Shinsuke Torisawa

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.

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.

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.

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.

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

Satellite Tagging of Blue Sharks (Prionace glauca) in the Gulf of Lions: Depth Behaviour, Temperature Experience and Movements – Preliminary Results

The lack of reliable fishery-dependent data and fundamental understanding of the biology of most shark species causes concern for the sustainable management of shark populations in the Mediterranean Sea. The blue shark (Prionace glauca), a wide-ranging shark occurring in all tropical and temperate seas, is also one of the most heavily fished sharks in the world and so in the Mediterranean. In other oceans, its distribution is depicted as complex with spatial and temporal segregation by sex and size. In the Mediterranean Sea, movements and behaviours of this species are poorly known. The study aims at investigating on habitat occupancy, residency times and migratory pathways as well as providing behavioural data on temperature experience and swimming depth of the blue shark. This study strives to identify habitats and regions that are essential for the survival of sharks, while also determining when and where sharks are most vulnerable and will assist in the conservation of the species. The use for the first in the Mediterranean Sea of different types of satellite tags is proposed to investigate the ecology of this species. The preliminary results of the “smart tags” deployed on eight individuals are presented.

How many fish in a tank? Constructing an automated fish counting system by using PTV analysis

Because escape from a net cage and mortality are constant problems in fish farming, health control and management of facilities are important in aquaculture. In particular, the development of an accurate fish counting system has been strongly desired for the Pacific Bluefin tuna farming industry owing to the high market value of these fish. The current fish counting method, which involves human counting, results in poor accuracy; moreover, the method is cumbersome because the aquaculture net cage is so large that fish can only be counted when they move to another net cage. Therefore, we have developed an automated fish counting system by applying particle tracking velocimetry (PTV) analysis to a shoal of swimming fish inside a net cage. In essence, we treated the swimming fish as tracer particles and estimated the number of fish by analyzing the corresponding motion vectors. The proposed fish counting system comprises two main components: image processing and motion analysis, where the image-processing component abstracts the foreground and the motion analysis component traces the individual’s motion. In this study, we developed a Region Extraction and Centroid Computation (RECC) method and a Kalman filter and Chi-square (KC) test for the two main components. To evaluate the efficiency of our method, we constructed a closed system, placed an underwater video camera with a spherical curved lens at the bottom of the tank, and recorded a 360° view of a swimming school of Japanese rice fish (Oryzias latipes). Our study showed that almost all fish could be abstracted by the RECC method and the motion vectors could be calculated by the KC test. The recognition rate was approximately 90% when more than 180 individuals were observed within the frame of the video camera. These results suggest that the presented method has potential application as a fish counting system for industrial aquaculture.

Mathematical and Experimental Analysis of Schooling Behavior During Growth in Juvenile Chub Mackerel: Considerations of Population Density and Space Limitation

Population density and space limitation have proven important considerations for both fisheries management and aquaculture, resulting in intense interest in the development of new techniques and technologies for management and hatchery applications. To investigate the effects of space limitation and population density on the schooling behavior of fish, we examined the schooling behavior of captive juvenile chub mackerel. Three groups of fish were collected; collections were made at 18, 42, and 73 days post-hatch (dph) at which mean body lengths were 2.28, 8.83, and 11.4 cm, respectively. The two-dimensional movement of individuals during 200-s observation periods was digitized and processed. A mathematical model based on Newton’s second law of motion was used to quantify the forces dominating schooling behavior. The forces of swimming motion were quantified for each fish in a school as the propulsive force due to the swimming ability of each fish, the interactive force used to keep the proper distance and maintain similar velocity with neighboring fish, and the repulsive force used to keep a proper distance from the wall. The magnitude of the repulsive force was minimized in the 18-dph school, maximized in the 42-dph school, and decreased in the 73-dph school. The magnitudes of the propulsive and interactive forces increased with growth. Thus, the interactive force, which was critical for school formation and maintenance appears to reduce the importance of the repulsive force and causes the decline in the repulsive force between 18 and 73 dph.Copyright