Looking out on the gulf, everything seems fine whatever the weather – blue water, studded with green islands, fringed with sandy beaches, seemingly teeming with marine life.
It’s what brings hundreds of thousands of visitors to Waiheke and an estimated 220,000 fishers out on the water each year.
But the gulf’s beauty masks several human lifetimes of abuse. Scientists estimate that the Hauraki Gulf now supports less than half the biomass (or volume of marine species) it supported in 1925, the Sea Change report says. A number of fish stocks, particularly snapper and crayfish, are well below optimum level. Other species causing concern are John Dory, porae (blue morwong), gurnard and trevally, while there is a lack of information for other harvested species.
Declining abundance may be contributing to declines in seabird populations, with fewer bait fish driven to the surface in “boil-ups”, presenting fewer prey for birds.
A notable effect of over-fishing is the creation of kina barrens through the phenomenon of “trophic cascades” – the reduction in top predators including snapper and crayfish leading to the collapse of an eco-system. Marine Biologist Roger Grace says the seabed off northern Waiheke contains some of the worst kina barrens he has seen.
A more profound effect of previous trawl fishing was the almost complete removal of extensive beds of green shell mussel and horse mussels in the gulf. Other habitats probably greatly reduced in extent and abundance include bryozoans, sea pens, sponges, gorgonians, and hard corals, the report says.
Shellfish in the intertidal zone, especially cockles (tuangi) and pipi, are under pressure from over-harvesting, increased sedimentation and muddiness.
For crayfish, there is anecdotal evidence of a widespread decline in abundance.
On bottom species, Sea Change quotes Niwa research findings that major reductions in benthic primary production have almost certainly occurred. This stems from declining water quality, increased turbidity in the water column, and sediments smothering plants and covering potential settlement surfaces.
“Some habitats, such as subtidal seagrass meadows and benthic green-lipped mussels, are now effectively functionally extinct in the Hauraki Gulf Marine Park…
“The present day Hauraki Gulf Marine Park is less diverse and productive than it historically was.”
Excessive sediment runoff from land is the main cause of degraded marine habitats in estuaries, harbours and the inner gulf. Nutrients from farms and sewage treatment plants which leach into waterways then into the gulf can cause algal blooms and lead to the development of anoxic conditions. Oxygen depletion and water acidification have been measured in autumn in the outer Firth of Thames. Dissolved nitrogen levels in the outer firth have risen over the past 15 years despite total nitrogen loads in rivers draining into the firth being stable or increasing only slowly.
The report proposes wide-ranging measures to reverse the trends – addressing harmful fishing practices, sedimentation, nutrient loadings and pollution.
• Geoff Cumming