Anne Mortimer: Cam-trawl, July 14, 2011


NOAA Teacher at Sea
Anne Mortimer
Onboard NOAA Ship Oscar Dyson
July 4 — 22, 2011 

Mission: Pollock Survey
Geographical area of cruise: Gulf of Alaska
Date: July 14, 2011

Weather Data from the Bridge
Conditions: sunny and windy
Air Temperature: 10.1 ⁰C
Sea Temperature: 7.6 ⁰C
Wind direction: 237 ⁰C
Wind speed: 20 knots
Wave height: 2-3 ft.
Swell height: 5-6 ft.

Science and Technology Log

My last blog I said that I would talk more about the cam-trawl. This technology was created by scientists working on the pollock survey. The purpose behind the cam-trawl is to be able to put a net in the water with an open cod-end (basically a net with an opening at the end), and have images of the number, species, and size of fish that went through the net. Of course, sometimes some fish would have to be brought on deck so the otoliths and stomachs could be taken back to the lab in Seattle. Overall, this could eliminate taking so many research-based fish and/or invertebrate samples. When cam-trawl is used on acoustic-trawl surveys, the echograms can be matched up with the stereo-camera  images which can provide more data about the distribution of fish or other marine organisms in the water.

How the cam-trawl works: it is a stereo-camera system that takes snapshots of whatever comes through the net. These images allow the research team (including me on this leg) to determine the approximate number, species (some, not all), and size of fish that go through the net.

cam-trawl image

This still image from the cam-trawl shows a salmon and pollock against a black “curtain.”

The pictures are taken at the same time, but because of the slight difference in camera position, they look similar but not identical. You can mimic this with your eyes by looking at an object with only your right eye, then switching to looking with only your left eye. Did you see the same object but from a slightly different perspective? This is called disparity, or parallax (astronomers often use parallax to estimate the distance of far-away stars or other celestial objects). The program that was written for the cam-trawl (also by this research team) can then calculate the approximate size of the fish based on their relative positions.

In this photo, I’m using the cam-trawl measuring program to measure a sample of fish.

This screen shot shows the stereo-images and the yellow measurements that I’ve added. Using the lengths that I’ve chosen for the program, it calculates the approximate length (in meters) of the fish.

Personal Log

After several windy days with lots of swell, I’m happy to be in calmer waters. I’ve been working on the computer for some of the time which doesn’t go well with swell. I have also found it to be very tiring and tense on my body to be in constant motion and prepared to grab whatever I can to stay upright. I can’t tell you how hard it is to use a treadmill or take a shower in rough seas! BUT, for the time being, it’s calm and I just watched a great sunset over Kodiak island with a few humpback whale blows in the distance. If you are still wondering about the salmon in the picture above, it’s a chum!

Species Observed
humpback whales
northern fulmars
tufted puffins
black-footed albatross
storm petrels
porpoises (yesterday)

One response to “Anne Mortimer: Cam-trawl, July 14, 2011

  1. I have really enjoyed reading your blog. It looks like the experience of a lifetime.

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