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Fish Dine by Carefully Monitored Light

Photonics Spectra
Oct 1997
Kathleen G. Tatterson

Do the gilled inhabitants of Georges Bank, an important feeding and spawning ground just east of Cape Cod, Mass., prefer to take their meals at high noon, or are their appetites whetted by the ambiance of a sunset or the moonlight?

Researchers mount International Light's photodiode irradiance sensor on a conductivity temperature-depth rosette frame for deployment.

Researchers at Woods Hole Oceanographic Institute in Woods Hole, Mass., are using International Light's photodiode irradiance sensor to study the feeding habits of larval cod as part of US GLOBEC, a project funded by the National Science Foundation and the National Oceanic and Atmospheric Administration. The program seeks to determine how global climate change can affect the distribution, abundance and production of animals in the sea. Jeff Van Keuren and Scott Gallager at the institution are looking at how light quality and quantity within the water column affect how larval cod capture prey.

Light conditions vary with the position of the sun and climatic factors such as amount of cloud cover, wave conditions and timing of stratification. Gallager and Van Keuren turned to International Light Inc. for equipment to monitor these changing light conditions. The research team is using International Light's CEDE silicon and SED005 GaAsP photodiode detectors and an IL1700 radiometer to do the job.

"International Light's line of photodiode detectors and radiometers were selected for use due to their accuracy, large dynamic range, ease of use and cost," said Van Keuren. "The modular nature and small size of the sensors and the wide selection of available filters and accessories also factored into the selection of these detectors." Special filters added to each sensor limit the measured irradiance to the relative amounts of the wavelengths that the fish can see -- a waveband centered near 520 nm.

To measure light attenuation, the researchers attach these sensors to a frame and lower them through the water column while recording how the irradiance level drops off as the depth changes. To continuously monitor surface irradiance within this same spectral waveband, a matched set of sensors sits as high up on the research vessel as possible to avoid any shadows from the ship itself.

Data collected thus far suggests that the fish have an optimal range of irradiance levels within which they can use their vision to seek out prey. In addition, light shifts caused by the time of day, the amount of cloud cover and the season all appear to play a role in feeding ability. The scientists are confident that these results will provide clues about how changes in the Earth's climate affect marine species.

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