Abstract or Summary
More than 80,000 juvenile freshwater mussels were produced in a hatchery and reared in a variety of aquatic habitats and aquaculture gear within Kent and New Castle County, Delaware, to identify suitable locations and practices that will be needed for expanded mussel restoration in the future. The main species used in this study was the native Alewife Floater (Utterbackiana implicata) because it can be a functional dominant of natural mussel assemblages of the lower Delaware River Basin, and its large size and abundance can materially contribute to improved water quality. A small number of gravid females were collected from the tidal Delaware River to serve as broodstock for this study. Mussel larvae from these broodstock were then used to produce transformed juvenile mussels in partnership with a United States Fish and Wildlife Service hatchery in Virginia. These juvenile mussels were then grown in a variety of environments and systems. Juvenile mussels of a different species, the Eastern Pondmussel (Ligumia nasuta), were provided by a complimentary study, enabling interspecific comparisons in addition to main factors of rearing location and culture gear type. Water quality and seston food resources were also monitored at the rearing locations. Mussel growth and survival was then compared species, site, gear type, and ambient water quality to deduce best rearing practices that will guide future mussel propagation, rearing and recovery in the State of Delaware. Due to the excess mussel production and access to a second species, outcomes from this study exceeded the original objectives. With one exception, all rearing sites tested were found to sustain positive growth and moderate to high survival of native freshwater mussels.
In some cases, mussels grew at an extraordinary pace, increasing from <1 mm to >50 mm in a span of <6 months. The site that did not support juvenile mussels was a man-made aquaculture pond at Delaware State University, and the mortality at that location is unclear but could have been due to the use of herbicides to control algae in the vicinity. Elsewhere, suitable rearing locations included lentic systems such as ponds at Winterthur Gardens and Bellevue Lake, and lotic systems such as Wilson Run, White Clay Creek and Red Clay Creek. Although seasonally and spatially variable, water quality and food conditions at all sites was found to be acceptable for mussels. Interestingly, we had successful survival and growth of extremely young <1 mm juveniles in floating basket cultures held in ponds. Typically, juvenile mussels perish when moved to field sites at such a small size, or they can be easily washed out of the baskets. We did not originally intend to test outplanting of these Class-I mussels, but the surplus seed production provided an opportunity to test whether this could work if the mussels were very carefully added to the baskets, following guidelines from USFWS. Although mortality was considerably higher than when mussels were outplanted at a larger Class-II size, many of these mussels survived and achieved much larger sizes than their cohorts that were moved to ponds later. Presumably, this result was due to richer and more suitable natural food resources in the pond, compared to the hatchery. This survival-growth trade-off will be useful to guide appropriate rearing protocols for future projects that have different needs (e.g., large animals versus large numbers). In general, aquaculture systems developed for marine shellfish such as oysters supported equivalent or better survival and growth of mussels as compared to floating baskets used by USFWS and in previous PDE mussel studies. The floating baskets are preferable for small, young mussels because of tighter controls of mesh sizes and the opportunity to provide sediments which juvenile mussels benefit from. Once mussels are more than 2.5 cm in shell height, they can be moved into floating aquaculture systems that have larger mesh openings and which have much higher capacity due to the tiered racks. The off-bottom aquaculture cages also supported mussel growth and survival, but they were not as easy to maintain and had lower capacity compared to the floating culture systems. Juvenile mussels that were relocated to streams fared very well when they were held in silo pens that supply ample water flow but which protect the animals from predators and flood associated wash-out. The only exception was that silos placed into White Clay Creek were themselves damaged or washed out, either by stormwater flooding or human vandalism. Mussels held in silos in Red Clay Creek and Wilson Run grew very quickly and exhibited high survival. Until the silos were lost, White Clay also supported good mussel growth. In contrast, mussels that were free-released into these same streams either perished or were washed downstream. As we have found in earlier studies, most streams in northern Delaware are very flashy and mussels are rarely retained for very long at the point where they are released because of high bed transport rates. This was the case in this study in Red Clay and White Clay Creeks, where retention quickly dropped to 0%. In Wilson Run, our free-released juveniles were retained where they were released, but they appeared to all be preyed upon since the juveniles were found dead with pierced or cracked shells. The poor retention and survival of the free-released juvenile mussels, compared to the protected mussels held in silos, reinforces the belief that mussels should be reared to larger sizes (e.g., in ponds) prior to being relocated to restoration sites in streams and rivers. Predators appear to focus on smaller sized mussels (and Asian clams). For mussel restoration projects in small streams, it might also be advised to focus on a mussel species that has a thicker shell and is more adapted to streams, such as Eastern Elliptio (Elliptio complanata). The primary mussel we tested here was the Alewife Floater, which has a more ovoid shape, thinner shell, and more typically inhabits deeper, slower moving rivers and tidal tributaries.
This study demonstrated that native freshwater mussel species can be propagated and reared to support mussel restoration projects in Delaware, which will directly support the state’s clean water goals. This study focused on just one mussel species, which will be useful for restoration projects in the types of waters that mussel species is adapted for. With over a dozen native mussel species that are adapted to different types of aquatic habitats, future studies should expand on this project’s success by developing similar hatchery and rearing methods for the others, especially Eastern Elliptio, which is the dominant mussel in streams and rivers. More work is also needed to understand the habitat conditions upon which mussels depend, and to pair mussel restoration efforts with stream restoration projects that include mussel bed habitat in the project goals. Many of our state’s streams are impaired by stormwater or contain too much mobile fine sediments, and the carrying capacity for mussels in systems with high bed transport rates will remain low to nil until the root problems are addressed or until those systems are enhanced with refugia for mussels.