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    Re: Noisy Sea Surface
    From: Trevor Kenchington
    Date: 2003 Nov 15, 11:11 -0400

    Interesting thread. (I hope that those who are not contributing can
    excuse those who are for our straying off topic.)
    
    The ocean is full of sound -- some caused by physical processes, some
    biological and too much man-made. (Cousteau's "Silent World" was a
    misnomer based on the low efficiency of the ears of immersed human
    divers.) I have not heard anything like the sounds described on this
    thread but from what has been posted here, the most likely explanation
    seems to be small crustaceans. To answer Keith's inquiry: Crustaceans
    certainly have hard mouth parts which click together or grind against
    one another as the animal eats. (Look at a crab or lobster feeding to
    see the same things at much larger scale.)
    
    John mentioned the sounds caused by herring adjusting their buoyancy:
    
    > The other night on Daily Planet (see text and video clips on www.exn.ca)
    > they talked about a strange popping or scratching sound that herring make
    > in schools.  This seems to be mainly at night.  It is caused by release of
    > gas from the swim bladder, through a duct connecting the bladder to the
    > anus of the fish.  It occurs when the school changes depth in the water.
    > It sounds a bit like a ticking.  When the fish are in large numbers, it
    > could be a scratching sound, presumably.
    
    Few types of fish have functional ducts between their swim bladders and
    guts. Most have to absorb excess gas into their blood and then expel it
    through their gills. Moreover, the release of gas by herring is usually
    when they move upwards in the water column (which is why it shows up at
    night), just as divers dump air out of drysuits when surfacing and for
    the same reason. Unless you are anchored in a very deep harbour, it
    isn't likely that such a release of gas could account for the
    observations noted on this thread.
    
    
    
    What I do not understand about these reports is how relatively
    high-frequency sound (George described the source as "clickers")
    generated underwater can then be heard above the surface. Sound
    transmission from air to water and vice versa is very poor and I would
    not expect the sound of feeding crustaceans, or those signalling for
    mates, to be audible out of the water. Is it possible that boats are
    acting as sounding boards, the vibrations passing from water to boat, to
    air and so to human ears? That would fit with the observations of a
    couple of contributors to the thread who have said that the sound is
    heard _inside_ a boat, rather than on deck. If it is so, I'd guess that
    the animals in question are directly on the hull, feeding on the fouling
    organisms, else trying to feed on the hull itself or perhaps (as Jose
    suggested) attaching themselves to the hull. (I wonder how much noise
    a growing barnacle makes as its skeletal plates are forced outwards
    across whatever it is attached to, so as to accommodate the beast within?)
    
    Alternatively, could these be animals that climb above the surface on
    cool nights to feed on intertidal fauna and flora?
    
    
    The source of the sound will, of course, be different species in
    different places -- there is nothing living that is common in the waters
    of the English Channel, Florida and New Zealand, from all of which areas
    these sounds have now been reported. Indeed, quite different animals may
    be involved in different places, though small crustaceans seem the most
    likely in every case.
    
    The name "krill" has been suggested. That may be as good a generic term
    as any but it can be a source of confusion. "Krill" can mean just
    _Euphausia_superba_, the hugely-abundant plankton species of the
    Southern Ocean which feeds the great whales and so much else of the
    ecosystem south of the Antarctic Convergence. It can mean that and all
    other planktonic euphausids (crustaceans one step down in bodily
    complexity from the familiar decapods -- shrimps, crabs, lobsters etc.).
    However, it is also used locally for other crustaceans: New Zealand "red
    krill" are squat lobsters, for example. If anyone wants to use it for
    the small animals munching along their waterlines, just be sure that you
    don't confuse those with the open-sea plankton that the name is also
    used for.
    
    Keith asked for a "definitive answer". Unfortunately, that would be
    specific to the area in question, might vary with the seasons, and could
    prove to involve multiple species at the same time and place. Somebody
    may already have investigated the phenomenon scientifically but, if so,
    I haven't ever heard the results. So getting the answers may involve
    putting a diver over the side at a time that the sound is audible and
    having him or her gather samples of whatever is on the hull. Then keep
    those beasties in captivity and see which ones can be provoked into
    making the sounds. It would make an interesting project for a
    high-school biology class or even a university honours thesis.
    
    
    Trevor Kenchington
    
    
    --
    Trevor J. Kenchington PhD                         Gadus{at}iStar.ca
    Gadus Associates,                                 Office(902) 889-9250
    R.R.#1, Musquodoboit Harbour,                     Fax   (902) 889-9251
    Nova Scotia  B0J 2L0, CANADA                      Home  (902) 889-3555
    
                         Science Serving the Fisheries
                          http://home.istar.ca/~gadus
    
    
    

       
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