About 40% of the cereals and legume grains produced every year are used to feed farm animals.
Many commentators argue that this is highly resource inefficient as around 70% of the human edible energy produced is lost in this process due to metabolic losses in the animals (Papargyropoulou et al., 2014). 
For example, typically 4 kg of cereals and legumes is needed to produce one kg of edible poultry meat (Röös et al., 2014), although the quantity varies by system. 
By contrast raising livestock on by-products that humans can’t or don’t want to eat or waste is often considered to be a resource efficient way of producing protein for human consumption, as is raising livestock on biomass from grasslands unsuitable for the production of human edible foods e.g. natural or semi-natural pastures or other marginal grasslands. 
This approach of letting the ecological resource capacity be the constraining factor for livestock production, i.e. to restrict livestock production to feeds not suitable for human consumption has been around for some time and was coined as ‘producing livestock on ecological leftovers’ in a paper by FCRN’s Tara Garnett in 2009 and explored further as one of the scenarios presented in Gut Feelings (2015). 

This concept of restricting livestock production is attractive in several ways. 
From a food security perspective it has been argued that diverting grains from animal feed to human food would increase available food supply. 
Further, the concept is compatible with principles of agro-ecology (e.g. adjusting production to context-specific conditions and making use of local resources) and makes use of pastures for food production which a vegan diet would not. 
It also yields a diet which would contain meat and dairy; products that most people appreciate in their diets. 
However, despite the popularity of framing sustainable meat production in this way, there are few studies that have looked into how a diet based on this concept would actually look like and what the environmental impacts of such a diet would be. 
This said, recently Schader et al. (2015) modelled various scenarios of global livestock production in which reliance on food-competing feed crops was progressively reduced. 
They found that greenhouse gas emissions and other environmental impacts would be reduced (compared with the reference scenario), and enough food would be supplied although only 11% of protein would come from animal sources, as compared with 38% in the reference scenario. 

In a study published late last year my colleagues and I made an attempt to apply this concept to Sweden and see what kind of diet that would give (Röös et al., 2015). 
As a first step we formalised the general ecological leftover (EL) principles a bit further. 
These are: 
. Arable land should primarily be used for the production of plant-based food for humans. 
. Livestock should be fed biomass not suitable for or wanted by humans. 
. Grasslands should be used for livestock production if grazing can be justified by reasons other than meat and milk production, e.g. biodiversity conservation, providing a livelihood for vulnerable populations etc.

Since 14% of seafood production goes to non-food uses, I was thinking how useful could it be to transfer this principles to seafood as for example:
. Ocean waters should primarily be used for the production of seafood for humans (not to feed other animals).
. Fish from aquaculture production (including tuna farming - fattening) should be fed biomass not suitable for or wanted by humans (tuna farming in the Mediterranean are fed with sardine and mackerel*).
. Marine resources should be used for aquaculture production if this resources can be justified by reasons other than human food, e.g. production of certain species that cannot be fished or do not have environmental negative impacts, use of aquatic area (3 dimension) to produce animals instead of land (there is no more wild land available to grow food).

Picture from the only set-net tuna fishery and fattening in Portugal taken from here.

* Bluefin tuna are fed mainly with a mixed diet composed principally of a variety of smallpelagic species including sardine (Sardinella aurita), pilchard (Sardina pilchardus),round sardinella, herring (Clupea harengus), mackerel (Scomber japonicus), bogue (Boops boops) and squid (Illex sp.). The proportion and volume of the feed variesamong the different countries and from farm to farm...Feed conversion ratios (FCR) are generally high around15–20:1 for large specimens and 10–15:1 for smaller fish.