Evaluating factors affecting restoration of an endangered marine broadcast-spawning invertebrate using an individual-based model of white abalone

Restoration and modeling of endangered species is challenging when the available data are sparse and the environmental conditions are stochastic. Using white abalone Haliotis sorenseni, a critically endangered marine broadcast spawner with spatially patchy remnant populations in southern California (USA), as an example, we developed an individual-based model that describes density-, temperature-, and habitat-dependent biological processes. Using the model, we evaluated hypothetical stocking strategies including spatial distribution and size of stocked abalone and initial density of resident abalone. Our results show that stocking strategies for the modeled white abalone populations benefited most when stocking locations had resident abalone and when stocked abalone were large in size. We also examined the potential influence of the spatial structure of the habitat (i.e. percentage of suitable habitat and spatial distribution of habitat cells), ocean warming, and poaching on restoration outcomes. We found that the percentage of suitable habitat greatly affected population dynamics, with more suitable habitat resulting in a slower decline in population growth and recruitment, as well as a higher and more stable proportion of intermediate-sized abalone. The spatial distribution (random or clustered) of habitat cells and of stocked abalone had less of an impact on model outcomes. Elevated temperature and poaching both had strong negative impacts on abalone population growth in the model. Given the spatial structure of the habitat and population, ocean warming and poaching are considered critical factors in the development of restoration strategies for endangered broadcast spawners.