As the environmental and financial costs of red meat, and white meat for that matter (reed: pork, beef, lamb, venison etc. for red meat and chicken, turkey and other birds for white meat) continue to rise globally, seafood of all kinds are becoming more popular in order to maintain a healthy diet and obtain essential minerals like omega-3 fatty acids and zinc without resorting to supplements.
Pescetarianism (pes-e-ter-I-an-ism) is a specific diet which is similar to vegetarianism except with the addition of fish and seafood, but not, of course, other types of animal. Prominent practitioners of this diet include X-files actor David Duchovny, Felipe VI's wife, Queen Sofia, and Smashing Pumpkins lead singer Billy Corgan. This diet was also practiced by many orders of monks around the world in medieval times, who did not eat other meat for religious reasons. Catholics were once required to adopt pescetarian diets on Fridays, and still do so on Ash Wednesday and every Friday during Lent. This diet has been catching on considerably in the last few years, although it doesn't receive as much publicity as moves towards vegetarian and vegan diets in Britain and elsewhere do.
Fishy diets restrict eating options less whilst at the same time enabling weight loss by drastically reducing saturated fat content and cholesterol levels, preventing exposure to common meat-borne diseases such as BSE and salmonella, and increasing intake of essential minerals such as iron, phosphorous and selenium (found frequently in molluscs like mussels). This, in turn, reduces heart disease and cancer rates, even compared to vegetarian diets, and allows for better muscle-build up at the same time.
However, you can manage this by being pescetarian some days and vegetarian/vegan some days, as I do. I keep one day per week, sometimes two days per week, as a vegetarian/vegan-only day which is also useful for cutting costs, as some kinds of seafood (especially crustaceans like crabs and lobsters) can get very expensive, even in the UK which has a strong "fish and chips" tradition. You should also choose your seafood carefully within a pescetarian diet, and choose fish with low levels of mercury and high levels of omega-3. This includes not only (wild) salmon but also such things as scallops and halibut; the intake of cod and mackerel, in particular, should be limited, especially when cod stocks are being depleted around the world faster than they can replenish. A balanced seafood diet also ensures a balance of essential minerals in addition to those you get from fruit and vegetables, and most importantly variety so you don't end up being bored with a fish-heavy diet.
In restaurants, fishy meals also contain higher quantities and qualities of vegetables; peas are a stable of British fish and chips meals but meals based on salmon and sea bass are often served with such vegetables as seaweed and potatoes rather than chips. So it's also healthier for when you eat out on special occasions as well as during your everyday life.
A pescetarian diet also lowers your carbon footprint compared to a standard meat-filled diet. White meat, and red meat more so, are big offenders in terms of greenhouse gas emissions, and are notoriously inefficient in agricultural terms. It can take as much as 450,000 litres of water per tonne of beef or pork, and beef production also contributes to increasing methane emissions, which are five times more harmful than carbon dioxide emissions. Not only that, but cows, sheep, pigs, and chickens are generally cruelly raised in factory farms which produce a breeding ground for disease. Meanwhile, most seafood can and should be bought wild (avoid farmed salmon-they are just as bad in these respects) which will lower carbon footprints, water usage, and grain usage dramatically, as long as acidification of our oceans is stopped and water pollution is tackled.
Therefore, if you don't fancy going fully vegan or vegetarian, and want a healthier diet than your current one which is still enjoyable, a pescetarian diet is definitely one to try.