Marine microfibres: less plastic than predicted
They don't mention it explicitly. But they don't classify further than "natural" and "synthetic". And, may I say, animal husbandry as well as agricultural use of the producers of such natural fibres (e.g. jute, wool) are under human control as well. They are introduced into the ocean via waste-water, rivers, wash from the continents.

The study used samples from surface waters, if I understand it correctly from reading superficially. They say that too many fibres are prematurely seen as "microplastic" while they may actually be natural. 

Other studies have come to the conclusion that the missing plastic-fibre in surface waters rests deeper in the water column, or may come in small(er) sizes than sampled.