Sunday, January 23, 2011

Is Fish Growth Inherently Limited by Oxygen?

New theory suggests fish size limited by ability to take up oxygen:

Quite a long article, but worth reading for people interested in fish.
“One day, I went for a walk along the embankment after a fight with my girlfriend, and suddenly it hit me: fish need oxygen to grow, so the theoretical surface problem and the breathing problem that von Bertalanffy was talking about had to be the gills.”
I think I see his problem with his girlfriend.  He was thinking about fish.  I sympathize.
"As fish grow, he proposed, they accumulate tissues, which must be supplied with oxygen if they are to remain alive – so their oxygen requirements are proportional to volume, he realised. To be useful, oxygen must be inside the fish body, which it can only occur if it has penetrated through a surface – the gills. But gills don’t grow as quickly as the body volume.
OK, I see that, but is there any experimental evidence for this?  One consequence of this could be decreased oxygen saturation in tissue with size, all other factors being held constant.  Theories are nice; but data to support them are required.

Anyway, the theory does have some interesting outcomes; spawn size for a fish species could be determined by the size at which it starts having difficulty keeping itself adequately oxygenated:
"It also predicts how a fish knows when to spawn. How does a Beluga sturgeon know that it should start spawning at 18 years of age, an Atlantic cod at five and a Peruvian anchovy at one? Scientists, notably Beverton and Holt, knew that the ratio of length at first maturity to maximum length is somewhat constant in fish, ranging between 0.4 in species that get big to 0.6 in species that stay small.

But this only restated the problem: how do they know when to spawn? The answer provided by Pauly’s theory is simple: when they start running out of breath. As fish grow larger, their maintenance metabolism – the amount of oxygen they need just to survive – increases, leaving less and less for other activities. Thus, when their breathing rate declines to a set point of 1.4 times maintenance metabolism, all species of fish – whether they are guppies, cod or Beluga sturgeon – will spawn.
 Or why fish growth slows substantially above a certain size for most species:
The maximum tolerable gasping size is also the signal for the fish to change its metabolism and stop growing. This is why your guppy remains tiny, although you feed it nutritious food twice a day. “If you want bigger guppies,” Pauly says, “keep them in water as cold as they can tolerate and make sure your tank is well aerated.”
So, this seems like a reasonable theory, with some evidence, but nothing I see that would clinch the deal.  Remember, not all you read is true...

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