Hokuto Shirakawa

Lamprey larvae as ecosystem engineers: physical and geochemical impact on the streambed by their burrowing behavior

Larval lampreys are thought to have a significant impact on freshwater ecosystems because of their high densities and broad distributions. However, the significance of their role in the stream ecosystem remains unclear. In general, it is reported that filter feeders alter their habitat conditions (oxygen, organic matter, and nutrients levels). We evaluated the impacts of two lamprey species larvae on the physical and geochemical factors in streambed by their burrowing and feeding behavior using in situ enclosures in a mountainous river located in northern Japan. During a 21-day field experiment, we found that the larvae treatments compared to the controls (no larvae) caused increased oxygen levels in the streambed, maintained the relative softness of the streambed and increased the FPOM abundance on the streambed surface. The substantial impact of the lamprey larvae on the physical and geochemical conditions in the streambed may be attributable to their burrowing and feeding behavior. Hence, our findings contribute to understanding the importance of the conservation and management of lamprey populations in order to maintain the freshwater ecosystem. Additional experiments using lamprey larvae are needed to reveal their impact on biotic factors (aquatic insects, algae, etc.) in and around the larval habitats.

Stage-specific distribution of Japanese sea cucumber Apostichopus japonicus in Maizuru Bay, Sea of Japan, in relation to environmental factors

The Japanese sea cucumber Apostichopus japonicus plays important ecological and economic roles in the coastal areas of Japan. Here, we aim to reveal the relation between habitat characteristics and density of juvenile and adult sea cucumbers in Maizuru Bay. Density of adult sea cucumbers was estimated by underwater visual surveys and bottom trawls and that of newly settled individuals by seedling collectors distributed in the bay. The physical environment of the sea bottom was evaluated from median diameter, total organic carbon, total nitrogen, carbon-to-nitrogen ratio, carbon isotope ratio, nitrogen isotope ratio, depth, slope, and distance from the coastline. Adult sea cucumber density was higher in the east side than the west side of Maizuru Bay, the former having a no-take zone for sea cucumbers. Juvenile sea cucumbers were most abundantly collected in the central part of the bay. High organic matter from the sea, short distance from the coastline, and high seabed gradient (steepness) had a positive effect on the density of adult sea cucumbers, and depth had positive effects on their body size. We suggest that sustainable management of sea cucumbers can be achieved by a combination of the establishment of no-take zones and increased protection of nursery habitats.

Assortative mating and divergent male courtship behaviours between two cryptic species of nine-spined sticklebacks (genus Pungitius )

Assortative mating based on ecologically divergent traits is a major driver of speciation among three-spined sticklebacks, however, little is known about reproductive isolation and variations in courtship behaviour among nine-spined sticklebacks. Here we demonstrate assortative mating and divergent courtship behaviours between two cryptic species of nine-spined sticklebacks using no-choice mate trials and kinematic analyses. Strong assortative mating was demonstrated in our tank experiments, highlighting the importance of prezygotic reproductive isolation in these species. Kinematic analyses showed that the freshwater type exhibited aggressive courtship behaviour with frequent ‘Attacking’ and spent more time exhibiting displacement activities. In contrast, the brackish-water type demonstrated a higher frequency of the ‘Zigzag-dance’ and ‘Nest-showing’. Our study highlights the value of nine-spined sticklebacks as a comparative system for the study of speciation and paves the way for future studies on the role of courtship behaviours and assortative mating in the evolution of sticklebacks.

Acoustic tracking in shallow coastal areas on spawning migrations of pacific herring (Clupea pallasii) using ultrasonic telemetry

The behavioral ecology of small pelagic fish has been studied by acoustic method for a long time. In particular, on pacific herring (Clupea pallasii), the behavioral ecology in shallow coastal areas which is observed spawning migration should be understood for the resource management and reproduction protection. In this study, supersonic telemetry was used for behavioral tracking of herring at shallow coastal areas. For the tracking, herring of regional group (n = 23) at Akkeshi bay (Depth ave. 5.78±2.15 m) in east Hokkaido were used. On the horizontal aspect, herring showed two migration patterns (pattern1: staying at the bay, pattern2: migration to Akkeshi lake (ave. 0.84±0.13 m) which is brackish water lake adjacent the bay). The lake is the main spawning grounds for herring, therefore the pattern 2 is indicated to be for spawning, and the pattern1 is before or after spawning. On the vertical aspect, diurnal distribution patterns were shown at the bay (day: ave. 5.41±3.78 m, night: ave. 3.83±2.60 m). B...

Annual change in the distribution of Sargassum horneri using a quantitative echo sounder in Yamada Bay, Iwate, Japan

Sargassum horneri, a type of seaweed, has attracted attention as a new fisheries resource in Japan because it contains large amounts of functional ingredients. Yamada Bay (16 km2), in Iwate, Tohoku, Japan, was seriously damaged by the 2011 tsunami, and a sustainable new fisheries resources would aid in the reconstruction of local industries. In this study, we estimated annual change in the distribution of S. horneri using a quantitative echo sounder for sustainability of S. horneri in Yamada Bay. Surveys were conducted in spring (during the growth season) 2013, 2014, and 2015. The acoustic data were obtained using a 120 kHz quantitative echo sounder (KCE300, Sonic Co.). S. horneri were detected on the rope of aquaculture facilities and the rock reef in the coastal sea area. Growth conditions were better in 2013 than those in 2014 and 2015. In 2013, water temperature in Yamada Bay was lower than in 2014 and 2015 because of the influence of cold water from the strong Oyashio Current. It was concluded that p...

An inter-individual communication biotelemetry system using Pseudo-random noise (Gold code)

Recently, bio-logging techniques involving electronic data-storage tags (data-logger) and acoustic transmitters have been increasingly used to understand migratory fish movements and behaviors. The monitoring of migratory fish can provide important insights into the marine ecosystem dynamics. The number of tags data-loggers used, however, is normally limited due to cost and the data-logger recovery rate is usually low. Therefore, we developed a new system that combines the acoustic communication and data logger. In other words, a data-logger that can share data with each other. One of the challenges is to develop an inter-individual communication biotelemetry system based on hydro-acoustic methods. In this system, we used the phase modulation of pseudo-random noise (Gold code). At the final stage of development, we will be doing conduct a field test of this inter-individual communication biotelemetry system on wild herrings and bonitos in the open sea. We expect this new technology to help overcome the bo...

Accuracy verification of tilt-angle estimation by split-beam echosounder: Comparison with acceleration data-logger data

Fish tilt-angle is one of the most important factors for fish target strength (TS) estimation, because the little change varies TS value dramatically. Split-beam echosounder collects a fish position with 3-D coordinates. Hence, if fish swim within acoustic beam continuously, the tilt-angle can be estimated from the gap between two positions easily. However, the accuracy of the estimation depends on performance of the echosounder. We verified accuracy of the estimated tilt-angle by comparing high-resolution angle from an acceleration data logger. Experiment was conducted at a large experimental tank (W:10m x D:5m x H:6m) in HRCFO. A 2-axis acceleration data logger (φ15 mm, L: 53 mm, M190L-D2GT, Little Leonardo, Inc.) was tethered into the tank, and towed at respective angle from -50 to 50 degree (1 degree pitch). Positions of the logger echos (from KCE300, Sonic Co., 120kHz) were exported by algorithms of Echoview7.1. After this process, the tilt-angles were calculated. 28 logger echos were identified from...