The big fish review: part three, the open ocean

The slightly overdue third and penultimate part of my sweeping survey of all things fishy. There are two remaining ecological disaster zones that are as yet unmolested by my withering gaze, the open ocean fisheries (us fishy types call them pelagic) that hold our swordfish, tuna and marlin as well as the silty mangrove swamps in which baby fish and coastal prawns grow up before commuting out to the big sea. Both are critical ecosystems in slightly different ways, and both are being decimated by two very different types of fishing practice. Todays focus is on the open ocean so here's the scoop (or should I say trawl...oh dear):

1) Blue and yellowfin tuna...not so dolphin friendly

A rare shot of a bluefin tuna that isnt rushing past at 20 knots

These are the big guys of the fish world, the top predators in a similar way to wolves in Europe and lions in Africa. Sleek fast, hydrodynamic, with fast metabolisms to keep them active when hunting. Indeed tuna have warm blood and control their temperature in similar ways to land mammals. Being at the top of many open ocean food chains, these rapid hunters act as keystone predators. They influence the numbers and diversity of nearly all other smaller fish in the ecosystem.

Now sadly for these fish they taste great (although that can be said for most animals depending on who you ask) and we have been fishing them for as long as we had boats capable of getting out to their mid ocean hunting grounds (and for as long as we have had nets and lines capable of catching them). The issue here is similar to that discussed with sharks, these are large slow growing top predators, as such it takes time for them to recover from population losses caused by fishing. Now tuna due produce millions of eggs (as opposed to the handful of shark pups) , but as they are released into the open ocean, only a tiny fraction don't become a whale snack.

As such we have seen dramatic declines in these species. I would also like to point out the folly of buying dolphin friendly tuna, which is a sad story showing that having your heart in the right place doesn't always mean that you get the intended results.

People like dolphins, especially environmentalist types. The traditional method of catching tuna was with a purse siene net, it looks like a big coin purse, and is pulled around a shoal of tuna before being zipped up by another boat, just as you would pull the cord on such a purse. The best way to target the shoal of tuna was actually to follow the pods of dolphins that ate the same small fish as the tuna. The sad fact that a few dolphins were being caught prompted Greenpeace and a few other charities to campaign to get people only to eat dolphin friendly tuna. Purse siening was a very selective method of fishing, apart from the fact it caught a few dolphins, the replacement method long lining was much worse.

Long lines are similar to regular pole and line fishing, the idea is the same, put out a baited hook and wait for the fish to take the bait.
The difference is in scale, long lines are kilometres long and hang millions of baited hooks in the water. These are towed slowly or simply left to wait in the sea, they subsequently catch anything and everything (except dolphins, who don't really go in for baited hooks), this includes critically endangered turtles, regular old endangered sharks, hordes of non target species that are thrown back as bycatch and oceanic birds such as the albatross. Long lining bycatch rates range from 30-60%, much less selective and ultimately much more damaging than the original method! So while dolphins were saved (a collection of species that were not threatened in most of their ranges ) everything else in the sea suffers.

Fortunately we have started to realise the error of our ways and sustainable pole and line caught tuna is making a comeback. If you can't live without it go for skipjack, albacore or blackfin (American south east asian readers should also avoid pacific bigeye) and try to make sure it is pole and line caught. Do this and fish everywhere will thank you, furthermore, you will no longer be an accessory to turtle murder!

2) Swordfish, king mackerel and marlin...heavy death metal

Beautiful animals these guys...and shown with spiffingly brillinat British accents in finding nemo

These charismatic chaps follow very much the same rules as tuna, indeed they are top predators and have been decimated by the practices of long lining. Yet there is an extra piece to fit in this puzzle box, poisonous mercury.

Mercury is a rather nasty heavy metal, in liquid form it looks cool, but inside your body even in relatively small amounts it does some serious harm. Hair loss, seizures, trembling, loss of liver function and kidney shutdown, in extreme doses brain damage and death heads up a symptom list aggressive cancer would be proud of.

Top predators such as swordfish naturally build up mercury in their systems, each tiny fish eaten has a small amount, and as it takes a very long time to get out of their (and our) bodies it builds up. When We eat swordfish steak we get a big dose of this mercury. Indeed swordfish is so loaded with the stuff, one portion puts a regular man up to his weekly safe mercury threshold and is considered a risk to an unborn child. The food and agriculture organisation for the UN recommends that pregnant and breast feeding mothers consume absolutely no swordfish, marlin or king mackerel, as even one portion is loaded with this poison.

Now this problem has been around for a while, but it is getting worse. Incidents of mercury poisoning are increasing year on year in Japan (eat lots of fish) and there have been cases reported of new York lawyers losing their hair and having seizures due to a steady diet of high grade sashimi. The problem stems from our increased industrial activity. Mining for gold and platinum produces mercury waste that ends up being dumped into rivers and then slowly makes its way to the sea. Mercury pollution is also driven by sales of batteries, so if you want to help out the swordfish (and maybe one day eat one that's not loaded with a deadly poison) switch to rechargeable batteries.


If unsure about the levels of mercury in fish you can consult this handy chart, if you are pregant or breast feeding (or thinking about it), this list if quite important.