Benthic fauna thrives in fishing exclusion zone

Research has revealed key differences in seafloor communities and habitats inside and outside the Separation Point trawl fishing exclusion zone in Tasman Bay. These have important implications for valuable benthic fisheries in the area.

The Ministry of Fisheries closed off 146 square kilometres of Tasman Bay to fishing from commercial trawlers and dredgers in December 1980, making it one of the oldest fishing exclusion zones in New Zealand. Their aim was to preserve beds of bryozoa – small colonial invertebrates that form large branching colonies, also known as ‘lace corals’. These are thought to act as fish nurseries, providing habitat structure for juvenile fish – including tarakihi and snapper – and their prey.

Between 2003 and 2010, we made the first comparisons of habitats and fauna inside and outside this protected area. We initially used sidescan sonar to examine the distribution of complex habitat either side of the exclusion zone boundary. We then took sediment grab samples to analyse the abundance, productivity (based on size distributions), and feeding modes of sediment-dwelling species.

Exclusion zone supports complex habitat

“The sidescan sonar images revealed that commercial fisherman have carefully avoided the exclusion zone,” says Dr Sean Handley, who led the project. Biogenic cover (habitats created by animals, including bryozoa) was up to four-fold greater inside the zone. And there was no evidence of fishing-related disturbance, such as trawl marks, that were common outside the protected zone.

After 30 years of protection, the overall productivity and biomass of sediment-dwelling species is higher inside the exclusion zone and sediments are coarser. Surprisingly, we found higher biodiversity of sediment-dwelling organisms in patchy shell-gravels present in the exclusion zone, than in the bryozoan beds. “This highlights the importance of large filter-feeding bivalves, whose shells are the source of these coarse shell-gravels in Tasman Bay,” says Dr Handley.

Habitat changes may affect value of the harvest

Within the exclusion zone, there were higher numbers of small (less than 2 mm) organisms, which may be good food for juvenile tarakihi and snapper. But in the fished habitat, we found greater numbers of medium-sized (2–11 mm) organisms – especially worms – which are favoured by flounder. This suggests that disturbance associated with trawling and dredging may stimulate productivity of some organisms, as predicted by ecological theory.

Seafloor disturbance by commercial trawling and dredging appears to reduce filter-feeder numbers and homogenise the sediments, making them muddier at the surface. This may be good for flounder, but could create a double whammy for higher value species like tarakihi and snapper, removing their preferred habitat and food sources.

The significance of these findings to the broader ecosystem deserves further investigation, as Tasman Bay and Golden Bay support several benthic fisheries of considerable value.

Contact: Dr Sean Handley