Is it wise to ‘mow’ our kelp forests???

By Rebecca McLeod 24/09/2009

Submissions closed on Monday the 21st for a proposal by the Ministry of Fisheries to enter giant kelp (Macrocystis pyrifera) into the Quota Management System. There is demand for giant kelp on the international market, and at least one Canterbury-based business is keen to develop a harvest of live kelp in the near future. On first thoughts, this might seem like a great idea — the kelp can be used for so many things: pharmaceuticals, food products, fertilizers, feed for aquaculture and farm animals… the possible applications appear to be limited only by the imagination.

However, as the story unfolded on Campbell Live on Monday night , it became clear that there are two sides that are clashing over the potential for kelp to be commercially harvested. One side comprises fishermen, scientists and environmentalists who are concerned that harvesting could lead to a loss of kelp forests and the multitude of species that live amongst them. The other side seems to be made up of a small group of entrepreneurs, who can see only benefits when it comes to kelp harvesting. Financial benefits. This is an unusual situation: it isn’t often that environmentalists and fishermen are united in their views. I feel that it is time to inject some science to the debate, to gauge whether the concerns of the objectors are legitimate.

There are currently two MFish proposals that are undergoing public consultation. In the North Island, it has been proposed that drift kelp (that which is floating unattached, or stranded on beaches) be allowed to be collected from all beaches by commercial operators under permit. Whilst such collection certainly has the potential for ecological impact, it is the other proposal that is attracting the most attention from marine scientists – the entry of live growing giant kelp into the Quota Management System, effectively opening the door to commercial harvesting of kelp forests.

Giant kelp grows up through the murk in search of light
Giant kelp grows up through the murk in search of light

In other countries, kelp forests are cut to one metre below the surface either by hand, or using ships equipped with cutting blades. It might seem appropriate then for the practise to be likened to mowing the lawns. Supporters of kelp harvesting here say that giant kelp grows one metre a day, therefore harvesting it will be totally sustainable. But Dr. Chris Hepburn, a marine botanist at Otago University, believes that growth at that rate, in New Zealand at least, is a myth. Despite many years researching Macrocystis growth around the South Island, and publishing on the topic1, he has never come across the original source of that figure, nor measured growth rates anything like it. It is quite likely that the one metre per day figure is an exaggerated value from research in California, where expansive forests are harvested. ’Frond elongation rates of 2-10 cm per day are typical for Macrocystis, while maximal growth rates of 40 cm a day may be possible when a kelp frond first grows up through the water column under perfect conditions. Frond rates really have little relevance to how Macrocystis will respond to harvesting – if you chop the top of a frond it stops growing. It’s the rate that a kelp individual can produce new fronds that matters from a harvest recovery perspective.’

Throughout the MFish proposal, references are made to the operation in California, with a suggestion that it provides a model upon which to base any future harvesting here. But is it wise to make comparisons between New Zealand and Californian kelp forests in this way? Dr Nick Shears is a marine ecologist who has recently returned to work in New Zealand from California. Having expertise in kelp forest ecology2, he is in an ideal position to make comparisons between the two. ’The giant kelp forests off California are world renowned for their size and productivity – forests span for miles along the coast, grow to depths of 30 m, and individual plants grow up to 60 m in length. In comparison, New Zealand kelp forests are a fraction of the size, the plants grow slower, are smaller and are less dense’. Dr Shears believes that given the differing sizes of kelp forests in California and New Zealand it is difficult to envisage a viable and ecologically sustainable fishery for kelp in New Zealand.

Kelp forests are highly productive, in that they provide a food source and a habitat for a diverse range of marine species, including many fish and invertebrates that form the basis of large commercial, recreational and cultural fisheries. For example, it is thought that crayfish are closely associated with kelp both as adults, and as juveniles, as larvae recruit into the canopy of the forest, and then make their way down the fronds to the seafloor. Blue cod, moki, trumpeter, paua, kina, and greenbone also inhabit kelp forests. Dr Hepburn says that this provision of habitat for other species is a key difference between giant kelp and every other marine species managed under the Quota Management System. ’To harvest a habitat is very shortsighted. Imagine a fishery is supported by a coral reef – does it make sense to allow someone to come in and remove the coral for commercial use and risk the fishery? Harvesting Macrocystis risks high value fisheries and ecosystem services provided by the kelp for an unproven product. At this stage it’s a no-brainer – Macrocystis is worth more in the water than spread over paddocks as fertilser, and we simply don’t know enough about how kelp forests will respond to harvesting.’

A multitude of animals live on and among the forest habitat that giant kelp provides
A multitude of animals live on and among the forest habitat that giant kelp provides

Despite the Californian kelp forests having a century-long history of harvesting, questions are now being raised as to the sustainability of the practise. The annual takes reported by the California Department of Fish and Game have regularly exceeded 150,000 tonnes, but have dropped to less than 5,000 tonnes per year since 2006. This sudden decline reflects the major harvester ceasing operation due to a lack of economic viability, which is caused by competition from exporters in Mexico and China. The exit of this harvester from the market will probably be of much relief to environmentalists in the region, who have observed a dramatic reduction in the extent of kelp forests over the last 35 years3. Scientists from the Scripps Institution of Oceanography have identified many environmental pressures that could be causing this decline, including changes in currents linked to the El Ni̱o РSouthern Oscillation, increased abundances of sea urchins due to overfishing of their predators, sedimentation, pollution and kelp harvesting. When it comes to kelp forest health, teasing apart the importance of these different factors is proving to be an incredibly difficult task3. One thing is for sure, kelp forests are under pressure, and they are threatened in many places around the globe. In Tasmania, a more than 50% decline in extent of giant kelp over the past 60 years has prompted the State Government to consider listing Macrocystis kelp forests as an endangered habitat type. They are already listed alongside the Great Barrier Reef as a Special Marine Area. Whether kelp has suffered similar declines in New Zealand has not been monitored by scientists, although there is anecdotal evidence of kelp forest loss in the Otago region.

Back in New Zealand, Dr Hepburn has many concerns about what might happen if you harvest kelp. Unlike California, the East Coast of the South Island has a lot of sediment in the water, which acts to limit the amount of sunlight reaching kelp, and frequent storms act to resuspend the sediment into the water column. The growth of Macrocystis kelp is limited by the amount of sunlight1, meaning that in times when there is a lot of sediment in the water, such as following rainfall or a storm, kelp growth rates slow. ’My concern is that were a harvesting event to coincide with a period of high sediment load in the water, the kelp would not receive enough light to promote regeneration and the forest would therefore be at risk of dying off.’ There is also the issue of the invasive kelp Undaria. This highly opportunistic species may take advantage of a temporary reduction in giant kelp, and out-compete the species. Such a shift in kelp assemblages, would not necessarily correct itself over time. There are a lot of uncertainties about how kelp, and the species associated with the forest habitat it creates, will respond to harvesting, yet surprisingly little research that could help to address these concerns. The only relevant study was conducted in Akaroa Harbour — a wave sheltered environment that is very different from the exposed coastlines along the extent of the proposed harvesting areas. To progress with harvesting along the East Coast of the South Island appears to be incredibly risky, particularly when the size of associated fisheries are considered. Is it wise to risk the multi-million dollar blue cod and crayfish fisheries, for a market that is yet to be tested?

Many prominent marine scientists have voiced their concerns with the Ministry of Fisheries about the current proposal, along with fishermen, environmental groups and iwi. This large, diverse group of objectors is at odds with one small group of supporters, whom stand to benefit financially from a positive outcome. It will certainly be interesting to see the outcome of this process of public consultation — will the Ministry listen to the voices and experience of the opposition and consider the lack of scientific testing of the sustainability of kelp harvesting when they make their decision? Furthermore will the Ministry adhere to their own management objectives – to protect and maintain habitats that are significant for fisheries? It certainly seems to me that there is a lot at stake.


1. Hepburn CD, Holborow JD, Wing SR, Frew RD, Hurd CL (2007) Exposure to waves enhances the growth rate and nitrogen status of the giant kelp Macrocystis pyrifera. Marine Ecology Progress Series 339:99-108

2. Salomon AK, Shears NT, Langlois TJ, Babcock RC (2008) Cascading effects of fishing can alter carbon flow through a temperate coastal ecosystem. Ecological Applications 18(8): 1874-1887

3. Dayton PK, Tegner MJ, Edwards PB, Riser KL (1998) Sliding baselines, ghosts, and reduced expectations in kelp forest communities