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Originally Posted by dogsbody70
How long before the seas are completely empty?
I also wondered about the way Herons? are used to catch fish-- the way the bird is roped so it cannot swallow the fish?
It seems very skilful but how ethical?
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Cormorants, not herons. In Japan, from what i've read, you need special permission to fish using a cormorant, and the few who do (it's not exactly a thriving industry) rear their own birds and do it to maintain tradition. The bird has a flexible elastic band put on it's neck for the duration of the fishing, completely harmless and no worse than the rings we put on birds legs for tracking, which stop it from eating ~big~ fish. It can still eat small ones, and the birds aren't starved regardless. They get fed after the fishing is done. China I cannot vouch for, but really, if you're making a living off of selling tourist photos of the bird, you can't present one that is scabby and skinny.
Quote:
Originally Posted by cranks
There is no conceivable reason for smaller whales to accumulate more mercury. If anything, I'd guess it'd be the other way around.
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Actually, I think there is, and quite logically. Dolphins and those categorized as "small whales" tend to be toothed; this means they eat fish rather than krill and shrimp etc. Fish retain much more mercury than krill, (in fact, shrimp have among the lowest levels of mercury absorption of all sea creatures), which is then in turn absorbed by the secondary and tertiary predators. Unlike large toothed whales, dolphins and small whales don't hunt at great depth, so they encounter more contamination in their prey. Dolphins particularly will hunt in shallows, greatly increasing the risk of contact with mercury from waste. Besides which, the main diet of large toothed whales tends to be squid, which generally have less mercury than fish anyway.
Basically it builds up so that a young dolphin could easily have a higher concentration of mercury than a young whale, although a very old whale will probably have more than an old dolphin, simply by having lived for much. much longer.