I H
D C B E F G
K L A J I H G
D C B A
14 July 17 July 25 July 28 July 4 August 8 August 18 August 18 August 18 August 21 August 21 August 29 August 31 11 12 18 22 25 26 2
August September September September September September September October
>12.5 mm
<7.0 mm
7.0 to 12.5 mm
1,000 50 540 2 1,400 4,650 100 400 3,090 11,300 6,630 67
100 100 98.0 100 99.9 96.8 82.0 82.7 74.5 7.1 2.0 26.9
0 0 2.0 0 0.1 2.4 13.0 12.3 22.4 55.8 49.8 29.9
0 0 0 0 0 0.8 5.0 5.0 3.1 37.1 48.2 43.2
1,240 300 63 140 0 0 1 1
18.6 0 0 0 0 0 0 0
55.6 14.5 0 4.4 0 0 0 0
25.8 85.5 100 95.6 0 0 100 100
(No.l1,000
Date
m3)
greatest abundance (4,500 larvae per 1,000 m 3) at Darlington (B) on 8 August (Table 4). Pickering was sampled following what appeared to be the peak hatching period in early August and the importance of this site as a spawning area was probably underestimated. Highest densities of intermediate and large larvae occurred in the Wellington (K) - Sandbanks (L) area on 21 August (Table 4). Larvae at these two sites consisted primarily (95 %) of fish ~ 7.0 mm, while at sites near Colborne (E, F, G), which were sampled 3 days earlier, only 24070 of the catch was comprised of these larger larvae. Greater abundance of the larger larvae at sites G to L suggests an accumula-
tion or residence of alewife larvae in the eastern portion of the study area. Water transparency, determined by Secchi disc, varied considerably in August and September, particularly close to shore (4-m contour), although values were generally comparable for eastern and western sites (Balesic 1979). This would exclude the possibility that net avoidance accounted for differences in catch between the two areas. Distribution of larvae from the three size categories was compared, based on catch data for the intensively sampled sites (B, D, H, I) (Table 5). Early post-hatch larvae « 7.0 mm) were most abundant in surface waters over the 3-m contour
TABLE 5. Distribution of alewife larvae (percent) of three size categories at all stations (stratum-depth contour) sampled during 14 July-2 October 1978. Data averaged for intensively sampled sites (B, D, H, I). Strata Sampled (m) 0.5 6 12
Larvae 7.0 to 12.5 mm
Larvae < 7.0 mm
Larvae > 12.5 mm
3m
7m
13m
3m
7m
13m
3m
7m
13m
62.8
34.2 0.4
1.3 0.9 0.4
25.1
38.2 3.9
28.9 3.9 0.0
16.3
48.9 0.5
31.7 2.1 0.5
278
T. G. DUNSTALL
and their numbers decreased rapidly both offshore and in deeper strata. Intermediate-sized larvae (7.0 to 12.5 mm) were nearly equally distributed in surface waters over all depth contours sampled. However, their densities also decreased rapidly from surface to bottom strata. Large larvae (> 12.5 mm) likewise were concentrated in surface waters with somewhat higher densities over the 7- and 13-m contours. Distribution of alewife larvae was similar at other sites. DISCUSSION Highest densities of rainbow smelt larvae occurred in the Proctor Point-Presqu'ile region of Lake Ontario where substantial variation in both shoreline aspect and gradient exists; lowest densities occurred along the relatively exposed, featureless shoreline west of Colborne. Similarly, in western Lake Huron, O'Gorman (1983) found that lowest numbers of rainbow smelt larvae were associated with uniform stretches of shoreline, moderate densities were reported for irregular portions of shoreline, and highest densities occurred in bays and harbours. In the North Channel of Lake Huron, highest densities of rainbow smelt larvae occurred in shallow protected areas close to stream mouths (Loftus 1979a). The contribution of streamspawned larvae to observed lake densities in the present study was unknown, but was believed to have been minimal. Rapid disappearance of rainbow smelt larvae from the nearshore pelagic zone had previously been observed in the vicinity of Ontario Hydro's Pickering Generating Station during 1975-1977, with post-yolk-sac larvae being entrained only occasionally (Dunstall 1981). In Lake Michigan, rainbow smelt larvae were also reported to disperse offshore soon after hatching (Tin and Jude 1983). Developing rainbow smelt larvae, present inside the 15-m contour in Lake Michigan up to the middle of July, were rarely collected in 363-JLm mesh nets after July, although this was attributed mainly to net avoidance by larger larvae. Along the western shore of Lake Huron, O'Gorman (1983) found that density of larval rainbow smelt within the 5- to lO-m contours was greatly reduced by mid-July. Young-of-the-year rainbow smelt in Lake Erie moved into deeper water during July with continued movement offshore until late October, when they were completely mixed with adults at a depth of about 45 m (Ferguson 1965). Rainbow smelt larvae have been described as
being closely associated with the bottom in June and July, particularly during the day (Jude et al. 1980). A similar pattern was observed in Georgian Bay, with larvae moving into upper water strata at night (Emery 1973). O'Gorman (1983) reported significantly higher densities of rainbow smelt larvae in strata deeper than 1 m, over the 5.5- and 9.2-m contours, than in near-surface waters. Young-of-the-year rainbow smelt were concentrated near bottom within the 15-m contour in Lake Michigan during late August (Tin and Jude 1983). In Lake Erie, young-of-the-year rainbow smelt entering the offshore submerged intakes of a power plant at night represented about 720/0 of the total number of smelt entrained (Teleki 1976). In contrast, rainbow smelt larvae in the nearshore waters of Lake Ontario were concentrated in surface waters (0.5-m stratum) immediately following hatching, which may be characteristic for larvae at this stage of development, prior to their active feeding. Alewife was the dominant species in the pelagic ichthyoplankton along the north shore of Lake Ontario from July to September. Densities of alewife larvae immediately following hatching were highest in the 0.5-m stratum with numbers decreasing in that stratum when proceeding from the 3- to the 13-m contours. Similarly, in the Bay of Quinte region of Lake Ontario, Lam (1977) found greater abundance of larvae in near-surface waters than at the 3-m stratum. In western Lake Huron, however, alewife larvae were more abundant in the 1- to 3-m strata than in surface waters or in the 4- to 6-m strata (O'Gorman 1983). Along the relatively featureless portion of shoreline west of Colborne there was a rapid decline in abundance of small alewife larvae « 7.0 mm) from the nearshore zone (3- to 13-m contours), a pattern consistently observed during 1975-1977 near Pickering Generating Station (DunstallI981). Post-yolk-sac alewife larvae were rarely entrained in the surface intake of this station. Variation in topography within the nearshore zone, in the area east of Colborne, provides a relatively sheltered environment which appears to serve as a better nursery ground for developing alewife larvae than areas of uninterrupted shoreline. The larger alewife larvae (~7.0 mm) at the eastern sites may have persisted in the region following hatching or accumulated as a result of immigration from other areas of the lake. These larvae were concentrated in the surface stratum with highest densities along the 7- and 13-m contours. Spatial distributions of
DISTRIBUTION OF RAINBOW SMELT AND ALEWIFE
alewife larvae and fry, similar to those found in the present study, have been described for Lakes Michigan (Wells 1973, Jude et al. 1980), and Erie (Mizera et al. 1981). ACKNOWLEDGMENTS This study was funded by Ontario Hydro. The field program was conducted by L. P. King and E. Checko aboard the Tug Seagull, operated by E. Semple. Identifications and measurements of specimens were made by E. Checko. H. Balesic contributed many useful suggestions during the course of this study. REFERENCES Balesic, H. 1979. Nearshore environment of the north shore of Lake Ontario-1978 intake location study interim report. Ontario Hydro Res. Div. Rep. 79300-K. Cooper, C. L., Heniken, M. R., and Herdendorf, C. E. 1981. Limnetic larval fish in the Ohio portion of the western basin of Lake Erie, 1975-1976. J. Great Lakes Res. 7:326-329. Cooper, J. E. 1978. Identification of eggs, larvae, and juveniles of the rainbow smelt, Osmerus mordax, with comparisons to larval alewife, Alosa pseudoharengus, and gizzard shad, Dorosoma cepedianum. Trans. Am. Fish. Soc. 107:56-62. Dunstall, T. G. 1981. Ichthyoplankton entrainment at Pickering GS 'A", 1975-1977. Ontario Hydro Res. Div. Rep. 81-211-K. Emery, A. R. 1973. Preliminary comparison of day and night habitats of freshwater fish in Ontario Lakes. J. Fish Res. Board Can. 30:761-774. Ferguson, R. G. 1965. Bathymetric distribution of American smelt Osmerus mordax in Lake Erie. In Proceedings of the 8th Conf. Great Lakes Res., pp. 47-60. Internat. Assoc. Great Lakes Res. Jude, D. J., Heufelder, G. R., Tin, H. T., Auer, N. A., Klinger, S. A., Schneeberger, P. J., Rutecki, T. L., Madenjian, C. P., and Rago, P. J. 1979. Adult, juve-
nile and larval fish in the vicinity of the J. H. Campbell Power Plant, eastern Lake Michigan, 1978. Great Lakes Res. Div. Spec. Rep. No. 73, Univ. Mich.
279
_ _ _ _ , Heufelder, G. R., Auer, N. A., Tin, H. T., Klinger, S. A., Schneeberger, P. J., Madenjian, C. P., Rutecki, T. L., and Godun, G. G. 1980. Adult,
juvenile and larval fish populations in the vicinity of the J. H. Campbell Power Plant, eastern Lake Michigan, 1979. Great Lakes Res. Div. Spec. Rep. No. 79, Univ. Mich. Kelso, J. R. M., and Milburn, G. S. 1979. Entrainment and impingement of fish by power plants in the Great Lakes which use the once-through cooling process. J. Great Lakes Res. 5:182-194. Lam, C. N. G. 1977. Distribution and abundance of the early developmental phases of alewife, Alosa pseudoharengus (Wilson) and other fishes in the Bay of Quinte. M.Sc. dissertation, Univ. of Guelph, Guelph, Ontario. Lippson, A. J., and Moran, R. L. 1974. Manual for -
identification of early development stages of fishes of the Potomac River Estuary. Tech. Center Martin Marietta Corp., Baltimore, Maryland. Loftus, D. H. 1979a. North Channel larval fish survey: Joe Dollar Bay to Aird Bay, May 1979. Lake Huron Fish. Assess. Unit, Ontairo Min. Nat. Res. Rep. 279. _ _ _ _ . 1979b. Larval fish sampling in Lake Huron, 1979. Lake Huron Fish. Ass.ess. Unit, Ontario Min. Nat. Res. Rep. 3-79. Mizera, J. J., Cooper, C. L., and Herdendorf, C. E. 1981. Limentic larval fish in the nearshore zone of the western basin of Lake Erie. J. Great Lakes Res. 7:62-64. O'Gorman, R. 1983. Distribution and abundance of larval fish in the nearshore waters of western Lake Huron. J. Great Lakes Res. 9: 14-22. Teleki, G. C. 1976. The incidence and effect of oncethrough cooling on young-of-the-year fishes at Long Point Bay, Lake Erie: A preliminary assessment. In Thermal Ecology II, ed. G. W. Esch and R. W. McFarlane, pp. 387-393. Nat. Tech. Inform. Ser. Conf.-750425, Springfield, Virgo Tin, H. T., and Jude, D. J. 1983. Distribution and growth of larval rainbow smelt in eastern Lake Michigan, 1978-1981. Trans. Am. Fish. Soc. 112:517-524. Wells, L. 1973. Distribution of fish fry in nearshore
waters of southeastern and east central Lake Michigan, May-August 1972. Admin. Rep. Great Lakes Fish. Lab., US Fish. Wildl. Serv., Ann Arbor, Mich.
DISTRIBUTION OF RAINBOW SMELT AND ALEWIFE LARVAE ALONG THE NORTH SHORE OF LAKE ONTARIO
Thomas G. Dunstall Ontario Hydro Research Division 800 Kipling Ave. Toronto, Ontario M8Z 584
ABSTRACT. Distribution and abundance of pelagic larval fish were determined for 12 sites on the north shore of Lake Ontario between Pickering and Wellington. Alewife (Alosa pseudoharengus) and rainbow smelt (Osmerus mordax) larvae represented over 99.9% of the total catch in the nearshore zone between the 3- and 13-m depth contours. Yolk-sac alewife and rainbow smelt larvae were widespread, while most post-yolk-sac alewife larvae occurred in sheltered waters east of Colborne; developing alewife larvae moved away from shore in upper water strata. Rainbow smelt larvae were concentrated in surface waters shortly after hatching, with a subsequent rapid decline in abundance as larvae moved away from shore to deeper strata. ADDITIONAL INDEX WORDS: Fish populations, nearshore processes, power plants.
INTRODUCTION Fish entrainment and impingement at electrical generating stations on the lower Great Lakes have been greatest among schooling pelagic fish, particularly rainbow smelt (Osmerus mordax), alewife (Alosa pseudoharengus), and gizzard shad (Dorosoma cepedianum) (Kelso and Milburn 1979). Cooling water intake location is likely to influence the extent of fish entrapment, and knowledge of the distribution, abundance, and behavior of nearshore fish may provide a basis for selecting the most appropriate location for water withdrawal. Rainbow smelt and alewife spawn in nearshore waters along the north shore of Lake Ontario. Distribution of larvae of these species following hatching, and prior to adopting adult behavioral patterns is, however, not well known. Previous studies have described larval fish distribution in portions of Lakes Michigan (Jude et al. 1979, 1980; Tin and Jude 1983; Wells 1973), Huron (Loftus 1979a, b; O'Gorman 1983), and Erie (Mizera et al. 1981, Cooper et al. 1981). In Lake Huron, nursery grounds for rainbow smelt occurred where topographical features of the shoreline provided calm water, while alewife larvae were uniformily distributed (O'Gorman 1983). The purpose of the present study was to determine the nearshore (bottom depth 3 to 13 m) distribution and abundance
of rainbow smelt and alewife larvae along a 170km portion of Lake Ontario and to determine if the occurrence of larvae bore any relationship to physical features of the environment. STUDY AREA Twelve 2-km-square sites between Pickering, just east of Toronto, and Wellington, Prince Edward County (Ontario, Canada) were studied (Fig. 1). The lack of prominent headlands in the region encompassing the four western sites (A to D) exposes the inshore zone to wave action and longshore currents generated by winds from the west, south, and east. Gentle to moderate increases in depth, without sharp drops, are common. Bottom gradient averages about 10 m per km. Substrate in this region, determined by closed-circuit television adapted for underwater use and examination of ponar grab samples, consists of a heterogeneous mix of glacial sediment ranging from the base material clay and sand to boulders. At remaining sites, east of Colborne, variation in topography provides a degree of protection from wind-generated waves and currents. Included in this area are two large bays, Presqu'ile and Wellers, a headland, Presqu'ile, and three islands. Five shoals are found within the 20-m depth contour. Bottom gradient is more variable than in the west-
273
T. G. DUNSTALL
274
43 o ' - -
US_A
--l
PRESQU'ILE
BAY .../ PORT HOPE
I
~
LAKE
ONTARIO
5-m DEPTH CONTOUR
20-m DEPTH CON TOUR
o
km
25 I
!
TORONTO •
TOWNS
FIG. 1. Location of larval fish collection sites (boxed letters) on the north shore of Lake Ontario (see stippled area in inset), 1978-1979.
ern portion of the study area, ranging from 5 m per km near Wellers Bay to 15 m per km east of Nicholson Island. Substrate at sites E and F consists of glacial sediment while farther east, sand (sites G, I, K, L) and bedrock (sites H, J) predominate, particularly offshore at depths greater than 4 to 7 m. METHODS Larval fish were collected during five sampling cycles between 23 May 1978 and 1 June 1979 (Table 1). At sites B, D, H, and I, samples were taken from the surface stratum (0.5-m depth) along the 3-, 7-, and 13-m bottom contours, approximately 1 m off bottom along the 7- and 13m contours, and at mid-depth (6 m) over the 13-m contour. Collections at remaining sites consisted of surface tows at the 3- and 13-m contours and bottom tows at the 13-m contour. In the first three cycles some sites and collections at the 6- and 12-m strata were omitted. Day samples only were taken and each site was not visited more than once per cycle. A series of three tows was taken parallel to shore at evenly spaced intervals over a distance of about 2.4 km at each depth contour and stratum sampled. Two 0.5-m diameter conical nets with a length-to-diameter ratio of 3: 1 and mesh size of 363 /Lm were used during each tow, resulting in six replicate collections for each bottom contour and
stratum sampled. Towing time was 5 min at a boat speed, determined with an Ott current meter, of approximately 1.2 m . s -1. Upon retrieval of nets, the catch was preserved in formaldehyde (5070 by volume). Surface lake water temperatures were recorded for each collection. Nets used for surface water collections (0.5-m depth) were mounted flush to a metal support frame suspended from the bow on the port and starboard sides of the boat, thereby sampling ahead of the wake created by the boat. A second frame holding two nets, separated by a distance of 1 m, was used to sample at depths of 6 and 12 m. Guy wires attached to the outside corners of the upper support frame, two 22.7-kg depressors mounted on the lower side of the frame, and the flush mount of nets within a double-ring swivel assembly ensured unobstructed presentation of nets at a right angle to the tow direction. Nets were open when lowered to depth, which took about 15 sec, but were pulled closed from the boat with auxiliary ropes prior to retrieval. Volume of water filtered per tow, as determined using General Oceanics current meters positioned in the center of the nets, averaged 60 m 3• This value was used in calculations of larval fish densities, expressed as numbers per 1,000 m 3 of lake water, assuming 100070 filtering efficiency of the nets. Species identifications, made with the aid of a
DISTRIBUTION OF RAINBOW SMELT AND ALEWIFE
275
TABLE 1. A summary oftimes, sites, species composition, and catches of larval fish collected in Lake Ontario during 1978-1979. See Figure 1 for identification of sites. Sample Cycle
2
Approximate Duration (Weeks)
Taxa
23 May-15 June 1978
3
Rainbow Smelt
882
All
O.5-m depth only
14 July-29 August 1978
6.5
Alewife Rainbow Smelt Threespine Stickleback
15,856 7
All I
All, except sites E, F, K, and L (O.5-m depth only)
Dates
3
31 August-2 October 1978
4 5
23 April-30 April 1979 28 May-l June 1979
4.5
Total Number Caught
Sites Where Species Caught
Stations Sampled
4
F, G, H
Alewife
1,666
A,B,G H, I, J
All at sites A, B, C, D, G, H, I, J. (Sites E, F, K, and L, omitted).
Coregonid Rainbow Smelt
1 5,542
J All
All All
stereomicroscope, were based primarily on Lippson and Moran (1974) and Cooper (1978). Total length of larvae was measured to the nearest 0.5 mm for complete samples which had fewer than 30 fish or for subsamples of 30 fish in samples with greater abundance. RESULTS Species Composition Alewife and rainbow smelt comprised 730,10 and 27%, respectively, of the 23,958 larvae collected (Table 1). Four threespine sticklebacks (Gasterosteus acu!eatus) and a single coregonid (species not determined) were also collected. Rainbow Smelt Rainbow smelt larvae were taken in late May to mid-June collections at all sites in both years of study. Continual recruitment and rapid dispersal of larvae during this period were indicated since there was no overall increase in total length with time. Mean total lengths of larvae for the individual sites sampled over the 2 years of study varied between 5.3 and 6.3 mm. The coefficient of variation for fish larvae length data averaged 8.4% for individual sites. Avoidance of nets by larger larvae was likely although seven fish with a mean total length of 9.8 mm (range 6.0 to 23 mm) were taken at site I on 17 July 1978. No rainbow smelt larvae were taken after mid-July, although three other sites were sampled in the last half of that month. At remaining sites, larger rainbow smelt larvae may have been missed since no samples were taken
during the mid-June to mid-July period. Abundance of rainbow smelt larvae was not related to lake water temperatures at time of sampling, although highest densities were found at temperatures ranging from 6 to 14°C. Sampling at sites between Darlington and Wesleyville (B to D) was conducted from 29 to 31 May in both years (Table 2) and may have preceded the period of greatest larval rainbow smelt abundance. At Proctor Point (H) and Presqu'ile (I), abundance of larvae was low in 1978 and exceptionally high in 1979. The difference between years may have reflected time of sampling relative to peak hatching period. Temporal variability associated with
TABLE 2. Mean density or range in mean density (number per 1,000 m 3) of rainbow smelt larvae in the 0.5-m stratum over the 3- and 13-m contours for Lake Ontario sites (see Fig. 1) sampled during 23 May-15 June 1978, and 28 May-l June 1979. Site
Density or Density Range (No/1,000 m 3)
I,H,D,B,C
23-31 May 1978 13-83
A,E,F,G K,J,L
7-15 June 1978 160-250 19-63
A,B,C,D,K,L,G J,E,F,
28 May-1 June 1979 10-81 180-210 2,900
H I
1,800
276
T. G. DUNSTALL
sequential sampling of the sites obscured differences in densities among sites during 1978 when the sampling cycle exceeded 3 weeks. Extreme variability in spatial distribution was evident in 1979, when all sites were visited within a 5-day period. During 1979 larvae were most abundant (> 1,000 fish per 1,000 m 3) in the Proctor Point (H) - Presqu'ile (I) region with moderately high abundance of larvae at sites immediately east (J) and west (E, F) of this area. Densities were lowest « 100 fish per 1,000 m3) along relatively exposed portions of shoreline (A to D) and in the Wellington (K)-Sandbanks (L) area. No apparent relationship existed between larval rainbow smelt density and substrate type for the 12 sites sampled. Densities of rainbow smelt larvae at a particular site were generally greatest in surface waters over the 3-m depth contour (Table 3). In 1979, decreased density of larvae with increasing bottom depth and, to a lesser extent, from surface to bottom, was characteristic of larval fish distribution at the Proctor Point-Presqu'ile sites (H, I), which accounted for 84% of all larvae caught, and most other sites. The decline in larval fish density with bottom depth was not as pronounced in 1978, possibly the result of differences between years in time of sampling relative to peak hatching. Densities of rainbow smelt larvae over the 7- to 13-m contours exceeded densities over the 3-m contour at Darlington (B) and Wesleyville (D) in both years, at Robinson Point (1) in 1978, and at the Sandbanks (L) in 1979.
gest that after hatching, rainbow smelt larvae dispersed offshore to deeper strata. Alewife Alewife larvae, collected in two sampling cycles between 14 July and 2 October 1978, showed a progressive increase in mean length (Fig. 2). Small larvae ( < 7.0 mm) comprised the bulk of the catch up to 18 August and were present in samples collected up to 31 August. Water temperatures at which alewife larvae were collected ranged from 13 to 21°C, although densities of larvae were generally highest in August when temperatures were approximately 18°C. 35,-----------------------,
665 152
30
25 58 331
E E J: f<.:J
51
84
20 508
Z W
...J ...J
« fa
15
f-
10 H
36 131
i'l + t J
I
H
C
K E L
B
G
AJ
F
G
TABLE 3. Distribution of rainbow smelt larvae (percent) at all stations (stratum-depth contour) sampled during 23 May-I5 June 1978, and 28 May-I June 1979. Values averaged for intensively sampled sites (B, D, H, I) and all sites (see Fig. 1). Sites B, D, H, I
All Sites
Strata Sampled (m)
3m
7m
13 m
3m
13m
1978
0.5
49.7
36.7
13.7
76.8
23.2
1979
0.5 6 12
73.4
16.6 2.0
5.6 1.8 0.6
89.9
8.9
Year
1.2
Mean total length of larvae for each contour and strata sampled was generally 5.6 mm. In 1979 larvae in mid-depth and bottom samples along the 13m contour averaged 5.8 and 6.1 mm, respectively. Although no statistically significant differences in size of fish among locations existed, the data sug-
20 JULY
10
20 AUGUST
10 SEPTEMBER
FIG. 2. Total length (mm) of alewife larvae collected during 1978 with means, standard deviations (vertical box), and ranges (vertical lines) indicated. Associated letters and numbers represent sites sampled (see Fig. 1) and number of larvae measured, respectively.
To evaluate movement of alewife larvae following hatching and identify potential nursery areas, larvae were divided into three size categories and catch data were compared for all surface collections from the 3- and 13-m depth contours. Small larvae ( < 7.0 mm), which included all yolk-sac larvae, comprised 36.70/0 of this total catch, while intermediate-sized larvae (7.0 to 12.5 mm) and large larvae (> 12.5 mm) represented 36.2 and 27.1 % of the catch, respectively. Small alewife larvae were present at all sites with
DISTRIBUTION OF RAINBOW SMELT AND ALEWIFE
277
TABLE 4. Mean densities (number per 1,000 m 3) and size composition (percent) of alewife larvae in the 0.5-m stratum over the 3- and 13-m contours for Lake Ontario sites (see Fig. 1) samples during 14 July-2 October 1978). Size Composition (Percent)
Density Site J I H
D C B E F G
K L A J I H G
D C B A
14 July 17 July 25 July 28 July 4 August 8 August 18 August 18 August 18 August 21 August 21 August 29 August 31 11 12 18 22 25 26 2
August September September September September September September October
>12.5 mm
<7.0 mm
7.0 to 12.5 mm
1,000 50 540 2 1,400 4,650 100 400 3,090 11,300 6,630 67
100 100 98.0 100 99.9 96.8 82.0 82.7 74.5 7.1 2.0 26.9
0 0 2.0 0 0.1 2.4 13.0 12.3 22.4 55.8 49.8 29.9
0 0 0 0 0 0.8 5.0 5.0 3.1 37.1 48.2 43.2
1,240 300 63 140 0 0 1 1
18.6 0 0 0 0 0 0 0
55.6 14.5 0 4.4 0 0 0 0
25.8 85.5 100 95.6 0 0 100 100
(No.l1,000
Date
m3)
greatest abundance (4,500 larvae per 1,000 m 3) at Darlington (B) on 8 August (Table 4). Pickering was sampled following what appeared to be the peak hatching period in early August and the importance of this site as a spawning area was probably underestimated. Highest densities of intermediate and large larvae occurred in the Wellington (K) - Sandbanks (L) area on 21 August (Table 4). Larvae at these two sites consisted primarily (95 %) of fish ~ 7.0 mm, while at sites near Colborne (E, F, G), which were sampled 3 days earlier, only 24070 of the catch was comprised of these larger larvae. Greater abundance of the larger larvae at sites G to L suggests an accumula-
tion or residence of alewife larvae in the eastern portion of the study area. Water transparency, determined by Secchi disc, varied considerably in August and September, particularly close to shore (4-m contour), although values were generally comparable for eastern and western sites (Balesic 1979). This would exclude the possibility that net avoidance accounted for differences in catch between the two areas. Distribution of larvae from the three size categories was compared, based on catch data for the intensively sampled sites (B, D, H, I) (Table 5). Early post-hatch larvae « 7.0 mm) were most abundant in surface waters over the 3-m contour
TABLE 5. Distribution of alewife larvae (percent) of three size categories at all stations (stratum-depth contour) sampled during 14 July-2 October 1978. Data averaged for intensively sampled sites (B, D, H, I). Strata Sampled (m) 0.5 6 12
Larvae 7.0 to 12.5 mm
Larvae < 7.0 mm
Larvae > 12.5 mm
3m
7m
13m
3m
7m
13m
3m
7m
13m
62.8
34.2 0.4
1.3 0.9 0.4
25.1
38.2 3.9
28.9 3.9 0.0
16.3
48.9 0.5
31.7 2.1 0.5
278
T. G. DUNSTALL
and their numbers decreased rapidly both offshore and in deeper strata. Intermediate-sized larvae (7.0 to 12.5 mm) were nearly equally distributed in surface waters over all depth contours sampled. However, their densities also decreased rapidly from surface to bottom strata. Large larvae (> 12.5 mm) likewise were concentrated in surface waters with somewhat higher densities over the 7- and 13-m contours. Distribution of alewife larvae was similar at other sites. DISCUSSION Highest densities of rainbow smelt larvae occurred in the Proctor Point-Presqu'ile region of Lake Ontario where substantial variation in both shoreline aspect and gradient exists; lowest densities occurred along the relatively exposed, featureless shoreline west of Colborne. Similarly, in western Lake Huron, O'Gorman (1983) found that lowest numbers of rainbow smelt larvae were associated with uniform stretches of shoreline, moderate densities were reported for irregular portions of shoreline, and highest densities occurred in bays and harbours. In the North Channel of Lake Huron, highest densities of rainbow smelt larvae occurred in shallow protected areas close to stream mouths (Loftus 1979a). The contribution of streamspawned larvae to observed lake densities in the present study was unknown, but was believed to have been minimal. Rapid disappearance of rainbow smelt larvae from the nearshore pelagic zone had previously been observed in the vicinity of Ontario Hydro's Pickering Generating Station during 1975-1977, with post-yolk-sac larvae being entrained only occasionally (Dunstall 1981). In Lake Michigan, rainbow smelt larvae were also reported to disperse offshore soon after hatching (Tin and Jude 1983). Developing rainbow smelt larvae, present inside the 15-m contour in Lake Michigan up to the middle of July, were rarely collected in 363-JLm mesh nets after July, although this was attributed mainly to net avoidance by larger larvae. Along the western shore of Lake Huron, O'Gorman (1983) found that density of larval rainbow smelt within the 5- to lO-m contours was greatly reduced by mid-July. Young-of-the-year rainbow smelt in Lake Erie moved into deeper water during July with continued movement offshore until late October, when they were completely mixed with adults at a depth of about 45 m (Ferguson 1965). Rainbow smelt larvae have been described as
being closely associated with the bottom in June and July, particularly during the day (Jude et al. 1980). A similar pattern was observed in Georgian Bay, with larvae moving into upper water strata at night (Emery 1973). O'Gorman (1983) reported significantly higher densities of rainbow smelt larvae in strata deeper than 1 m, over the 5.5- and 9.2-m contours, than in near-surface waters. Young-of-the-year rainbow smelt were concentrated near bottom within the 15-m contour in Lake Michigan during late August (Tin and Jude 1983). In Lake Erie, young-of-the-year rainbow smelt entering the offshore submerged intakes of a power plant at night represented about 720/0 of the total number of smelt entrained (Teleki 1976). In contrast, rainbow smelt larvae in the nearshore waters of Lake Ontario were concentrated in surface waters (0.5-m stratum) immediately following hatching, which may be characteristic for larvae at this stage of development, prior to their active feeding. Alewife was the dominant species in the pelagic ichthyoplankton along the north shore of Lake Ontario from July to September. Densities of alewife larvae immediately following hatching were highest in the 0.5-m stratum with numbers decreasing in that stratum when proceeding from the 3- to the 13-m contours. Similarly, in the Bay of Quinte region of Lake Ontario, Lam (1977) found greater abundance of larvae in near-surface waters than at the 3-m stratum. In western Lake Huron, however, alewife larvae were more abundant in the 1- to 3-m strata than in surface waters or in the 4- to 6-m strata (O'Gorman 1983). Along the relatively featureless portion of shoreline west of Colborne there was a rapid decline in abundance of small alewife larvae « 7.0 mm) from the nearshore zone (3- to 13-m contours), a pattern consistently observed during 1975-1977 near Pickering Generating Station (DunstallI981). Post-yolk-sac alewife larvae were rarely entrained in the surface intake of this station. Variation in topography within the nearshore zone, in the area east of Colborne, provides a relatively sheltered environment which appears to serve as a better nursery ground for developing alewife larvae than areas of uninterrupted shoreline. The larger alewife larvae (~7.0 mm) at the eastern sites may have persisted in the region following hatching or accumulated as a result of immigration from other areas of the lake. These larvae were concentrated in the surface stratum with highest densities along the 7- and 13-m contours. Spatial distributions of
DISTRIBUTION OF RAINBOW SMELT AND ALEWIFE
alewife larvae and fry, similar to those found in the present study, have been described for Lakes Michigan (Wells 1973, Jude et al. 1980), and Erie (Mizera et al. 1981). ACKNOWLEDGMENTS This study was funded by Ontario Hydro. The field program was conducted by L. P. King and E. Checko aboard the Tug Seagull, operated by E. Semple. Identifications and measurements of specimens were made by E. Checko. H. Balesic contributed many useful suggestions during the course of this study. REFERENCES Balesic, H. 1979. Nearshore environment of the north shore of Lake Ontario-1978 intake location study interim report. Ontario Hydro Res. Div. Rep. 79300-K. Cooper, C. L., Heniken, M. R., and Herdendorf, C. E. 1981. Limnetic larval fish in the Ohio portion of the western basin of Lake Erie, 1975-1976. J. Great Lakes Res. 7:326-329. Cooper, J. E. 1978. Identification of eggs, larvae, and juveniles of the rainbow smelt, Osmerus mordax, with comparisons to larval alewife, Alosa pseudoharengus, and gizzard shad, Dorosoma cepedianum. Trans. Am. Fish. Soc. 107:56-62. Dunstall, T. G. 1981. Ichthyoplankton entrainment at Pickering GS 'A", 1975-1977. Ontario Hydro Res. Div. Rep. 81-211-K. Emery, A. R. 1973. Preliminary comparison of day and night habitats of freshwater fish in Ontario Lakes. J. Fish Res. Board Can. 30:761-774. Ferguson, R. G. 1965. Bathymetric distribution of American smelt Osmerus mordax in Lake Erie. In Proceedings of the 8th Conf. Great Lakes Res., pp. 47-60. Internat. Assoc. Great Lakes Res. Jude, D. J., Heufelder, G. R., Tin, H. T., Auer, N. A., Klinger, S. A., Schneeberger, P. J., Rutecki, T. L., Madenjian, C. P., and Rago, P. J. 1979. Adult, juve-
nile and larval fish in the vicinity of the J. H. Campbell Power Plant, eastern Lake Michigan, 1978. Great Lakes Res. Div. Spec. Rep. No. 73, Univ. Mich.
279
_ _ _ _ , Heufelder, G. R., Auer, N. A., Tin, H. T., Klinger, S. A., Schneeberger, P. J., Madenjian, C. P., Rutecki, T. L., and Godun, G. G. 1980. Adult,
juvenile and larval fish populations in the vicinity of the J. H. Campbell Power Plant, eastern Lake Michigan, 1979. Great Lakes Res. Div. Spec. Rep. No. 79, Univ. Mich. Kelso, J. R. M., and Milburn, G. S. 1979. Entrainment and impingement of fish by power plants in the Great Lakes which use the once-through cooling process. J. Great Lakes Res. 5:182-194. Lam, C. N. G. 1977. Distribution and abundance of the early developmental phases of alewife, Alosa pseudoharengus (Wilson) and other fishes in the Bay of Quinte. M.Sc. dissertation, Univ. of Guelph, Guelph, Ontario. Lippson, A. J., and Moran, R. L. 1974. Manual for -
identification of early development stages of fishes of the Potomac River Estuary. Tech. Center Martin Marietta Corp., Baltimore, Maryland. Loftus, D. H. 1979a. North Channel larval fish survey: Joe Dollar Bay to Aird Bay, May 1979. Lake Huron Fish. Assess. Unit, Ontairo Min. Nat. Res. Rep. 279. _ _ _ _ . 1979b. Larval fish sampling in Lake Huron, 1979. Lake Huron Fish. Ass.ess. Unit, Ontario Min. Nat. Res. Rep. 3-79. Mizera, J. J., Cooper, C. L., and Herdendorf, C. E. 1981. Limentic larval fish in the nearshore zone of the western basin of Lake Erie. J. Great Lakes Res. 7:62-64. O'Gorman, R. 1983. Distribution and abundance of larval fish in the nearshore waters of western Lake Huron. J. Great Lakes Res. 9: 14-22. Teleki, G. C. 1976. The incidence and effect of oncethrough cooling on young-of-the-year fishes at Long Point Bay, Lake Erie: A preliminary assessment. In Thermal Ecology II, ed. G. W. Esch and R. W. McFarlane, pp. 387-393. Nat. Tech. Inform. Ser. Conf.-750425, Springfield, Virgo Tin, H. T., and Jude, D. J. 1983. Distribution and growth of larval rainbow smelt in eastern Lake Michigan, 1978-1981. Trans. Am. Fish. Soc. 112:517-524. Wells, L. 1973. Distribution of fish fry in nearshore
waters of southeastern and east central Lake Michigan, May-August 1972. Admin. Rep. Great Lakes Fish. Lab., US Fish. Wildl. Serv., Ann Arbor, Mich.