Does sonar bounce off jellyfish?

[SwanseaBoat]

South Wales seeing the invasion of the giant jelly jellyfish. zillions. like phallic dustbins.
Any ways, as I pootle at displacement speed the old depth gauge starts flashing from 7.5m down to 0.9m. several times. Bit disconcerting as we were close in on the Wormshead looking for seals, and not wanting to become a permanent navigational mark.
So was the signal bouncing off the jellies?
Thought someone might know about sound frequency propagation in jellified bodies, or something?
Dan

 

[rbcoomer]

Hi Dan,

I'm no expert in sonar, but as nobody has yet replied I thought I'd push your post up a bit! I would guess not however as Jellyfish have no bone or swim bladder that I'm aware of and thus they probably have little or no discernible difference in density to the surrounding water. Although I suspect sophisticated sonar could perhaps detect small fluctuations in density, I doubt that the basic stuff fitted to everyday marine echo-sounders would! Interesting question however... :encouragement:

Regards,

Robin

 

[SwanseaBoat]

Is there a radiologist out there? Medical ultrasound bounces off pretty much everything except air. Babies are little more than jellyfish, anyway, and they show up on the obstetric fishfinder....

 

[Deleted User YDKXO]

All I can add is that when we boated in the Balearics, we would regularly come across swarms of jellyfish in bays, although generally small ones, and I don't remember the depth sounder misbehaving. I was far more concerned about blocking engine cooling water strainers and other intake pipes although to be fair, we only experienced this once when the seawater intake for the aircon blocked but I'm not even sure that was due to a jellyfish anyway

 

[Grampus]

Fish finders get their return signal when bouncing of the gas in the swim bladders of fish. Consequently you cannot 'see' things like sharks, rays, jelly fish etc.

Interestingly, those big white barrel jelly fish are the favourite food of the Leather-back turtles that we find along the western shores, Irish Sea and the Hebrides.

If you see something that looks like a 6 foot upturned saucer with a football shape at its head you might have spotted one of our rarer visitors, some of which have come all the way from the Caribbean just for lunch.

 

[Nick_H]

Fish finders get their return signal when bouncing of the gas in the swim bladders of fish. Consequently you cannot 'see' things like sharks, rays, jelly fish etc.

Interestingly, those big white barrel jelly fish are the favourite food of the Leather-back turtles that we find along the western shores, Irish Sea and the Hebrides.

If you see something that looks like a 6 foot upturned saucer with a football shape at its head you might have spotted one of our rarer visitors, some of which have come all the way from the Caribbean just for lunch.

I'm no expert on this, so may be wrong, but as I understand it 50 khz fishfinders can't spot fish without swim bladders, but 200 khz finders can. Depth sounders typically work at 200 khz, so it does seem feasible to me that you could get a false depth reading if cruising over a swarm of giant jellyfish.

 

[Grampus]

200khz fish finders have limited capability as generally their higher frequency is subject to greater attenuation by absorption in the water column. Their higher frequency does mean greater resolution but also a narrower beam and thereby smaller search cone.

50khz is an obviously good choice, it's the one nature uses after millions of years of evolution. It is the frequency that harbour porpoises employ as well a other cetaceans. Some dolphins operate around 80khz as do some commercial fish finders, which sometimes provide a range of frequencies from 15 khz up.

I'm pretty sure that jelly fish and sharks are transparent to 200 khz, been in water with lots of both, large and small and even a basking shark just under the boat does not show up, they just look like water.

Typically, sharks, rays and jelly fish are mostly water and since fish finders need to transmit through the water column, anything that looks like water will not create a suitable return reflection back to the transducer, so sharks and jellies just look like part of the water through which the beam transmits.

Rissos dolphins use sonar to detect cuttle fish which might appear to be similar, but cuttlefish have a cuttlefish bone with voids that creates the back reflection, in this case Rissos operate at around 83 khz if I recall.

 

[hlb]

My experience is, that sounders can bounce off nearly anything. I've often had them showing 0.9 metres crossing the Irish sea. Things improved when I also got a fish finder. Then I found that at least one of them would be giving me the correct depth.

 

[Davy_S]

IMHO, a good fishfinder will show jellyfish. Forget the swimbladder theory, or bone structure. It will also show a fishing weight being lowered to the bottom, or a jigger being worked, weed on the bottom, thermoclines, the list goes on.

 

[MapisM]

My experience is, that sounders can bounce off nearly anything. I've often had them showing 0.9 metres crossing the Irish sea. Things improved when I also got a fish finder. Then I found that at least one of them would be giving me the correct depth.

Absolutely +1. Well, aside from the Irish Sea bit, where I never went boating...
In all echosounders I've seen, bar none, the numeric display can occasionally show 0.9m (or whatever, depending on the brand/setting), not because they spot something 0.9m under the transducer, but just because in absence of a "solid" signal, they simply show the default lower depth, just in case.
Imho, it's much better to look at the bottom profile (assuming of course to have a graphic sounder) rather than the depth digits alone.

 

[NoIllusion]

Absolutely +1. Well, aside from the Irish Sea bit, where I never went boating...
In all echosounders I've seen, bar none, the numeric display can occasionally show 0.9m (or whatever, depending on the brand/setting), not because they spot something 0.9m under the transducer, but just because in absence of a "solid" signal, they simply show the default lower depth, just in case.
Imho, it's much better to look at the bottom profile (assuming of course to have a graphic sounder) rather than the depth digits alone.

Spot on MapisM. A accurate sonar return is very much dependent on the platform from which it was "pinged". Transducers are affected by a number of issues, not least stability. Cavitation will also seriously affect returns. During my time/training by the RN as a Anti Submarine Warfare Controller/Director, associated issues with poor returns had to be taken into consideration when searching for "enemies". In poor sea conditions sonars were often useless, bear in mind we were transmitting from a transducer that was 12/14 ft beneath the surface. The kit we had could register accurate returns up to 8 nautical miles away.......on a good day!!!.......but was bloody useless if it was rolling/pitching around in a F8!. Can't claim to have ever pinged a jellyfish......had they been near the transducer when pinging active, the power would have cooked 'em!...........but did pick up whales on a very regular basis......unfortunately the whales we picked up during the Falkland Crisis were treated as "enemies" and duly dispatched........apologies to all if you of a tree hugging/fish kissing disposition!?
So......will a sonar on a leisure boat pick up jellyfish??.......don't think so!......most probably the transducer trying to work out what the hell it's being asked to do!............

 

[hlb]

So, just like radar, sonar has found a target to bounce off. On it's own, it does not mean much.

Presuming that you have a chart, a chart plotter or GPS, even a basic tide table, you should know if the figures are false.

 

[NoIllusion]

So, just like radar, sonar has found a target to bounce off. On it's own, it does not mean much.

Presuming that you have a chart, a chart plotter or GPS, even a basic tide table, you should know if the figures are false.

HLB.......yes!!..........HM "droggies" (those that produce charts) are absolutely brilliant!........I worked with them for a while charting the Goodwin Sands ( and wrecks) and believe me they are professional. Every seafarer needs to learn and understand charts/depths etc...........and never rely on a electronic devices...............

.....which brings us back to "What would you do if all of your electronics failed??

 

[SwanseaBoat]

Yes yes thread drift. I quite agree, nearest bit of land under the boat, etc.. Admiralty charts not just cartographical drinks mat fading in the sun!
Thanks All.D