Well, it depends on the seafood. Every trophic level upwards means a 90% loss of the previous foodweb.
Don’t understand? Let The Conversation explain.
A comparison between the way we exploit marine and terrestrial food webs can illustrate the problem. The efficiency in the transference of organic matter up the food webs is typically below 10% (i.e. an organism grows in weight by less than 100 g for each kg of food ingested) (note 1). This means that production is dissipated as it moves up in the food web, so for every ton of plant production introduced in the food web, we can harvest up to 100 Kg of herbivores, 10 Kg of carnivores feeding on herbivores, and so on. On land, we eat largely plants and herbivores, with a few omnivores and very few carnivores (e.g. dogs in some Asian countries). Accordingly, the mean food production on land has a mean weighted trophic level of 1.008, where 1 is a plant, 2 is a herbivore, etc.
In contrast, we eat many large predatory fish, such as tuna or sharks, that sit high up in the marine food web, which has many more steps than the terrestrial food web does. For instance, tuna has a trophic level of about 5 (i.e. four other steps in between plankton production and tuna), which is unparalleled in terrestrial food webs, equivalent to imaginary monsters eating wolf-eaters (Duarte et al. 2009).
This means that the production of 1 Kg of tuna (trophic level 5) requires about 100,000 tons of plankton production, which is equivalent to the annual primary production of 5 hectares of ocean surface. If we, however, consume 1 kg of small pelagic fish, such as anchovies (trophic level 3), we are effectively harvesting the annual production of an ocean surface 100 fold smaller.