Seagrasses vital for fish stocks

Fish stock habitat a weak spot
ScienceAlert 4 Mar 09;

NIWA scientists have discovered that nearly all snapper on the west coast of the North Island come from nurseries in just one harbour.

Snapper is New Zealand’s largest recreational fishery, and one of the country’s largest coastal commercial fisheries with an annual export value of $32 million (2008).

But in recent years some stocks have failed to recover from historical overfishing, with some commercial catch quotas for snapper being cut recently to protect the species.

In 2003, NIWA scientists collected juvenile snapper from seven estuaries along the west coast of the North Island.

By testing their ear bones (otoliths) for eight different chemical elements, scientists were able to create a ‘chemical signature’ to identify which estuary the fish came from.

Four years on, a sample of 140 adult snapper was collected from commercial catches, from Ninety Mile Beach to Mana Island in Wellington.

The analysis of these snapper using the ‘chemical signatures’ established is now complete. The scientists found that 98 percent of the adult snapper were originally juveniles from Kaipara Harbour.

NIWA Fisheries Ecologist Dr Mark Morrison said the discovery is a significant breakthrough for scientists working on ways to maintain and potentially increase snapper and other fisheries stocks.

“These findings show how fragile some New Zealand snapper and other coastal fish stocks could be. It highlights the importance of protecting natural habitats, like Kaipara Harbour.”

“Any negative impacts on the production of juvenile fish in the harbour will cascade through into a much larger coastal ecosystem, ultimately having a huge effect on the abundance of fish over a 700-kilometre coastline,” Dr Morrison says.

Kaipara Harbour is under threat from human activities – particularly land uses which causes sedimentation, eutrophication, and changes in water quality. These effects can all damage the biogenic (living) nursery habitat of snapper (usually seagrasses and horse mussel beds). It is likely other west coast harbours were also once important nurseries.

NIWA is now working on ways to restore and recover the habitats. This work includes using artificial seagrass habitats to understand why the fish value that environment, as well as the possible consequences on snapper numbers if the habitat was lost or destroyed.

Early indications from a recent experiment in Whangapoua Estuary, Coromandel, suggest a strong positive correlation between increasing seagrass blade densities and juvenile snapper (and other fish species) abundance. Reductions in seagrass blade densities are a sign of environmental degradation of seagrass meadows, which may reduce their value to small fish.

“Now that we know more about where the important nurseries are, we need to know why snapper larvae settle there, and how we can stop degradation of their habitat. Our aim is to be able to advise coastal resource managers on the likely consequences of different habitat management to fish stocks so that we can ensure that recreational, customary, and commercial fishing can continue in the future.”