Lake Erie beaches: diel variation in fish assemblage structure and implications for monitoring

Scott M. Reid 0 Nicholas E. Mandrak 0 Handling editor: J. Trexler 0 0 S. M. Reid (&) N. E. Mandrak Great Lakes Laboratory for Fisheries and Aquatic Sciences , Fisheries and Oceans Canada, Burlington, ON , Canada L7R 4A6 Knowledge of temporal variation in nearshore Laurentian Great Lakes fish assemblages is important for understanding species-habitat associations, how abiotic and biotic influences vary temporally, and when sampling should occur. Using spring and fall seining data from Lake Erie beaches, we compared day and night fish assemblages and tested for differences among sampling periods. Beaches were utilized by a diverse collection of Lake Erie basin fishes (one-third of known species). During all sampling periods, catches were dominated by cyprinid species (53-91%), and by invertivores and planktivorous fishes. Diel differences were detected in abundance, species richness and assemblage structure. Multivariate analyses (canonical analysis of principal coordinates) indicated that season had a larger influence on fish assemblage structure than diel period. Given observed temporal variation in assemblage structure, studies of Laurentian Great Lakes beach fishes should be restricted to a single time period (e.g. day-time spring sampling), or adopt sampling designs that permit diel period and season to be included as factors in analyses. Second, the large seasonal variation in assemblage composition combined with higher night species richness indicates that night sampling during both spring and fall would be the most efficient and comprehensive approach for beach fish inventory. - Nearshore areas of the Laurentian Great Lakes are of special interest because of their linkages and interactions with large pelagic offshore habitats (Randall et al., 1996), and their important role in fish production and supporting biodiversity (Goforth & Carman, 2003; Wei et al., 2004). It has been estimated that approximately 80% of Laurentian Great Lakes fishes use nearshore areas for at least part of the year (Chow-Fraser & Albert, 1999). Beaches account for approximately 20% of the Laurentian Great Lakes shoreline (Wei et al., 2004), and provide important reproductive and nursery habitats for fishes (Heufelder et al., 1982; Wei et al., 2004). Groynes, jetties and breakwaters have extensively modified these habitats; leading to the loss of nearshore sand deposits and reduced size of beaches (Meadows et al., 2005). However, compared to coastal wetlands and other habitats, beach fish assemblages and the abiotic and biotic factors influencing their composition have been rarely investigated. Diel variation in the composition of fish assemblages has been detected in the shallow, nearshore areas of a variety of freshwater and marine environments (Sanders, 1992; Pierce et al., 2001; Morrison et al., 2002; Blackwell & Brown, 2005). Day and night differences in species associations with shallow, nearshore areas have been attributed to environmental gradients (i.e. oxygen concentration), feeding activity and predator avoidance (Emery, 1973; Helfman, 1981; Morrison et al., 2002; Wolter & Frehof, 2004). Diel patterns may also reflect differences in vulnerability to sampling gear (Pierce et al., 2001; Gries & Letcher, 2002). Seining (the sampling gear employed in this study) is a common method for determining species distributions, and assessing the abundance and richness of small fishes in the nearshore areas of lakes (Blackwell & Brown, 2005). However, compared to well-documented differences in day and night electrofishing catches, few studies have investigated the influence of diel period on beach seining results (Pierce et al., 2001). The objective of this study was to compare day and night Lake Erie beach fish assemblages. Lake Erie is the 11th largest lake in world (surface area: 25,700 km2). It is the most biologically productive of the Laurentian Great Lakes; supporting the highest diversity of fishes (Coon, 1999). However, the Lake Erie ecosystem has been severely altered as a result of the combined effects of eutrophication, overexploitation of fishery resources, habitat degradation, pollution and invasive species (Koonce et al., 1996). Quantification of diel differences in Lake Erie beach fish assemblages is important for the: (1) characterization of how species and lifestages utilize different habitats (Morrison et al., 2002); (2) description of how abiotic and biotic influences on fish assemblage composition vary with time of sampling (Rundle & Jackson, 1996); and (3) identification of appropriate sampling schedules for impact assessments and longterm monitoring of fish populations. Using data from spring and fall seining of beaches across the north shore of Lake Erie, we tested for diel differences in fish abundance, species richness and beach fish assemblage structure. As diel variation in shallow habitat use can reflect patterns of feeding activity and predation risk, and feeding guilds are incorporated into indicators of aquatic ecosystem health, we also investigated temporal variation in the representation of feeding guilds. Materials and methods In 2005 and 2006, 24 beach sites were sampled along the north shore of Lake Erie (Fig. 1). Beaches were distributed across the three Lake Erie basins: eastern basin (n = 6), central basin (n = 13) and western basin (n = 5). Most beaches sampled were located in municipal, provincial and federal parks, which ensured that sites were accessible for both day and night beach access. Sites sampled were a mix of sand and sand/gravel beaches with maximum sampled water depths between 0.5 and 1.5 m. The ranges of spring and fall water temperatures were similar: 12.9 to 27.0 C and 12.5 to 25.0 C, respectively. Measurements of conductivity and water clarity (secchi depth) ranged from 214 to 330 ls cm-1 and 0.04 to 1.2 m, respectively. At each site, 250 m of beach habitat were sampled with five consecutive 50 m long seine hauls pulled parallel along the shores edge. The bag seine was 15.2 9 2.4 m with a 2.4 9 2.4 9 2.4 m bag. Mesh size was 6.4 mm for the wings and 3.2 mm for the bag. Day and night sampling at each site was completed in the late spring (June 5th to 21st) and fall (September 27th to October 12th). Night sampling (after twilight) commenced after 2130 EDT in the spring, and after 1930 in the fall. Sites were sampled only during one season each year (i.e. spring 2005 and fall 2006, or spring 2006 and fall 2005). We were prevented from summer sampling because of high recreational beach use, and excessive amounts of filamentous algae (Cladophora) that quickly filled the bag seine. For each season, we tested the effect of diel period on total fish abundance with a paired sample t-test (Zar, 1999). Diel differences in overall feeding guild structure were tested with a paired sample Hotellings t-test (Hammer et al., 2001). A multivariate test was used because small-bodied species are expected to alter habitat use in the presence of (...truncated)