Tuesday, April 6, 2010

Nested Sets of Sushi

This colorful opinion piece in Time by Josh Ozersky followed the 175-nation Convention on International Trade in Endangered Species (CITES); the outcome of which was, to be charitable, an unmitigated disaster for science-based fisheries management. At issue were US- and EU-backed proposals to ban trade in a number of marine species including sharks and bluefin tuna, which are being overfished vastly beyond the ability of these large, slow-growing fishes to replenish their populations.

It is difficult to deliberately fish a widespread, open-ocean species into extinction. The commercial inviability of continuing to target vanishing fishes will in most cases spare them from true extinction. Where these beasts can get in trouble is if they're hit by one or more extra liabilities: for example, going through a bottleneck in their life history where it's easy to find and fish them all up, or if they command exorbitant prices sufficient to keep fishing pressure intense to the point of complete stock exhaustion. The Atlantic bluefin tuna Thunnus thynnus bears both of these albatrosses. These voracious, silver leviathans can reach ten feet and 1400 pounds, taking decades to reach maturity. Populations are harvested wholesale when they gather in the Mediterranean, which is one of their exactly two spawning grounds. And to top it off, they are tasty. Very tasty.

Tuna being shaped at Tsukiji fish market, Tokyo.

The Japanese in particular are crazy about these things. A single, epic fish sold for $175,000 in Tokyo last year. It was no surprise that Japan spearheaded the effort to torpedo the proposed CITES ban on bluefin trade, along with a number of fishing nation allies including Canada, Indonesia, Venezuela, UAE and friends. The proposed ban was destroyed by a vote of 68 to 20, with 30 abstentions. That is a go-limping-home kind of whooping, crowned by a strident Libyan denunciation of sound fisheries science as "lies." Nice.

But this post isn't about conservation. It's about a curious twist in Mr Ozersky's mostly rational and impassioned call to boycott his once-beloved o-toro, bluefin sushi, in response to the shenanigans of Japan et al. Ozersky writes:

"It's been around for more than 400 million years, which means it is older than the trees, older than the Himalayas, older than the Atlantic Ocean itself. ... But either way, the loss of a creature that has been living here since before the continents formed won't be on my hands."

Wait. Huh?

Four hundred million years is old. Even in evolutionary time, this goes back to approaching the rise of large animal life on the planet. Paired fins and jaws had just made the scene. At this time near the beginning of the Devonian period, jawed fishes were finally getting their act together and beginning to diversify while their jawless cousins began a slow decline into obscurity. Most of these fishes were armored oddities called placoderms that have no living descendents. The first dinosaurs, lithe little things, wouldn't trod the earth for another 170 million years. So... is the Atlantic bluefin nearly twice as old as dinosaurs??

To answer this strange puzzle, let's consider the nature of life as a series of nested sets. This is the pattern of evolution by cladogenesis, in which an ancestral species gives rise over time to two distinct daughter species. This also has the nice consequence of producing a hierarchical arrangement of organisms. For example, all ferrets belong to a larger group called mustelids, along with weasels and sea otters and wolverines. All mustelids are placental mammals. All placental mammals are vertebrates, and all vertebrates are animals. Therefore, you don't predict to find weasels before mammals evolved, or mammals before the advent of animals.

Now how about tunas? Tunas are in a family of fishes called scombrids along with mackerels, bonitos, and other tasty things. They are nested well within the "set" of fishes called teleosts, which comprise over 99% of all living bony fishes, and the next larger group we'll worry about are the ray-finned fishes. So we can't have tunas without teleosts, we can't have teleosts without ray-finned fishes. Here's the problem.

Divergence times for groups containing bluefin tuna.

Teleosts, the crown group of bony fishes from marlins to mollies, date to the early Triassic period at the dawn of the dinosaurs, sometime around 240 million years ago (with some uncertainty). Right off the bat, we can see that there is no way that the bluefin can be older than that - you can't be your own grandpa, so to speak. What about ray-finned fishes? They crop up around 420 million years ago in the period right before the Devonian. Could this be what Ozersky meant? Even given that benefit of the doubt, does it matter? His heart is in the right place, but conservation may be legitimately justified by appealing to the maintenance of viable commercial markets, or ecosystems, or even an organism's aesthetic value (which I submit the bluefin has). Lemurs are worth protecting because they represent the entirety of a major offshoot of the primate family tree, not because they are vertebrates or animals or organisms with a discrete nucleus.

Species longevity is an interesting concept that gets at the heart of the pattern of "punctuated equilibrium" characterizing the fossil record of many groups. Punctuation is itself intriguing: why did this plant/animal/fungus' morphology change rapidly in a short period of time? But the equilibrium often gets lost in punctuation's spotlight: why do so many species not visibly change over long periods of time?

Across vertebrate taxa, species longevity tends to be on the order of only a few million years; perhaps 2-5. Even without knowing more about the timing of the scombrid fishes' radiation, we would not predict that the Atlantic bluefin is much older than that. The number of "famous" animals that buck the trend and appear to have changed very little over time can be counted on one hand, but are usually overblown. We'll return to the coelacanth, sharks, and similar cases in the next post, with a revelation or two that may surprise you.

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