Evaluation of caging designs and a fingernail clam for use in an in situ bioassay

Two cage designs and fingernail clams (Sphaerium fabale) were evaluated for their suitability for use in in situ bioassays to assess the ecological co ndition of a stream and predict ecological recovery potential. One design ( referred to as tray design) was a modified plastic tray about one-fourth fu ll of small gravels and covered with 1 mm fiberglass mesh. The second desig n (referred to as tube-plates) consisted of short plexiglass tubes about on e-third full of small gravels and attached horizontally to a plexiglass pla te. One end of each tube faced into the current; both ends were covered wit h mesh. Cages containing clams were deployed at reference and impacted (tes t) sites for periods of 70 to 135 d. Growth and survival were the primary e ndpoints evaluated, but the tube-plates allowed isolation of individual cla ms so that natality also could be evaluated as an endpoint. Results of bent hic macroinvertebrate surveys, performed for another study, were included t o help validate bioassay results. Both cage designs yielded good quantitati ve, site-specific results for clam survival and growth; results for natalit y, though, were less conclusive. Clam survival and growth results were in g ood general agreement with the results for the benthic macroinvertebrate co mmunity surveys. At a site where the macroinvertebrate community was the mo st depauperate, clam mortality was always rapid. At a site where the condit ion of the macroinvertebrate community was only slightly less impacted than the most impacted site, clam growth was almost always significantly lower than at reference sites. Survival of clams was significantly reduced in < 2 5 d at this site in some trials, but in other trials there was little morta lity. At a minimally impacted site, clam survival was similar to that found at reference sites, and differences in clam growth were not detectable unt il after 40 to 50 d of exposure. The tube-plate design was easier to use, a llowed more flexibility in selection of response parameters, and required l ess handling time of test animals, thus, this was the preferred design. Our results demonstrated that either in situ bioassay design can be used to au gment monitoring and assessment programs. Their use as a predictor of ecolo gical recovery, however, requires further evaluation.