Abiotic conditions are not sufficient to predict spatial and interannual variation in abundance of Ciona intestinalis in Nova Scotia, Canada.
The vase tunicate Ciona intestinalis is an invasive sea squirt that poses several challenges for coastal marine ecosystems and human activities. Despite its widespread distribution, temporal and spatial variability in population abundances is high. We tested whether this variation could be explained by 4 abiotic variables: temperature, salinity, pH and water movement. We repeatedly measured these 4 variables and relative abundance of C. intestinalis at 11 sites along the Atlantic coastline of Nova Scotia, Canada, each month between May and October 2014 and 2015. Using nonlinear mixed-effects modelling, we found that salinity moderately explained abundance of C. intestinalis across sites in 2014, while in 2015 there were no strong associations between abundance and any measured abiotic variable. The combination of little explanatory power within years and inconsistency between years led us to conclude that none of the measured abiotic conditions was useful in predicting the abundance of C. intestinalis. This finding contrasts with previous studies in which temperature and salinity were effective at predicting the presence or absence of the species. Thus, we suggest that tolerances of these 2 factors may determine whether C. intestinalis can survive in a location, but that other factors predict the rate of population growth. Given that increased abundance exacerbates the negative effects of this invasive species, we advocate further study of alternative factors that lead to higher abundances of C. intestinalis, to help inform management decisions.