Monday, March 16, 2009

Is eating fish good or not?

That grammatically simple question is anything but when you consider the different understandings of the consequences of humans eating fish. Here I will consider three points-of-view: a medical POV, a risk assessment POV, and an ecologist POV.

Many times the justification for eating fish comes to the benefits of Omega-3 fatty acids. Doing a simple search on the PhysOrg website for “omega-3” yields twenty-two still-accessible news stories about this or that finding about the benefits of this fishy fatty acid:
  • Help prevent medical complications due to obesity (Feb 12, 2009);
  • Minimize menopause depression (Jan 28, 2009);
  • Good with wine (Dec 4, 2008);
  • Lower rates of eye disease (Jun 9, 2008);
  • Protects against Parkinson’s Disease (Nov 26, 2007);
and so on…

Furthermore, studies in the medical literature also point out that North American diets are low in Omega-3 fatty acids, so… since they are so useful to us, we should eat more of it, and one easy way is to eat fish. Okay. I get it. You get it. A three-year-old can get it. However, the medical POV isn’t the only one around.

Over the past several decades, we have learned that there are different types of chemical compounds in our environment, some natural, others man-made. Some of these (mainly metallic compounds) are nasty, but don’t accumulate in the food chain (with the major exception of mercury, which – when methylated – has this nasty habit of bioaccumulating). Others (mainly hydrophobic nonmetallic compounds) are not only nasty, but bioaccumulate in the foodchain. The problem with bioaccumulation is that an organism absorbs the toxin at a rate greater than that in which the toxin is lost. Applying this principle to a food web/chain concept, an organism absorbing toxins is usually ingesting organisms at a lower trophic level (i.e., at a lower level on the food chain). Therefore, bioaccumulation leads to biomagnifications of a toxin as one climbs the foodchain to … you guessed it: FISH! What’s more, all those tasty fishes we human cultures are so amenable to gobbling up like there's no tomorrow (like tuna and cod) tend to be near the top of the food chain. (There are dangers of eating generalists like catfish and carp, but I won’t get into those here…) So what does that mean? Well, from a risk assessment POV, if you are catching fishes in polluted waters – or catching top-of-the-food-chain fishes who spend time in polluted waters (because, hey, fish swim around) – chances are increased that you will increase your ingestion of bioaccumulative biomagnified substances, like methylated mercury, PCBs, etc. These substances… which have medically been shown to be not good for you, leading to (among other things) retardation, immune disorders, gender-bending, and cancer. Well, so you can always just harvest the omega-3 fatty acids from fish, devoid of all those toxins that have bioaccumulated in the... oh, yeah. In the fatty tissue of fish...

But regardless of the touted medical benefits and the touted toxic risks, fishing is also a way in which man interacts with his environment. Indeed, some projections of harvest rates (against estimates of standing stock in the world's oceans) place economic extinction of fish species near the middle of this century (around 2042 if memory serves). What is the up-shot of this? Well, if you remove top-predators from a food chain, you have shifted the pressure to lower levels of the food chain. Since top predators have a direct impact on the number of lower-trophic animals (and those on still-lower trophic levels), the removal of a top predator (i.e., those fishes we love to eat due to their tasty pink-to-red flesh whose musculature is so suited for chasing down other fish and eating them) means that second-tier fish populations boom and crash, causing cascades of knock-on effects in a predator-controlled food chain. This leaves (basically) a system that doesn't function in a stabilizing manner. However, what is happening alongside these systems are the rather non-vertical foodchains of generalists (i.e., those fishes and alternate sea-life we don't really find that tasty). Since they do not suffer as greately from the loss of a top predator, their numbers increase beyond the ability of previously dominant food webs to manage (through marginalization, usually). In time - evolutionary timescales - these generalists will start to fill in niches that are left open by a loss in a predator-dominated foodweb. However, by the time this happens, humans are likely to be long past a stage that we would recognize them.

In the meantime, what do we have? We have doctors saying eat fish, because it's good for you (especially for pregnant women). We have risk assessors saying don't eat fish, because it's laced with toxins that are bad for you (especially for pregnant women). And we have ecologists saying don't eat fish, because it's leading to the worldwide collapse of fish stocks, meaning that the (possibly) toxin-fed babies carried by pregnant women today won't be able to have the benefits of fish the sea.

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