Two main areas of interest: how to keep fishing is sort of obvious. But here are some notes on sustainable marine vegetarianism. (see also sustainable seafood finance initiative)
It seems reasonable that, in the same way that eating land animals to a great extent is generally unsustainable, so is the eating of water animals. Add to this the (non) existence of the “moral barrier of the water's surface” one can think that marine vegetarianism might be useful.
There is the claim that the Australian King Island Dairy products are so excellent because the cattle are (partially) fed on kelp. Seaweed is offered as a nutritional supplement for cattle, as well as being used in chemical processes.
There is a whole chapter on using algae (Ch 6, page 125) in Sailing the Farm. Link
There are two Dutch enterprises that have received some attention.
Other harvesters / farmers:
A short list of edible weeds with notes, initially extracted from Sea spaghetti in the supermarket: the unstoppable rise of seaweed and extended with information as it has arisen. Note that Sailing the Farm has a forager's guide for seaweeds. Black and White, but okay!
=Green seaweeds= Sea Lettuce: Leafy and sorrel-like, it can be added fresh or rehydrated to salads, or dried and used in butter to accompany fish dishes.
Gutweed: Bright green with tubular fronts, it can be fried to create Chinese crispy salad.
Spirulina is not a seaweed, but an algae. However very protein rich (70 percent dry weight, higher than canned tuna)
=Red seaweeds= Dulse: Maroon-coloured with a smoky, meaty flavour when dried, it can be included stews and soups for added depth. Dronfield suggests toasting flakes in a pan, then crushing and sprinkling over nuts or hummus.
Purple Laver: Also called nori, it’s the star ingredient in laverbread. The Cornish Seaweed Company suggest combining it with yoghurt to create a salad dressing or dip.
Carrageen: Varying in colour from a purple-red to yellow, it’s primarily used as a thickener and setting agent in everything from panna cotta to seafood mousses. It can also be used to flavour beer.
=Brown seaweeds= Sea spaghetti: Relatively mild, this makes a good addition to pasta dishes, and could even be a gluten-free substitute to pasta. Andy Appleton recommends deep-frying it, fresh or rehydrated and lightly coated in semolina, to use as a garnish for fish dishes.
Kombu: With thick, dark green leaves and a savoury, umami flavour, kombu is an essential ingredient in Japanese dashi stock. The Cornish Seaweed Company recommends turning dried flakes into crisps by soaking them and baking them in the oven, or using them in pesto.
Channel wrack: Also known as channelled wrack, this is green and sprig-like. McKellar suggests blanching it and serving it as a vegetable accompaniment to meat. Tom Kitchen has a recipe for poached turbot with saffron broth and channel wrack, while Loubet includes it in a recipe for seaweed butter.
Bladderwrack: So-called because of the bulbous “bladders” that run along its dark green leaves, it can be added to stocks or stews – or, Mckellar suggests, stir-fries.
Kelp powder is apparently a fine salt replacement.
Desmarestia contains a significant amount of sulphuric acid.
Lyngbya is toxic. Very fine hairs, often wrapped around other seaweeds.
Hijiki / Hiziki can have significant levels of non organic arsenic. LINK