The New Zealand government has just introduced ‘fast track’ legislation to bypass the usual checks and balances for environmental protection. This means less voice for nature in the application and approval process. To help address that gap, I’ve been looking into the impact a proposed sand mine might have on the tipa / scallop population in Bream Bay. I shared the report with quota owners who have been fishing the area, they agreed with my findings and have sent the report to Ministers.
This is an experimental idea for restoring severely degraded seafloor ecosystems in the Hauraki Gulf Marine Park. It was inspired by reforestation efforts in China, where desertification is being tackled using tools like sandbag tubes and straw fences.
The idea I tested was based on the concept of straw fences and how they stabilise loose substrates. Underwater, a similar structure might:
Stabilise the seafloor by reducing sediment movement
Improve water clarity by slowing currents and allowing sediment to settle
Provide structure for marine organisms to attach to or shelter in
To test this, I needed to see whether natural fibres could be planted in the seafloor and remain in place. I chose tī kōuka (cabbage tree) leaves because they are tough, fibrous, and slow to decompose on land.
20 tī kōuka (cabbage tree) leavesThe triangle marker on the Ōkahu Breakwater
I collected ten live leaves and ten dead ones, plus two extras to bind them into bundles. The leaves were 67–80 cm long. I kayaked to the triangle marker on the Ōkahu Breakwater (-36.84498185012416, 174.8125985293282) and selected a site 10 metres north of this point, where the substrate shifts from broken shell (common around the piles) to soft mud.
On a calm, high-tide day 25 March 2025, I dived to 5-6 metres and planted the leaves vertically into the mud using a 2 × 2 cm, 40 cm wooden stake, driving each leaf 15–20 cm into the sediment.
The live leaves were planted in a cluster with 5–10 cm spacing. The dead leaves were placed in a similar cluster 1 metre east of the live group.
10 live leaves, Note the substrate was a little firmer here than were I planted the dead leaves.10 dead leaves, 1 meter east of the live leaves.
My speech to commissioners in the Environment Court on including fishing controls in Waikato Regional Council’s Coastal Plan. There was some mention of tubeworm mounds beforehand so I also gave the commissioners an off-the-cuff intro to these ones which was appreciated.
The ecological imbalance caused by overfishing kōura (spiny rock lobster) in CRA 2 has led to the proliferation of kina barrens, devastating kelp forests along Northland’s east coast.
Kelp forests in the Hauraki Gulf could be worth up to USD 147,100 per hectare annually, far exceeding the $10.17 million export value of CRA 2. Kina barrens, by contrast, provide no ecological or economic value.
Fisheries New Zealand’s reliance on biased data, such as Catch Per Unit Effort (CPUE), underestimates kōura depletion. Independent research shows kōura populations, even in marine reserves, are well below natural levels.
The proposal to close commercial and recreational kōura fishing in the inner Gulf for 10 years is the largest fisheries closure ever suggested for the Hauraki Gulf Marine Park. However, fisheries independent data shows it’s not enough.
A new biomass target is precedent-setting and a significant step for Ecosystem-Based Management initiated by Sea Change – Tai Timu Tai Pari. A 3x BR target is essential to control kina populations, halt the spread of kina barrens, and restore productive kelp forests.
Independent data must be prioritised, and a precautionary approach adopted, including a full closure of the CRA 2 fishery. Further delays will only worsen environmental and economic losses.
I don’t know how to solve this problem, so I am writing it up as a public brief for people smarter than me.
Brief for restoring extremely degraded seafloor ecosystems
Soft sediment marine ecosystems support diverse and productive biogenic habitats like shellfish beds, sponge gardens, tubeworm fields, and bryozoan mounds. Direct impacts such as mobile bottom contact fishing, and indirect impacts such as sediment pollution, reduce the function of these habitats. Stopping or reducing the impacting activities can help the habitats recover naturally over many decades. Active restoration (like mussel and oyster seeding) can be done in areas where the habitat is not recovering naturally; however, some environments can be too degraded for these methods.
Problem
In my opinion, at least tens of square kilometres of the Hauraki Gulf Marine Park are too degraded to restore with known techniques. In these extremely degraded areas, the seafloor is very soft, deep mud. It’s not lifeless; there are burrows and infauna present. But the areas would be more diverse and productive if the seafloor was less soft with less sediment in suspension. Even when pollution input has been reduced in these extremely degraded areas, legacy sediment is constantly resuspended – choking filter-feeding animals and smothering photosynthesising plants. It is difficult to visually convey the condition of these ecosystems, as visibility is usually less than 30 cm on a good day.
Solutions
To increase biodiversity in these areas, the benthic enhancement method must be low-cost at scale. This means traditional erect concrete and steel structures are not likely to be the solution. In my opinion, resurfacing the seafloor with demolition rubble or quarried aggregate is too extreme because it kills all the infauna. Anything heavy deployed will immediately sink into the mud, anything lighter than the mud will quickly be covered by sediment. A smarter solution might contain one of these elements:
Local pits or trenches to collect the most mobile sediment.
Dispersed erect artificial shellfish (think horse mussels) to slow benthic currents and allow sediment to fall out of the water column in fields or fences.
Regular deployments of waterlogged woody debris.
Biological concrete structures that grow using elements from the local environment.
Hardened local seafloor sediments (think mudbrick or mudcrete).
Growing dense algae at the surface which will 1) slow currents and surge to reduce resuspension 2) drop fragments for sequestration, feeding invertebrates, collecting sediment and seafloor hardening.
Caution
While these solutions will restore some ecosystem function they will not restore the original ecosystems. Hard surfaces will likely be first colonised by invasive species and the new habitat will offer more ecosystem services but be novel / new. We must first halt the destructive activities that degraded the seafloor ecosystems.
I made these graphics for Auckland Council to show people how to make healthier and more resilient waterways. It’s awesome to see informed leadership on attitudes to rural and urban land use.
Artificial reefs have significant potential to boost fish populations, even surpassing pre-fished levels or what is possible in marine reserves. However they have a checkered history overseas, with many reefs:
Failing to restore native biodiversity to levels of those of conserved natural reefs (Bracho-Villavicencio et al. 2023).
Creating hard surfaces which are favoured by invasive species (these species often travel to new areas on hard structures) (Gauff et al. 2023).
Created as a convenient way to dispose of something which pollutes the marine environment (E.g. UnderwaterTimes.com 2006). They can also attract polluting activities (Zhang et al. 2019).
When considering building an artificial reef, it is crucial to determine whether it will provide additional habitat to support reef communities or merely function as a Fish Aggregation Device (FAD). Like artificial reefs, FADs are man-made structures which are attract fish to a specific area by providing habitat and shelter for marine life. The problem with FADs is that they decrease local fish populations by concentrating them in one area where they are easily targeted by fishers (Cabral et al. 2014) as illustrated below.
To define the size of the habitat required to avoid FAD functionality, you could base it on the home range of each fish species you want to increase. For example:
The most studied fish in the Hauraki Gulf is the tāmure / snapper, which show high site fidelity to reef habitats. Tāmure in deep soft sediment habitats are quite mobile, with a median distance of 19 km, and some movements up to 400 km. In contrast tāmure in shallow rocky reef habitats have restricted movements, with a median distance of 0.7 km (Parsons et al. 2011). You can see this in small marine reserves with shallow rock reefs, such as the 5 km² Cape Rodney – Okakari Point Marine Reserve (Goat Island), which effectively increase the size and abundance of this species. Additionally, tāmure around mussel farms have been found to be healthier than those in surrounding soft sediment habitats (Underwood 2023). The studied mussel farms were near rocky reefs and covered about eight hectares (200 x 400m).
This means fished artificial reefs should be deployed at hectare scales to avoid acting as population sinks. For tāmure, an area about the size of eight rugby fields is a considerable undertaking, but to avoid your reef functioning as a FAD for fishing, it is essential to spread your structure over a large area. If this sounds more like ‘habitat enhancement’ than an artificial reef, then perhaps that is a better way to frame your design. Of course, this consideration is unnecessary if your artificial reef is not fished.
REFERNCES
Bracho-Villavicencio et al., 2023 https://doi.org/10.3390/environments10070121 A Review of the State of the Art and a Meta-Analysis of Its Effectiveness for the Restoration of Marine Ecosystems. Environments.
Gauff et al., 2023 https://doi.org/10.1016/j.jembe.2023.151882 Unexpected biotic homogenization masks the effect of a pollution gradient on local variability of community structure in a marine urban environment.
Underwood et al., 2023 https://www.aquaculturescience.org/content/dam/tnc/nature/en/documents/aquaculture/AquacultureHabitatComparativeReport.pdf Habitat value of green-lipped mussel farms for fish in northern Aotearoa New Zealand
In an article titled “Shane Jones sets sights on killer kina – An industrial grade problem” the Fisheries Minister Shane Jones is quoted as saying “Some ecologists say it’s related to overfishing but the evidence is not conclusive in that regard. From my perspective as Fisheries Minister, I can provide some practical tools through the law, to allow local communities to go and cull them.”
The consultation document on the proposed tool provides a definition of kina barrens and clearly explains they are formed by a “low abundance of predator species“.
I was concerned that our Fisheries Minister could be so poorly informed so I asked Fisheries New Zealand for:
Any recent New Zealand research findings that corroborate the Minister’s statement that the cause of kina barens is inconclusive.
Any ministerial advice provided to the Minister that supports his statement about the formation of kina barrens.
Fisheries New Zealand have clearly supplied conclusive evidence that kina barrens are related to overfishing. They have not supplied any research findings that corroborate the Minister’s statement that the cause of kina barens is inconclusive. I don’t know where Shane Jones is getting his alternative facts.
Of interest (and to his credit) Shane Jones has asked staff to increase the pace on starting pre-engagement to identify voluntary and/or regulatory measures to support increased large rock lobster abundance in areas with kina barrens (p25, detail on p21). Progress on this workstream seems to have been withheld and its clearly delayed as the other (extractive) measures mentioned have been consulted on. Two new measures that staff have suggested include a Maximum Legal Size Limit for Lobster and ‘Catch Spreading’.
While we wait for government action, millions of kina relentlessly devour our kelp forests.
