A toenail cowrie feeding on soft corals at Cook Island. (Photo Steve Smith)
A decline in the number of molluscs found off Cook Island, near Fingal Head on the Tweed Coast, has sounded alarm bells for Southern Cross University (SCU) researchers involved in a long-term marine monitoring program.
Associate Professor Steve Smith and his team from the university’s National Marine Science Centre have been monitoring reefs between Port Macquarie and Tweed Heads since 2005, in a program supported by the Northern Rivers Catchment Management Authority.
‘We have been monitoring 35 reefs between Port Macquarie and Cook Island since 2005. We do assessments of fish and molluscs, and also look at the evidence of human impact in the form of debris,’ Professor Smith said.
At Cook Island, the reefs have shown a continued downward trend in biodiversity from 2009 to 2012.
‘We witnessed a loss of indicative species and at the same time we noticed considerable amounts of fine silt coating the sea floor. We think that was related to run-off through the Tweed River or Cudgen Creek.
‘The fish seem to be okay, but some of the species of shells disappeared completely.’
Following the most recent visit to the site in June, there is some good news.
‘Early indicators are that conditions have picked up over the last year. Fine sediment was absent from the reefs and mollusc diversity was substantially higher than in the 2012 surveys,’ said Professor Smith.
‘Conditions have improved a lot in 12 months, and this could be related to periods of heavy seas that have flushed the habitats.
‘However, some of the indicator species are still missing.’
Professor Smith said molluscs were a good indicator of the health of a marine environment.
‘We do find quite substantial variations in biodiversity, even between sites 600 metres apart, and over time. But this is the first time we have seen an obvious decline in biodiversity at any of our monitoring sites,’ he said.
Also of concern, was the fact marine debris loads were the highest recorded from Cook Island to date.
‘Almost all of the debris comprised fishing line, which we found entangled around corals,’ Professor Smith said.
‘This is an important management issue for this site, which is an Aquatic Reserve from which fishing is banned,’ he said.