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How to weigh a catch?

Discussion in 'Boat Electronics' started by Dave Baker, Nov 24, 2005.

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  1. Dave Baker

    Dave Baker Guest

    This isn't "rec", but it is "boats.electronics".

    I am wondering how the weight of a catch on a fishing trawler could be
    measured while the vessel is at sea. I'm thinking that the weight of the
    whole vessel might need to be taken into account as there will be variables
    such as fuel, water, etc.

    Anyone know how this would be done?

    Maybe rather than weighing while in the hold it would be weighed in the net
    before dumping into the hold?

    Thanks,

    Dave
     
  2. I would think after a few trips you could tell pretty close just by looking
    in the hold. You would get a pretty good feel for how full it has been in
    the past and what it weighted out to at the dock.

    Some winches have load gauges on them but that doesn't take trash fish
    (which can be a lot) into account.

    --
    Glenn Ashmore

    I'm building a 45' cutter in strip/composite. Watch my progress (or lack
    there of) at: http://www.rutuonline.com
    Shameless Commercial Division: http://www.spade-anchor-us.com
     
  3. In the North Pacific, the fish is sorted by specie into totes and then
    weighed with hanging scales before it goes down to the hold on some
    boats. On others, the fish isn't weighed untill it is off loaded to a
    tender, and then weighed in hopper scales, or brailers. Most skippers
    have a good idea how much fish is in the hold just by looking. Some
    of them can guestimate to within a few hundred pounds.

    Bruce in alaska
     
  4. Dave Baker

    Dave Baker Guest

    Thanks to everyone who has replied so far - the main thing everyone has made
    me realise is that it's not as easily as I thought, and actually I didn't
    think it would be easy anyway! :)

    For a bit more info, what we are trying to do is to automatically quantify
    how much is being caught on the fishing boats, and to transmit that
    information back along with position, speed, etc as part of Fisheries
    monitoring.

    This is in Asia, and there are some problems with vessels selling the fish to
    other countries instead of locally, not to mention selling of their
    subsidized diesel, so we can't really rely on information from the crew.

    The vessels are maybe 30' to 40' wooden trawlers. Not sure what they catch,
    but from looking at the sorting areas in port I believe they just take
    everything they catch & don't do any sorting on board, nor throw anything
    away (unless it's maybe a turtle or something big that they don't really
    want). Undersize individual fish don't appear to be a problem either - seems
    they are more prized actually!

    Weather could be anything from dead calm to typhoon, though I'm guessing they
    probably don't fish much in anything more than 2m of seas.

    It actually sounds like a tricky task - weight on winch would probably be
    highly inaccurate & too dependent on sea state & vessel speed. Hold weight?
    Maybe also difficult to instrument? Especially as these are relatively cheap
    hand-made wooden fishing boats that just go out with loads of ice to pack the
    catch in & have no refrigeration or no standard size or position holds.

    And then we have the problems of low bandwidth to transmit the information
    back to shore, and the very high potential for sabotage..... :-(

    Dave
     
  5. Dave Baker

    Dave Baker Guest

    Trawlers, so nets.
    This is Asia - basically anything that gets into the net stays in the net,
    and goes into the hold. From minnows to marlin. Sorting is done onshore when
    the boat gets back to land. In general they are aiming for fish of 10kg
    downwards.
    Because the government wants to know so they can stop their fishermen selling
    their catch overseas.
    Does this change the way it would be done?
    Apart from the fact that my father is a commercial fisherman, I agree that I
    have little experience in this field. Which is why I am asking questions.

    It's not so much of an inability to describe the situation, but a reluctance
    to do so as this is a project which has certain sensitivities & even possibly
    commercial considerations. I am happy to give information that is pertinent
    to the technical problem at hand though.
    I am pretty sure I mentioned this before on a previous post. To stop the
    fishermen selling their catch in neighbouring countries as they are using
    subsidized fuel, and nobody wants to subsidize fishermen from one country to
    have them sell their catch in another country.

    Permission - Yes - by law.

    Dave
     
  6. Dave Baker

    Dave Baker Guest

    Thanks for the links - I will definitely look into them.
    That is basically what is happening - the idea is that the boats will only be
    given the diesel subsidy (which is quite significant) in return for being
    tracked. The tracking will actually be done in real time.

    The idea is to incorporate (if possible) a catch monitoring function into the
    tracking equipment, so not only do you know where they are, but whether their
    nets are in or out, and whether their nets are full or empty.
    Yes, all of it. :)
    If the government is going to subsidize the fishermen's fuel, it want to make
    sure the fish come back to be sold locally.
    There is talk of aircraft, though there are bigger problems around here like
    pirates & smugglers, so tracking fishermen is a pretty low priority for these
    valuable aircraft resources.
    That's a good question - at the moment it's the other way around. They want
    us to come up with a proposal & a price, and they'll say yes or no.

    We are looking at providing the tracking, and we are very experienced in this
    side of things, but the catch monitoring is another kettle of fish, if you'll
    pardon the pun.
    "pop and egg"? No Google hits. I've just ordered my subscription to National
    Fisherman though. Trawl sonar brought up some hits though - some fancy gear
    at http://www.wesmar.com/trawl.html
    Pretty sure it's price - no fuel subsidy in neighbouring countries, so fish
    price is expensive. Subsidy here, but no compulsion to bring back. Well, the
    law is there (I think), but insufficient enforcement.

    Unfortunately I'm just an engineer working for a company providing tracking
    equipment. The legalities & moral consequences of how it is used is decided
    higher up than me. :)
    That's the current problem - they fill up with fuel, disappear for a week,
    come back empty, asking for more fuel. The unknown is whether they are
    sailing around the corner & selling their fuel to foreign vessels, or
    catching fish & selling them to neighbouring countries, or both. Real-time
    tracking will answer most of these questions, but catch monitoring will also
    help.
    Not so easy in developing countries with scarce enforcement resources.
    Me? None! :)

    The client - not sure. They want us to include an option for detecting catch
    amounts with our tracking system. If it's within their (unknown) budget, then
    they will hopefully take the option. Presumably if they are at all interested
    we will have to do some trials first.

    Dave
     
  7. Terry Spragg

    Terry Spragg Guest

    You might need to compensate for fuel load and, for the suspicious,
    ballast tankage. A long term average might actually compensate for
    heave, etc.

    And displacement vs drag vs speed vs fuel flow vs windage might help
    refine the reading. But still, you got bycatch to worry about, eh,
    and disposable anchor blocks, what else?

    Sounds pretty invasive to me. Pretty vulnerable to cheating, too.

    Terry K
     
  8. Dave Baker

    Dave Baker Guest

    Unfortunately our bandwidth is EXTREMELY limited - 8 bytes per hour (or
    transmission) in fact! :)

    However, having an onboard camera doing local logging & offloading the data
    back in port sounds like a possibility. I notice that there are digital
    cameras coming out now with built-in WiFi - it would be great if one of those
    could be set up to take a picture every X minutes into a big memory card,
    then automatically dump the pictures into a WiFi system as soon as the
    fishing boat came back to port!
    Unfortunately these boats are pretty rough & ready - something like this:
    http://www.drewish.com/photos/2002/thailand/08_krabi/DSC01055.jpg
    although this is actually a squid boat I think, with all the light globes.
    Their fuel tanks might be a 44 gallon drum & their lines to their engines
    might be plastic garden hoses in the worst of cases. Seals tend to be a very
    small nuisance that is easily bypassed somewhere along the fuel line.
    I'll definitely give them (2) as an option.
    Probably hard to find observers that would want to go out on these sort of
    boats! :) And they couldn't do it on every voyage for every boat anyway, so
    the fishermen would just change tactics if the observer were to come onboard
    for a voyage. We are after an electronic method that can find patterns of
    abuse of the system to allow fuller investigation by humans later.

    Thanks for all the good ideas from the participants in the thread though -
    it's been most educational.

    Dave
     
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