Release of Juvenile Mussels into a Fish Hatchery Raceway: a Comparison of Techniques

by Hanlon, Shane David

Recent efforts to restore depressed or extirpated populations of freshwater mussels have focused on artificial propagation as an effective and practical conservation strategy. Although artificially cultured juveniles have been produced and released to the wild, no study has investigated the best time of year or the best developmental stage to release these juveniles. Several experiments were conducted to gain knowledge of appropriate culture and release techniques. Juvenile wavyrayed lampmussels (Lampsilis fasciola) were artificially propagated in the laboratory and subsequently release into a fish hatchery raceway during June, September and March. Juveniles released in June experienced a gradual decline in survival rate, with 50% survival after 72 days and stable survival thereafter until 200 days. Juveniles released in September and March experienced high mortality within the first month, and were unsuccessful in surviving the cold water conditions typical of those seasons. Temperature was strongly associated with growth; thus, juveniles released in June exhibited considerably greater growth than those released in September and March. Survival was positively correlated with shell length in the first 32 days post-metamorphosis. Fall and spring survival values, shell length-frequency data, and a significant increase in overwinter mean shell length (p = 0.045) suggest that overwinter survival is size-dependent. Survival rates of juveniles released in June exceed those of previous culture studies reported in the literature. From these results, I suggest that the best time for release of propagated juvenile freshwater mussels should be at the beginning of the growing season in late spring, when water temperatures exceed 15 degrees C (plus/minus 1°C).

Offspring of four, gravid female L. fasciola were tested to evaluate the extent of variability in growth and survival among brood stock. Comparison of growth and survival of progeny showed significantly lower performance of only one female (p < 0.05). Although results indicate variability in growth and survival among broods, it is not known whether heritabilities or physiological fitness caused this variability.

Infested fish (IF), newly metamorphosed juveniles (NMJ) and juveniles cultured for 1 month (CJ) were released to a fish hatchery raceway in order to determine the most appropriate developmental stage to release juveniles. Significant differences were recorded among the three release methods, with CJ attaining the greatest growth (2.47 mm plus/minus 0.02), NMJ with the next best growth (1.86 mm plus/minus 0.02), and IF exhibiting the least growth (1.34 mm plus/minus 0.02) (p < 0.0001). Survival among release methods was not statistically different because of high variability within each release method. High mortality from predacious fish was presumably the cause of this variability. With minimal predation, L. fasciola experienced 82.2% (plus/minus 3.6) survival at 90 days. Survival of hatchery-reared juveniles was comparably higher than laboratory-reared juveniles, suggesting that culturing freshwater mussels in a hatchery raceway is a preferable alternative to laboratory culture.

An experiment was conducted to compare growth of confined and unconfined juvenile L. fasciola released to a fish hatchery raceway. After 72 days, juveniles confined to small open dishes within the raceway (1.04 mm plus/minus 0.08) exhibited significantly less growth than juveniles released to the raceway that were not held in containers (2.15 mm plus/minus 0.07) (p < 0.0001). Results of additional comparisons with compiled growth data suggest that juvenile growth is retarded when cultured in small dishes.