Virtually all the snapper caught in a recent survey along the west coast of the North Island grew up in the same nursery. This is the finding of research conducted by NIWA, in which scientists examined the carbon and oxygen isotopes of snapper ear bones, known as otoliths. The snapper came from a wide range of places, from Ninety Mile Beach to Mana Island off the Kapiti Coast. Yet, as fisheries ecologist Mark Morrison discovered, 98 per cent of the fish had a distinctive chemical signature that can be matched to the environmental profile of Kaipara waters.
The study is part of a programme to understand the country’s ailing $32 m snapper fishery, which has suffered quota cuts as a result of over-fishing. Morrison is now exploring a link between the poor health of the Kaipara Harbour and low snapper numbers at sea. The impacts of land management—such as forest clearance and fertiliser use—have fl ow-on effects for shallow harbours, he says, silting up waterways and stifling the growth of seagrass beds vital to young fish. “These findings show how fragile some New Zealand snapper and other coastal fish stocks could be.”
As you might expect, a low number of juvenile fish in the harbour will impact upon the larger coastal ecosystem. Morrison also notes that other west coast harbours were probably once important snapper nurseries that have since collapsed. “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”.