Northern Prairie Wildlife Research Center
Spring and fall diets. Ninety-seven percent (31 of 32) of Lesser Scaup upper GI tracts contained at least one zebra mussel (Table 1); therefore, no statistical comparisons of frequency of occurrence between seasons, ages or sexes were made. Frequency of occurrence of zebra mussels did not differ between Lake Erie and Lake St. Clair (Fisher's Exact, n = 32, P = 0.156).
|Table 1. Frequency of occurrence and aggregate percent of foods in the upper gastrointestinal tract (esophagus and proventriculus) of five species of diving ducks from the western end of Lake Erie and Lake St. Clair during fall and spring, 1992 and 1993.|
|Species, location, food taxa (n)||Frequency of occurrence (%)||Aggregate %|
|Lesser Scaup (32)|
|Lake Erie (27)|
|zebra mussels||27 (100)||98.6||0.94|
|Amnicola walkeri||1 (4)||0.2||0.19|
|Physella integra||1 (4)||0.9||0.93|
|Sphaeriidae||1 (4)||< 0.1|
|Caecidotea sp.||2 (7)||0.2||0.13|
|Lake St. Clair (5)|
|zebra mussels||4 (80)||54.4||19.92|
|Amnicola walkeri||3 (60)||2.6||1.58|
|Pleurocera acuta||2 (40)||20.2||19.95|
|Physella integra||1 (20)||1.0||1.00|
|Valvata sp.||1 (20)||0.8||0.80|
|Gyraulus sp.||1 (20)||1.2||1.20|
|Ferrissia parallela||1 (20)||0.1||0.10|
|Hyalella/Gammarus spp.||4 (80)||14.4||12.21|
|Caecidotea sp.||1 (60)||3.0||1.48|
|Fish||1 (20)||< 0.1|
|zebra mussels||9 (60)||23.5||8.83|
|Amnicola walkeri||1 (7)||0.6||0.60|
|Hyalella/Gammarus spp.||10 (67)||46.6||10.42|
|Caecidotea sp.||3 (20)||8.8||6.72|
|Hydropsyche sp.||1 (7)||4.8||4.80|
|Tricoptera adult||1 (7)||< 0.1|
|Enallagma sp.||1 (7)||3.6||2.45|
|wildcelery winterbuds||1 (7)||6.1||6.13|
|Polygonum sp. seeds||1 (7)||6.7||6.67|
|Common Goldeneye (6)|
|zebra mussels||5 (83)||79.2||16.35|
|Physella integra||1 (17)||0.3||0.33|
|Hyalella/Gammarus spp.||2 (33)||5.8||5.44|
|Caecidotea sp.||2 (33)||14.8||11.03|
|Lake Erie (3) and Lake St. Clair (8)|
|zebra mussels||1 (9)||9.1||9.09|
|wildcelery winterbuds||10 (91)||90.9||9.09|
|Lake Erie (4)|
|zebra mussels||4 (100)||50.5||28.58|
|Potamogeton sp.||2 (50)||49.5||28.58|
|Lake St. Clair (6)|
|zebra mussels||6 (100)||1.3||0.98|
|Potamogeton sp.||6 (100)||98.7||0.98|
The aggregate percent zebra mussels in the upper GI tract of Lesser Scaups in Lake Erie (98.6%) was greater than in Lake St. Clair (54.4%) (Table 1, F1,29 = 17.97, P < 0.001). Aggregate percent of zebra mussels in the upper GI tract did not differ between ages or seasons (age, season, and interaction, F1,23 = 0.56, < 0.01, 2.39; P = 0.463, 0.993, 0.136, respectively) or between sexes (F1,25 = 1.48, P = 0.235) for Lesser Scaups in Lake Erie. Food habits of Lesser Scaups in Lake St. Clair were more variable than food habits in Lake Erie; six taxa comprised >95% of the food items in Lake St. Clair compared with only one taxon, zebra mussels, comprising >95% of the food items in Lake Erie.
Sixty percent of Buffleheads contained one or more zebra mussels (Table 1). Frequency of occurrence of zebra mussels did not vary by sex (Fisher's Exact, P = 1.00, n = 15) or season (P = 0.136, n = 15).
No one food item dominated in the upper GI tracts of Buffleheads (Table 1). Approximately 47% of Bufflehead food items consisted of amphipods. Nearly 24% of Bufflehead diet was composed of zebra mussels; aggregate percent did not differ by sex or season (sex, season, and interaction F1,5 = 0.08, 0.29, < 0.01; P = 0.793, 0.613, 0.954, respectively).
Five of six Common Goldeneyes contained at least one zebra mussel (Table 1). Zebra mussels dominated in the diet (79%); isopods and amphipods also were consumed.
Wildcelery winterbuds (Vallisneria americana) were the dominant food (91%) in Canvasback upper GI tracts (Table 1). Only one Canvasback, collected from Lake Erie, contained zebra mussels in its upper GI tract. This individual was collected while it was dabbling along the shoreline during a period of low water caused by a seiche (Bookhout et al. 1989). One Canvasback and one Lesser Scaup were collected from a mixed feeding flock of Lesser Scaups and Canvasbacks. The Canvasback had only wildcelery winterbuds and the Lesser Scaup had only zebra mussels in their upper GI tracts.
All 10 Redheads contained at least one zebra mussel in their upper GI tracts (Table 1). The difference in aggregate percent of zebra mussels between Lakes Erie and St. Clair approached significance (F1,8 = 3.51, P = 0.098). This lack of difference resulted from the large variance of Lake Erie Redheads; two of four Redheads contained 99% vegetation, whereas the other two Redheads contained 100% zebra mussels. The zebra mussels that eight of 10 Redheads were eating (six of six Redheads from Lake St. Clair and two of four Redheads from Lake Erie) were attached to the stems of vegetation.
Only one Greater Scaup (immature female from Lake Erie) contained food in its upper GI tract. Ninety-nine percent of the food in its upper GI tract was zebra mussels and 1% was a snail, Pleurocera sp.
Size of zebra mussels. Size of zebra mussels consumed did not differ between male and female Lesser Scaups (Table 2, F1,18 = 2.69, P = 0.118) or between male and female Buffleheads (Table 2, F1,7 = 1.12, P = 0.325). There were no seasonal differences in size of zebra mussels eaten by Lesser Scaups (Table 2, F1,18 = 2.57, P = 0.126) or Buffleheads (F1,7 = 1.21, P = 0.308). Lesser Scaups from Lake Erie ate larger zebra mussels than Lesser Scaups from Lake St. Clair (Table 2, F1,30 = 16.19, P < 0.001). Using Lake Erie data only, Lesser Scaups, Buffleheads and Common Goldeneyes consumed zebra mussels of similar size, and Lesser Scaups and Goldeneyes consumed larger zebra mussels than Redheads (F3,42 = 3.51, P = 0.023).
|Table 2. Size (mm) of zebra mussels consumed by six species of diving ducks in the western end of Lake Erie and Lake St. Clair during fall and spring, 1992 and 1993.|
|Location and species||SE||na||Minimumb||Maximumb|
|Common Goldeneye||12.0 Ac||1.55||5||1.6||23|
|Lesser Scaup||10.7 A||0.60||27||0.8||23|
|Lake St. Clair|
| a Number of
ducks used to calculate mean zebra mussel shell lengths.
b Minimum and maximum sizes determined from all zebra mussels consumed by each species.
c Means with same letter are not significantly different (F = 3.51; df = 3,42; P = 0.023, Bonferroni mean separation).
The size of zebra mussels consumed by Redheads did not differ between Lake Erie and Lake St. Clair (Table 2, F1,8 = 0.70, P = 0.426). The average size of zebra mussels eaten by Redheads when zebra mussels were the only food present in the upper GI tract was 10.1 ± 0.27 mm (n = 2). Average size of zebra mussels attached to vegetation in the Redhead's upper GI tract was 2.9 ± 0.43 mm (n = 8) .
Although the mean size of zebra mussels eaten by Buffleheads and Lesser Scaups did not differ, the size distribution of zebra mussels consumed by Lesser Scaups was larger than those consumed by Buffleheads (Kolmogorov-Smirnov, P < 0.05) (Fig. 2). The median size of zebra mussels eaten by Buffleheads was 5.5 mm compared with 10.3 mm for Lesser Scaups.
|Figure 2. Size distribution of zebra mussels from the upper gastrointestinal tracts of Lesser Scaups and Buffleheads collected from Lake Erie during fall 1992 and spring 1993.|
Winter season. Eight of nine Lesser Scaups collected in an area of open water influenced by the hot water discharge of a power plant contained 100% gizzard shad (Dorosoma cepedianum); one Lesser Scaup contained 100% zebra mussels. One Canvasback collected from the same area contained 100% gizzard shad also.