Cavendish Corporate Finance has been instructed to handle the matter, and NESI's major customers have been informed.
Although customers have been told, it is understood that no presentations have been made to potential investors yet.
NESI is believed to be open to how much of its business it will ultimately sell, with deals expected to start around the 25% mark.
NESI supplies both fresh and frozen fish to the UK multiples, and is one of the UK's largest importers of fresh tuna. Its key retail customers include Sainsbury's, Waitrose and Marks & Spencer.
Cavendish senior partner Howard Leigh said he was "delighted" to be working with a company that was as "fast growing" as NESI, especially as Cavendish had recently carried out similar work for other businesses.
NESI did not comment.