Reflections on the mechanical harvesting of kelp: science, environmental change, wider thoughts and a way forward

by Juliet Brodie

9th November 2018

Revised 19th June 2019


Kelp forests are biologically remarkable marine habitats.  They occur in cold and temperate shallow coastal waters and are one of the most productive habitats on the planet (Birkett et al., 1998). Supporting an immense diversity of organisms (Christie et al., 2003), acting as nurseries for fish and dampening the erosive effect of the waves, these underwater forests are vital for the functioning of these shallow water ecosystems. Yet, these wild populations are highly sought after and the main source (with the exception of some farmed kelp) of raw material for alginates. These polysaccharides (carbohydrates) are used in a wide range of applications, including textiles, food additives, and demand is increasing with interest in their use in the biomedical and bioengineering fields (Peteiro, 2018). At the same time that the demand for alginates in increasing, kelp forests are declining in many parts of the world due to environmental change and anthropogenic activities. It is almost certainly true to say that nothing in the marine environment is pristine but unharvested kelp forests, which are considered to be the underwater equivalent to terrestrial rain forest (Birkett et al., 1998), might be one of the best examples of a natural habitat.

In view of increasing pressure to mechanically harvest wild kelp from kelp forests around Britain and Ireland, in this article I summarise scientific evidence relating to the impact of mechanical harvesting of the kelp beds, reflect more widely on the subject and present a way forward.

Studies on the effects of mechanical harvesting

Much research that has studied the effects of mechanical harvesting on kelp has been undertaken in Norway where kelp has been mechanically harvested since the 1970s.  Harvesting takes place on a five-year cycle which does not allow kelp forests to fully regrow before the next harvest.  In one study (Christie et al., 1998), evidence showed that the kelps made a new generation within the five years.  However, neither the kelp epiphytes nor the resident amphipods and isopods, which are important food sources for gadoid fish (e.g. cod), had fully recovered before the next trawl.  In another study (Lorentsen et al., 2010), newly harvested areas had 92% less small gadoid fish compared to unharvested areas.  Kelp harvest removes the habitat for coastal fish and their food base which suggests that the number of juvenile fish remain low for several years after harvest (Lorentsen et al., 2010).  In summary, the harvesting cycle is too short to allow for full recovery of the kelp forest (Steen et al., 2016).

A study undertaken in Scotland between 1991 and 1994 (Bunker, personal communication), which monitored the effects of mechanical harvesting trials, concluded that recovery of the forests varied depending on the environmental conditions of a particular area.  Sand scour at one of the study sites was noted as a particular impediment to the survival and establishment of small kelp plants left behind after harvesting.

Effect of repeated harvesting

Kelp forests are very much a part of the marine landscape in the rocky coastal waters of Britain and Ireland.  Here, Laminaria hyperborea, the species that dominates the underwater forests, is in the goldilocks zone, right at the heart of its distribution in the North East Atlantic (Yesson et al., 2015a).

Repeated harvesting of the kelp forest will have the tendency to homogenise the gene pool, making the kelp forest potentially less resilient/resistant to environmental change.  This maybe further exacerbated by increasing storminess as a result of climate change. There is very little data on the population structure of kelp species around Britain.  A small, pilot population study undertaken by my research group on populations of the related kelp, Laminaria digitata, indicated that, although there was gene flow within populations in the south of Britain, there was little or no gene flow between the populations. If this is also the case for L. hyperborea, then we risk losing genetically distinct populations (as has already happened for other organisms; see Brodie et al., 2009 and references therein; Yesson et al., 2015b) and these might be the ones that have the ability to adapt and survive to climate change.

Effect of environmental change on kelp

Sea surface temperatures around the coast of British Isles have, on average, increased by approximately 2°C over the last c. 40 years (Yesson et al., 2015b) and it has been predicted that kelps will retreat at their southern edges in the NE Atlantic (see Brodie et al., 2014 and refs therein). Recent evidence unequivocally demonstrates that this is occurring in this region in response to both climate and non-climate related stressors (e.g. Smale et al., 2013). Analysis of changes in the abundance of large brown seaweeds around the British between 1974 and 2010 revealed significant declines in the south for kelp species (Yesson et al., 2015b). Laminaria hyperborea was one of those species, although the data indicated that it was stable or increasing in the north.

Here, it is also important to consider the different stages of the life history of these species in relation to temperature. The large kelp plants are the sporophytes.  These produce microscopic zoospores which develop microscopic male and female gametophyte that produce gametes which after fertilization grow into new sporophytes (Birkett et al., 1998).  Laminaria hyperborea is fertile in the winter and early spring and the different parts of the life history develop optimally at different temperatures (Müller et al., 2008, Lüning, 1990).  Some aspects of the life history may favour warmer temperatures but other parts maybe negatively affected.

Wider thoughts

A recent remark that I had made that I was heartbroken when I read of a proposal to mechanically harvest the kelp off the coast of Scotland was not deemed to be a scientific reaction.  That’s okay, I’m a human being. My response is that when I read how the kelp would be harvested it reminded me of all the devastating impacts humans have made on the planet.  I thought about Rachel Carson and the fierce opposition her work engendered when she published Silent Spring in 1962. I was also prompted to think about the length of time it has taken on the land in Britain for conservation and the environment to be taken as mainstream.  My hope is that we would no more contemplate destroying kelp forests than we would chopping down ancient woodlands on the land. And whilst the designation of Marine Protected Areas now is a huge advance on the three Marine Nature Reserves following the 1981 Wildlife and Countryside Act, we are so far behind in caring for the marine environment.

The way forward

There is a way forward to obtain the products we need. We know how to grow kelp and we know that integrated kelp aquaculture can ensure a sustainable supply of raw material, and be good for the environment and the economy. The technology is here and with thought to the environment and people, sensible investment, vision, innovation and patience there is great potential for an aquaculture industry without having to destroy even a small part of these precious kelp forests.


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Peteiro, C. (2018). Alginate production from marine macroalgae, with emphasis on kelp farming. In: Alginates and their biomedical applications (B.H.A Rehm, M.F. Moradali eds). Springer Series in Biomaterials, Science and Engineering 11. Springer Nature Singapore Pte Ltd.

Smale, D.A., Burrows, M.T., Moore, P.J., O’Connor, N. & S. J. Hawkins. 2013. Threats and knowledge gaps for ecosystem services provided by kelp forests: a northeast Atlantic perspective. Ecology and Evolution 3: 4016–4038.

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Yesson, C, Bush, L., Davies, A., Maggs, C.A. & Brodie, J.  (2015b). Large brown seaweeds of the British Isles: evidence of changes in abundance over four decades.  Estuarine and Coastal Shelf Science 155: 167-175.


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