predation on zebra mussels by freshwater drum and al_1997.pdfj. great lakes res. 23(2): 177-189...
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J. Great Lakes Res. 23(2): 177-189 Internat. Assoc. Great Lakes Res., 1997
Predation on Zebra Mussels by Freshwater Drum and Yellow Perch in Western Lake Erie
T.W. Morrison, W.E. Lynch Jr. & K. Dabrowski
School of Natural Resources The Ohio State University 2021 Coffey Rd. Columbus, Ohio 43210
Abstract. Although considerable research has been done regarding zebra mussel (Dreissena polymorpha) expansion in the Great Lakes, information on fish species preying on zebra mussels is lacking. We examined diets of freshwater drum (Aplodinotus grunniens) and yellow perch (Perca flavescens) collected in western Lake Erie, 1992. Stomach contents were quantified in May, July, and October to examine the importance of zebra mussels in the diets and to determine if either fish species exhibited size-selective feeding. Zebra mussels were consumed by freshwater drum and yellow perch once they reached 250 mm and 150 mm total length, respectively. Consumption by freshwater drum was highest in May and July and lowest in October. Most yellow perch consumption occurred in May. Chesson 's alpha indicated that freshwater drum less than 350 mm TL and yellow perch less than 200 mm TL selected small zebra mussels and generally rejected larger individuals. Larger fish exhibited less selectivity, consuming zebra mussels in proportion to their estimated availability in western Lake Erie. Small fish just beginning to prey on zebra mussels may be physically limited to small sizes or clumps by their pharyngeal gape and musculature. Larger freshwater drum and yellow perch are restricted more by the sizes of zebra mussels available on the surface of the substrate and possibly the size of clumps which they remove, rather than by their physical abilities to crush any one size. This may explain the strong selection for small zebra mussels by both species even though they are capable of eating larger sizes.
INDEX WORDS: Zebra mussel, yellow perch, freshwater drum, Lake Erie.
Introduction Zebra mussels (Dreissena polymorpha) were first collected in the Great Lakes in Lake St. Clair, 1988. The introduction probably resulted from the discharge of freshwater ship ballast around 1986 (Hebert et al. 1989). This prolific spawner has colonized much of the Great Lakes, attaining densities of up to 341,000 individuals m-2 in wes-tern Lake Erie (MacIsaac et al. 1991). The ability of zebra mussels to form high-den-sity colonies has led to a myriad of research efforts in recent years, ranging from their effects on ecosystems (Wu and Culver 1991) to methods for their control (Fisher et al. 1991). Consumption of zebra mussels by European fish species has been documented (Stein et al. 1975, Olszewski 1976, Prejs 1976, Martyniak et al. 1987), but few studies have examined the feeding ecology of fishes that eat zebra mussels in North America. French (1993) noted that at least six North American species are potential predators because they possess pharyngeal teeth and/or chewing pads for crushing mollusk shells. However, he also noted that zebra mussels are found in the diets of several Great Lakes species lacking teeth or pads. Exotic species such as the round goby (Neogobius melanostomus) have also made use of zebra mussels in their diets (Jude et al. 1995). In Lake Erie, freshwater drum (Aplodinotus grunniens) and yellow perch (Perca flavescens) consume zebra mussels although only freshwater drum have phary-ngeal teeth (French and Bur 1993, French 1993). One of our objectives is to quantify seasonal diets of freshwater drum and yellow perch and qualitatively compare our 1992 results with diet information for freshwater drum from French and Bur's (1993) 1990 data. This comparison may provide insight into whether diets of these two im-portant fish species changed in response to the continual progression of zebra mussel colonization. It is unknown whether North American fish species exhibit size-selective predation on zebra mussels similar to that exhibited by roach (Rutilus rutilis) in Europe (Olszewski 1976, Prejs et al. 1990). In the initial stages of the zebra mussel colonization of Lake Erie, French (1993) found that freshwater drum consumed zebra mussels less than 25 mm long. Continued colonization may result in increased availability of larger zebra mussels and allow molluscivores to selectively prey on these larger sizes and maximize energy return. Prejs et al. (1990) found that roach optimally foraged on zeb-ra mussels, concentrating on numerous, easily accessible size classes. We examined whether freshwater drum and yellow perch selectively consumed specific sizes of zeb-ra mussels and explored how mouth and throat morphology influenced their predation.
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Freshwater drum and yellow perch were collected in conjunction with Ohio Depart-ment of Natural Resources, Division of Wildlife's scheduled surveys in May, July, and October, 1992. Survey sites were randomly selected from grid squares in western Lake Erie. Each grid was a 2.5-by 2.5-min latitude x longitude section of Lake Erie. Grid sites were unevenly distributed across the following depth strata: < 3 m; 3-6 m; 6-9 m; and 9 m (Fig. 1). Neither fish species was evenly sampled at every site, therefore we were unable to evaluate diet across depths and sites. Rather, fish were kept as they were encountered. Fish were collected using a semi-balloon bottom trawl with a 10.7-m headrope and 6.4-mm mesh in the cod end. All tows averaged 0.8 ms-1 and lasted 10 min. We kept a maximum of 25 fish each month from each of four size groups of yellow perch (50-mm intervals; 100-250+ mm) and five groups of freshwater drum (50-mm intervals; 150-350+ mm). All fish retained for stomach analysis were quick-frozen on dry ice to stop digestion.
To estimate the size distribution of zebra mussels available to fish, we used SCUBA in May-July to randomly collect rocks with zebra mussels in the Marblehead (mainland) and Green Island (offshore) areas of western Lake Erie. All zebra mussels were remo-ved from the rocks and separated from each other, measured with calipers, and assig-ned to 1-mm size classes.
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Fish were thawed and measured (mm total length, TL). Pharyngeal gape was measured (mm) for all freshwater drum (N = 66) and yellow perch (N = 61) collected in May using wood/plastic dowels, a method described by Wainwright (1987). Dowel diameters ranged from 3.2 to 25.4 mm with intervals of 1.6 mm. Dowels were inserted into the pharyngeal gape until we encountered a dowel that would not fit. The diameter of the largest dowel that fit was the pharyngeal gape measurement. The zebra mussel's anterior abductor muscle attaches to two internal septa, which are usually undamaged when ingested by fish (Prejs et al. 1990). We measured right septa and corresponding shell length, width, and height from 100 Lake Erie zebra mussels ranging in length from 3 to 35 mm. Measurements of septa were made using a dissecting microscope at 40X magnification; length, width, and height measurements of whole zebra mussels were made using calipers. We generated regression equations of septa length versus shell length, width, and height and used them to calculate di-mensions of zebra mussels ingested by fish.
The entire gastrointestinal tracts were removed and preserved in 95% alcohol. Sto-mach contents were identified under a dissecting microscope (up to 40X). Fish were identified to species, mollusks to genus, and other invertebrates to order when possib-le. High abundance of zooplankton in July samples necessitated subsampling, using methods described by Edmondson (1974). Random 2-mL subsamples were analyzed until either 50 Leptodora sp. or 75 cladocerans of the most abundant genus were counted and measured. Except for fish, a reference body part (usually head capsule or total length) was measured with an ocular micrometer for each ingested prey item. Prey TL was calculated using equations provided in Appendix 1. These TLs were then used to calculate individual dry weight of prey items eaten (Appendix 2). For fish, sta-ndard or backbone lengths were recorded when possible and converted to total length using equations from Knight (1983). Wet weight of individual fish was calculated from TL with equations developed by Hartman and Margraf (1992). Dry weight of fish was 23% of wet weight (Dabrowski 1979). Because stomachs often yielded few zebra mussels for assessing size-selective feed-ing, we also used mussels in the intestines, calculating their size from the zebra mus-selsepta length equation. No attempt was made to identify other material in the inte-stine, thus intestinal contents were not used for assessing diet composition.
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To compare zebra mussel populations between our two collection sites, we used Kolo-mogorov-Smirnov's two-sample test (Hollander and Wolfe 1973). Similar frequencies would allow us to combine sites for use in evaluating size-selective predation by fish. To evaluate size-selective predation, diets of freshwater drum and yellow perch were analyzed using Chesson's alpha (Chesson 1978, 1983), treating individual fish within a length group as replicates. Chesson's alpha is calculated as follows:
pi is the proportion of zebra mussel size class i from our estimated zebra mussel size distribution, and ri is the proportion of zebra mussel size class i in the fish's diet. Pre-ference for various 1-mm mussel size classes was determined by comparing mean ( 1 SE) alpha values for a size class with the alpha expected had that size class been eaten in proportion to that size's availability. Expected alphas are the reciprocal of the number of size classes (N = 35) w