Importance of Lake Ontario Embayments and Nearshore Habitats as Nurseries for Larval Fishes with Emphasis on Alewife (Alosa pseudoharengus)

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1 J. Great Lakes Res. 29(1): Internat. Assoc. Great Lakes Res., 2003 Importance of Lake Ontario Embayments and Nearshore Habitats as Nurseries for Larval Fishes with Emphasis on Alewife (Alosa pseudoharengus) Robert A. Klumb 1,4, Lars G. Rudstam 1,*, Edward L. Mills 1, Clifford P. Schneider 2,5, and Paul M. Sawyko 3 1 Department of Natural Resources Cornell University Biological Field Station 900 Shackelton Point Road Bridgeport, New York , USA 2 New York Department of Environmental Conservation Cape Vincent Fisheries Research Station Cape Vincent, New York 13618, USA 3 Rochester Gas and Electric Corporation 89 East Avenue Rochester, New York , USA ABSTRACT. Productive embayments, areas morphometrically sheltered from open lake physical processes, were compared to less productive, exposed shorelines of the nearshore as nursery habitats for larval alewives (Alosa pseudoharengus), the principal forage fish in Lake Ontario. Species composition and densities of pelagic larval fishes were assessed with towed plankton nets at three embayments and adjacent nearshore habitats located along the New York shore, two sites in 1997 and a third site in Peak nearshore densities of larval alewives in 1997 and 1998 were compared to 1977 and 1978, a time period prior to major phosphorus reductions and the establishment of exotic dreissenid mussels in Lake Ontario. Seasonal changes in the pelagic larval fish assemblage were generally similar in the embayments; yellow perch (Perca flavescens) were prevalent in late-may shifting to co-occurrence of alewives and Lepomis spp. in June and July. At all nearshore sites, the larval fish assemblage throughout summer consisted mainly of alewives (> 65%). Densities of larval alewives peaked earlier in embayments but generally at lower densities than observed later in the nearshore. Because the amount of nearshore habitat greatly exceeds that of embayment habitat in Lake Ontario, nearshore waters served as the principal nurseries for larval alewives. However, densities of all larval fish species other than alewives (principally yellow perch and Lepomis spp.) were usually greater in embayments than the nearshore. Compared to three nearshore sites in 1977 and 1978, timing of peak densities of alewives in 1997 and 1998 were similar and temperature dependent; highest densities occurred in late July to early August after nearshore waters were 18 C. Peak median nearshore alewife densities (larvae/100 m 3 ) in 1997 (47) and 1998 (60) were similar to those in 1977 (8 to 100) and 1978 (34 to 49). This similarity suggested a limited effect of lowered phosphorus and food web changes on peak larval alewife densities in Lake Ontario. INDEX WORDS: Embayments, nearshore, nursery habitat, larval fish, alewife, Lake Ontario. * Corresponding author. lgr1@cornell.edu 4 Current address: U.S. Fish and Wildlife Service, Great Plains Management Assistance Office, 420 South Garfield Avenue, Suite 400, Pierre, South Dakota 57501, USA 5 Retired INTRODUCTION The Lake Ontario ecosystem is changing due to lake-wide phosphorus reductions from pollution control efforts (Stevens and Neilson 1987), the in- 181

2 182 Klumb et al. vasion of two exotic dreissenid mussels, Dreissena polymorpha and D. bugensis (Griffiths et al. 1991, May and Marsden 1992), and invasion by two predatory cladocerans Bythotrephes longimanus, previously B. cederstroemi (Lange and Cap 1986), and Cercopagis pengoi (MacIssac et al. 1999). These changes are anticipated to affect dynamics of zooplankton and the major planktivore the alewife (Alosa pseudoharengus). The abundance of alewives influences sustainability of the salmonid sport fishery of Lake Ontario (Rand and Stewart 1998, O Gorman and Stewart 1999). Once an exotic nuisance that died off in massive numbers (Smith 1892, Pritchard 1929, Colby 1971), management now focuses on maintaining alewife abundance as forage for Lake Ontario salmonids (Brandt 1986). High salmonid stocking levels concurrent with high young-of-theyear (YOY) alewife mortality from a severe winter could reduce alewife numbers and slow salmonid growth (O Gorman et al. 1987, Jones et al. 1993). In 1993, numbers of salmonids stocked in Lake Ontario were reduced to lower predation demand on alewives approximately 50% from that of the late 1980s (O Gorman and Stewart 1999). So far, the management of Great Lakes salmonids has been based on estimating their predatory demand on alewife populations (Stewart and Ibarra 1991, Jones et al. 1993, Rand and Stewart 1998). Less attention has been focused on factors regulating the supply of alewives. Alewives in Lake Ontario migrate inshore in spring to spawn at night (Graham 1956) and the entire New York nearshore zone has been considered suitable spawning and nursery habitat (Goodyear et al. 1982). However, the relative importance of protected embayments and exposed nearshore habitats as nursery areas for larval alewives in Lake Ontario has never been quantified. Also, comparisons of larval fish assemblages between Lake Ontario embayment and nearshore habitats do not currently exist. Seasonal changes in the pelagic larval fish assemblage have only been studied in one Lake Ontario embayment, Hamilton Harbour, a severely degraded habitat (Leslie and Timmins 1992), or in shallow wetland habitats (Stephenson 1990). There is also a need to compare the contemporary seasonal occurrence of alewife larvae in the nearshore with historical observations. Since colonization of Lake Ontario by Dreissena spp., spring depth distributions of adult alewives shifted to deep waters (O Gorman et al. 2000). A by-product of this depth shift could be a delay in spring spawning migrations by alewives leading to a shortened growing season for YOY and, potentially, poor growth and recruitment. It is possible that embayments are more conducive to larval fish growth and survival than the nearshore because embayments are productive habitats, morphometrically sheltered from open lake processes. Lake Ontario embayments have higher total phosphorus, chlorophyll-a concentrations, zooplankton densities, and warmer water temperatures than nearshore habitats (Hall et al. 2003). Therefore, the objectives of this study were to: 1) compare species composition and densities of pelagic larval fishes in embayment and nearshore habitats, and 2) compare the historical magnitude and timing of peak alewife densities to assess whether ecosystem changes in Lake Ontario have affected alewife density and seasonal occurrence. METHODS Habitat Definitions and Study Sites Currently a specific limnological definition does not exist for embayment and nearshore habitats in the Great Lakes (Hutchinson 1957, Wetzel 1983). For this study, embayments are defined morphometrically, as a water body formed by invagination of the shoreline that has a narrow connection (inlet) to the main lake. Embayments are distinguished from bays by having higher shoreline development factors and narrower inlets, which restrict water exchange with the lake. The nearshore habitat is arbitrarily defined to extend out to the 30-m depth contour, an area of dynamic thermal gradients. Within this area the thermocline forms in early summer behind the thermal bar (Rodgers 1968) and intersects the lake bottom. The intersection of the thermocline with the bottom actively moves throughout the growing season and segregates fish species (Brandt et al. 1980) or life stages of alewives (Brandt 1980). The three embayments studied along the New York shore of Lake Ontario, Chaumont, Sodus and Irondequoit, differed in size and degree of isolation from Lake Ontario due to their inlet s width and aspect to prevailing westerly winds (Fig. 1). The largest embayment, Chaumont (6,166 ha), has a mean depth of 5 m (maximum depth 20 m), and is connected to Lake Ontario by a 1,560-m wide inlet oriented to the southwest. Sodus Bay covers 1,354 ha, has a mean depth of 5 m (maximum depth 15 m), and is connected to Lake Ontario by a 140-m wide inlet oriented to the north. The smallest em-

3 Lake Ontario Larval Alewife Habitat 183 FIG. 1. Map showing three Lake Ontario, New York embayment and nearshore habitats where larval fishes were collected in 1997 (Chaumont and Sodus) and 1998 (Irondequoit). Nearshore larval fish collections at Chaumont occurred near Galloo Island. Stars indicate sites of nearshore larval fish collections by the Rochester Gas and Electric Co. in 1977 and bayment, Irondequoit (698 ha), has a mean depth of 7 m (maximum depth 24 m) and is connected to Lake Ontario by a 110-m wide inlet oriented to the north. The three nearshore sites were adjacent to each respective embayment. Two sites, Irondequoit and Sodus, were located in the open waters of Lake Ontario. The 30-m contour lies 3.3 to 6.3 km from shore at Irondequoit and 3.7 to 4.4 km from shore at Sodus. At Irondequoit and Sodus, all larval fish sampling occurred within 3.8 km of the shoreline. Nearshore samples at Chaumont occurred approximately 12 km from the mainland near Galloo Island (Fig. 1). At Chaumont, the 30-m contour lies 2.8 to 5.9 km from the shore of Galloo Island and all larval fish sampling occurred within 2.9 km of Galloo Island. Seasonal occurrence and densities of larval fish were assessed at two sites in 1997 and one site in Chaumont and Sodus were sampled biweekly in 1997 from May through August with each embayment sampled once in September. In 1998, Irondequoit was sampled six times from June through August. Paired embayment and nearshore samples occurred within 1 day of each other and effort was greater for embayments than for nearshore sites due to higher waves along the exposed shoreline. Historic occurrence and densities of larval alewives in the nearshore of Lake Ontario prior to phosphorus reductions and the establishment of exotic mussels and zooplankters were determined from larval collections made by Rochester Gas and Electric Corporation (RG&E) during 1977 and Larval fish sampling occurred in nearshore waters along the New York shore at three sites named Sterling, Ginna, and Russell (Fig. 1). At Sterling, samples were collected on 16 dates from 27 April to 30 September 1977 (RG&E 1977), and on 17 dates from 25 April to 8 November 1978

4 184 Klumb et al. (RG&E 1978a). Nearshore larval fish collections at Ginna occurred on seven dates from 13 June to 8 September 1977 (RG&E 1978b) and on 10 dates from 25 May to 20 September 1978 (RG&E 1979a). In the nearshore at Russell, larval fish collections occurred on six dates from 15 June to 25 August 1977 (RG&E 1978c) and on eight dates from 23 May to 21 August 1978 (RG&E 1979b). Larval Fish Collections In 1997, larval fish were collected during the day (0910 to 1645) with 1-m diameter, 4-m long, conical plankton nets of 500-µm and 750-µm nylon mesh. The two nets were concurrently towed for 10 minutes just below the surface, approximately 25 m behind the boat. The 750-µm net was used throughout the season and the 500-µm net was used after 16 June. Generally, five tows were conducted in each habitat (range = 2 to 6) over bottom depths ranging from 3.8 to 12.0 m in embayments (mean = 6.7 m) and from 2.5 to 25.0 m in the nearshore (mean = 7.6 m). Constant boat speed was maintained by adjusting engine revolutions per minute; average speed determined from a global positioning system (GPS) was approximately 4.2 km/h. Coordinates at the start and end of each tow were recorded with a GPS. In both years, water temperatures were recorded at the surface and at 1 and 2 m with a YSI model 57 temperature-dissolved oxygen meter; temperatures reported in this study were means of the three observations. In 1998, larval fish were collected during the day (1050 to 1645) with two 0.5-m diameter, 1.5-m long, conical zooplankton nets of 505-µm nylon mesh. The boat available for sampling in 1998 could not safely tow the 1-m diameter nets and this necessitated the change to the 0.5-m diameter nets. The two nets were towed concurrently just below the surface, 25 m behind the boat, at approximately 4.0 km/hr. Tow duration was reduced to 5 minutes to increase sample size (generally 6 to 12 tows per habitat). Surface collections occurred over bottom depths ranging from 1.8 to 24 m in the embayment (mean = 14.0 m) and from 6.9 to 24.2 m in the nearshore (mean = 14.1 m). Larval fish densities for the two replicate tows were averaged. Fish were also collected at Chaumont on one date in 1998 (6 July) to compare larval alewife densities and assess spatial distributions of larvae in the nearshore. All larval fish were preserved in the field using 70% ethanol, identified to species using the keys in Auer (1982), and densities (fish/100 m 3 ) calculated by dividing the catch by the volume of water sampled. For larval tows, the total water volume strained was calculated using the net opening area and the tow distance obtained from the GPS coordinates. Assumptions made for density calculations include 100% net efficiency, tows were in a straight line, lake currents were insignificant, and water volume filtered deploying and retrieving the nets was negligible. The three early developmental phases of fish, yolk-sac larvae, larvae, and juvenile, were distinguished using the definitions in Auer (1982). The yolk-sac phase lasts from hatching to complete yolk absorption, the larval phase lasts from yolk-sac absorption to the complete formation of all fin rays, and the juvenile phase lasts from complete fin ray development to sexual maturity. Larval pumpkinseeds (Lepomis gibbosus) and bluegills (L. macrochirus), prior to fin ray formation, could not be reliably distinguished based on myomere counts or pigmentation and are hereafter called Lepomis spp. Densities (fish/100 m 3 ) of all alewife, yolk-sac alewife, and all other species (excluding alewife) in embayments were compared to those in nearshore habitats using a permutation procedure (Manly 1997). Specifically, alewife yolk-sac larvae were compared because the habitat of capture was the likely spawning habitat used by the adults; yolk-sac larvae are less than 1 to 4 d of age (R. Klumb, personal observations in the laboratory). Non-parametric permutation tests were used because small sample sizes (N = 3 to 12) precluded testing for normality. The Wilcoxon rank-sum statistic was permutated 5,000 times to minimize effects due to patchiness; outliers of high larval abundance were observed, and mean densities were also skewed by zero catches. Therefore, median densities are reported in this study along with the minimum and maximum density observed on each date. All permutation tests were performed with S-Plus 2000 (Mathsoft 1999) and significance was set at α = 0.1. Seasonal mean densities (June through August) were calculated to compare summer relative abundance among sites in each habitat. Mean peak densities of larval fish were also reported in this study to facilitate comparisons with historical studies of larval fish abundance in Lake Ontario (Lam 1977, Dunstall 1984, Leslie and Timmins 1992) and the other Great Lakes (Bimber et al. 1984, O Gorman 1983, O Gorman 1984, Nash and Geffen 1991, Leslie and Timmins 1994). Larval collections were stratified by depth and by distance from nearest shoreline in the nearshore

5 Lake Ontario Larval Alewife Habitat 185 habitats of Chaumont and Irondequoit during 1998 to describe the spatial distribution of larval alewives (all life stages) and alewife yolk-sac larvae. Data for Irondequoit Bay collected on 10 June, 24 June, and 15 July were combined. Mean larval densities, were calculated for three depth categories 5 to 10, 10 to 15, and > 15 m. Distances from shore were divided into two categories at Chaumont (< 500 m and 500 m) and three categories at Irondequoit (< 1,000 m, 1,000 to 2,000 m, and > 2,000 m) so sampling effort among categories was similar. The Chaumont nearshore site was located approximately 12 km from the shore; however, samples were collected near a large island, Galloo. Therefore, at Chaumont, distances were calculated from the sample location to Galloo Island. Methods of larval fish collections made in the nearshore of Lake Ontario off Sterling, New York by RG&E during 1977 and 1978 were similar to our methods and are briefly summarized here. Triplicate larval fish samples were collected just below the surface during the day along two transects (3.2 km apart) over the 2-, 5-, 8-, 11-, and 14-m depth contours. A Hensen-style plankton net (1-m diameter, 7-m long) with a nylon mesh of 505 µm was towed approximately 35 m behind the boat at 2.8 to 4.6 km/hr for 2 minutes in order to filter a desired volume of approximately 100 m 3. Boat speed was measured with an air gauge marine speedometer and filtered water volume was determined with an impeller type flow meter (General Oceanics, Inc., Miami, FL) mounted slightly off-center in the net s mouth. All samples were preserved in 10% buffered formalin and densities of alewife yolk-sac larvae were distinguished from later life stages. The median density of alewife larvae at each depth contour was calculated from the six samples (three replicates at two transects). For each site, the overall median peak density in the entire nearshore was calculated across the five depth strata on the date with the highest median density observed that year at any depth stratum. Water temperatures at each transect were measured at the 2- and 11-m depth contours in 1-m increments; mean nearshore temperatures were calculated from measurements at 0, 1, and 2 m. Sampling methods in 1977 and 1978 at the Ginna and Russell sites differed from those used at Sterling. Single larval fish samples were collected just below the surface during the day along one transect over the 2-, 5-, 8-, and 11-m depth contours. A conical plankton net (0.5-m diameter, 1.5-m long) with a nylon mesh of 571 µm was towed behind the boat at 5.8 km/hr for 6 to 7 minutes in order to filter a desired volume of approximately 100 m 3. Filtered volume was measured with the same impeller type flow meters used at the Sterling site. All samples were preserved in 10% buffered formalin and only densities of all larval alewife life stages combined were determined. For each site, the overall median peak density in the entire nearshore was calculated across the four depth strata on the date with the highest observed density that year at any depth stratum. Water temperatures were measured at the four depth contours in 0.5-m increments; mean nearshore temperatures were averaged from the five measurements made from the surface to a depth of 2 m. RESULTS Larval Fish Assemblages Numbers of pelagic larval fish species ranged from 3 to 6 in the embayments (Table 1) and the assemblage changed seasonally. At Chaumont and Sodus in 1997, yellow perch were prevalent in catches in May or early June, the assemblage then shifted to a combination of alewives and Lepomis spp. in early July (Fig. 2). At Irondequoit in 1998, a similar change from the early prevalence of larval alewives in June to Lepomis spp. in July was observed. At all three embayments, alewife larvae peaked in June or early July and then declined through late July and August. Although catches at Chaumont and Sodus were lower in the 750-µm mesh net, seasonal trends in the composition of the pelagic larval fish assemblage were similar to that in the 500-mm mesh net. Numbers of pelagic larval fish species observed in the nearshore at all three sites (4 to 6 species) were similar to those in the embayments but the principal species was alewife (> 65%) (Fig. 2). Emerald shiner (Notropis atherinoides) larvae were caught in mid-july at Sodus in 1997 and Irondequoit in 1998 (Table 1). Stickleback larvae (likely Gasterosteus aculeatus) were only captured in nearshore habitats. Embayments vs. Nearshore Habitats Densities of larval alewives peaked sooner in embayments but generally at lower densities than observed later in the nearshore (Tables 2 and 3). Peak abundance occurred in both habitats after surface water temperatures were 18 C. At Chaumont in 1997, significantly more larvae were caught in the

6 186 Klumb et al. TABLE 1. Peak densities (larvae/100 m3) of all pelagic larval fish species other than alewife and the dates when they occurred in three embayment and nearshore habitats of Lake Ontario, New York during 1997 and (Number of larvae captured shown in parentheses.) Towed plankton nets were of 500 and 750 µm mesh in 1997 and 505 µm mesh in Species Date Peak density Mesh size Chaumont embayment 1997 Rainbow smelt (Osmerus mordax) 5 Jun 0.2 (5) 750 Yellow perch (Perca flavescens) 5 Jun 4.0 (104) 750 White perch (Morone americana) 16 Jun 1.0 (30) 500 Common carp (Cyprinus carpio) 1 Jul 0.1 (1) 750 Lepomis spp. 1 Jul 8.5 (170) 750 Freshwater drum (Aplondinotus grunniens) 11 Aug 0.2 (3) 750 Chaumont nearshore 1997 Common carp (Cyprinus carpio) 30 Jun 0.2 (5) 750 Lepomis spp. 30 Jun 0.1 (2) 500 Unidentified darter (Percidae) 30 Jun < 0.1 (1) 750 Emerald shiner (Notropis atherinoides) 12 Aug < 0.1 (1) 750 Sodus embayment 1997 White sucker (Catostomus commersoni) 19 May < 0.1 (1) 750 Yellow perch (Perca flavescens) 19 May 11.7 (294) 750 Rainbow smelt (Osmerus mordax) 23 Jun 0.1 (1) 750 Common carp (Cyprinus carpio) 23 Jun 0.1 (2) 750 Banded killifish (Fundulus diaphanus) 23 Jul < 0.1 (1) 500 Lepomis spp. 23 Jul 6.3 (168) 500 Sodus nearshore 1997 Mottled sculpin (Cottus bairdi) 24 Jun < 0.1 (1) 500 Stickleback a (Culaea or Gasterosteus sp.) 24 Jun 0.1 (3) 500 Common carp (Cyprinus carpio) 8 Jul < 0.1 (1) 750 Lepomis spp. 8 Jul 0.1 (2) 500 Unidentified darter (Percidae) 8 Jul 0.1 (3) 750 Emerald shiner (Notropis atherinoides) 22 Jul 0.8 (26) 500 Irondequoit embayment 1998 Gizzard shad (Dorosoma cepedianum) 3 Jun 0.3 (1) 505 Lepomis spp. 3 Jun 3.6 (18) 505 Brook silversides (Labidesthes sicculus) 10 Jun 0.1 (1) 505 Irondequoit nearshore 1998 Emerald shiner (Notropis atherinoides) 15 Jul 3.3 (45) 505 Lepomis spp. 15 Jul 0.1 (1) 505 Rainbow smelt (Osmerus mordax) 15 Jul 0.1 (1) 505 Stickleback a (Culaea or Gasterosteus sp.) 15 Jul 0.1 (1) 505 a Culea and Gasterosteus sp. cannot be distinguished prior to dorsal fin spine formation (Heufelder 1982). embayments on 1 July (750-µm mesh) and in the nearshore on 14 July (500-µm mesh) and 11 August (both mesh sizes). For the 500-µm mesh net at Chaumont, the mean peak density of alewives (72 larvae/100 m 3 ) in the embayment on 1 July was 2.2 times greater than the peak observed later in the nearshore on 14 July (32.8 larvae/100 m 3 ). At Sodus, significantly more larvae were caught in the nearshore on 4 August and 18 August than in the embayment with the 500-µm mesh net. At Sodus in 1997, the mean peak density of alewives (73 larvae/100 m 3 ) caught with the 500-µm mesh net in the nearshore on 4 August was 28 times greater than the peak observed in the embayment on 7 July (2.6 larvae/100 m 3 ). At Irondequoit in 1998, significantly more larvae were caught in the embayment

7 Lake Ontario Larval Alewife Habitat 187 FIG. 2. Seasonal composition of the larval fish community in Lake Ontario, New York embayment and nearshore habitats: A) Chaumont using a 750 µm mesh net, B) Chaumont using a 500 µm mesh net, C) Sodus using a 750 µm mesh net, D) Sodus using a 500 µm mesh net, and E) Irondequoit using a 505 µm mesh net. Chaumont and Sodus were sampled in 1997 and Irondequoit was sampled in Total number of larvae captured shown above bars.

8 188 Klumb et al. TABLE 2. Median densities (larvae/100 m 3 ) of alewife larvae (all life stages), alewife yolk-sac larvae, and all species other than alewife captured in Lake Ontario embayment and nearshore habitats of Chaumont and Sodus, New York during 1997 with a towed 750 µm mesh net. For each site, densities of all alewife life stages, alewife yolk-sac larvae, and all species other than alewife that are significantly different (permutation test, α = 0.1) between habitats on a given date are indicated by asterisks. Also shown is the mean temperature ( C) of the top 2 m of water. Minimum and maximum densities observed on each date are in parentheses, N = number of tows, and ns = not sampled. Embayment Nearshore Alewife Alewife Other Alewife Alewife Other Date N C all life stages yolk-sac species N C all life stages yolk-sac species Chaumont 12 May ns 5 Jun (0, 0.1) (0, 9.8)* * 16 Jun (0, 7.0) 0 (0, 0.2) 0.2 (0, 4.1) Jul (0, 97.5)* 0.5 (0, 9.4) 4.2 (0.2, 23.3)* (0, 1.7)* 0.2 (0, 0.6) 0.2 (0, 1.0)* 14 Jul (0.3, 2.8) 0 (0, 1.1) 0.9 (0.3, 2.5)* (0, 1.5) 0 (0, 0.2) 0* 29 Jul (0, 2.0) (1.0, 3.8)* (0, 0.3) 0 0* 11 Aug (0, 0.2)* (0, 1.0)* (0.4, 3.4)* 0 0 (0, 0.2)* Sodus 19 May (0, 28.4) ns 30 May (0.2, 3.2) ns 9 Jun Jun (0, 1.5) (0, 1.2) 0.2 (0, 0.6) 0 (0, 0.2) 7 Jul (0.3, 2.2) (0.2, 1.1)* (0, 3.1) 0 (0, 0.9) 0.2 (0, 0.4)* 23 Jul (0.3, 2.2) (0.3, 6.4)* (0, 2.6) 0 (0, 0.3) 0* 5 Aug (0, 0.9) (0.2, 18.9) Aug (0, 1.5) Sep ns on 10 June and there was significantly more larvae in the nearshore on 23 Jun. The mean peak density of alewives (85 larvae/100 m 3 ) in the nearshore of Irondequoit on 18 June was over six times greater than the highest density observed in the embayment on 3 June (13.5 larvae/100 m 3 ). For the one sampling date in 1998 at Chaumont on 6 July, densities of larval alewives were significantly greater in the nearshore (median = 17, range: larvae/100 m 3 ) than in the embayment (median = 0, range: larvae/100 m 3 ). At all three sites, densities of yolk-sac alewife larvae in the nearshore were similar to, or significantly greater than, densities found in the embayments (Tables 2 and 3). Yolk-sac larvae first appeared in early June in both habitats although surface water temperatures were cooler in the nearshore (15 to 17 C) than in the embayments (18 to 21 C). Densities of yolk-sac larvae generally peaked in the embayments during June or early July. In the nearshore, densities of yolk-sac larvae generally peaked in mid-july when surface water temperatures were 18 C and persisted in the catches at Chaumont and Sodus from mid-june through mid-august. The densities of all pelagic larvae other than alewife (principally yellow perch and Lepomis spp.) were significantly greater in the embayments than in the nearshore at all three sites on all dates with paired samples (Tables 2 and 3) except for 11 August at Chaumont (500-µm net) and 23 June at Sodus (both nets). At all sites the highest mean density observed in embayments was 12 larvae/100 m 3 whereas in the nearshore, densities of all other species only exceeded 1 larvae/100 m 3 on one occasion (Table 1). The seasonal (June through August) mean density of larval alewives differed among the three embayments but the mean densities of all other species excluding alewife were similar. Comparing only the

9 Lake Ontario Larval Alewife Habitat 189 TABLE 3. Median densities (larvae/100 m 3 ) of alewife larvae (all life stages), alewife yolk-sac larvae, and all species other than alewife captured in Lake Ontario embayment and nearshore habitats of Chaumont, Sodus, and Irondequoit, New York. Chaumont and Sodus were sampled in 1997 with a 1-m-diameter net of 500 µm mesh, whereas Irondequoit was sampled in 1998 with a 0.5-m-diameter net of 505 µm. For each site, densities of all alewife life stages, alewife yolk-sac larvae, and all species other than alewife that are significantly different (permutation test, α = 0.1) between habitats on a given date are indicated by asterisks. Also shown is the mean temperature ( C) of the top 2 m of water. Minimum and maximum densities observed on each date are in parentheses, N = number of tows, and ns = not sampled. Embayment Nearshore Alewife Alewife Other Alewife Alewife Other Date N C all life stages yolk-sac species N C all life stages yolk-sac species Chaumont Jun (0, 2.5) 0 (0, 0.2) 0 (0, 8.5) Jul (2.3, 139.0) 1.5 (0.2, 17.4) 5.3 (0.2, 19.0)* (3.4, 38.1) 7.4 (3.4, 22.9) 0 (0, 0.4)* 14 Jul (0.8, 16.7)* 0 (0, 1.4)* 1.3 (0.6, 12.6)* (4.1, 62.0)* 12.5 (0.9, 28.4)* 0 (0, 0.2)* 29 Jul (0, 1.0) (0, 5.3)* (0, 1.2) 0 (0, 0.2) 0.2 (0, 0.3)* 11 Aug (0, 0.2)* 0* 0 (0, 1.5) (0.2, 2.0)* 0.2 (0, 0.6)* 0 (0, 0.2) Sodus Jun (0, 4.4) 0 (0, 0.4) 0.8 (0, 10.4) (0, 2.0) 0.3 (0, 1.8) 0 (0, 0.9) 7 Jul (0.4, 4.2) 0* 1.2 (0.6, 2.9) (0.6, 79.6) 1.0 (0, 40.9)* 0.2 (0, 0.6) 23 Jul (1.1, 4.6) 0* 6.4 (1.3, 14.4)* (0.2, 171.3) 0.5 (0, 20.1)* 0 (0, 3.6)* 5 Aug (0, 0.3)* 0* (3.9, 195.2)* 2.6 (0.5, 5.4)* 0 18 Aug * 0 0 (0, 0.2)* (0, 1.2)* 0 (0, 0.4) 0* 25 Sep ns Irondequoit Jun (2.4, 32.9) 0.5 (0, 4.1) 4.2 (1.2, 6.2) ns 10 Jun (0.7, 12.1)* 0.3 (0, 0.9) 0.3 (0, 3.4)* (0, 4.0)* 0 (0, 1.3) 0* 24 Jun (0, 0.8)* 0* 0 (0, 5.1)* (1.1, 85.1)* 2.8 (0, 53.9)* 0 (0, 0.7)* 15 Jul ns (0, 262.5) 9.6 (0, 84.1) 1.6 (0, 14.5) 23 Jul (0, 12.2) ns 6 Aug ns 12 Aug (0, 0.7) ns 500- and 505-µm nets, Chaumont had the highest mean density of alewife larvae (15.4 larvae/100 m 3 ) compared to 2.3 larvae/100 m 3 for Irondequoit and 1.4 larvae/100 m 3 for Sodus. Overall mean summer densities of all other species at all three embayments ranged from 1.4 to 3.4 larvae/100 m 3. At the three nearshore sites, the seasonal (June through August) mean density of alewives and all other species exclusive of alewife were generally similar. Comparing means only from the 500- and 505-mm nets, Irondequoit had the highest mean density of alewife larvae (30.2 larvae/100 m 3 ) compared to 25.0 larvae/100 m 3 for Sodus and 11.9 larvae/100 m 3 for Chaumont. Overall seasonal mean densities of all other species at all three nearshore sites were low, 1.4 larvae/100 m 3 at Irondequoit, 0.3 larvae/100 m 3 at Sodus, and 0.1 larvae/100 m 3 at Chaumont. In general, alewife larvae were uniformly distributed in the nearshore at Chaumont and Irondequoit in 1998 out to the 15-m contour (Fig. 3). Densities declined with increased distance from shore and at depth strata > 15 m. High densities of larvae (> 25 larvae/100 m 3 ) were present at distances greater than 2,000 m from shore at Irondequoit. Yolk-sac larvae were captured in surface waters out to the 24-m contour, 2,300 m from shore at Irondequoit and out to the 23-m contour, 2,800 m from the shore of Galloo Island at Chaumont.

10 190 Klumb et al. FIG. 3. Spatial distribution of all larval alewives and yolk-sac larvae collected with surface tows (505 µm mesh) in the Lake Ontario nearshore at Chaumont and Irondequoit in 1998: A) mean densities (± 2 SE) of larvae within each of three depth strata and B) mean densities (± 2 SE) of larvae at various distances from shore. Distances at Chaumont were measured from Galloo Island; sample location was 12 km from the mainland. Historical Comparisons In 1977, alewife larvae first appeared in the nearshore of Lake Ontario in early June and densities peaked at all three sites in late-july or early August when water temperatures were 18 C (Table 4). During peak larval alewife abundance, median surface densities across all depth strata in the nearshore were 100 larvae/100 m 3 at Sterling on 28 July, 84 larvae/100 m 3 at Ginna on 8 August, and 8 larvae/100 m 3 at Russell on 10 August. Corresponding mean surface densities on these dates were 112 larvae/100 m 3 at Sterling, 133 larvae/100 m 3 at Ginna, and 79 larvae/100 m 3 at Russell. High densities (> 25 larvae/100 m 3 ) were found out to the 14-m bottom contour (approximately 2.5 km offshore) at the Sterling site from 19 July to 2 August. For all three sites, median densities > 5 larvae/100 m 3 persisted for 2 weeks above most depth strata in the nearshore during late July to early Au-

11 Lake Ontario Larval Alewife Habitat 191 TABLE 4. Median densities (larvae/100 m 3 ) of larval alewives captured in surface tows over various depths in the New York nearshore of Lake Ontario at three sites in At Sterling, six tows were made over each depth contour while only single tows were made at Ginna and Russell. Mean nearshore temperatures ( C) were calculated from five measurements made at 0.5-m increments from the surface to a depth of 2 m. Minimum and maximum densities observed on each date are in parentheses, blanks indicate no larvae captured, and ns = not sampled. Site locations are indicated in Figure 1. Depth contour Date 2 m 5 m 8 m 11 m 14 m C Sterling 27 Apr 7 3 May ns 17 May 12 3 Jun 0 (0, 1.0) 9 13 Jun Jun 0 (0, 0.9) Jun 0 (0, 6.4) 0 (0, 1.2) 19 6 Jul 3.3 (0, 14.6) 0.6 (0, 2.1) 0.4 (0, 9.9) 9.5 (1.6, 16.1) 2.1 (0, 8.7) Jul 2.1 (0, 7.1) 1.2 (0, 3.4) 3.1 (0, 11.1) 1.9 (0, 9.4) 2.8 (0.8, 7.1) Jul 10.1 (0, 51.1) 5.5 (0, 20.7) 8.1 (2.7, 15.5) 9.4 (2.7, 40.9) 29.3 (12.7, 40.0) Jul (0, 236.3) (49.5, 246.2) (10.4, 364.4) 18.5 (13.9, 36.1) 77.7 (13.8, 223.0) 22 2 Aug 84.4 (9.7, 172.7) 61.5 (78.3, 206.0) 62.2 (0, 213.9) 0 (0, 0.9) 7.1 (0, 129.2) ns 16 Aug 1.0 (0, 3.8) 1.0 (0, 2.3) 4.5 (0, 6.8) 1.1 (0, 3.9) 4.4 (1.4, 7.9) Aug 0.5 (0, 2.7) 2.2 (0, 4.0) 1.8 (0.7, 7.9) 1.3 (0, 2.8) 4.0 (0, 31.3) Sep 0 (0, 1.0) 0 (0, 1.1) 0 (0, 0.9) ns 30 Sep 14 Ginna 13 Jun ns Jun ns Jul ns Jul ns 22 8 Aug ns Aug ns 19 8 Sep ns 18 Russell 15 Jun ns 16 5 Jul ns Jul ns Jul ns Aug ns Aug ns 18 gust. At the Sterling site, alewife yolk-sac larvae were first caught on 27 June, persisted in the catches from 19 July through 2 August and mean densities peaked at 4.2 larvae/100 m 3 over the 2-m contour and 4.9 larvae/100 m 3 over the 14-m contour on 2 August In 1978, alewife larvae first appeared in the nearshore of Lake Ontario in mid-june and densities peaked at all three sites in mid-july or early August when water temperatures were 18 C (Table 5). During peak larval alewife abundance, the median surface density across all depth strata in the nearshore was 36 larvae/100 m 3 at Sterling on 12 July, 49 larvae/100 m 3 at Ginna on 5 September, and 34 larvae/100 m 3 at Russell on 25 July. Corresponding mean surface densities on these dates were 154 larvae/100 m 3 at Sterling, 48 larvae/100 m 3 at Ginna, and 44 larvae/100 m 3 at Russell. Over the 11 and 14 m contours at Sterling on 8 August and Ginna on 5 September, median densities of larval alewives increased within 1 week from < 6 larvae/100 m 3 to > 50 larvae/100 m 3 in the surface

12 192 Klumb et al. TABLE 5. Median densities (larvae/100 m 3 ) of larval alewives captured in surface tows over various depths in the New York nearshore of Lake Ontario at three sites in At Sterling, six tows were made over each depth contour while only single tows were made at Ginna and Russell. Mean nearshore temperatures ( C) were calculated from five measurements made at 0.5-m increments from the surface to a depth of 2 m. Minimum and maximum densities observed on each date are in parentheses, blanks indicate no larvae captured, and ns = not sampled. Site locations are indicated in Figure 1. Depth contour Date 2 m 5 m 8 m 11 m 14 m C Sterling 25 Apr ns 5 11 May 9 23 May Jun ns 20 Jun Jun 0 (0, 13.2) 0 (0, 2.3) 0 (0, 3.5) ns 6 Jul 38.1 (9.6, 156.5) 45.1 (17.1, 88.9) 78.3 (26.3, 235.0) 73.9 (29.3, 143.2) 35.0 (0, 46.6) Jul (87.8, 522.6) (53.0, 961.6) 35.7 (6.6, 67.0) 12.8 (0, 34.7) 12.9 (0, 42.9) Jul 30.0 (12.1, 106.4) 46.6 (32.4, 90.1) 42.5 (23.6, 91.6) 3.7 (0, 45.7) 4.7 (0, 18.5) Jul 13.4 (0, 23.8) 9.8 (3.2, 30.3) 17.4 (5.4, 44.7) 18.5 (7.8, 30.2) 8.9 (3.7, 27.7) ns 1 Aug 44.0 (2.0, 119.6) 15.9 (3.5, 22.7) 2.5 (0, 3.4) 2.0 (0, 13.8) 5.5 (0, 8.6) 21 8 Aug 51.6 (0, 151.1) 46.0 (7.0, 99.2) (9.5, 209.5) 89.1 (26.6, 194.6) 71.2 (55.3, 103.7) Aug 0 (0, 5.5) 0.7 (0, 3.5) 1.8 (0, 7.3) 2.3 (0, 4.4) 2.7 (0, 7.7) Sep 0 (0, 1.3) 0 (0, 3.2) 0 (0, 1.8) 0 (0, 1.5) 6 3 Oct 0 (0, 1.8) Oct 1.3 (0, 4.0) 10 8 Nov 10 Ginna 25 May ns 11 1 Jun ns Jun 1.0 ns Jun ns Jul ns Jul ns Aug ns Aug ns 22 5 Sep ns Sep ns 12 Russell 23 May ns May ns Jun 1.0 ns ns 26 Jun 2.0 ns Jul ns Jul ns 23 8 Aug ns Aug ns 22 waters above the 8 to 14 m contours. For all three sites, median densities > 10 larvae/100 m 3 persisted 2 to 4 weeks above most depth strata in the nearshore during July and August. At the Sterling site, alewife yolk-sac larvae were first caught on 27 June and peaked in the surface waters above all contours on 6 July. Greatest mean densities of yolksac larvae at Sterling were found over the 2-m contour (18 larvae/100 m 3 ) with densities of 1 larva/100 m 3 found out to the 14-m contour.

13 Lake Ontario Larval Alewife Habitat 193 DISCUSSION Larval Fish Assemblages Pelagic larval fish assemblages differed in embayment and nearshore habitats. Alewives were not always the main larval fish species captured in the embayments but they always constituted greater than 65% of the catch in the nearshore (Fig. 2). Dunstall (1984) found that the nearshore pelagic larval fish assemblage along the north shore of Lake Ontario was 73% alewives. Changes in species composition for embayments mirrored the different spawning seasons for particular species; yellow perch spawn in late spring (April to mid- May) whereas alewife and Lepomis spp. spawn throughout the summer (Smith 1985). Stephenson (1990) found the same co-occurrence by larval alewife and pumpkinseeds in Lake Ontario coastal marshes. However, larval alewives were the main species caught in the open waters of Hamilton Harbour, a 2,194-ha Lake Ontario embayment with degraded water quality and modified (urbanized) shorelines (Leslie and Timmins 1992). Embayments vs. Nearshore Habitats In general, peak densities of larval alewives were observed 2 to 4 weeks earlier in the embayments than in the nearshore, which likely resulted from shallow, protected embayment waters warming sooner than the deeper thermally well-mixed nearshore (Hall et al. 2003). Larval alewives have been observed to occur earlier in warm shallow protected habitats compared to the cool deep waters of Lake Ontario (Lam 1977, Leslie and Timmins 1992), Lake Huron (O Gorman 1983), and Lake Michigan (Mansfield 1984, Bimber et al. 1984). The generally similar or lower densities of alewife larvae in embayments compared to nearshore habitats indicated that the small embayments we studied (combined surface area < 8,600 ha) were not major sources to the lake-wide population. The combined surface area of the embayments at Chaumont, Sodus, and Irondequoit is approximately 0.5% of the total surface area of Lake Ontario. The entire nearshore habitat, from the shore to the 15-m depth contour encompasses approximately 15% of the whole lake surface area. In Lake Michigan, Brandt et al. (1991) found that 30% of the increase in lake-wide alewife and rainbow smelt Osmerus mordax numbers in summer was from YOY produced in Green Bay, Lake Michigan s largest embayment (> 220,000 ha). Inner Saginaw Bay (155,400 ha) had the highest densities of larval alewives in Lake Huron (O Gorman 1983). However, Lake Ontario s largest embayment, the Bay of Quinte (25,400 ha), prior to large reductions in phosphorus loading (Robinson 1986), had mean summer densities of alewife larvae in 1974 (range 3 to 68 larvae/100 m 3 ) (Lam 1977) similar to those observed in the embayment at Chaumont and all three nearshore habitats during this study (Table 3). Perhaps the relatively shallow (< 40 m) northeastern basin of Lake Ontario, collectively serves as a nursery habitat that can significantly contribute to the lake wide alewife population. Approximately 39% of Lake Ontario depths 15 m lie within the northeastern basin. In the surface waters over the shallow depth strata sampled in this study (< 20 m), densities of alewife yolk-sac larvae were generally an order of magnitude higher in the nearshore than in the embayments. Higher densities of alewife larvae in the nearshore likely did not result from drift of larvae out of the embayments. Alewife eggs generally hatch within 2 3 days at temperatures greater than 20 C (Edsall 1970). Therefore, the habitat of capture for this life stage likely was the habitat the fish hatched in. Water temperatures during our nearshore sampling were always greater than 14 C indicating upwelling events, which would reduce larval densities, did not occur (Heufelder et al. 1982). High densities (> 25 larvae/100 m 3 ) of alewife larvae in surface waters at depth strata greater than 15 m in 1998 (Fig. 3), and at the three sites used by Rochester Gas and Electric Co. in 1977 and 1978 (Tables 4 and 5), indicated that nearshore nursery habitat should at a minimum extend to the 15-m contour or 3 km from the south shore. Larval alewives were also captured out to the 13-m contour along the north shore of Lake Ontario (Dunstall 1984). Entrainment samples collected from the intake of RG&E s Russell Power Station near Rochester, NY captured larval alewives (3 to 36 larvae/100 m 3 ) in bottom waters at the 10-m depth contour, 1.2 km from shore, from 23 July to 11 August 1998 (P. Sawyko unpublished data). The data in this study support the conclusions of Goodyear et al. (1982) that the entire nearshore of Lake Ontario serves as nursery habitat for alewives. Although densities of alewives were highest in inner Saginaw Bay of Lake Huron, O Gorman (1983) found no statistical difference in densities among three habitat types: bays, irregular shorelines, and exposed shorelines. However, for the north shore of Lake

14 194 Klumb et al. Ontario, Dunstall (1984) found highest densities of larval alewives in more sheltered areas with irregular shorelines. Contrary to the initial hypothesis, the productive, sheltered embayments were not major contributors of larval alewife production; however, densities of all other species combined were almost always higher in the embayments compared with exposed nearshore habitats. Excluding alewives, rainbow smelt, and yellow perch, O Gorman (1983) found low densities (< 10 larvae/100 m 3 ) of all other combined species in open waters of Lake Huron; inner Saginaw Bay had the highest densities of yellow perch. All three embayments in this study have coastal wetlands associated with their major inlet s stream mouth and densities and numbers of species of YOY fishes can be very high in wetland habitats (Chubb and Liston 1986, Stephenson 1990). In this study the number of species was similar in the embayments and nearshore (Table 1) but lower than the 18 to 31 species documented in Great Lakes wetland habitats (Chubb and Liston 1986, Stephenson 1990). Leslie and Timmins (1994, 1995) found species diversity of YOY fishes was lowest in open water habitats compared to the littoral zone in Severn Sound, Lake Huron. Abundance of larval alewives in embayments and the nearshore may be higher than estimated from daytime surface tows used in this study. Catches of larval alewives are generally greater at night (Heufelder et al. 1982, Cole and MacMillan 1984) due to diel changes in alewife depth distributions. Many studies have found alewife larvae distributed mainly near the surface during the day (Lam 1977, O Gorman 1983, Nash and Geffen 1991). However, in Lake Michigan during 1977 to 1979, Heufelder et al. (1982) reported similar alewife densities (> 100 larvae/100 m 3 ) near the surface and within the water column at 15-m depths during the day and at night in the absence of upwelling. Differences in the horizontal distribution of larval alewives also exist with very high densities (> 1,000 larvae/100 m 3 ) captured in waters < 5 m deep (Bimber et al. 1984; Leslie and Timmins 1994, 1995). Leslie and Timmins (1993) found the horizontal distribution of alewives to be age dependent with more yolk-sac larvae found in shallow littoral habitats. The pelagic larvae that were captured in the nearshore likely drifted from shallow habitats along the shore (which were not sampled in this study) rather than originating from the embayments. Also, larval alewife densities could have been biased low because clupeid larvae may have passed through the mesh sizes used in this study (Tomljanovich and Heuer 1986). Patchy distributions of larvae and the > 2-week sampling interval used in this study also affected the density estimates. Variability in the density estimates was high due to many zero catches; two standard errors were generally greater than the mean. Small sample size and likely non-normal distributions necessitated the choice of non-parametric tests (Manly 1997). Although the statistical power was low, significant differences and general patterns of larval abundance could be distinguished in the two habitats. Peak densities of alewives during the 1997 and 1998 reported in this study are likely conservative because sampling at 2-week intervals or longer may have missed the peak. Comparisons of larval alewife densities between years in the embayments and nearshore habitats are hampered in this study by the reduction in plankton net diameter from 1.0 m in 1997 to 0.5 m in Although less volume was sampled with the 0.5-m diameter nets, which likely reduced catch rates, results from simultaneous gear comparison studies are equivocal. In a study comparing catches of clupeid larvae using 0.5- and 1.0-m diameter towed plankton nets (500- and 571-µm mesh, respectively), catch rates were lower (10 to 65%) for the 0.5-m diameter net but seasonal trends in abundance between the nets were similar (Hale et al. 1995). However, Gallagher and Conner (1983) found no consistent trend in clupeid larval densities between 1.0 and 0.5 diameter nets (571-µm mesh) fished simultaneously; differences between nets ranged from 14 to 40%. Avoidance of other larval species such as yellow perch were likely due to the low towing speeds (< 4.5 km/h) used in this study compared to the reduction in net diameter (Noble 1970). Historical Comparisons Abundance of adult alewives influences recruitment (Brown 1972, O Gorman et al. 1987) by cannibalism (Edsall 1964, Rhodes et al. 1974) or intraspecific competition (O Gorman et al. 1997) because diets of YOY and adults are similar (Strus and Hurley 1992). Methods used to collect larval alewives in 1977 and 1978 were similar to those employed in 1997 and 1998, which provided an indication of historical relative abundance. However, 1977 may have been an atypical year because the larvae collected were produced by the relatively

15 Lake Ontario Larval Alewife Habitat 195 few adults who survived a mass over-winter mortality (O Gorman and Schneider 1986). At low population levels, alewife recruitment to age-1 in spring was directly related to spawner abundance (O Gorman et al. 1987). Abundance of adult alewives (age-2 and older) in 1978 was only slightly lower than in 1997 and was similar to 1998 (Owens et al. in press) which may explain the generally similar peak densities of larval alewives observed in the late 1970s and late 1990s. Similar low July catches of yolk-sac larvae in the nearshore during the late 1970s at the Sterling site (4 to 18 larvae/100 m 3 ) and in the late 1990s at Chaumont, Sodus, and Irondequoit (4 to 19 larvae/100 m 3 ) lend additional support to this stock-recruitment relation. Although abundance of adult alewives in the late 1970s and late 1990s was similar, mean summer (May through October) total phosphorus, chlorophyll-a, and zooplankton have declined 40 to 80% in the Lake Ontario nearshore since the 1970s. Mean total phosphorous in the nearshore during the 1970s, was 22 µg/l (Gregor and Rast 1982, Mills and Forney 1982) compared to 11 µg/l during 1995 to 2000 (Mills et al. 2000). Nearshore chlorophyll-a concentrations in Lake Ontario during the 1970s ranged from 4.1 to 6.6 µg/l (Gregor and Rast 1982, Mills and Forney 1982) and has declined to 1.4 µg/l during the 1990s (Mills et al. 2000). Mean crustacean zooplankton densities, excluding nauplii, in the nearshore of Lake Ontario in the 1970s ranged from 45 to 69 organisms/l (Czaika 1974, Mills and Forney 1982) whereas the mean density observed from 1995 to 2000 was 25 organisms/l (Mills et al. 2000). Although Czaika (1974) collected zooplankton with an 80-µm mesh net compared to the 153-µm mesh net used by Mills and Forney (1982), mean zooplankton densities observed in the two 1970s studies were similar. Effects of bottom-up forces generally weaken at progressively higher trophic levels in pelagic food webs (McQueen et al. 1986). Despite apparent reductions in phosphorus, chlorophyll, and zooplankton in the Lake Ontario nearshore since the 1970s, this study suggests lowered zooplankton densities have not been sufficient to result in a detectable reduction in densities of larval alewives. With the exception of shallow habitats and samples collected at night, the timing and magnitude of peak densities of larval alewives documented throughout the Great Lakes in the 1970s to the early 1990s were generally similar to those observed in this study. Larval alewives persisted in catches from May through October but peak densities mainly occurred in late-june, July, and August (Lam 1977; Bimber et al. 1984; Dunstall 1984; Leslie and Timmins 1992, 1993). Along the north shore of Lake Ontario, mean density in mid-july through August in 1978 was 234 larvae/100 m 3 but densities as high as 1,130 larvae/100 m 3 were observed (Dunstall 1984). In a Lake Ontario embayment, the Bay of Quinte, Lam (1977) found a peak density of 24 larvae/100 m 3 in Peak densities of larval alewives in the 1970s ranged from 117 to 198 larvae/100 m 3 in Lake Michigan (Bimber et al. 1984). Highest alewife densities observed in Lake Huron in the 1970s ranged from 10 to 160 larvae/100 m 3 (O Gorman 1983, 1984); however, fish were not collected throughout the summer. In the 1980s, peak alewife densities in Hamilton Harbour, Lake Ontario were 328 larvae/100 m 3 (Leslie and Timmins 1992) and 128 larvae/100 m 3 in Lake St. Clair (Leslie and Timmins 1993). The variable occurrence of alewife larvae and timing of peak densities were most likely due to inter-annual climatic influences on water temperature. The smaller mesh sizes (< 390 µm) used to collect alewife larvae in all these earlier studies, except Lam (1977), also potentially resulted in higher densities compared to larger mesh sizes ( 500 µm) used in this study (O Gorman 1984). Despite effects of gear selectivity and sampling intervals that varied from 1 to 3 weeks, past densities observed in the Great Lakes were generally within a factor of two to four times the densities seen in this study. In general, Lake Ontario larval alewife densities in the late 1970s and 1990s were within the same order of magnitude and within the observed variability on a given date and among years. Detecting potential changes in recruitment using densities of larvae is difficult because alewives are very fecund (Norden 1967) and have a long spawning season. It was shown in this study that alewives are generalists and used both sheltered embayments and the exposed nearshore as nursery habitats. However, the preponderance of available nursery habitat for alewives is in the Lake Ontario nearshore. Lower nearshore zooplankton densities during 1995 to 2000 (Mills et al. 2000) compared to the 1970s (Czaika 1974, Mills and Forney 1982) implies a lowered probability that alewife larvae in Lake Ontario encounter zooplankton patches of sufficient density and size for growth and survival in the 1990s. Yet the 1998 year class of alewives at age-1 was the strongest on record since 1978 (Robert O Gorman, personal communication, US Geological Survey, 17 Lake Street, Oswego, New York, 13126;