NIWA creates designer homes for fish
Seagrass meadows – vital nursery grounds for young fish – are vanishing at an alarming rate worldwide.
In New Zealand, young snapper, trevally, and other popular fish rely on subtidal seagrass for shelter and food, in many areas.
Much of the New Zealand seagrass was lost last century largely owing to sediment washing into estuaries and harbours from land development. With environmental degradation, subtidal seagrass habitat is one of the first things to be lost. “If you degrade a habitat, then you reduce the density of the fish that habitat can support. It is the subtidal seagrass that is most important to fish. If the subtidal seagrass is lost, then the juvenile fish production goes with it. It has a cascade effect,” says NIWA fisheries ecologist Dr Mark Morrison.
“Places like Rangaunu and Parengarenga harbours, in Northland, still have abundant subtidal seagrass extents, which are huge nurseries teeming with juvenile fish. Other places have lost much of their subtidal seagrass, such as Whangarei Harbour where seagrass has gone from twelve to fourteen square kilometres (including significant subtidal areas) in the 1960s to virtually none, and Tauranga Harbour, which lost 90 percent of its subtidal seagrass between 1959 and 1996. By association, all of the juvenile fish production dependent on those lost habitats has gone as well, reducing the numbers of adult fish we have today.”
Seagrass has thin, olive-green leaves that lie flat on the sand when exposed, and float upright when submerged in water. As well as providing shelter for fish, seagrass improves water clarity – the leaves trap fine sediments and reduce particle loads in the water by slowing water movement and encouraging particles to settle to the seafloor. Seagrass plants absorb nutrients from the sea and the seabed. They also release oxygen from their leaves and roots, which is beneficial for other marine life, and stimulates nutrient cycling.
Designer homes for fish
In a New Zealand first, and with funding from the Foundation for Research, Science and Technology, NIWA scientists created their own artificial seagrass beds on a relatively large scale. In 2009, at Whangapoua Estuary, Coromandel, a team lead by Dr Morrison used artificial seagrass mats to look at how habitat quality effects fish densities and habitat use.
“We made them with tantalising long blades of artificial grass,” says Dr Morrison, “the things that fish really go for. We designed and created the mats so that they could attract fish, and we could measure the effect of increasingly dense seagrass on fish abundance.” Denser seagrass provides better habitat quality for fish.
The plants were made from plastic fronds, tied to wire frames to form the artificial mats. The artificial mats mimicked six density levels of seagrass from sparse to very dense. Using specially designed seine nets, NIWA scientists counted how many fish, and what species, were associated with each density level of seagrass.
“Fish numbers reached their highest towards the higher seagrass densities, with a plateau in numbers at the highest density. Dr Morrison says this is probably due to “food limitation kicking in”, as fish form an active feeding front on the up-current side of the mats, visually picking out zooplankton (tiny crustaceans) as they are swept through the mats by the current.
“What we found is that the fish are really looking for shelter and seagrasses provide good protection to fish. Nineteen species of fish were found overall in the artificial mats including lots of juvenile snapper and trevally.” The research clearly shows the benefits of seagrass restoration in those areas where it has been lost.
In a follow up experiment being conducted this summer, NIWA scientist Dr Darren Parsons will tag several individual species of fish, including juvenile snapper, and then release them back into the artificial seagrass mats. After several months these will be sampled to measure how their survival and growth rates, as well as small-scale movements, vary in relation to seagrass mat density.
This knowledge will further increase our understanding of what changes to habitat quality ultimately mean in terms of fish (and associated fisheries) population sizes, and the economic and environmental benefits we derive from them.