Shellfish. Considered a delicacy by some, a necessity of life by others. Some societies are entirely reliant on shellfish farming, where simple animals such as mussels and oysters are st the base of the economy. And we have been told shellfish farming is a relatively sustainable activity and these simple animals are an optimal healthy source of proteins and essentials fats.
Yet there is an aspect nobody really think of when enjoying that pot of mussels alla marinara or slipping down that lemony oyster: what happens to their shells??
Shells are made by the animals as they grow and not only they need a great energetic investment to be made, energy that is taken away from other processes, but also take up resources such as minerals from the surrounding waters.
So... a waste product for us which is, well, a waste to throw away to landfills!
In a recent review paper (1) current shells usage are highlighted (although most shells still do end up in landfills so let’s not be fooled by the words ‘current uses’ - it is still fairly limited!)
Livestock feed supplement: okay, this to me personally seems a bit weird, but shells are a great source of calcium carbonate that can be added to poultry diets to enhance growth capacity and egg laying..
Agricultural liming agent: again, due to the high calcium carbonate, shells can be used as an alkalinising agent (reducing acidity) in agricultural soils. It is also allowed as part of organic agricultural practice in some countries.
Construction material (aggregates): of course, due to safety regulations, not all materials can be replaced by shell aggregates but can be incorporated in some simple concrete structures..
Biofilter medium: can be used to treat wastewaters as they can absorb heavy metals such as zinc, lead and cadmium.. although this is not yet used widely and research is still widely ongoing
The same review also proposes other uses that could be employed in the future such as:
Green roofing substrate
What about returning shells to the marine environment?
Not only oceans are becoming increasingly more acidic- thus shells return could contribute to alkalinisation by returning some of the lost calcium into the water as shells dissolve, but could serve another important function which I would like to discuss below
Provide hard substratum: in a world where we have been destroying (yes, really, destroying) seafloors everywhere with a significant loss of 3D complexity, shell addition could contribute important hard substratum to kickstart settlement and development of a diverse community.
Moreover, this hard substratum, providing some level of complexity, might be harvested back by the same aquaculture practitioners. For example mussels are known to use complex substratum for attachment which could protect them from hydrodynamic and biological stressors, and could be used as an advantage to relay initially lower densities or seeds on the sea floor (thus minimising initial costs) while obtaining high returns! And that is just using own waste.... talk about recycling eh!
Similarly it could be used for restoration projects of biogenic reefs, either in aggregates form or direct, depending on the species to be restored and requirements in term of substratum, bringing great savings to these expensive projects!!
So yes, all in all shellfish can be a sustainable font of proteins, but as always there is room for improvement of the practice!
1. Morris JP, Backeljau T, Chapelle G (2018). Shells from aquaculture: a valuable biomaterial, not a nuisance waste product. Reviews in aquaculture 0:1-16
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