Mussels: a font of sustainable protein

Last night I have decided to celebrate and treat myself to dinner out, and, when in Zeeland, mussels are a must!
You should also know that mussels are basically my job here too - so I can say that I have eaten my work! (What a weird thing to say... I know!)

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Anyway...
First of all let's have a look at mussels nutritional values

Per 100g of uncooked mussels (source Nutrition data)

Kcal 86
Carbohydrates 3.7g
    of which sugars 0g
    of which fibers 0g
Fat 2.2 
    of which saturated 0.4
                     monounsaturates 0.5
                     polyunsaturates 0.6
Proteins  11.9 g

They are also an important source of Omega3 and Omega6 as well as many minerals and vitamins, although be careful of their cholesterol content (28mg)
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Mussels are primarily cultivated, on ropes or as bottom cultures. Cultivating mussels does not require the provision of a feed, and it is therefore different to other farming or cultivation methods such as fish farming. Mussels filter the water for food, which also contributes to keeping our water clean. Useful food!! ​

How exactly are they cultivated?
Seeds (or baby mussels) are collected using spat collectors: for rope culturing usually spat is collected directly on the ropes where they will grow out to a commercially suitable size, while for bottom culturing seed is relayed on the seabed, and sometimes moved between suitable locations at different life stages of the mussels. Normally seed will take two years to mature. 
In certain locations, seed is collected from natural seed beds forming naturally on the seafloor. 

Is it really sustainable?
It is mostly sustainable. Seed collecting from areas of natural seafloor could cause some environmental impacts to adjacent habitats: for example if the seed bed is bordering an important habitat such as a biogenic reef. 
Moreover, rope culturing could cause impacts on the adjacent seafloor due to high organic input from the mussels to the bottom, and could cause aggregation of predatory species that would not otherwise be present in the area.
However, mussel farming can be MSC certified and I would recommend to get your mussels from a trusted source. 
I personally believe that even with these effects in mind it can be more sustainable than many other protein sources, including vegetarian sources such as soya beans! 

Also, I was very happy to hear that mussels producers over here (and maybe other places but I am unaware) are investing to study and find ways to limit the environmental impacts! It's great to see scientists and the fishing industry getting along :)