Thursday, July 10, 2014

How many giant squid? More than you thought

Indeed, more than you wanted to know about.  Giant squid (the true giants, Architeuthis dux), are among the most famous yet most mysterious of marine denizens. They are eclipsed in size among all invertebrates only by their bulkier relative, the colossal squid Mesonychoteuthis hamiltoni. Given that we've only recently (2012) videotaped a giant squid underwater and have seen very few of them alive at all, the question of how many there are seems unanswerable.
However, there is an estimate, and it's mind-blowing. Since large-scale sperm whaling continued until 1998, and dead specimens are often necropsied, we have a good idea about what they eat. (Bear with me. I'm getting there.) The majority of sperm whale diets are squid, but they are overwhelmingly small to medium-sized: the fabled battles between the giant squid and the sperm whale, never witnessed by a human observer, are relatively rare.  (Sperm whales also take a lot of fish, some developing a knack for nibbling them off hooks, and there is one known case of sperm whales attacking one another of the ocean's moist mysterious denizens, the megamouth shark.)

OK, enough meandering. The money shot: 

The paper "Unanswered Questions About the Giant Squid Architeuthis (Architeuthidae) Illustrate Our Incomplete Knowledge of Coleoid Cephalopods," by Clyde Roper and Elizabeth Shea (full paper not online), as quoted here by squid biologist Danna Staaf, says,  
“If the estimated 360,000 sperm whales remaining in the world’s oceans eat one giant squid per month, then the giant squid population consumed must be over 4.3 million individuals per year. If the number is one per week, then the consumed population would be over 18.7 million individuals consumed per year. Estimates based on actual samples taken from sperm whale stomachs are much larger still. Clarke (1980) suggested that approximately 1% of the 700–800 squids a female sperm whale eats each day and the 300–400 squids a male eats each day are Architeuthis specimens. If true, that yields the astonishing number of over 3.6 million giant squids consumed per day, and a yearly total over 131 million giant squids.”

Staff figures in her blog Squid a Day that this equates to a giant squid being eaten every one-fortieth of second.  Wow. Seriously, think about 131 million giant squid eaten per year. Playing with almost non-existent data here, with female squid topping out around 275 kg and males about 150, and assuming the sex ratio is 50-50 (which we don't know) and that all the squid eaten are adults averaging maybe 200kg, gives us 26.2 billion kilograms of giant squid being processed every year into whale poop.  The actual number is a good bit lower, since only a minority of the squid involved are likely to be full-grown when eaten, but it's still stupendous.  Indeed, given that sperm whales have been reduced by roughly (very roughly) two-thirds in the last two centuries by whaling, it's surprising the depths are not simply full of giant squid bumping into each other.  Human activity, which has reduced the population of large fish up to 90% or more, might be part of the reason they are not.  But wondering where all those missing squid are makes for a heck of a SyFy movie premise, doesn't it?

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