Every year, great white sharks are becoming, on average, less great in length and in girth, according to Lindsay Gaskins at The Story of Size. Rather than the sharks themselves shrinking, however, the drop in average size over the years more likely reflects decimation to great white populations. Sharks are not living long enough to reach the whopper sizes of their ancestors, and instead are reaching an untimely end in a fishing net.
Gaskins explains why this is a big deal:
Though a slightly lower size wouldn’t be a big deal on a functional level for an individual Great White Shark, on a population level, the decline in size indicates a shift in the percentage of sharks that are of reproductive age. Therefore, as the average size decreases, so do the number of sharks that can pup each year. As a species, these organisms are vulnerable as a result of their slow growth and whopping 15-year wait until maturity.
The IUCN summarizes some of the activities that are likely contributing the the species shrinking body size:
The species is targeted as a source for sports-fishing, commercial drumline trophy-hunting (for jaws, teeth and even entire specimens preserved), sporadic human consumption or merely as the piscine whipping-boy of individuals pandering to shark attack paranoia.
The majority of annual captures worldwide being made incidentally through commercial fisheries operating longlines, setlines, gillnets, trawls, fish-traps and other gear.
Additionally, great white sharks are actively hunted for their fins--one of the most expensive seafood items in the world. More than half of those fins are imported through Hong Kong, and 145 countries are involved in this trade. Conservationists consider this trade a threat to the great white's survival. As the IUCN points out, "Nowhere is the Great White Shark abundant and productive enough to sustain long-term directed fisheries."
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