exceptions, freshwater <strong>mussel</strong> recruitment is dependent upon <strong>the</strong> availability <strong>of</strong> an appropriate fish host (Lefevre and Curtis 1911, Zale and Neves 1982, Barfield and Watters 1998). Therefore, stream fish data may be useful to identify potential hosts, or <strong>the</strong> lack <strong>the</strong>re<strong>of</strong>, for <strong>Maryland</strong>’s <strong>mussel</strong>s. Describing and ultimately correlating <strong>mussel</strong> distribution to chemical, physical, and biological variables will provide <strong>the</strong> <strong>Maryland</strong> Natural Heritage Program (NHP) with critical information necessary to effectively manage this imperiled faunal group. The goals <strong>of</strong> this report are to: 1) incorporate results from sampling during 2009 <strong>by</strong> MANTA with previously <strong>collected</strong> freshwater <strong>mussel</strong> data; 2) describe environmental conditions found coincident with <strong>mussel</strong>s and throughout <strong>the</strong>ir observed range; and 3) investigate <strong>the</strong> co-occurrence <strong>of</strong> potential and known fish hosts for <strong>mussel</strong>s. METHODS Since 1995, MANTA has sampled for freshwater <strong>mussel</strong>s at more than 1,600 sites and Corbicula fluminea Müeller 1774 (Asian clam) at 3,000 sites. At MBSS sites, we visually searched for live individuals or discarded valves <strong>of</strong> freshwater <strong>mussel</strong>s and C. fluminea in suitable habitat, animal middens, and on stream banks for ≥15 minutes. At <strong>the</strong> remainder <strong>of</strong> sites sampled, we recorded incidental observations <strong>of</strong> <strong>mussel</strong>s if we did not conduct a visual search. We identified <strong>mussel</strong>s to species in <strong>the</strong> field and noted <strong>the</strong>ir condition (live or dead). When a live specimen was encountered, it was identified to species and returned to <strong>the</strong> location where it was <strong>collected</strong>. Representative voucher shells were retained and sent to taxonomical experts for verification <strong>of</strong> field identifications. Distributional maps <strong>of</strong> <strong>mussel</strong> species within 8-digit watersheds <strong>of</strong> <strong>Maryland</strong> (Appendix I) were created in GIS (ESRI ArcMap 9.3) and include <strong>records</strong> from all MANTA surveys where observations <strong>of</strong> freshwater <strong>mussel</strong>s were made. While <strong>the</strong> methods used to collect chemical, physical, and biological data from sample sites were standardized, <strong>the</strong> variables <strong>collected</strong> <strong>of</strong>ten changed. As a result, we limited our analysis <strong>of</strong> environmental conditions to <strong>the</strong> MBSS sampling events from 2007-2009 (N = 652). Water chemistry grab samples were <strong>collected</strong> at sites during spring base-flow (March - April) and analyzed for acid neutralizing capacity (µeq/L), chloride (mg/L), sulfate (mg/L), total nitrogen (mg/L), nitrate (mg/L), nitrite (mg/L), ammonia (mg/L), total phosphorus (mg/L), orthophosphate (mg/L) and dissolved organic carbon (DOC) (mg/L), using methods described <strong>by</strong> <strong>the</strong> U.S. EPA (1987) and APHA (1998). Slope (%) was calculated from <strong>the</strong> change in water surface height over <strong>the</strong> 75-mlong stream segment using a surveyor’s level and metric stadia. Benthic macroinvertebrates were <strong>collected</strong> with a 540 µm D-net from 20, 1 ft² areas <strong>of</strong> proportionally available optimal habitat to calculate a benthic index <strong>of</strong> biotic integrity (B- IBI) (Stribling et al. 1998). At each site, water temperature was recorded at 20 minute intervals from June to September with Hobo data-loggers (Onset Corporation). From <strong>the</strong>se data we calculated an average <strong>of</strong> <strong>the</strong> daily mean temperature (N ≈ 92). During summer base-flow conditions, in-situ water chemistry was measured with a multiparameter meter for pH, dissolved oxygen (mg/L), and specific conductance (µs/cm). We <strong>collected</strong> stream fish within each 75 m segment using a two-pass depletion method with backpack electr<strong>of</strong>ishing units (one unit per 3 m <strong>of</strong> stream width) to calculate a fish IBI (F-IBI) (Roth et al. 1998). From stream fish data we calculated species abundance, 4
species richness, host abundance, and host richness. We visually estimated physical habitat metrics (instream habitat, epifaunal substrate, velocity depth diversity, pool-glide quality, and riffle-run quality) from within sample reaches using five metrics scored on a 0-20 scale. Riffle embeddedness was determined <strong>by</strong> estimating <strong>the</strong> percentage <strong>of</strong> gravel and larger substrates that were surrounded <strong>by</strong> fine sediment (
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