Broad-band versus narrow-band irradiance for estimating latitude by archival tags |
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Authors: | Hisham A Qayum A Peter Klimley Ronald Newton John E Richert |
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Institution: | (1) Cooperative Institute for Marine Resource Studies, Hatfield Marine Science Center, Oregon State University, 2030 S.E. Marine Science Drive, Newport, OR 97365, USA;(2) Department of Wildlife, Fish, and Conservation Biology, University of California, Davis, One Shields Avenue, Davis, CA 95616, USA;(3) Tahoe Research and Development, 2230 Damon Road, Carson City, NV 89701, USA |
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Abstract: | The relative effectiveness of different bands of irradiance to estimate the latitude of archival tags was evaluated. These
tags are placed on fishes in order to describe their movements during long distance migrations. Measurements were recorded
of broad-band irradiance with and without a cosine collector and narrow-band irradiance of seven narrow bands with 50% attenuation
30 nm on either side of their central wavelength of 400 (violet), 450 (blue), 500 (blue–green), 550 (green), 600 (yellow),
650 (orange), and 700 nm (red). A holographic, cosine collector was used to reduce the vertical transmission of irradiance
to the sensor and to increase horizontal transmission of irradiance so the sensor detected more of the diffuse irradiance
penetrating the water at dawn and dusk. Daily measurements were made during seven periods of 1–2 days each, beginning 28 June
(after 21 June solstice) and ending on 6 October 1999 (after September 23 equinox) of submarine irradiance at 15-s intervals
at a fixed depth (10 m) and location (38.31°N; 123.08°W) in Horseshoe Cove, California. Irradiance transmission at this site
is intermediate between the clearest offshore waters, where blue irradiance (450 nm) penetrates farther with depth than green
irradiance (550 nm) and most oceanic and coastal waters, where green penetrates farther than blue irradiance. Two algorithms
were used to estimate latitude, the maximum slope method and the maximum logarithmic difference method. The broad-band, cosine-corrected
light, excluding those deployments near the equinox when error is highest, produced an estimate of latitude of 38.30° for
both methods and a latitudinal error of ±34.4 km for the former and ±27.2 km for the latter. The mean latitudinal error for
non-cosine-collected, broad-band irradiance was ±190.9 km, using the slope algorithm and ±184.8 km using the difference algorithm.
The blue band of irradiance, which attenuates least with increasing depth in clear, oceanic water, also produced a comparatively
high-latitudinal error of ±163.8 km error for the former algorithm and ±170.4 km for the latter algorithm. Tag designers should
consider using cosine-collectors over the irradiance sensors on their archival tags to increase the accuracy of position estimates. |
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