*New Worlds: Scientists fish for reason larvae die quickly*

Tel Aviv University researchers have discovered the biological flaw that is responsible for the death of 90 percent of fish larvae.

By JUDY SIEGEL-ITZKOVICH
Fresh fish (photo credit: MARC ISRAEL SELLEM/THE JERUSALEM POST)
Fresh fish
(photo credit: MARC ISRAEL SELLEM/THE JERUSALEM POST)
Tel Aviv University researchers have discovered the biological flaw that is responsible for the death of 90 percent of fish larvae just days after they have hatched – contributing to the decimation of the world’s fish supplies.
“An end to seafood by 2050?” and “Fish to disappear by 2050?” were some of the sensational media headlines resulting from the UN Environment Program Report in 2010 that declared over-fishing and pollution – coming on top of the natural survival of only 10% of larvae – had nearly emptied the world’s fish stocks. That scarcity portends disaster for over a billion people who are dependent on fish as their main source of protein.
Now, a new study by Dr. Roi Holzman and Victor China of the zoology department at TAU’s George S. Wise Faculty of Life Sciences has uncovered the reason the vast majority of fish die so soon after hatching. With this understanding of the mechanism, said Holzman, “we can help find a solution to the looming fish crisis in the world.”
The research, published in the Proceedings of the [US] National Academy of Science (PNAS) and conducted at Eilat’s Inter-University Institute for Marine Sciences suggests that “hydrodynamic starvation,” or the physical inability to feed due to environmental incompatibility, is the reason so many fish larvae perish.
“By focusing on the constraints placed on larvae survival, we have a better chance of producing higher quality mariculture,” a specialized branch of aquaculture involving the cultivation of marine organisms for food and other products in the open ocean, said Holzman. “If we can produce better fish, this will have huge implications for our ability to maintain fish populations.”
Holzman based his study on the problematic nature of fish reproduction.
Nearly all fish species reproduce externally, releasing and abandoning their sperm and eggs into the water and providing no parental care. The fertilized eggs then hatch in the water within a couple of days and the hatching larvae must sustain themselves. When attached to a yolk sac (a membranous sac attached to an embryo that provides early nourishment in the form of yolk), these premature organisms can survive for a period of two or three days. However, once the larvae, with poorly developed fins and gills, open their mouths, they start dying in droves.
“We thought that something was going on during this period. Our goal was to pinpoint the mechanism causing them to die.”
The physical structure of the larvae and their flawed interaction with the physical environment provided the answer the researchers were looking for. Over the course of two years, they observed fish larvae at three significant points in their development (at the beginning, middle, and end of that “critical period” – eight, 13 and 23 days old). They found that the “stickiness” of the water – the viscosity of the surrounding ocean water – was hampering the larvae’s attempts to feed.
“All that determines the larvae’s feeding ability is viscosity, not age or development.
Because the water molecules around you have weak electrical bonds, only a thin layer sticks to your skin – a mere millimeter thick. If you’re a large organism, you hardly feel it. But if you’re a three-millimeter-sized larva, dragging a millimeter of water across your body will prevent you from propelling forward to feed. So really, it’s all about larval size and... ability to grow fast and escape the size where it feels the water as viscous fluid.”
The researchers found that in less viscous water, the larvae improved their feeding ability. In theory, they can be expected to increase their survival rate.
“We conclude that hydrodynamic starvation is the reason for their death,” Holzman. “Imagine eating soup with a fork – that’s what it’s like for these larvae.
They’re not developed enough at the critical point to adopt the constrained feeding strategy of adult-sized, better-developed fish.”
Armed with this knowledge of the fish egg’s biological flaw, the researchers are currently patenting a solution to maintain higher survival rates among fish larvae populations.