Archive | October, 2012

How we search for an acoustic tag on a winter flounder

To find the acoustic tags we’re using three different methods. One is a passive method to see if the flounder are coming into the estuary, and the other two are active methods searching for the flounder out in the ocean.

1) Passive Hydrophones placed at the entrances of the Manasquan River, Shark River, and Navesink River.

We’ve placed these hydrophones at narrow points in the rivers where the acoustic signal can stretch the whole distance across the river.  This way if a winter flounder decides to come into these estuary areas, if they pass through that section of the river then the hydrophone will pick up the tag signal.  Motz likes to describe them as working like an EZ-pass would with your car, if you drive through the array then the sensor picks up your tag.  So much like we have these EZ-pass arrays on the major roads we’ve put hydrophones at the major rivers to track fish passage.



2) Actively searching for tags by towing hydrophones off the stern of a boat.

We’ve been searching an expanded area of where we deployed the tagged fish to see where they have moved over the last few weeks.  The fish have dispersed outward from the drop point in all directions.  Finding them out on the ocean shelf has already turned troublesome since the area has expanded and we can only cover so much ground.  We tie a cannonball weight to the end of the load bearing rope coupled to the cable so that the hydrophone sinks trailing behind the boat.  The rope gets tied to the boat, and then we just sit and wait for the hydrophones to pick up a tag.  We drive the boat in circles, loops, or zip-zags about 500 meters or a little over a quarter of a mile apart to optimize how far the hydrophone can hear and to triangulate a position by picking the fish up from different angles.

Ken lower the cable in the water

Motz and Mike lower the hydrophone cable into the water








3) Searching for tags with a hydrophone attached to the nose section of an autonomous underwater vehicle.

Searching with the REMUS (Hydroid’s Remote Environmental Monitoring Units)  and the towed hydrophones behind the boat expands our search area.  This way the vehicle can search one area while we search another.  Additionally the REMUS can fly a certain distance off of the sea floor which can detect the tags at a closer distance vertically than a boat on the surface.  This can come in handy when there might be a thermocline (the water is stratified by temperature) which might disrupt the acoustic signal from communicating between tag and hydrophone, or if the winter flounder is buried in the mud on the sea floor, being closer to the tag might have an advantage over detecting from the sea surface.

The hydrophone is on the nose cone and protected by a cage in case we crash into something

REMUS in the water









Expanding on the use of tags, the REMUS has a radio tag attached to the top of it so that we can hear when it comes to the surface.  This comes in handy if its been at the surface for too long then we can track it down with its GPS point, but in case of drift we can find it with a radio antennae.

The radio tag on top of the REMUS

Rose searches for REMUS with binoculars while Motz gets a direction with the antennae



How to attach an archival tag to a flatfish

Tagging flatfish can be a little different than tagging a round fish.  Below is a description of how we placed the archival tags on the winter flounder.

Step 1) Catch the fish.  For this we used a commercial trawl boat, the Viking II which unfortunately just sank.  The trawl catches bottom fish and brings it onto the boat for us to sort through and find the winter flounder.

Step 2) Put the fish in a live-well which is a large plastic box that holds water to keep the fish alive.  Here we can asses to see how healthy the fish is.  We only want to tag the healthiest fish.

Step 3) Measure the length of the fish.  We aimed for greater than 30cm because the tags are rather large.

Step 4) Next we place the fish on a board with hollow needles fastened in it.  These needles put holes in the fish which we can then thread wire through the body to attach the tag.  This may sound rough, but the majority of the fish don’t even bleed from the procedure. Its a lot like piercing someones ear. We want to place the fish as close to the center as possible without being on the vertebrae so that the center of the weight is balanced, which is best for the fish to swim normally.

Step 5) Once the wires are through we can flip the fish over on its underside.  On this side of the fish we thread a second piece of plastic on the underside so that the wires don’t rub the skin.  The cool cloth over the fish’s head helps to calm it.

Step 6) We use a standard ruler as a separator between the body of the fish and the plastic, then bend the wires in place.  The ruler helps to make sure the wires are not too tight on the fish to make it as comfortable as possible.  We can now twist the wires, bend them down, and trim the ends so they don’t scratch the fish. The fish now goes back into the live-well to relax before its release.

Step 7) Now the moment that the fish have been waiting for, their release.  For this we use long handle nets to get them as close to the water as possible.  We record the location of their release to see how far and in what direction they move.

Step 8) Now we just have to wait for the fish to be re-captured.  From the tags we can asses where the fish like to live.  The tags record temperature, salinity, and depth over time.  The graph below show depth vs. time with the colored points as temperature.  In the two weeks this fish was over you can see that once the water warmed up it moved into deeper cooler water.

This is the fish with tag 6663 which supplied the information for the graph above.

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