This portal tracks the research and sea-going activities of the Fisheries Engineering and Acoustic Technologies (FEAT) Team from NOAA¿s Northwest Fisheries Science Center. Follow us as we use acoustics, trawling, and oceanographic sampling to learn about the Northeast Pacific Ocean.
We collect lots of things at sea – otoliths (“ear bones”) for ageing fish, fin clips to see if our hake are from the same genetic stock, ovaries/testes to see how close to spawning our fish are, stomachs to see what our hake just ate, and tissue samples to see what a hake average diet is like over time.
Perhaps the coolest thing we are collecting from fish on this trip is an x-ray image of them. Dr Dezhang Chu (NWFSC-FRAM) is leading this work. Unlike humans, the fish x-rays aren’t to look at bones, but rather to look at their swimbladders.
(Break for a quick review of acoustics. In fisheries acoustics, also known as “active acoustics,” we send a pulse of sound into the water. That sound energy bounces off anything with density and sound speed that are different than water. The more different the object is than water, the stronger the returned energy back to our equipment. A fish’s swimbladder, which is often filled with air, is very different than water.)
Chu is taking x-ray images of fish (see the adult hake example) to see:
1. How large the swimbladder is, as this affects the amount of energy that is returned
2. How the swimbladder sits in the fish’s body relative to how the fish usually swims. If the swimbladder is very flat in a “normal” swimming orientation, it will reflect more sound than if the swimbladder is at a sharp angle in the body
How can we use this information? The main application is for estimating fish density/biomass. If we understand how much sound energy an individual fish reflects, we can more accurately calculate how many fish are present – simply, the total amount of energy from all fish divided by the average energy we expect to get from one fish. Chu is also interested in understanding how variable swimbladders are from fish to fish, and species to species, to improve models of expected energy returns.
Another application of the x-ray images is to determine how sound energy reflection changes as a fish grows. It is typically assumed that energy return (i.e. target strength) increases with fish size, but that may not be the case for all species. Relationships between target strength and fish length are sometimes developed based on measurements from dissected fish, but the dissection process itself may affect the size/shape of the swimbladder. The x-ray images that Chu is taking will help to more quantitatively describe the swimbladders of hake, as well as other fish species.