日本財団 図書館

Flatfish have been thought as a laterally compressed, benthic fishes that spend their adult lives lying on one side of their bodies on the seabed. However very little quantitative information of duration of a flatfish spends lying on the bottom is available. In the present study, four types of behaviors of the flatfish were recognized using acceleration profiles simultaneously with speed and depth data that was agreeable with the result of Olla et al. (1972). The acceleration data-loggers allowed us to distinguish between 'active' (swimming and burying) and 'inactive' phases (gliding and lying on the bottom of the aquarium). The burying behavior displayed by most flatfishes has been thought to be a way to both reduce the risk of predation and enhance the ability of the fish to catch prey (Ansell and Gibson, 1993). Our results also indicate that flatfish seem to rely on a swim-glide strategy (Fig.3). In the case of negatively buoyant, fish this behavior has been suggested to be an efficient way to save energy for swimming long-distances (Weihs, 1973).
The effects of captivity and instrumentation appeared to have little influences on the behavior of flatfish. Previous investigations of the swimming abilities of captive Japanese flounders showed that the maximum sustainable swimming speed was 1.0 and 1.2 BL/sec at 18.5 and 19.4℃, respectively (Hashimoto et al., 1996). These values are similar to the maximum swimming speeds recorded in other flatfish species e.g. 0.95 and 1.50 BL/sec at 15℃ (Duthie, 1982). Previous studies have demonstrated that tail beat frequency is related to swimming speed (Bainbridge, 1958; Nashimoto, 1980). Taking the equation, we could estimate the swimming speed using the tail beat frequency. The preferred tail beat frequency of captive Japanese flounders (1.45-2.29 Hz), corresponded to a 'preferred' swimming speed of 0.6- 1.2 BL/sec. However, when compared with the range of values obtained by Duthie, the Japanese flounders in our experiments rarely swam in excess of 1.2 BL/sec. Thus, it might be inferred from these results that we could find no clear differences in swimming speed and tail beat frequency between instrumented and non-instrumented fish.
Most telemetry studies have estimated a rate of fish activities or 'rate of movement' (speed over-ground) by measuring the distance traveled over certain duration of time intervals (Greer-Walker et al., 1978; Kakimoto et al., 1979). However, such methodology does not take into consideration fine-scale movements, and therefore, the actual rate of activity may be underestimated. Indeed, 'rate of movement' measured from instantaneous swimming speeds, using speed meters and from tail beat frequency using acceleration data-loggers (mean, 0.79- 1.10 BL/sec, N=4), are 1.8-15.7 times faster than those estimated by methods using the 'rate of movement' (mean, 0.07-0.43 BL/sec, N=3, Kakimoto et al., 1979). The instantaneous swimming speed, according to the acceleration data-logger, is consisted to be more similar to the 'rate of movement' if a fish only swims in straight lines because of its direct measurement nature. Similar observations were made by Gruber et al. (1988) using water speed sensors to determine the instantaneous swimming speeds of free-swimming lemon sharks. However, water speed sensors could not indicate whether an animal was continually swimming or gliding in order to save energy (Weihs, 1973). Thus, to estimate energy expenditure precisely, one needs monitoring of both the swimming speed and tailbeat activity of free-swimming fish simultaneously. Acceleration data-loggers appear to be a useful and reliable tool for accurate recording of the tail beat frequency of freely swimming flatfish and for estimating their activity. Future improvements in the miniaturization of the logger would allow experiments on smaller fish.
We would like to thank the following people for their cooperation: Toshihiro Watanabe and Shintaro Yamasaki of Fishing Technology Division, National Research Institute of Fisheries Engineering; Seiji Otani, Takashi Kitagawa, Ken Yoda and several other students for their assistance with these experiments, two anonymous referees and M.F. Cameron for constructive criticism of the manuscript. This study was supported by Grant-in-aid for scientific Research (C) from the ministry of Education, Science, Sports and Culture (No.C1266017).
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