Fish provides about one-fifth of the world's animal protein of which marine fisheries is the largest contributor. Fish is a larger proportion of the supply of animal protein in less developed countries, but there are very wide variations from country to country. However, the per person consumption is higher in nations that are better able to afford its higher cost. As populations grow, wealthier countries are able to purchase fishery products from around the world, making less available to those who cannot afford it. There is a nutrition impact, and climate change could exacerbate health and nutrition problems in some coastal nations that may endure decreased production from the sea.
Several types of natural toxins are found in the ocean. These include "red tides" which get their name from the toxic dinoflagellates that become so abundant that they give a red color to the sea. Sometimes they do not need to be so concentrated to discolor the sea before they cause problems. Consumers of clams, mussels and oysters are the occasional victims of this toxin. Ciguatera refers to a toxin that is produced in algae and then is concentrated by algal grazers. As predators concentrate the toxin further at each step in the food chain, it can get to dangerous levels. Ciguatera is usually associated with warm waters, and larger fish such as groupers and barracuda. These types of toxins can reach lethal doses or cause other temporary or permanent damage, when sufficiently concentrated. Marine mammals and birds are often affected in the same manner as humans. These toxins cannot be reduced through cooking. Resource managers and local fishers usually are able to avoid the harvesting of seafood containing these toxins. Global warming could lead to these toxins becoming more widely distributed and may cause changes in where they occur to the extent that local knowledge will not be sufficient to protect against their harvest.
Rising sea levels are a concern to the healthfulness of the food supply. In coastal areas flooding septic tanks or landfills or other disposal sites may lead to contamination of seafood production areas. Temporary impacts of this nature are often found around the world when large storms strike the coast. Local governments can be successful in protecting against the harvesting of unsuitable products. In some cases, the marine seafood species are also susceptible to unfamiliar pathogens and to heavy metals.
Cholera, Vibrio cholerae, can be distributed around the world in ship ballast water. Warmer temperatures enhance the ability for new strains of Cholera to move to and colonize new areas. Cooking can destroy cholera, but many people prefer to eat clams and oysters raw, thus increasing their susceptibility to the disease.
As stocks are redistributed with climate change, local fishers will likely pursue them from their usual port for some time. As the distances grow, the seafood will lose some of its freshness while in transport back to port. In some cases, particularly where refrigeration is rare or expensive, the quality of the seafood may decrease. This may lead to both unhealthy seafood in the market and to consumer resistance, once a bad meal is encountered.