Tomonori Hiraishi

Validity and layout of “NaLA”: a net configuration and loading analysis system

We have developed a system for determining fishing net configuration and for loading analysis, called NaLA, which uses 3D dynamic visualization; it is available for general and practical use with computer-aided design systems. This paper examines the validity of the numerical model employed in NaLA by using flume tank tests, and the model is then refined by modifying the drag parameters to improve the accuracy of estimates. There is close agreement between experimental and theoretical values for net shape configuration, and experimental values for tension loads agree well with calculated results when the drag coefficients are treated as a function of the Reynolds number. Our numerical method, in which four adjacent meshes are grouped into one large mesh, is effective in reducing computational effort.

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.

Application of NaLA, a fishing net configuration and loading analysis system, to bottom gill nets

The net-shape and loading analysis system (NaLA) was developed to determine fishing net configuration and load in a previous study. The system has since been applied to general gill nets and aquaculture nets, and its validity has been proven through model experiments in tanks. In this study, the system was applied to estimate the dynamic behavior of a bottom gill net for walleye pollock, to test the system’s applicability of the system to gear operations in the field. To obtain in situ data, four bottom gill net operations were performed in February 2004 off the coast of Sawara, Hokkaido, Japan. During operations, vertical displacements of the bottom gill net’s float and sinker lines were measured as representative values of gear behavior, and ocean current direction, and speed at the gear position were observed simultaneously. Then, bottom gill net behavior was simulated using NaLA, incorporating observed environmental conditions and gear specifications. The resulting calculated behavior was compared to measured behavior in terms of the relationship between net height and environmental or setting conditions. Agreement between the calculated and measured net behavior was found. Thus, it is believe that our NaLA calculation model has the potential to simulate the dynamic behavior of bottom gill nets in situ.

Comparison of visual acuity and visual axis of three flatfish species with different ecotypes

The visual acuity, visual axis and visual accommodation of pointhead flounder, slime flounder, and red halibut were determined to obtain basic knowledge for developing appropriate fishing gear and fishing methods for sustainable fisheries. Each of these species has a different ecotype in terms of habitat, depth and prey species. Thus, it was hypothesized that they may differ in terms of visual acuity, visual axis and visual accommodation. Few studies have compared these characters in flatfishes from different ecotypes. We used histological methods to determine visual acuity (i.e. cone cell density) and visual axis (i.e. cone cell distribution) in each of these species. The maximum visual acuity was 0.127 in pointhead flounder (total length, TL 344 mm), 0.092 in slime flounder (TL 372 mm) and 0.109 in red halibut (TL 336 mm). Based on the cone cell distribution in the retina, the visual axis was upward and forward in pointhead flounder, forward and downward in slime flounder, and downward in red halibut. Finally, the mean angle of lens movement was −2° in pointhead flounder, −13° in slime flounder and −32° in red halibut. This measurement of lens movement indicated that the average near-point distance was 0.87× TL in pointhead flounder, 0.65 × TL in slime flounder and 1.02 × TL in red halibut. At similar TL (336–355 mm), the visual acuity of these species differs depending on the direction in which they are looking.

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.

Audiogram measurement based on the auditory brainstem response for juvenile Japanese sand lance Ammodytes personatus

In this study, the auditory thresholds for juvenile Japanese sand lance Ammodytes personatus were measured based on its auditory brainstem response (ABR). The amplitude of the ABR waveforms to a sound stimulus were larger than that of the electric background noise caused by general brainwaves and myogenic signals after the averaging procedure. Japanese sand lance responded to low frequency sounds between 128 Hz and 512 Hz with a sound pressure level of 115–125 dB. As the test frequency decreased, so did the auditory threshold level, and the level was about 116 dB at 128 Hz and 181 Hz. These results indicate that Japanese sand lance can detect low frequency sound but are less sensitive than other fish species. These high thresholds are probably caused by the lack of a swim bladder.

Method for underwater measurement of the auditory brainstem response of fish

The auditory brainstem response (ABR) of fishes is commonly measured by bringing the heads of the fishes out of the water in a small tank. However, this method is inapplicable to experiments for large fishes that are economically important in large spaces such as the sea or in a large tank. This paper describes a method of recording, the ABR for fishes in water, without exposing the fish heads to air, by using a waterproof, insulated electrode. To evaluate the effectiveness of this method, the goldfish Carassius auratus was investigated, and the ABR waveform and auditory thresholds measured in water were compared with those measured on the surface. Both ABR waveforms and auditory thresholds showed similar trends between the two methods. The underwater ABR method is useful to measure the auditory thresholds of larger fish in natural or on-site environments such as the sea, net enclosures and lare aquaria in which precise positioning of the fish is not possible. However, more improvement is needed to apply this method to large fishes.

The Fundamental Study on the Optimum Design of Shrimp Pot for the Coon-Stripe Shrimp (Pandalus Hypsinotus Brandt) in Hokkaido

This study was conducted to develop an optimally designed pot that is capable of controlling catch selectively. To achieve the optimum design for the shrimp pot, tank experiments were conducted to investigate the catch mechanism and catch selectivity of differently shaped pots. In this study, the catch selectivity of pots was estimated with different slope lengths. The experiments were conducted using model shrimp pots with five different slope lengths (24, 34, 44, 54, and 64cm) in an experimental tank (2 m × 2 m × 1 m). The catch mechanism was analyzed through a video recording system and catch selectivity was estimated using the SELECT model and master curve method. Two models using the likelihood ratio test and Akaike’s information criterion (AIC) evaluated the goodness of fit of the selectivity curve. That results, the slope length with 50% retention of individuals with carapace length of 25 mm (about 1 year old) was 60.1cm, which is about 16 cm longer than the commercial pot design.© 2007 ASME