No, they actually don't work... unless by "work" you mean "kill lots of animals other than sharks"?
Have you got some free time today?
From the Wikipedia article on Drum Lines:
utilised on Australia's eastern coast and in South Africa where the numbers of attacks reduced dramatically.
It has two references. One is a dead link, the other is this:http://www2.dpi.qld.gov.au/extra/pdf/fishweb/sharksafetyreport.pdf
which contains this:
It is recommended that: the effectiveness of shark, mullet and other baits be evaluated in
controlled statistical trials.
There are many likely factors affecting local shark populations including: reduction in food due to
declining fish stocks; increased boat traffic; water quality and climatic conditions (most notably the
drought); the effectiveness of the SSP in local areas; and overall pressures on shark populations
from legal and illegal fishing. It is extremely unlikely that the declines in catch rate of some species
(particularly tiger sharks), most notable in the southern part of the State, are solely related to a
change in effectiveness of the SSP in these areas. It should be appreciated that trends in catch, of
shark or by-catch, manifest over long timeframes. Accordingly to endeavour to establish cause or
causes of trends, it will be necessary to determine a series of trigger events e.g. peak catch, low
catch which automatically prompt a detailed review of the operation of the SSP in a particular area.
So, the wikipedia article implies that drum lines has reduced the number of sharks, when the referenced article says there has been a reduction, but doesn't say it's because of the drum lines and recommends study to find causal links.
I think the Wikipedia article needs to be adjusted.
If this is such a big thing, surely we can come up with better solutions than nets and baited hooks. What about sonar based early warning systems? FFS, for a couple of hundred bucks, you can get sonars that detect fish for fishermen, why can't they develop sonars that detect sharks, warn people, get out of the water, wait until it passes, and then go back in. No harm to the shark and safer for people in the water.
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