Artificial reefs increase fish abundance in habitat-linked estuaries

Human activities have reduced the carrying capacity of many estuarine systems by degrading and removing habitat. Artificial reefs increase estuarine rocky‐reef habitat, but our understanding of their ecological impact was limited.

In a boost for both recreational fishing and the environment, new research shows that artificial reefs can increase fish abundance in estuaries with little natural reef. Researchers installed six manmade reefs per estuary studied and found overall fish abundance increased up to 20 times in each reef across a two-year period. 

The research was a collaboration between UNSW Sydney, NSW Department of Primary Industries (DPI) Fisheries and the Sydney Institute of Marine Science (SIMS).

Professor Iain Suthers, of UNSW and SIMS, led the research, while Dr Heath Folpp, of NSW DPI Fisheries, was lead author.  

Co-author Dr Hayden Schilling, a SIMS researcher said the study was part of a larger investigation into the use of artificial reefs for recreational fisheries improvement in estuaries along Australia’s southeast coast.

“Lake Macquarie, Botany Bay and St Georges Basin were chosen to install the artificial reefs because they had commercial fishing removed in 2002 and are designated specifically as recreational fishing havens,” Dr Schilling said.

“Also, these estuaries don’t have much natural reef because they are created from sand.  So, we wanted to find out what would happen to fish abundance if we installed new reef habitat on bare sand.

“Previous research has been inconclusive about whether artificial reefs increased the amount of fish in an area, or if they simply attracted fish from other areas nearby.”

“The research team 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 before artificial reef deployment,  1 year after and 2 years after.

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.

“Fish find the reef balls attractive compared to the bare sand: the holes provide protection for fish and help with water flowing around the reefs,” Professor Suthers said. The researchers observed a wide variety of fish using the artificial reefs, but the study was specifically monitoring for species popular with recreational fishermen, including snapper, bream and tarwhine.

“These species increased up to five times and, compared to the bare sand habitat before the reefs were installed, we found up to 20 times more fish overall in those locations.

“What was really exciting to see was  that on the nearby natural reefs, fish abundance went up two to five times overall”.  Dr Schilling said their findings provided strong evidence that purpose-built artificial reefs could be used in conjunction with the restoration or protection of existing natural habitat to increase fish abundance, for the benefit of recreational fishing and estuarine restoration of urbanised estuaries.

“Our results validate NSW Fisheries’ artificial reef program to enhance recreational fishing, which includes artificial reefs in estuarine and offshore locations,” he said.

“About 90 per cent of the artificial reefs are still sitting there and we now have a student researching the reefs’ 10-year impact.”

The study was funded by the NSW Recreational Fishing Trust.

Read the study in the Journal of Applied Ecology: https://doi.org/10.1111/1365-2664.13666