Will Shark Decline Cause Squid Swarms?
    By Danna Staaf | September 9th 2011 07:01 PM | 4 comments | Print | E-mail | Track Comments
    About Danna

    Cephalopods have been rocking my world since I was in grade school. I pursued them through a BA in marine biology at the University of California...

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    An interesting fellow is doing an interesting thing. Scott Cassell aims to "raise awareness around the detrimental state of the ocean along the Southern California coastline" with a specific focus on the decline of sharks. Says Cassell,
    If we lose an apex predator from the food chain it causes other species to then have population explosions. For example, Sharks and Tuna are the natural predators of the Humboldt squid. If you kill off all the sharks, the squid population (each female can potentially have 20 million "babies") will begin to overpower the part of the food chain below them. They will eat anything and everything. As they swarm for food they exhaust feeding grounds and fisheries as they move.
    Cassell doesn't have a scientist's reluctance to make absolute statements, a feature that many journalists probably appreciate. But of course I can't resist jumping in with all the caveats . . . 

    Firstly and obviously, not all shark species eat Humboldt squid. The blue shark, on which Cassell is particularly focused, is one of the sharks that does, but only rarely (see second page of Field et al. 2007). Far more terrifying from the adult squids' point of view are sperm whales, and from the baby squids' point of view--well, anything from chaetognaths to other squid. Those millions of babies are awfully vulnerable! So, while some sharks do eat some squid, I don't think they're a critical check on the population that, if eliminated, will lead to overpowering swarms of squid.

    Will Humboldt squid eat "anything and everything"? Of course not, but hey, we should all recognize that as hyperbole! They are capable generalist predators.

    Do Humboldt squid "exhaust feeding grounds and fisheries"? Well, despite anxiety from hake fishermen, market squid fishermen, and salmon fishermen, it hasn't happened yet.

    All this doesn't mean I'm indifferent to the idea of killing off sharks! There are numerous reasons to be concerned about shark decline, from the aesthetic to the economic, and my love for squid doesn't extend to the extermination of their predators.

    Even those terrifying sperm whales.


    Bonny Bonobo alias Brat
    Will Shark Decline Cause Squid Swarms? If it does then we might start seeing some pretty bloated looking dolphins swimming around, because many of them also eat squid but according to this Yahoo Q&A , dolphins have to eat a lot more squid than fish to get the same calorific value.
    My article about researchers identifying a potential blue green algae cause & L-Serine treatment for Lou Gehrig's ALS, MND, Parkinsons & Alzheimers is at
    Here is a table of calorific values from Wikipedia: I can’t get the column layout to copy properly, so the figures to the left are Energy density kJ/g    while those to the right are Energy density kcal/g

    Food component
    Ethanol (alcohol)297
    Organic acids133
    Polyols (sugar alcohols, sweeteners)102.4

    So the dolphins would have to work harder to catch more squid.  I don’t think, however, that the difference is anywhere near so great that our cetacean friends would find themselves burning off more calories than they are consuming.
    Robert H. Olley / Quondam Physics Department / University of Reading / England
    Bonny Bonobo alias Brat
    Thanks for this information Robert. Lately there has been a bit of a fad around here where I live of dog owners putting their sick and/or old dogs on squid only diets and the dogs miraculously recovering and/or rejuvenating against all of the odds. I imagine a squid only diet for a few days might even be a healthy, relatively low calorie way for people to quickly lose weight while hopefully not dying of boredom at the same time, as they are pretty tasteless without any fattening garnish. I can remember that on holidays in Spain and Greece I repeatedly saw people thrashing what looked like squid on the craggy rocks by the beach presumably to tenderise them and not just beat them to death, before us eating them in restaurants, cooked in olive oil and served with garlic and lemon, yum.

    Anyway, back to the dolphins, I wonder how easy and energy consuming it is for them to catch the intelligent squid compared to catching the less intelligent fish? Whales and dolphins can use bubbles to force fish into tightly packed schools before darting in and taking a few mouthfuls but I doubt if this would work with squid. I wonder how they catch them? Google time, unless Danna knows?
    My article about researchers identifying a potential blue green algae cause & L-Serine treatment for Lou Gehrig's ALS, MND, Parkinsons & Alzheimers is at
    Danna Staaf
    Very interesting thoughts; thanks for the comments! The cooperative hunting trick of forcing a school of fish together does in fact seem to work on (at least some) squid, as documented in La Jolla in July
    In general, I'd be very hesitant to say that fish are less intelligent than squid. When we rave about how intelligent squids and octopuses are, it's usually in comparison to other invertebrates, rather than in comparison to vertebrates. But I also wouldn't say that fish are more intelligent than squid--I think the jury's still out. =)