The world's oceans are massively overfished and polluted, which the United Nations began warning about five years ago. The rapidly-dwindling stock of fish portends trouble for the roughly billion people dependent on the sea for protein.
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But what if the terrible mortality rate of the young'uns could be reduced?
It's the nature of the beast. Fish can lay thousands of eggs – almost a million, in the case of tuna for example. Only a very few reach maturity. Now Dr. Roi Holzman and graduate student Victor China of the Department of Zoology at Tel Aviv University's George S. Wise Faculty of Life Sciences say they've figured out why 99% of fish die within days of hatching.
Yes, as thought, the baby fish starve to death. But their misery can't be relieved by just adding more prey to their water, the scientists demonstrated. Clearly the problem lay elsewhere.
Fish hatchlings feed by suction. They swim towards their prey and when they reach it, they open their little mouths and expand their mouth cavity. Water flows in, drawing in the prey.
The bigger and older the larva, the better it works. The smaller and weaker the baby fish, the less suction they can create and the more likely the prey is to escape, the Israeli scientists found (see the video of a larva catching, then losing its meal).
But why can't the small baby fish feed properly? That was the question.
Almost all fish reproduce externally. The mother drops her eggs into the water and the male releases his sperm over them. Very few species provide anything like parental care to their helpless offspring. (This is especially true of sea-fish, explains Holzman: parental care is more common in fresh-water fish.)
The sea-fish eggs scatter in the water, as do the larvae. These hatchlings break out of the fish egg still attached to a yolk sac, which can sustain them for up to two or three days.
But once the little larvae, with their still under-developed fins and gills, start needing nutrients and trying to feed, they start dying in droves – even if prey organisms abound in the water.
Clearly something happens during the post-yolk phase: that's when the proportional number of larvae dying is greatest, explains Holzman: "Our goal was to pinpoint the mechanism causing them to die."
They turned to physics to find out the source of the problem, and worked on sea bream (denise in Hebrew), a commonly farmed fish that are highly representative of sea fish in general, says Holzman. The parents drop their eggs and sperm into the water and merrily swim off. Then the larvae hatch into the sea, and good luck to them.
Like swimming through honey
Over two years, Holzman and China observed bream larvae at three significant points in development (at the beginning, middle, and end of that critical period of high mortality - eight, 13; and 23 days old). To watch the fish properly, they developed a camera system that snapped the hatchlings at a terrific 1,000 images per second.
They found that larvae a few days old failed at most of their feeding attempts, while bigger, older ones succeeded 80% of the time. Why did the smaller ones fail to catch prey?
Because they were so tiny that the viscosity of water was gumming them up. "From their perspective, it felt like they were swimming through honey," says Holzman.
"All that determines the larvae's feeding ability is viscosity – not age, not development. Just their interaction with the surrounding water," says Holzman.
When a human dips in the sea, a thin layer sticks to the skin, about a millimeter thick. You wouldn't notice it at all if it weren't wet. "But if you’re a three-millimeter-sized larva, dragging a millimeter of water bonded to your body will prevent you from propelling forward to feed. So really, it's all about larval size, and its ability to grow fast and escape the size where it feels the water as viscous fluid," explains the scientist.
Lake water is almost as viscous as sea-water, by the way: the same principle applies to sweet-water fish that proliferate this way.
So, how can fish proliferation rates be improved?
"At first we searched a way to reduce the viscosity of water. But we couldn't find an appropriate substance because the agents that would reduce water viscosity are toxic, for instance containing alcohol, benzenes and so on," explains Holzman.
Another avenue of thought is to develop a food that might be easier for the larvae to eat. "We don't know at this stage what it might be – perhaps something heavier, or smaller, maybe a different shape that would stay in their mouths better. We can see in the video that the larvae take in food, but it often escapes," he says.
The most obvious way would seem to be engineering – not genetic, but selecting for bigger fish eggs and larvae. But here Mother Nature may have her own opinion.
"I'd like to find a way to make sure that the larva hatches at a bigger point. But it isn't simple," says Holzman. "If we look at egg size we see a very clear pattern – almost all fish eggs are very tiny, including of 3-meter tuna or 20-centimeter sardines. We suspect there are physical limitations to egg size."