Artificial reefs increase fish abundance in habitat-limited estuaries.
Human activities have reduced the carrying capacity of many estuarine systems by degrading and removing habitat. Artificial reefs may increase estuarine rocky-reef habitat, but our understanding of their ecological impact is limited. In particular, the question of whether fish on artificial structures are produced by the habitat or attracted from nearby natural rocky-reefs is of concern. We used baited remote underwater video at artificial reef sites and nearby natural reef sites to investigate the influence of artificial reefs on fish abundance in estuaries with low amounts of natural rocky-reef. We measured total fish abundance and the abundance of three species of fisheries importance (all in the family Sparidae) before artificial reef deployment (Reefballs®), 1 year after and 2 years after. This design was replicated in three widely separate estuaries over 4 years. During the 2 years post-deployment, abundance of Sparidae fish increased on both artificial and natural rocky-reefs, even when artificial reefs were deployed in different years and seasons. Total fish abundance increased at artificial reef sites with no evidence of change at natural rocky-reef sites. Our findings provide evidence that the fish seen on artificial reefs were not attracted from the nearby rocky-reefs and were likely 'produced' by the addition of artificial reefs in these estuaries. Artificial reefs can increase the carrying capacity in these estuaries by providing refuge that would otherwise be unavailable. Synthesis and applications. The increased fish abundance in three estuaries at both artificial reef and natural reef locations shows that purpose-built artificial reefs can be used in conjunction with restoration/protection of existing natural habitat, to increase estuarine carrying capacity and fish abundance. This may be for fisheries enhancement or estuarine restoration.