Improving large-scale population estimates and assessments of ecological importance of three epifaunal bivalve species by combining distribution and abundance models

Epifaunal bivalves include reef building organisms that provide several important ecological functions in coastal marine environments. Evaluating the distributional patterns and population sizes is key in assessing the total and relative contribution of species towards these functions and can aid in improving spatial planning and management. In this paper, the utility of combining distribution models and models of abundance to improve population size estimates of epifaunal bivalves on the Swedish west coast was investigated. Models were fitted to predict occurrences and densities of three, both native and invasive, ecologically important bivalve species (blue mussel, Mytilus edulis; Pacific oyster, Magallana gigas and European oysters, Ostrea edulis). Bootstrapping methods were used to estimate model performance and error margins around the final predictions. Results from the distribution models show considerable overlap in use of shallow habitat by the native blue mussels and the invasive Pacific oyster, while the European oyster resided mostly in deeper habitats. There were however differences in larger geographical distribution patterns in all species. The use of distribution and abundance models drastically altered population estimates in species where models performed well, compared to methods without the use of models or only distribution models. Importantly, estimates of total abundance when translated to biomass estimates indicated that the invasive Pacific oyster likely contributes the most to ecosystem functions associated with epibenthic bivalves in this area.