Zebra mussels generally prefer to settle on hard substrates. However, the fine, often muddy substrates of the Inner Bay, and ready availability of submerged macrophytes necessitated that they use aquatic plants as their primary substrate for attachment (Table 6.4). Zebra mussels were most abundant on musk grass, Eurasian milfoil, naiads, wild celery and species of pondweed. Maximum zebra mussel densities ranged from 23,650 individuals per mē in 1991 to 5,074 in 1995. This is substantially less than densities reported for the western basin of Lake Erie (300,00 to 700,000 per mē have been reported; see MacIsaac et al. 1991). The reason for this disparity is unknown. However, the fact that zebra mussels preferentially settle on hard substrates, which are not readily available in the Inner Bay, may explain the lower Inner Bay densities. Because the zebra and quagga mussel decline at Long Point did not correspond with a decline in aquatic plants, their major substrate (Chapter 5), reduced numbers of introduced mussels were probably caused by some other factor.

Knapton and Petrie 1998

Studies have reported that certain waterfowl species have altered their dietary intake, as well as their movement patterns, to take advantage of readily available zebra mussels on the lower Great Lakes (Wormington and Leach; Mitchell and Carlson 1993; Hamilton and Ankney 1994; Hamilton et al. 1994), and studies have reported minor to severe reductions in mussel densities where populations of these waterfowl species are high (Stanczykowska 1977, et al. 1990; Pedroli 1981; Suter 1982; Hamilton et al. 1994). Because Long Point Bay is one of the most important waterfowl staging areas in North America, Petrie and Knapton (1999) predicted that duck predation might be influencing the density or demographics of zebra and quagga mussels on Long Point Bay. In response to this, the dietary intake of 552 ducks (12 species) was sampled at the LPWMU waterfowl check station (1991-1995) to determine what species were consuming zebra mussels at Long Point. Of 12 species that were sampled, 6 were diving ducks and 6 were dabbling ducks. Only 5 of the 12 species analysed had consumed zebra mussels, and only Lesser Scaup (75-83% occurrence), Greater Scaup (67-82%), and Buffleheads (47-60%) consistently incorporated zebra mussels in their diet (Table 6.5). Given their inability to forage in deep water, and their normally herbivorous/granivorous diet, it was not surprising that dabbling ducks did not consume zebra mussels.

*All birds with zebra mussels or identified shells in their diet.

To determine if there were changes in the abundance of any of the species /species groups (Lesser and Greater Scaup are grouped as scaup spp. due to the difficulties of differentiating between these two species while doing aerial surveys), spring and fall aerial survey data from 1986, 1988 and every year between 1991 and 1997 (n = 75) were analysed. Waterfowl days for all species combined increased from 3.7 million in 1986 to 7.7 million in 1997. However, most of this increase can be attributed to those species that consume zebra mussels. While scaup spp. and Buffleheads were the only species that consistently incorporated zebra mussels in their diets, they were also the only species that exhibited significant increases in the use of Long Point since the colonization by zebra mussels (Figure 6.3). These increases were substantial, as waterfowl days for scaup spp. increased rapidly from 38,500 in 1986, prior to the colonization by zebra mussels, to 3.5 million in 1997 (a 92-fold increase), despite a substantial decline in the North American population of scaup spp. (Dilworth-Christie and Dickson 1997). Bufflehead days increased from 4,700 to 67,000 during the same period. Peak spring counts for scaup spp. increased from 3,761 birds in 1986 to 22,435 birds in 1997, while fall counts increased from 1,925 to 50,966 birds during the same period. Peak spring counts for Buffleheads increased from 31 birds in 1986 to 920 birds in 1997, while fall counts increased from 216 to 1,618 birds during the same time.

As scaup have responded most readily to the availability of zebra mussels, waterfowl day calculations for scaup spp. were multiplied by minimum and maximum daily mollusc consumption estimates/requirements for related species (200 - 1025 g/d; Longcore and Cornwell 1964; Nilsson 1969; Thompson 1973; Pedroli 1981; Suter 1982) to generate a crude annual estimate of what proportion of Long Point Bays zebra mussels were being consumed by these two species of waterfowl. Consumption of ready available zebra mussels generally requires a limited search and handling time. This is probably particularly true at Long Point, as most zebra mussels are attached to vegetation rather than to rocks and Unionid shells. When a prey item becomes super-abundant and it is easily exploited, waterfowl tend to concentrate their foraging efforts on that food source; other pochard species have been shown to consume zebra mussels exclusively when they are readily available (Pedroli 1981). If this is in fact the case for scaup spp. at Long Point, then these two species alone could have theoretically consumed between 22 and 112% of Long Point Bay's zebra mussels in 1991, and between 43 and 220% by 1995 (Table 6.6). Clearly, predation by these waterfowl species alone could have caused the observed decline in zebra mussel distribution and abundance on Long Point Bay. Importantly, several other species of waterfowl (e.g., Oldsquaw Clangula hyemalis, White-winged Scoter Melanitta fusca deglandi, American Coot Fulia americana) and native and non-native fish also reportedly consume zebra mussels, and probably also contributed to the declining zebra mussel population at Long Point (see French 1993 ; Hamilton and Ankney 1994; Morrison et al. 1997). However, despite high rates of waterfowl consumption probably suppressing the population of zebra mussels in Long Point Bay, the mussels high fecundity and dispersal capabilities will probably prevent their eradication from the Bay.

The reduced phosphorus loading to Lake Erie (Chapter 3) and enormous non-selective filtering capacities of zebra mussels (Nicholls and Hopkins 1993) has resulted in a reduction in the phytoplankton availability in Lake Erie, and probably also at Long Point (see Chapter 4). As phytoplankton is the primary food source of zebra mussels, this could have contributed to the zebra mussel decline at Long Point. However, the fact that a lack of hard substrates has prevented zebra mussels from reaching the high densities recorded in western Lake Erie (see bij de Vaate 1991; MacIsaac et al. 1991; Leach 1993), I suggest that waterfowl predation rather than food limitation was the principal cause of the zebra mussel decline.

Approximately 90% of zebra mussels during all years were <10 mm in length, while very few were >15 mm (Table 6.7). This can probably be attributed to size selective predation by foraging ducks. Waterfowl at Long Point concentrated most foraging effort on zebra mussels that are >8 mm long (Hamilton and Ankney 1994), and in western Lake Erie, waterfowl preferentially took medium and large mussels over the more common small ones (Hamilton et al. 1994). Studies have also reported that heavy predation rates result in entire size cohorts being missing from zebra mussel populations (Wisiniewski 1974; Pedroli 1977). The increased water clarity of Long Point Bay (Chapter 4) would facilitate visual prey selection, thereby increasing the capability of ducks to select larger size zebra mussels. Such selective predation would also suppress the reproductive capabilities of the population, as mussels do not reproduce until they are between 5 and 9 mm in length (Morton 1969; Stancykowska 1977; Mackie 1992), and female fecundity increases exponentially with age (Walz 1978).