Final Report U.S. Department of the Interior – Fish and Wildlife Service Arkansas Ecological Services Field Office DISTRIBUTION AND POPULATION STRUCTURE OF FRESHWATER MUSSELS (BIVALVIA: UNIONOIDA) IN MIDDLE DITCH, BIG LAKE NATIONAL WILDLIFE REFUGE - ARKANSAS Principal Investigator: Chris L. Davidson U.S. Fish and Wildlife Service Arkansas Ecological Services Field Office Conway, AR 72032 [email protected] January 2018 INTRODUCTION Big Lake National Wildlife Refuge (BLNWR), established in 1915, is comprised of 4,467 ha (11,038 ac) located in northeast Arkansas near Manilla, Mississippi County. BLNWR was a free- flowing river system until the New Madrid Earthquake of 1811-1812 shifted land and water flow to its present lake and swamp environment (https://www.fws.gov/refuge/Big_Lake/about.html). The watershed consists of approximately 6,475 km2 of agricultural land funneled into the refuge via several large drainage ditches within the St. Francis River basin (U.S. Fish and Wildlife Service 1976). In 1991, the U. S. Army Corps of Engineers constructed a series of water control structures to divert silt-laden floodwaters from the refuge and improve water quality and promote growth of natural aquatic vegetation. During the larval stage, mussels are obligate ectoparasites, attaching to the fins and gills of a host fish (Barnhart et al. 2008; Haag 2012). Larvae metamorphose into juveniles that excyst from the host fish and sink to the river bottom, where, if habitat is suitable, they continue their growth to adulthood. Some mussels are host generalists (i.e. using many species of fish), while others are host specialists (i.e. using one species of fish). Mussel dispersal and gene flow is a function of host fish movement. Changes in flow regimes of river systems affect dispersal potential and population structure of mussels and host fishes. Mussels are long-lived and relatively immobile. Therefore, they integrate environmental stressors occurring at multiple temporal and spatial scales (local to catchment) (Atkinson et al. 2012). Extensive systematic inventories of mussel fauna in the St. Francis River exist primarily as a result of efforts to determine the distribution and abundance of Fat Pocketbook (Potamilus capax) (Ahlstedt and Jenkinson 1987, 1991; Harris and Gordon 1987; Harris et al. 2009; Jenkinson and Ahlstedt 1987; Clarke 1985; Posey 1997). Mussel inventories have been conducted in selected sections of Stateline Outlet Ditch, which adjoins the eastern border of Big Lake Wildlife Management Area (abutting the eastern border of BLNWR) and Ditch No. 81 adjoining the western border of BLNWR, mostly to assess the potential effects of ditch cleanout and highway projects on mussels. In this report, I present the results of a mussel inventory from Middle Ditch. These data provide baseline information about the distribution and population structure of mussels within the portion of Middle Ditch that is located within the BLNWR boundary. BLNWR requested this information to better inform land and water management activities to conserve the mussel fauna at BLNWR. METHODS Study Area Middle Ditch diverges from Ditch No. 1 and traverses approximately 9.4 km of the “old” Little River channel before it converges with Big Lake (Figure 1). The northernmost point of Middle Ditch begins at a water control structure located on Ditch No. 1, approximately 16.7 km upstream of Arkansas Highway 18 (Figure 2). I used type of substrate, water depth, and 1 channel characteristics (e.g., meandering v. straight, channeling braiding or convergence, etc.) (Figures 2 – 7) to divide the study area into six sections. Section 1 was approximately 1,461 m in length and extended from Transect 1, near the water control structure, to the convergence of an overflow channel from Ditch No. 28 on river left facing downstream (35.98011, -90.11952; Figure 2). Mean width of wetted channel was 22.5 m. Water depth ranged from < 1 m to approximately 4 m. Substrate was dominated by hardpan clay near center channel and clay with silt near the margins of the wetted channel. Section 2 was approximately 1,179 m in length and extended from the terminus of Section 1 to Transect 53, where a noticeable sand bar extended nearly the entire width of the wetted channel (Figure 3). Mean wetted channel width was 23.1 m. Water depth ranged from 1 m near the lowermost third of this section to approximately 6 m in the upper two-thirds of it. Substrate in the deeper portion of this section was dominated by clay or sand with loose flocculent silt in the thalweg and clay with silt on descending banks. The shallower portion of this section was dominated by sand and clay with silt. Figure 1. Study area for Middle Ditch mussel survey at Big Lake National Wildlife Refuge, 2017. 2 Figure 2. Location of Section 1 and mussel survey transects in Middle Ditch, Big Lake National Wildlife Refuge, 2017. Uppermost transect is approximately 50 m downstream of water control structure. Section 3 was approximately 1,179 m in length and extended from the terminus of Section 2 to Transect 78, just past the point where the channel braids (35.96291, -90.13067) (Figure 4). Mean width of wetted channel was 6.4 m (during a water drawdown). Water depth was ≤ 1 m, and a substantive portion of the normal wetted channel was dry during water drawdowns. Substrate was dominated by sand and clay with silt. Section 4 was approximately 1,345 m in length and extended from the terminus of Section 3 to Transect 80, which begins Section 5 (35.954589; -90.13295) (Figure 5). At this point, the channel widens substantially from the constricted channel width common to Section 4. The majority of this section was comprised of small (< 100 m2), shallow (≤ 1 m) pools with constricted, shallow channels (< 1 m wide and < 0.2 m deep) connecting pools. As a result, we determined while flagging transects that qualitative sampling in this section would be more informative for locating isolated pockets of mussels than transect sampling. The wetted channel ranged from < 1 m to approximately 10 m. Because we qualitatively sampled this 3 section, we did not measure mean width of wetted channel. Substrate was dominated by clay with silt. Figure 3. Location of Section 2 and mussel survey transects in Middle Ditch, Big Lake National Wildlife Refuge, 2017. Figure 4. Location of Section 3 and mussel survey transects in Middle Ditch, Big Lake National Wildlife Refuge, 2017. 4 Section 5 was approximately 1,581 m in length and extended from the terminus of Section 4 (35.95251; -90.13295) to Transect 110, which begins Section 6 (35.94396; -90.13063) (Figure 6). Mean width of wetted channel was 22.8 m. Water depth ranged from < 1 to 4 m. Substrate was dominated by clay with silt to clay or sand with loose flocculent silt in deeper (generally ≥ 3m) thalwegs. Section 6 was approximately 2,618 m in length and extended from the terminus of Section 5 to Big Lake at Transect 159 (35.92175, -90.13207) (Figure 7). Mean width of wetted channel was 22.8 m. Water depth ranged from 1 to 4 m. Substrate was dominated primarily by clay with silt with pockets of sand. Figure 5. Location of Section 4 and qualitative mussel sample points in Middle Ditch, Big Lake National Wildlife Refuge, 2017. 5 Figure 6. Location of Section 5 and mussel survey transects in Middle Ditch, Big Lake National Wildlife Refuge, 2017. Figure 7. Location of Section 6 and mussel survey transects in Middle Ditch, Big Lake National Wildlife Refuge, 2017. 6 Survey Approach We surveyed approximately 9.4 km of Middle Ditch by searching bank to bank transects aligned perpendicular to flow. We spaced linear transects approximately 50 m apart in Sections 1 – 3 and 5 – 6 and measured width of wetted channel by stretching a weighted rope across it at each location. Using 1 m2 quadrats, two divers worked from the water’s edge towards the center of the channel to quantitatively sample each transect. Beginning at the upstream limit of the study area and extending to the uppermost margin of Big Lake, we numbered transect 1 to 159. We hand searched quadrats and then flipped them successively across the channel until they reached the opposite diver near the center of the channel (Figure 8). This method enabled us to quantitatively sample approximately 2% of the wetted area of Middle Ditch and qualitatively sample all available suitable habitat in Section 4. Our survey represents the first quantitative mussel data for Middle Ditch. Wetted Width 1 m 1 m Figure 8. Transect diagram. Mid line of each 1 m2 quadrat represents transect line positioned perpendicular to flow extending across wetted width of channel. We sampled these sections from August 14 – September 20, 2017. Within each quadrat, we collected mussels by excavating the substrate to a depth of approximately 10 cm. Each diver identified, counted, and reported mussels to a tender/boat operator, who recorded all data via a diver to tender, diver to diver three-way underwater communication system. Nomenclature followed Williams et al. (2017). We calculated population estimates for each species and assemblage total community numeric standing crop using modified methods from Sampford (1962) (Huebner et al. 1990 and Harris et al. 1993) where total number of mussels were calculated as follows: = � Where X is total number of mussels in a bed, i is the number of strata, yi is the sample total (total number of organisms encountered in the ni sampling units) and gi is the raising factor (gi = 1/fi,) where fi is the fraction sampled and is defined by ni/Ni, with ni being the number of sample 7 units counted in the ith stratum, and Ni the total potential number of sampling units in the ith stratum (Huebner et al.