Katsuaki Nashimoto

High-frequency depth recording reveals the vertical movement of flounder in the Tsugaru Strait of northern Japan

Analysis of high-frequency depth-recording data of adult Japanese flounder Paralichthys olivaceus (Temminck & Schlegel) with depth/temperature logging tags, released in the Tsugaru Strait of northern Japan, has yielded new insights into behavioural differences on vertical movement. Here, we document diel differences in fine-temporal-scale swimming behaviour observed from six flounder released during the no-spawning season. While the flounder remained on the seabed for the majority of the recording period, fish occasionally left the seabed, swam into the water column and then swam back to the seabed. The mean swimming duration per tagged fish ranged from 44 ± 49 to 94 ± 164 s, with a maximum observed swimming duration of 44.5 min. Vertical movements included one or more clear ascent and descent phases highlighted by a distinct peak. Our results reveal that in the no-spawning season nocturnal swimming is more active and frequent than during the day. Our results provide important information about diel differences in swimming behaviour with respect to vertical movement and also show the vulnerability of Japanese flounder to capture by bottom-fishing gear (e.g. trawling) during the day.

Color vision, spectral sensitivity, accommodation, and visual acuity in juvenile masu salmon Oncorhynchus masou masou

Color vision, spectral sensitivity, accommodation, and visual acuity were examined in juvenile masu salmon Oncorhynchus masou masou to obtain fundamental information about the visual system. Two types of S-potentials were recorded from 415 horizontal cells in isolated retinas from 34 cultured freshwater masu salmon (114–219 mm standard length, SL). Although horizontal cells recording S-potentials were not identified, the horizontal cells were confirmed because their responses were maintained while the stimulus remained. The variety of chromaticity (C)-type S-potentials indicated well-developed color vision. The analysis of the luminosity (L)-type S-potentials response to ultraviolet light. The direction and extent of lens movement induced by electrical stimulation was measured in 12 cultured masu salmon (99.0–142.5 mm SL). The results indicated that the visual axis was upward and forward, and that the range of accommodation was from 0.79×SL in front of the eye to infinity. In histological analysis of the retinas of five wild smolts (100–118 mm SL), the maximum cone densities (276–345 cones/0.01 mm2) were detected in the ventral to temporal regions. The visual acuities assessed by histological methods were 0.069–0.075.

Behavioral responses of arabesque greenling to trap entrance design

Fish catch efficiency can vary with the design of the trap entrance. To clarify how trap entrance design affects catch efficiency, the responses of fish to different trap entrance designs with different inclination angles or funnel lengths were observed. The passing ratio (i. e. catch) increased and then decreased with increasing inclination angle. An inclination angle of 34° had the highest passing ratio of 43%; the lowest passing ratio was 8% at an angle of 53°. A significant difference in passing ratio was found between two entrance designs (P=0.006). There were significant differences in passing ratio for entrance designs with different funnel lengths (P=0.01). The highest passing ratio was 38% for a funnel length of 29 cm. The approach and passing durations were also compared among the trap entrance designs. The results suggest that modifying the entrance shape can significantly improve the catch efficiency.

Visual acuity and spectral sensitivity of the elkhorn sculpin Alcichthys alcicornis

In this study, the visual acuity, visual accommodation, and spectral sensitivity of the elkhorn sculpin were determined. The elkhorn sculpin Alcichthys alcicornis is a typical groundfish that has conspicuously large eyes. They are caught by bottom gillnets, trawl nets, bottom line angling, and other fishing methods. Although fish behavior arising from visual stimuli is important in these fishing methods, there is little information available regarding the visual sensitivity of the elkhorn sculpin. Three specimens, with total lengths of 300–310 mm, were studied. Visual acuity was calculated, from the maximum cone density in the retina, to be between 0.10 and 0.14. The minimum separable angles were calculated to be 7.14–10.37 min. Measurement of lens movement in five specimens indicated that the near-point distance was between 0.87 and 1.53 times the total length. Lens direction was estimated to be 11–22° in the downward visual axis. Spectral sensitivity was also determined electrophysiologically by recording S-potentials. Three types of response were obtained from 21 specimens, including L responses that showed hyperpolarization at all wavelengths of stimulating light, and two C responses that showed wavelength-dependent hyperpolarization and depolarization. Based on these results, the elkhorn sculpin has color vision and its sensitivity is most acute in light with a wavelength of 554 nm.

Effects of entrance design on catch efficiency of arabesque greenling traps: a field experiment in Matsumae, Hokkaido

A field experiment was conducted in the Matsumae area of Hokkaido, Japan, during June and July 2002, to investigate the effects of different entrance designs on the catch efficiency of fish traps by fishing with commercial traps (entrance inclination angle [α]=37°; funnel length of entrance [Lf]=22 cm) and experimental traps. The experimental traps were of the same size and similar design as commercial traps, with different entrance inclination angles (trap E1: α=46°; E2: α=27°; E3: α=0°; all Lf=22 cm) or funnel lengths (E4: α=37°, Lf=8 cm). In total, 2200 fish during 200 trap hauls were captured. The catch was significantly higher using both traps E2 and the commercial trap than with trap E3 (P<0.05), and the catch of trap E2 was higher than that of the commercial trap. There were no significant differences in mean fish body length or the frequency distributions of body length among trap types (E1, E2, E3 and commercial). The funnel length of the entrance also affected the catch of traps. Trap E4 had significantly higher catches than the commercial trap (P=0.04) when traps were deployed for a 1-day soak time. Fish body length frequency distributions did not differ between trap E4 and the commercial trap. The results showed that catch can be greatly affected by trap entrance designs.

Auditory Threshold of Black Rockfish.

The auditory threshold of the black rockfish Sebastes schlegeli was determined by condition-ing method using a sound coupled with bait in a tank. The audio-singnals used were pure tones of 70 to 1400Hz. The mean size of the fishes used in this experiment was 6.53g in body weight and 75.3mm in body length. A positive response was defined by analyzing the swimming performance of the experimental fishes. The fish learned to come to a feeding point in response to the stimulus sounds after 7 to 14 days training. The threshold in sound pressure was 90.6 dB (re 1μPa) at 100Hz which was most stimulative to the fish.

Oscillations of fish reef blocks hung in the water.

A fish reef block model (a sphere and a cube were also used as the simple artificial reef models) was swung by a wire string from a fixed point with an initial angle in still water. Free-damped oscillation curves which were measured for different lengths of the string, were compared with the curves obtained by solving a differential equation of motion. In the solution a graphical method of trajectory on a phase plane was used.The experimental curves fitted well in the calculated ones for the first 1/4 period in every oscillation. Therefore, we applied this method to the actual fish reef block hung by a wire rope from a derrick barge and estimated the length of the wire rope resonant with the rolling period of barge while installing the fish reef block in sea water. The results suggest that the resonance hardly occurs when the fish reef block is in the water, but it may easily occur in the air.