The recommended intake of fish for health purposes is about 15kg a year. Currently there is enough fish (farmed and wild) for everyone in the world to eat 17kg each. A large and growing chunk of this is farmed fish which, as we will see below, has its drawbacks.
But New Zealanders swallow 25kg, well above our fair share. Interestingly, we don't eat seafood as regularly as recommended, so like most of our eating it's the portion size that's our problem - we scoff big slabs of seafood in one sitting. Ironically this actually diminishes the health benefits of omega 3 oils. The lesson? We'd help ourselves by eating smaller portions of high-quality seafood.
Of course quantity is only half the equation. We also need to be aware of which seafood we should eat.
This issue is a source of huge confusion, with environmental groups on the one hand telling us to steer clear of most fish, and the fishing industry on the other telling us if it is in the supermarket it must be sustainable.
Certification schemes such as the Marine Stewardship Council (MSC) are designed to bring some balance to this debate, although even the MSC system is not perfect.
There is room for a more balanced recommendation list in New Zealand that is a lighter touch than full MSC certification would require (and so less reliable) but useful as a rule of thumb.
We've had a go at applying the Monterey Bay Aquarium's (MB) Seafood Guide criteria to the fish in our supermarkets. .
The MB Seafood Guide's approach includes a "Best Choice" rating for all the seafood that gets through with no major red flags. Well-managed fish stocks that grow fast (so are less vulnerable to overfishing) and are caught with environmentally safe methods get this seal of approval.
Small fry such as sardines, anchovies and sprats as well as farmed mussels meet these criteria. For New Zealand we thought blue cod (except in the Marlborough Sounds), kahawai, crayfish (except near Gisborne) also earned this top rating.
The next rating is "Good Alternative". This rating effectively means you should consume with care, particularly if you're willing to give up the squeaky clean environmental high ground because you can't bring yourself to eat sardines.
This rating allows up to one major red flag with the fishery, but as long as it is otherwise well managed then it is deemed okay.
This seems to be a fairly pragmatic position. Most industrial-sized fisheries that provide us with cheap fish have at least one environmental downside, but then so do most food production methods, not least farming.
Big-name fisheries to make this grade include hoki, toothfish and snapper. Hoki and toothfish have both been the targets of environmental campaigns - hoki for bottom trawling and (more so in the past) bycatch, and toothfish because something that lives so long and takes so long to grow is inherently riskier to fish sustainably.
But apart from these problems these fisheries are both managed very carefully, so they are good enough to get this rating. Meanwhile, snapper would rate better if we could sort out the endless tug of war between commercial and recreational fishers and let the stock recover to truly healthy levels. We discussed that in an earlier column - licensing recreational fishers is a no-brainer.
Finally we come to the rating that should give the fishing industry the guilts - "Avoid". If a fishery has more than one red flag this is the result. No surprise that this rating is applied to orange roughy - a long-lived fish that has been overfished and is caught using destructive trawling methods.
Industry members still dispute the science on roughy (surprise, surprise), but the onus really lies on them to shell out for the research to prove this is now a well-managed fishery. Another fishery to get this rating was, surprisingly, Bluff oysters.
This fishery gets one red flag for the use of dredging, but has also unfortunately been struck by problems with disease in recent years which has reduced the population below target levels, similar to what happened in the Tasman Bay scallop fishery.
Hopefully the stock will recover quickly and oysters move up to a "Good Alternative" rating, but in the meantime it serves as a good illustration of managing our fisheries so close to the wire. Aiming to keep our fish stocks at higher levels would reduce the risks that these sorts of problems would take the stock below dangerous levels.
You are probably wondering why the supermarket and sushi lovers' healthy favourite, tuna, has no mention. This is where the hypocrisy of the health-conscious, sustainability-worshipping set is well exposed. Sure, it is all dolphin-friendly these days, but that means diddly squat.
There are actually many different kinds of tuna. The up-market bluefin tuna is a definite no-no - couldn't be worse. Along with whale and dolphin catching, we're best to leave it to the Japanese, the world's marine idiots.
As for the tuna that ends up in a can and in the cheaper sushi, it all depends on what tuna it is and where it was caught - and there's the rub. At present this sort of information is not available, so you could be supporting dodgy fishing practices by buying it.
Any responsible Greenie would hold back on tuna sushi until MSC-certified tuna appears on our shelves - which should be pretty soon. Only then will you know the tuna in your can is the prolific skipjack tuna, caught without greedy practices which lead to unnecessary bycatch of sharks, turtles and other rarer types of tuna.
Some tout aquaculture as the saviour of the world fisheries, but this logic leaks. Farmed vegetarian fish such as catfish, basa and tilapia are now feeding China and have even sneaked on to our shelves.
They are a reasonable source of protein, but don't cut the mustard when it comes to providing omega 3s. Think of them as cheap meat, like sausages. How nasty? Who knows?
Farming carnivorous critters such as salmon and prawns delivers more omega 3s, but these fish have to eat fishmeal and oil to grow - so can be just a middle-fish as you gaily destroy the world stock of wild fish. We'd do far less damage cutting out the middle fish and directly eating the sardines most farmed salmon and prawns are fed on.
Despite this, New Zealand salmon scrapes in with a "Good Alternative" rating because, unlike overseas, our stocks don't mix with wild salmon, and are not farmed so intensively they need antibiotics to stem disease.
But don't be conned that just because it is farmed means it is sustainable. Farmed mussels and oysters are really the only truly valuable contribution from aquaculture to global omega 3 intake, though they are not always welcome in our precious bays.
The sad truth is all of our food choices have some impact on the planet, and there are no easy answers. The question is how much you are willing to trade your conscience for taste. Informed consumers make choices that balance the issues. Bon appetit.
Gareth Morgan and Geoff Simmons are authors of Hook, Line & Blinkers: Everything Kiwis Never Wanted to Know About Fishing.
