Interactions Between Alewife (Alosa Pseudoharengus), Their Food, and Phytoplankton Biomass in the Bay of Quinte, Lake Ontario

J. Great Lakes Res. 18(4):709-723 Internat. Assoc. Great Lakes Res., 1992

Interactions between alewife (Alosa Pseudoharengus), their food, and phytoplankton biomass in the Bay of Quinte, Lake Ontario

Richard H. Strus and Donal A. Hurley Lake Ontario Fisheries Unit Ontario Ministry of Natural Resources R.R. #4 Picton, Ontario КОК 2ТО

Abstract. Alewife (Alosa pseudoharengus) diets were compared with the extant zooplankton community in two sections of the Bay of Quinte, Lake Ontario. In the shallow, eutrophic upper bay, cladocerans predominated both in the community and in the diet. Bosmina longirostris was avoided by alewife, while cyclopoid copepods were preferred. The sphaerical, pigmented, but small sized Chydorus sphaericus was more common in the diet than the larger B. longirostris. In the less eutrophic, deeper lower bay, cyclopoid copepods were more common in both the zooplankton community and in alewife diets than in the upper bay. Diets of both small (< 100-mm fork length) and large (> 100 mm) alewife showed a high degree of overlap indicating general similarity in diet composition. Alewife abundance declined significantly in the upper bay from 30 kg.ha-1 in 1972-1976 to 2.9 kg.hcr' in 1977-1988. Values for the lower bay were 28 kg.h-1 and 10.5 kg.ha-1, respectively. In several years alewife biomass exceeded the limit of 40 kg.ha-1 that has been proposed for the continuing presence of Daphnia galeata mendotae. However, the abundance of this zooplankter has increased from less than 0.01 mg.L-1 (dry weight) when alewife biomass was high to 0.30 mg.L-1 recently when alewife biomass was low. There was no significant correlation between the abundance of D. galeata mendotae and chlorophyll a concentration possibly because D. galeata mendotae could not utilize the filamentous diatoms and blue- green algae that formed the bulk of the phytoplankton. The zooplankton-phytoplankton portion of the top-down predation model continues to be elusive. INDEX WORDS: Alewife, Lake Ontario, diet, zooplankton.

Introduction Trophic interactions mediated through food web dynamics are major controlling factors for both abundance and species composition at several trophic levels (Carpenter et al. 1987). Within the Bay of Quinte ecosystem, there is strong evidence that predation by fish on crustacean zooplankton and benthic macroinvertebrates exerts significant control on phytoplankton biomass (Nicholls and Hurley 1989). Alewife (Alosa pseudoharengus) and white perch (Morone americana) abundance was positively correlated with phytoplankton biomass through their effects on phytoplankton consumers. Zooplanktivores, including alewife, small white perch, and yellow perch (Perca flavescens) are present in large numbers in the Bay of Quinte (Hurley 1986a) such that resident zooplankton populations undoubtedly experience a high level of selective predation (Hrbacek 1962, Brooks and Dodson 1965, Brooks 1968). Evans and Jude (1986) found in Lake Michigan that large Daphnia declined when planktivorous fish were abundant and that subsequently small cladocerans became predominant in the zooplankton community. McQueen et al. (1986) used enclosure (8 m diameter, 15 m deep) experiments to show a highly significant negative regression between planktivore and zooplankton biomass. McQueen and Post (1984) proposed a threshold of 40 kg.ha1 of planktivores above which Daphnia galeata mendotae could not survive. We examined this relationship in the Bay of Quinte and our results indicate that this threshold is too low. Attempts to continue the top-down interactions to lower trophic levels (zooplankton- phytoplankton) have not been very successful (McQueen et al. 1986, Nicholls and Hurley 1989), except when large daphnids were plentiful. This lack of interaction may result because filamentous algae are not readily used by zooplankton. Our results support the non- utilization hypothesis since we found substantial numbers of large daphnids in the zooplankton community when the biomass of filamentous diatoms (Melosira and blue-green algae) was high (Nicholls and Hurley 1989) and chlorophyll a concentrations were high. Hurley (1986b) found that small cladocerans were the most numerous items in the alewife diet, but were not being selected. This was a consequence of the abundance of these small forms and the possible filter feeding by alewife. This study examines the effect of changing alewife biomass on their diet, their preference for different food items based on alewife size and location within the Bay of Quinte, and the interaction between alewife and zooplankton biomass.

Methods

Sampling Locations Two sampling areas in the Bay of Quinte were selected. One was the upper bay that included four sites from Trenton to Deseronto (Fig. 1). This area is relatively shallow (3.2 m mean depth) and is subject to an annual phosphorus loads of 2.25-2.37 g P.m-2.yr-1 (Minns et al. 1986). Total P concentrations ranged from 60-78 µg.L-1 in the pre-phosphorus control period, 1972-1977, to 33-54 µg.L-1 in the 1978-1987 period (Hurley et al. 1986; Annual Reports Project Quinte 1985-87, Lake Ontario Fisheries Unit, R.R. #4, Picton, Ont., KOK 2TO). Thermal stratification does not persist for any extended period of time in the summer months. Conditions in this area ranged from hypereutrophic in the pre-1978 period to eutrophic after 1978. The second sampling area was in the lower bay and included three sites from Glenora to Lennox (Fig. 1). Here the mean water depth is 24.4 m. Summer thermal stratification persists -1 and dissolved oxygen falls below 6 mg O2.L from late July until turnover in late September (Minns and Johnson 1986). Total P concentration changed little as a result of phosphorus control programs in the upper bay and the values ranged from 17 to 33 µg.L-1 in the 1972-82 period (Hurley et al. 1986). Exchange of water also occurs between this area and Lake Ontario (Freeman and Prinsenberg 1986).

Alewife Samples Alewife samples were obtained using a 3/4 Western bottom trawl, 19 m long with 6-m wings, 18.3 m footrope, 13.7 m headrope, and 1.3-cm stretched mesh in the cod-end. This trawl was used each month from June through September and towed over a distance of 400 m at a speed of 1.1 m.s-1. The fork length and weight were recorded for a subsample of 30 alewife per trawl. The stomachs were removed from 10 of these fish and preserved in 10% neutralized formalin. Alewife biomass was calculated on a seasonal basis in the upper and lower bays separately by combining trawl tows in each area and calculating a geometric mean with 95% confi- dence limits (C.L.). The trawl swept 0.25 ha in each tow and biomass was expressed as kg.ha-1. Some degree of avoidance of the trawl probably occurred which would underestimate the biomass. Echographs made during trawl tows showed that fish concentrated fairly closely to the bottom. The trawl opening was about 2.5 m high which would encompass the region where the fish were most heavily concentrated.

Zooplankton Samples Zooplankton samples were obtained during weekly cruises carried out by Canada Depart- ment of Fisheries and Oceans, Great Lakes Laboratory for Fisheries and Aquatic Sciences, Burlington, Ontario (GLLFAS) from May through September using a 30-L Schindler-type sampler fitted with а 75-µm mesh net and bucket. Samples were preserved in 10% neutralized formalin. In the upper bay samples were taken at 1-m intervals beginning at 1-m depth to within 1 m of the bottom. At Conway samples were taken at 1-, 3-, 5-, 8-, 10-, 15-, 18-, 20-, 25-, and 30-m levels. Values were expressed as numbers per meter cubed as distributed with equal weighting of all strata sampled throughout the water column. Samples were counted at GLLFAS. Depending upon the density of zooplankton present, between 1/10 and 1/4 of the total was counted. Subsam-pling was done by mixing in a beaker and measuring out an aliquot to a graduated cylinder. At least 300 animals in any subsample were enumerated. Further details on technique can be found in Cooley et al. (1986).

Stomach Analysis Alewife stomach contents were identified and enumerated to species level, if possible, and were usually subsampled for a total count of at least 200 organisms. If less than 200 organisms were present, the entire contents were enumerated. If alewife sampling occurred between two weekly zooplankton sampling dates then mean zooplankton abundances from the two adjacent dates were calculated for that period and used in the calculation of electivity and preference indices. Species composition on an annual and daily basis was described in terms of percent numerical contribution to the total stomach contents. Taxonomic identification and enumeration of hard bodied cladocerans (chydorids, bosminids) was straightforward as these were rarely damaged in the stomachs. Soft bodied cladocerans (primarily daphnids), if not intact, were identified by means of head shape, the features thereon, and from postabdominal claw characteristics. Cope-pods were usually undamaged in the stomachs and were recognized by caudal rami features, fifth legs, antennal characteristics, and maturity of the genital segments. Copepods were also classified according to nauplius, copepodid, and adult life stages, with the latter stage yielding species determination.

Preference and Diet Overlap Measures

We used Ivlev's electivity index (Ei) (Ivlev 1961),

E i = (ri - pi ) / (г i + р i ) where ri and pi represent the proportion of food item i in the diet and in the environment, respectively. Preference measures were applied only to the upper bay data where the shallower depths ensured a more uniform interaction between alewife and zooplankton that would not be unduly influenced by depth related variations in abundance. In most instances the abundance of items in the water column was determined by averaging the values for the closest dates of alewife capture which could be an interval of 3 or 4 days. It is recognized that patchiness in zooplankton distribution would increase the variance; however, the value for each date was the mean of at least four samples at 1-m intervals which would smooth the differences in depth distribution of the zooplankton. The difference in abundance between the two sampling dates was generally less than a factor of two. The greatest differences generally occurred when abundances were less than 1,000 individuals.m-3. In those cases the difference was as much as five to seven times. Schoener's (1970) overlap index, a, was used to determine the degree of similarity existing between species composition in diet and zooplankton community, and for diet composition between groups of alewife. Values are calculated by:

a = 1 - 0.5 (Σ i │ Pxi - Pyi│) where Pxi = species i in group x,

Pyi = species i in group y, and n = number of species types.

Results

Alewife Abundance

Upper Bay of Quinte Alewife biomass declined dramatically in 1977 in the upper bay from a seasonal (June- September) mean of 30.4 kg.ha-1 in 1972-1976 to 4.7 kg.ha-1 in 1977-1982 (Fig. 2a). Variance was high in the 1983-1985 measurements such that the 95% C.L. included some of the higher values recorded in 1972-1977 (Fig. 2a). Part of this increased variance is the result of the small sample size. However, alewife biomass continued to decline from 1986 to 1988 which supports the contention that alewife biomass has not recovered to its previous level. The decline in adult alewife biomass may have resulted in an increase in the abundance of young-of-year (YOY) because of reduced predation from adults (Ridgway et al. 1990). For example, in 1975-1976, YOY alewife accounted for 21.5% of the total number of alewife captured in the trawl tows, while in 1979-1980 YOY formed 78.1% of the total numbers and in 1985-1986, 50.8%.

Lower Bay of Quinte Alewife biomass in the lower bay also declined dramatically in 1977 from a mean of 28.0 kg.ha-1 in 1972-1976 to 4.8 kg.ha-1 in 1977-1982 (Fig. 2b). After 1982 alewife abundance increased to approximately the 1972-1976 level. However, the variance increased substantially partly because of reduced effort but also because of increased seasonality in the catches. In 1988 when 12 trawl tows were made, variance was relatively high because the June catches ranged from 40 to 385 kg.ha-1 while August catches were zero. YOY alewife were not abundant in the lower bay in any sampling period. The maximum numbers occurred in 1979-1980 when they represented 5.7% of the total.

Zooplankton Abundance Shifts Mean seasonal abundance (May-September) of the major zooplankton species was regressed on year (1975-1988) for years that enumerations were made. In the upper bay significant increases in abundance were confined to Daphnia galeata mendotae (R2 = 0.56, P < 0.01), Cyclops bicuspidatus thomasi (R2 = 0.53, P < 0.01), and Mesocyclops edax (R2 = 0.61, P < 0.01). In the lower bay there were significant increases in Daphnia retrocurva (R2 = 0.51, P < 0.01) and Ceriodaphnia lacustris (R2 = 0.35, P < 0.05). When zooplankton abundance was regressed on alewife biomass, there was a significant negative regression on Eubosmina coregoni (R2 = 0.31, P < 0.05) and Mesocyclops edax (R2 = 0.39, P < 0.05) and a positive regression on Leptodora kindtii (R2 = 0.42, P < 0.05) in the upper bay. For the lower bay there was a significant positive regression on Diaphanosoma birgei (R2 = 0.47, P < 0.05) and Tropocyclops prasinus (R2 = P < 0.05). Neither of these latter two species was a major diet item for alewife. Zooplankton Composition and Alewife Diet

Upper Bay of Quinte In the water column of the upper bay cladocera contributed about 80% of the mean June, July, August abundance through all years of the study. Cyclopoid copepods accounted for 15 to 21% and calanoid copepods never greater than 4% of the total mean abundance (Fig. 3a). The alewife diet in the upper bay generally reflected the predominance of cladocerans found there but the percentages were similar only in 1979 and 1980 (Fig. 3b). Cladocerans were never less than 50% of the total diet. The major cyclopoid species eaten were Mesocyclops edax and Cyclops vernalis, comprising 10-40% of the diet. Among the cladocerans the predominant species present in the upper bay zooplankton community (Fig. 4a) — Eubosmina coregoni, Bosmina longirostris and Chydorus sphaericus—were also dominants in the alewife diet (Fig. 4b). However, B. longirostris was less well represented in the diet than the other cladoceran species (Fig. 4a). Daphnia became more important food items in 1979 and later, compared to 1975 and 1976 when alewife were abundant (Fig. 4b).

Lower Bay of Quinte In the lower Bay of Quinte, cladocerans tended to be the major group present (60-70%) except in 1979 when only 50% of the zooplankton were cladocerans (Fig. 5a). Cyclopoid copepods were more abundant than in the upper bay and formed about 30-50% of the total. Calanoid copepods never formed more than 3% of the total. Alewife diet in the lower bay was dominated by cyclopoid copepods (Fig. 5b) in marked contrast with the upper bay. The proportion increased from about 50% in 1975 and 1976 to about 80% in the later years (Fig. 5b). The increased presence of cyclopoids in the diets over the proportion found in the zooplankton community indicates positive preference for these forms. Cyclops bicuspidatus thomasi was the primary cyclopoid eaten. In the upper bay this species was never abundant in either the zooplankton population or in the alewife diet. Among the cladocerans in the lower bay, B. longirostris was the predominant species in all years (Fig. 6a). All other cladocerans made up less than 10% of the total. However, the alewife diet often contained higher percentages of cladocerans other than B. longirostris (Fig. 6b) and always under-represented this zooplankter in the stomach contents. E. coregoni was usually the most frequently eaten cladoceran with lesser proportions of C. sphaericus and Daphnia spp.

Food Item Preferences Mean annual values for Ivlev's electivity index for the upper bay show fairly consistent positive (preference) values for cyclopoid copepods and negative (avoidance) values for B. longirostris (Table 1). B. longirostris was the predominant cladoceran in the environment in 1976 and co-dominant in 1986 (Fig. 4a), but was not a major food item in either year (Fig. 4b) and was avoided according to the Ivlev index (Table 1).

During and after 1979, Daphnia galeata mendotae (usually a larger species than Daphnia retrocurva) became more abundant in the upper bay than in previous years (Cooley et al. 1986). Although too infrequent for preference calculations, D. galeata mendotae was present to a greater exent in the alewife diet than in the environment.

Cyclopoid copepods as a group and E. coregoni had the greatest number of positive electivity indies over the whole period 1975-1986 (Table 1). Although data are not presented here, adult cyclopid species (C. vernalis, M. edax) most often exhibited the highest preference values. Representation of cyclopoids as a group including sub-adults usually approxima- ted proportions found in the environment.

Generally, diet similarities outweighed differences. Applications of Schoener's (1970) overlap index (Table 2) to alewife diet and the zooplankton community at the species level (cladocera) and order-suborder level (copepods) yielded high similarities (0.77-0.85), where 1.0 would represent identical similarity, or complete overlap. No reduction in overlap was observed after 1976 when alewife abundance dropped substantially, indicating continuous extensive utilization of the taxa present.

Diet Related to Alewife Size While most Bay of Quinte trawl tows tended to capture either YOY or adult size classes on individual sampling dates, a few trawl tows captured sufficient numbers of each group so that comparisons could be made (Table 3a and 3b). YOY alewife were consistently under 100-mm fork length in the Bay of Quinte so that the separation into groups was made using this criterion.

The species composition and abundances within the diet of both YOY (< 100 mm) and adult (> 100 mm) alewife were inconsistent in pattern. In the upper bay increased abundances of the large adult cyclopoid copepod, M. edax, were found in small alewife in 1975 and 1979, but not another large copepod, C. vernalis (Table 3a). The small cladoceran, C. sphaericus, was consistently more abundant in larger alewife, but B. longirostris, also a small cladoceran, was not (Table 3a). Daphnia was an important diet item for larger alewife in 1979 after alewife numbers had decreased significantly. Schoener's (1970) overlap index, when calculated on both size ranges of fish within each sampling date in the upper bay, showed a considerable degree of overlap in two of the four years. Values of a > 0.60 are considered to indicate a high biological degree of overlap (Wallace 1981). In the lower bay cyclopoid copepods were major diet items while cladocerans tended to be less so, especially in 1985 (Table 3b). C. bicuspidatus thomasi was the most common cyclopoid encountered, but immature copepodites were the predominant diet items. Among the cladocerans, B. longirostris was important in 1978 but less so in 1980 and again in 1985 (Table 3b). In contrast to the upper bay, Daphnia were not important diet items for alewife in either size category in the lower bay (Table 3b).

The calculated overlap index showed a high degree of overlap between the two size groups in all 3 years (Table 3b). Examination of the total percent abundance of cladocera and cyclopoid copepods especially in 1980 and 1985, would account for overlap observed.

The greatest positive electivity indices for both small (< 100 mm) and large (> 100 mm) alewife were recorded for large adult copepods {Cyclops vernalis, Mesocyclops edax) (Tables 4 and 5). This reflects the fact that these forms were never very abundant (< 5,000 individuals.m-3) and yet were found in relatively large numbers in alewife stomachs (Tables 4 and 5). Copepodites, which would include immature stages of these species, had positive electivity values when the adults were positive. Bosmina longirostris always had large negative electivity values indicating avoidance that is reflected in the low numbers recorded in alewife stomachs even though this zooplankter was generally abundant (10,000-100,000 individuals.m-3). Eubosmina coregoni was generally a preferred item for smaller alewife (Table 5), but tended to switch from positive to negative among larger alewife (Table 4). This species was most abundant (35,000-190,000 individuals.m-3) in years when the preference indices were negative. The indices for Daphnia increased from negative in the early years to positive from 1980 onward (Tables 4 and 5).

There was a consistent increase in the mean numbers found per stomach except for 1979 and 1980 among small alewife (Table 5). The increasing number of daphnids consumed partially reflects the increased abundance of these forms from 1977 onward, but the consumption tended to increase more than did their numerical abundance. The presence of Chydorus sphaericus in stomachs of small alewife declined over the years (Table 5) in spite of the generally uniform abundance of this species. Among larger alewife, Chydorus sphaericus was a common diet item and were generally found in large numbers in alewife stomachs, although the shift in their abundance in the water column from year to year tended to be large (2,000-100,000 individuals.m-3).

Discussion

Feeding Preferences The avoidance of B. longirostris (Table 1) by alewives of all sizes is unexpected when the relative sizes of the food items are taken into account. С. sphaericus (0.2 mm) is smaller than B. longirostris (0.3 mm) but is more abundant in the diet than B. longirostris. Being highly pigmented (opaque) and spherical rather than flattened probably increases the visibility, and thus the vulnerability of С. sphaericus to alewife predation, compared to B. longirostris. Zaret and Kerfoot (1975) also found that the more heavily pigmented zooplankters were visually selected by zooplanktivorous fish. The numerical importance of small species in the alewife diet indicates that the lower limit of usable spherical food particle size exists below the size of C. sphaericus (0.2 mm). MacNeill and Brandt (1990) examined gill-raker spacing in alewife from 88 to 181 mm total length. They reported a significant (R2 = 0.98) regression between fish length and gill-raker spacing with a range of spacing between 0.13 mm and 0.22 mm over the lengths of fish they studied. Converting alewife total length to fork length for the Bay of Quinte fish (FL = 0.869*TL, r = 0.99) means that zooplankton as small as С sphaericus would be filtered by alewife up to about 157-mm fork length. C. sphaericus were more abundant in the diet of alewives over 100 mm in the upper bay than in alewives under 100 mm (Table 3a). None of the mean lengths reported in Table 4 was over 157 mm and the range exceeded this value in one-half of the years examined. The laterally flattened B. longirostris may allow escapement between gillrakers, especially if the plankters were aligned properly and sufficiently thin. Avoidance of Bosmina longirostris was also observed during population peaks of this species in the upper bay (Fig. 4). Gannon (1976) examined a single mid summer sample (107 fish) of alewife from southern Green Bay, Lake Michigan. The species composition of zooplankton in the environment was virtually identical to that observed in the Bay of Quinte. He reported similar relative preferences of E. coregoni, C. sphaericus, and B. longirostris, with alewife avoiding B. longirostris strongly. Large adult cyclo-poid copepods were also the most highly preferred food items in Gannon's (1976) study. The preference that alewife and other zooplanktivorous fish exhibit for large Daphnia has been described in this study and in others (Galbraith 1967, Brooks 1968). The consequent diminished size spectrum within zooplankton communities has also been documented (Bro- oks and Dodson 1965, Wells 1970, Evans and Jude 1986). The reduced alewife population during and after 1977 (Ridgway et al. 1990) would result in a relaxation in predation pressure on larger zooplankton to the extent that would favour greater Daphnia abundances.

Alewife-Zooplankton-Phytoplankton Interactions McQueen et al. (1986) quantified the relationship between planktivorous fish and D. galeata mendo-tae and found that this plankter could not survive predation from zooplanktivore biomasses over 40 kg.ha-1. The geometric mean June-September alewife biomass in the upper bay exceeded this limit during 1974 and 1976 and the 95% C.L. included this limit in 1972 and 1973 (Fig. 2a). D. galeata mendotae were found only in May 1976 before large numbers of alewife reached the upper bay (Hurley 1986b). Since 1977 alewife biomass has not exceeded 11 kg.ha-1 and D. galeata mendotae abundances have increased considerably from less than 0.01 mg.L-1 dry weight in 1975 and 1976 to 0.30 mg.L-1 in 1988 (J.E. Moore, DFO, Burlington, personal communication). It is noteworthy that D. galeata mendotae did survive in the upper bay in those years when alewife biomass exceeded 40 kg.ha-1 which suggests that this limit may be too low. The presence and relative abundances of large Daphnia spp. may well have value as indicator species in reflecting ongoing predation levels existing within the Bay of Quinte ecosystem.

Additional evidence for reduced top-down control at the lower trophic levels in eutrophic systems may be found in the interval between the alewife decline in 1977 and phosphorus control in 1978. Phytoplankton biomass declined most during 1978 when P control was initiated, not in 1977 when alewife biomass declined (Nicholls and Hurley 1989). Although the D. galeata mendotae increase coincided with the 1977 alewife decline, as evidenced by their increased presence in both alewife and white perch (Hurley, unpublished data) stomachs in that year, little visible effect on phytoplankton biomass was evident (Nicholls and Hurley 1989) when compared to previous years. McQueen et al. (1986) concluded that primary producers in eutrophic lakes are only affected by large Daphnia, suggesting a biomass of 0.06 mg.L-1 (dry weight) would be sufficiently large to lower chlorophyll a values. We examined the data from the upper bay for D. galeata mendotae biomass (J.E. Moore, personal communication) and chlorophyll a concentration (К.Н. Nicholls, Ontario Ministry of the Environment, Box 213, Rexdale, Ont., M9W 5L1, personal communication) for all years available (Table 6). We found that for three years in the series some of the highest chlorophyll a values occurred when zooplankton biomass was over 0.16 mg.L-1. There was no correlation (r = 0.05, P > 0.05) between the two variables for the 14 years of data. One explanation for this anomaly is the presence of long filamentous Melosira and blue-green algae that are not readily utilized by large daphnids (Infante and Litt 1985). The combined effects of reduced phosphorus levels and lower alewife predation imply opposite effects upon the zooplankton community (i.e., lower alewife densities allow zooplankton to proliferate, while reductions in phosphorus would reduce production and biomass). McQueen et al. (1986) speculated that "improved water quality will only be possible when planktivore yields are reduced to unacceptably low levels" and concluded that biomanipulation in eutrophic lakes "must be approached with caution." The impacts documented in our study did not result in predicably reduced phytoplankton biomass, lending support to the above statement. Additional "nutrient sinks" as may exist in the re-establish- ment of macrophyte beds in the bay (Nicholls and Hurley 1989) may have value in reducing P concentrations, in turn resulting in relatively greater control regarding the management of eutrophication problems.

Zooplankton Community Utilization On the basis of overall similarity of diet and food availability, the degree of utilization of the zooplankton community by alewife in the upper Bay of Quinte is extensive. Mean order and sub-order diet groupings closely match those found in the upper bay; the dominant cladoceran species existing in the upper bay were also found in stomach contents; cyclopoid copepod differences between upper and lower bay areas are reflected in respective diets. However, some discrepancies were noted—the reduced dietary abundances of B. longirostris, as well as the high selectivity for large adult copepods. B. longirostris was the only predominant zooplankton species to exhibit a dissimilar proportion between the environment and the diet. Copepod nauplii, the smallest crustaceans present, were only rarely found in the diet—their low abundances may reflect feeding avoidance as a result of their small sizes (frequently less than 0.1 mm). Gill-raker spacing in alewife down to 75-mm fork length would not retain organisms as small as 0.1 mm (MacNeill and Brandt 1990). Consequently, copepod nauplii were not included in preference calculations. White perch and yellow perch, as well as several other species, are facultative zooplanktivores during all or some portion of their lives, and were present in large numbers in the upper bay (Hurley 1986b). Although alewife, being obligate zooplanktivores, are most likely to feed more heavily on an individual basis, the predation pressure from large populations of larval and YOY fish, as well as other smaller species, may be of considerable impact. The nature of facultative zooplanktivory suggests opportunistic feeding for organisms of greater relative value (i.e., larger size), thereby favoring additional predominance of smaller zooplankton species. Inclusion of diet and feeding behavior of these fish will yield a more complete picture with regard to defining interactions that occur within these levels of the food web.

Acknowledgments J.E. Moore, DFO Burlington, provided the data on zooplankton abundance. John Casselman, Mark Ridgway, Joe Leach, Ken Muth, and an anonymous reviewer offered many helpful suggestions during revisions of the manuscript. Contribution No. 91-13, Ontario Ministry of Natural Resources, Fisheries Research Section.