Underwater visibility of the man-made spawning bed of oval squid Sepioteuthis sp. 1 (Aka-ika) in the coastal waters of Tanega-shima Island
2018
Okumura, J. (Mie University, Tsu, Mie (Japan). Graduate School of Bioresources) | Kato, A. | Yanagita, D. | Nakamura, T. | Ishikawa, A. | Kurashima, A. | Miyazaki, T.
In the coastal waters of Tanega-shima Island, man-made spawning beds for oval squid Sepioteuthis sp. 1. (Aka-ika) are set at the sea bottoms in depth of 30-50 m. In such a spawning zone, it is possible that the visibility of the squid may be limited due to a decrease in brightness with increasing water depth and an increase in turbidity sediment-derived in the sea-bottoms. To assess the underwater visibility of spawning beds by spawning squid, we investigated its visual accuracy and optical property of the water. Attenuate coefficient was estimated as 0.42-0.48 /m from the observed turbidity and transparency, and visible distance to a Ika-shiba spawning bed as a large black object was calculated to be about 10 m. Vertical profiles of photon flux and chlorophyll-a concentration showed that the light necessary for production of dissolved organic matter reach the sea floors in this area. Thus, it was inferred that the visibility of the spawning squid is not limited by the brightness even in Tanega-shima island's deeper spawning grounds during daytime. The visual accuracy of the parent Aka-ika was estimated to be 28.9 cycles/degree, which seems sufficient to visually recognize the man-made spawning bed.
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