Rapid re-establishment of an invasive bivalve following its apparent eradication

Abstract The invasive mytilid Arcuatula senhousia was thought to be eradicated from the Swan-Canning Estuary in temperate Western Australia in 2000 following an unseasonal heavy rainfall event. The novel use of a drop camera survey at 2,037 sites throughout the lower estuary over a 15 day period in February 2019 demonstrated A. senhousia was present at 212 sites, representing >10% of those surveyed. The presence of A. senhousia was most prominent in the upper half of the estuary basin and the lower Canning River, with those 1,288 sites accounting for 98% of detections. Of those, 75 sites had percentage covers of A. senhousia of 0 – 25 %, 66 sites coverage of 25 – 50 %, 19 had 50 – 75% and 12 sites had coverages of >75%. While the vast majority of A. senhousia occurred at depths of 2 to 8 m, a small number of detections occurred at 1-2 m, where percentage coverage was always <50%. The use of simple drop camera was cost-effective and facilitated the collection of data at a fine spatial scale and was shown to be suitable for benthic species such as A. senhousia that form benthic matts and whose spatial distribution and abundance may change markedly and so be missed by less exhaustive sampling.