LB 20-182 - Do natural habitats support better bivalve health relative to human-introduced habitats?

Estuarine habitats worldwide have been extensively modified by human-introduced structures such as seawalls. Human-introduced structures may favor non-indigenous species (NIS), but few studies have directly compared species health (i.e. condition index) on various habitat types. NIS Mytilus galloprovincialis is a dominant space-occupier on seawalls throughout Southern CA estuaries including Newport Bay. In a preliminary study, it had better condition on natural cobble vs. human-introduced seawall (2-way ANOVA, site*habitat, p=0.0001) but tidal elevation was not controlled, confounding our ability to discern a habitat effect. Lower tidal elevation should improve species condition due to longer immersion time, so for this follow-up study, we hypothesized that condition index (CI) would vary both across habitats and among tidal elevations. M. galloprovincialis were collected from a seawall and from cobbles at a site in upper Newport Bay, CA, and on the north and south-facing surfaces of an outfall pipe in San Diego Bay, CA, while recording tidal elevation for each individual. M. galloprovincialis experienced a nominally significant habitat*elevation interaction (2-way ANOVA, elevation*substrate, p=0.0595) with higher CI at lower elevations on seawalls only, and higher CI on cobble versus seawall but only at higher tidal elevations. Data from the outfall pipe are still being processed. Understanding habitat and tidal elevation effects on the health of a broader selection of both native and non-native bivalves will inform future management decisions about the use of natural and anthropogenic habitats.