Tsutomu Takagi

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.

Ontogenetic changes in schooling behaviour during larval and early juvenile stages of Pacific bluefin tuna Thunnus orientalis.

Schooling was first observed at 25-27 days after hatching (26. 2-33. 8 mm, total length) in the Pacific bluefin tuna Thunnus orientalis. At this time, the mode of swimming changed from intermittent sprinting to continuous cruising, and this allowed the fish to adjust to an inertial hydrodynamic environment.

Employing Relative Entropy Techniques for Assessing Modifications in Animal Behavior

In order to make quantitative statements regarding behavior patterns in animals, it is important to establish whether new observations are statistically consistent with the animals equilibrium behavior. For example, traumatic stress from the presence of a telemetry transmitter may modify the baseline behavior of an animal, which in turn can lead to a bias in results. From the perspective of information theory such a bias can be interpreted as the amount of information gained from a new measurement, relative to an existing equilibrium distribution. One important concept in information theory is the relative entropy, from which we develop a framework for quantifying time-dependent differences between new observations and equilibrium. We demonstrate the utility of the relative entropy by analyzing observed speed distributions of Pacific bluefin tuna, recorded within a 48-hour time span after capture and release. When the observed and equilibrium distributions are Gaussian, we show that the tunas behavior is modified by traumatic stress, and that the resulting modification is dominated by the difference in central tendencies of the two distributions. Within a 95% confidence level, we find that the tunas behavior is significantly altered for approximately 5 hours after release. Our analysis reveals a periodic fluctuation in speed corresponding to the moment just before sunrise on each day, a phenomenon related to the tunas daily diving pattern that occurs in response to changes in ambient light.

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.

Changes in the retinal cone density distribution and the retinal resolution during growth of juvenile Pacific bluefin tuna Thunnus orientalis

Tuna and marlin have well-developed vision. It has been suggested that the vision of the tuna is a major sensory element greatly affecting its behavior. In a behavioral experiment, Nakamura determined the visual acuity of adult skipjack tuna Katsuwonus pelamis. Kawamura et al. calculated the visual acuity of adult bluefin tuna Thunnus thynnus, histologically. Although there are developmental changes in the visual capability and increases in the visual acuity in many fish species with growth, juvenile Pacific bluefin tuna Thunnus orientalis has not been investigated. Furthermore, Fritsches et al. noted that the striped marlin Tetrapturus audax has different visual capabilities along different visual axes. The position of the area of high cell density, or the area centralis in the retina, is related to both habitat and the main visual axis of feeding behavior. Knowledge of developmental changes in retinal topography gives important clues to behavioral changes that occur with growth. To measure the distribution of retinal cone density at each growth stage, it is necessary to investigate specimens from each stage. Recently, full-cycle culture of the Pacific bluefin tuna Thunnus orientalis was achieved in the Fisheries Laboratory, Kinki University, Wakayama, Japan. Using full-cycle cultured specimens of this species, including juveniles, it was possible to investigate the cone density distribution at each stage in this species. In the present study, the authors determined the developmental changes in the cone density distribution, visual axis and minimum separable angle of cones with growth histologically. Specimens of Pacific bluefin tuna from six stages that were full-cycle cultured during 2003–2004 in the Fisheries Laboratory, Kinki University, were used: 30 (total length [TL] = 4.7 cm), 35 (TL = 6.2 cm), 41 (TL = 6.5 cm), 46 (TL = 11.1 cm), 80 (TL = 31.8 cm) days after hatching, and 1 yearold (TL = 102.6 cm). The eyes of the specimens were enucleated and fixed in Bouin’s solution for 24 h. Each retina was then divided into nine regions for 30to 46-day specimens, 25 regions for 80-day specimens and 33 regions for 1-year specimens, according to the eye size. After paraffin embedding, the retina was cut into sections parallel to the retinal surface using a microtome. The sections were stained with hematoxylin–eosin. For quantitative analysis, the sections were examined under light microscopy, photographed, and the number of cones in 0.01 mm of each region in the photomicrograph was counted. Photomicrographs of tangential sections of the retinal cones showed that twin cones form a regular mosaic of parallel rows, although there were occasional irregular single cones in each region from specimens at every stage. An example of a retinal tissue older than 1 year after hatching is shown in Figure 1. The total numbers of twin and single cones in 0.01 mm of each region were counted. Then, the density distributions of both cones determined from the right retinas of the specimens (30, 35, 41, 46, 80 days, and 1 year after hatching) were graded, drawn and displayed using contour lines at 25-cone intervals (Fig. 2). The specimens at even the youngest stage had already metamorphosed and were feeding on fish prey. The characteristics of the cone density distribution changed with each growth stage. In the specimen aged 30 days after hatching, the cone density in each peripheral area reached five times (668/132 = 5.03) that of the bottom region. Therewasnodefinitedirectionof acutevision, such as a visual axis, since no specialized region of maximum cone density existed. It seems reasonable to assume that the distribution at this stage *Corresponding author: Tel: 81-742-43-1511. Fax: 81-742-43-1316. Email: ns_torisawa@nara.kindai.ac.jp Received 10 November 2005. Accepted 27 March 2006. FISHERIES SCIENCE 2007; 73: 1202–1204

Schooling behaviour of juvenile Pacific bluefin tuna Thunnus orientalis depends on their vision development

The effects of vision development and light intensity on schooling behaviour during growth in juvenile Pacific bluefin tuna Thunnus orientalis were investigated using both behavioural and histological approaches. The schooling behaviour of three age groups [25, 40 and 55 days post hatching (dph)] of juvenile T. orinetalis were examined under various light intensities. Subsequently, schooling variables, such as the nearest neighbour distance (D(NN) ) and the separation swimming index (I(SS) ), were also measured under different light intensities. Furthermore, retinal indices of light adaptation in juvenile fish at each experimental light intensity and visual acuities in six stages (25-55 dph) of juveniles were examined histologically. During growth, the light intensity thresholds of I(SS) decreased from 5 to 0·05 lx, and D(NN) under light conditions (>300 lx) also decreased from 9·2 times the standard length (L(S) ) to 1·2 times L(S) . The thresholds of light intensities for the light adaptation of retinas in juveniles (25-55 dph) similarly decreased from 5 to 0·05 lx with growth. In addition, the visual acuities of juveniles developed from 0·04 to 0·17 with decreasing D(NN) . These data clearly indicate that the characteristics of schooling behaviour strongly correspond to the degree of vision development. Juvenile T. orinetalis also appear to be more dependent on cone rather than rod cells under low light intensity conditions, resulting in a relatively high light intensity threshold for schooling. These results suggest that juveniles can adapt to darker conditions during growth by developing improved visual capabilities.

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.

Numerical Analysis of Net Cage Dynamic Behavior Due to Concurrent Waves and Current

Large offshore net cages have been rather successful in reducing coastal contamination and developing aquacultural technology for raising large body-size species such as bluefin tuna. Deformation and shrinkage of net cages due to severe current and waves is one of the main causes of mortality of cultivated fish, and is thus of great concern for marine cage aquaculture. Even though the cage depth can be determined by pressure sensors at several locations on the cage, this is generally insufficient to measure the deformation and shrinkage. In this study, the dynamic shape and volume of a net cage under the influence of current and waves was analyzed using a numerical net geometry simulator previously validated by tank tests. The dynamic behavior of finite mass points distributed on the net cage under various wave and current conditions was simulated in detail, and reduction coefficients of the volume were calculated by the positions of these mass points on the cage. A drastic reduction of the cage volume occurred for a current velocity of 0.28–0.39 m/s. As the current velocity increased, the deepest point of the cage changed position, moving toward the downstream direction. The effective reduction of cage volume for a wave height of 3 m and a current velocity of 0.26 m/s was the same as that for a wave height of 5 m without any current. These results suggest that the volume deformation must be based on the actual measured depth of each part of the net cage, that the combination of wave levels and current velocities may have synergistic effects on the reduction of cage volume, and that our computational method is valid for estimating the volume reduction of a net cage under the influence of a concurrent wave and current field.Copyright

Seasonal distribution of adult crucian carp nigorobuna Carassius auratus grandoculis and gengoroubuna Carassius cuvieri in Lake Biwa, Japan

Seasonal habitat use by nigorobuna Carassius auratus grandoculis Temminck et Schlegel and gengoroubuna Carassius cuvieri (Temminck et Schlegel) in Lake Biwa was investigated using acoustic telemetry. Twenty-three nigorobuna and 11 gengoroubuna specimens caught using set-nets in the lake’s south basin were surgically fitted with acoustic transmitters and then released. Signals from the fishes were recorded by 23 receivers installed around the lake. Between April and June 2007, the first spawning season after release, signals were received from all tagged fishes. Thereafter, until the second spawning season, signals were collected from 26 and 45% of released nigorobuna and gengoroubuna individuals, respectively. Seasonal habitat preferences for these species were studied by distance-based analysis. The analysis revealed that nigorobuna tended to stay near their spawning area in the south basin of the lake throughout the year, whilst gengoroubuna tended to show a seasonal migration pattern between the north and south basins. After the spawning season, the latter species migrated to the north basin where it remained until the next spawning season, when it returned to the south basin. This is the first report of seasonal migration of nigorobuna and gengoroubuna in Lake Biwa.

Dynamics of Boat Seine Fishing Using a Net Geometry Simulator

Boat seine fishing is used to catch mainly demersal species; it has many benefits such as reduced energy and labor costs, and low damage to the sea bottom if it is controlled. This fishing technique has complex motion between the seine boat and the supple net over large operating areas; therefore, the dynamics of its operating processes during fishing are unknown. To investigate the dynamics of boat seine fishing in detail, we calculated its dynamics during a fishing operation using a net geometry simulation system that we developed. Two experimental surveys were carried out in January 2007 at Hokkaido, Japan. Latitude, longitude, and depth were measured in 10 places, from casting the anchor buoy to picking up the seine net. Depth sensors were installed at the top and bottom of the seine net mouth to record a time series of net shape during fishing. Using our net geometry simulator, we simulated net shape and the dynamics of boat seine fishing based on ship position and net depth. Very close agreement was observed between the experimental and simulated depths of the top and bottom of the net mouth during fishing. Three-dimensional visualization of the simulation results showed the capture processes of this fishing technique in a large scale operation. This study shows that our method is valid for modeling fishing operations to better implement smart fishing.Copyright