Age‐dependent distribution of juvenile southern bluefin tuna (Thunnus maccoyii) on the continental shelf off southwest Australia determined by acoustic monitoring

Juvenile southern bluefin tuna (Thunnus maccoyii, SBT) were monitored in nearshore waters off southwest Australia using acoustic tagging and monitoring over five austral summers (2002/2003–2006/2007) to determine patterns in age‐based distribution of SBT. A total of 20–70 receivers were deployed in early December along one to three cross‐shelf transects and at three inshore topographic features (lumps) where SBT are known to occur; and a total of 59–84 juvenile SBT (41–90 cm fork length) were tagged and released each year. After several months, receivers were recovered, and data extracted. In 2002/2003 and 2003/2004, 2‐yr‐old SBT were detected more frequently in nearshore areas than did 1‐yr old fish. Similarly in 2004/2005 and 2006/2007, it was the larger 1‐yr‐old SBT that were detected more frequently at inshore lumps (>85% of detections) while small 1‐yr‐old SBT were detected more widely over the study area. In 2005/2006, although large 1‐yr‐old SBT were still detected more frequently at inshore lumps than small 1‐yr old SBT, the percentage of all detections at lumps was lower at 31.5%, indicating wider distribution of both small and large age‐1 fish. The observed age‐dependent distribution pattern may be enhanced by the distribution of prey, which disperse or concentrate depending on the local oceanography.     

A new device for monitoring the activity of freely swimming flatfish, Japanese flounder Paralichthys olivaceus

ABSTRACT:   The tail beat and activity behavior of four captive Japanese flounder Paralichthys olivaceus , were monitored with acceleration data-loggers while the fish swam in an aquarium. Depth, swimming speeds and two-axis acceleration data were collected continuously for approximately 20 h per fish. Simultaneously, the swimming behaviors of the fish were filmed at different angles. Using the specific characteristic of the acceleration profiles, in tandem with other types of data (e.g. speed and depth), four behavioral patterns could be distinguished: (i) 'active' swimming; (ii) burying patterns; (iii) 'inactive' gliding; and (iv) lying on the bottom. Tail beat frequency ranged from 1.65 ± 0.47 to 2.04 ± 0.25 Hz (mean ± SD; n  = 4). Using the relationship between tail beat frequency and swimming speed, the 'preferred' swimming speed of the fish was estimated to be between 0.6 and 1.2 body lengths (BL)/s. Additionally, fish rarely swam faster than 1.2 BL/s. This study shows that the acceleration data-loggers represent a useful and reliable system for accurately recording the tail beat of free-ranging fish and estimating flatfish behavior.     

Interannual variation in summer habitat utilization by juvenile southern bluefin tuna (Thunnus maccoyii) in southern Western Australia

The spatial habitat utilization of juvenile southern bluefin tuna in southern Western Australia was investigated using automated acoustic receivers with acoustic transmitters implanted in tagged fish during three austral summers (2004/2005, N = 79 fish, 2005/2006, N = 81, 2006/2007, N = 84). Seventy acoustic receivers were deployed at three cross-shelf lines and three coastal topographic features (lumps) between December and May. We observed markedly different patterns of habitat utilization between the three seasons: (i) aggregation at lumps in 2004/2005 and 2006/2007, and (ii) wide distribution over the continental shelf (i.e., few occurring at lumps) in 2005/2006. Vertical profile by conductivity-temperature-depth casts showed these spatial shifts were caused by interannual changes in the presence of sub-Antarctic water. The sub-Antarctic water was present in the subsurface layer close to the continental slope only during 2005/2006, and the area had higher chlorophyll-a concentrations than the coastal areas, including at the lumps. These environmental characters, related to the nutrient rich sub-Antarctic water, appear to have a strong influence on fish distributions in 2005/2006, and may occur generally during La Niña events. Interannual fluctuations in habitat utilization will influence detection of fish in recruitment monitoring surveys and thus bias the resulting juvenile abundance indices.     

Naupliar copepod concentrations in the spawning grounds of Japanese sardine, Sardinops melanostictus, along the Kuroshio Current

Naupliar copepods were distributed at similar concentrations over the waters inshore and offshore of the Kuroshio Current off central Japan in early spring 1993 and 1994, overlapping with the distribution of early feeding larvae of Sardinops melanostictus . Although N, P, Si and chlorophyll a concentrations were higher in the waters inshore of the Kuroshio axis than in the offshore waters, the mean concentrations of nauplii were not statistically different between the two waters. Food availability for larval S. melanostictus did not seem to be different between the two waters in terms of the mean food concentrations. Using the critical food concentration (>9 nauplii L^–1) for 25% survival during 3 days after first feeding derived from a rearing experiment, percentages in number of stations or water samples with nauplii concentrations >9 nauplii L^–1 were higher in the inshore waters than in the offshore waters in both years examined. Considering that fish larvae may depend on small-scale patchiness of food for their survival, the inshore waters seemed to be more favourable for first-feeding larvae than the offshore waters.     

Variations in spawning ground area and egg density of the Japanese sardine in Pacific coastal and oceanic waters

The spawning ground of the Japanese sardine, Sardinops melanostictus (Schlegel), was distributed over the oceanic waters as well as the coastal waters along the Pacific coasts of western and eastern Japan during 1978–1992. The area of the spawning ground in the coastal waters on the continental shelf has ranged from 95 000 km² in 1992 to 143 000 km² in 1988, constituting 44–77% of the total area of the spawning ground. The area of the coastal spawning ground was relatively constant in spite of the large fluctuations in egg abundance, i.e. size of the spawning population, from 88 trillion (1987) to 668 trillion (1989) in the waters. Spawning adults seemed to extend over the coastal waters irrespective of the size of the spawning population. In contrast to the coastal waters, the spawning area in the oceanic waters offshore of the continental shelf increased from 31 000 km² in 1978 to 183 000 km² in 1988 and then shrank to 83 000 km² in 1992, as a function of the spawning population size. The egg distribution density in the coastal waters stayed less than 6000 m⁻² mo⁻¹, but it reached as high as 27 400 m⁻² mo⁻¹ in the expanded spawning ground in the oceanic waters. The oceanic waters seemed to function as a reserve spawning ground for the sardine in years of extremely high spawning population.     

Departure behaviour of juvenile southern bluefin tuna (Thunnus maccoyii) from southern Western Australia temperate waters in relation to the Leeuwin Current

The arrival of the warm tropical Leeuwin Current (LC) into southern Western Australia (SWA) may influence the movement timing and foraging habitat of juvenile southern bluefin tuna (SBT). Seasonal and interannual changes in the strength of the LC lead to thermal differences and potential changes in food availability between tropical and temperate waters in SWA. This phenomenon could influence the habitat utilization of SBT in these summer grounds. Movement characteristics determined from SBT tagged with acoustic transmitters (N = 244) using cross‐shelf lines of automated receivers for three summer‐autumn seasons (2004/2005, 2005/2006, 2006/2007) in SWA revealed interannual variability. Each year, the eastward movements of tagged fish increased as temperatures increased, and fish left the region when temperatures exceeded 20.0°C, a temperature indicative of the leading edge of the LC in SWA waters. Interannual fluctuations in the timing of movements were detected. When the LC was narrow and restricted to the shelf edge in 2004/2005, the distribution of SBT in shelf waters did not change before or after LC intrusion. In contrast, long distance eastward movements frequently occurred when the LC intrusion was spread wide over the continental shelf in 2005/2006 and 2006/2007. This suggests that, off SWA, juvenile SBT move quickly out of local foraging habitats defined by cool sub‐tropical temperate waters ahead of the tropical LC intrusion, despite these waters not being physiologically limiting. These results suggest that the behavioural response of SBT may be related to changes in prey availability as a result of changes in oceanographic conditions.     

Patchiness structure and mortality of Pacific saury Cololabis saira larvae in the northwestern Pacific

Patchiness structure and mortality rates of Pacific saury Cololabis saira larvae were estimated in the frontal area of the Kuroshio Extension in spring 2000 and 2001. Drogued GPS buoys, which were used as Lagrangian reference points, were deployed to mark dense patches of saury larvae and a series of samplings was conducted using a neuston net around the buoys during the night over a period of several days. A total of 225 tows were conducted and 11 690 specimens were sampled during four buoy tracking. Despite the occurrence of water diffusion, patchiness density, distance between adjacent patches and patch size estimated by geostatistical analysis did not drastically change during the buoy tracking and these parameters were robust for the difference of variogram models and the threshold larval density defining patchiness. Such a stability of the patchiness allowed the estimation of larval mortality from the decrease of larval density for different size classes with respect to individual growth based on the samples taken repeatedly during the buoy tracking. Although low or negative values were obtained for the larvae right after hatching, probably due to continuous hatching, the estimates of mortality then increased and became stable in the range of 23–37% day⁻¹ with a small standard deviation until their knob length reached to 36 mm. In situ information on the patchiness structure and larval mortality provides crucial parameters for simulating the recruitment process of small pelagic fishes in high-resolution hydrodynamic models.     

Effects of elevated pCO2 on the early development of the commercially important gastropod,Ezo abalone Haliotis discus hannai

We used an up‐to‐date, a high accuracy CO₂ manipulation system to investigate the sensitivity of organisms to CO₂ acidification, rearing marine calcifiers under elevated CO₂ in running water. We evaluated the effects of elevated partial pressures of carbon dioxide (pCO₂) in seawater on larvae of the commercially important marine gastropod Ezo abalone Haliotis discus hannai. In larval Ezo abalone, no effect of exposure to <1100 μatm pCO₂ seawater was observed in fertilization, malformation, or mortality rates until 15 h after fertilization. However, compared to control larvae in seawater (450 or 500 μatm pCO₂), the fertilization rate and the hatching rate (15 h after fertilization) decreased with increased pCO₂ exposure (1650 and 2150 μatm pCO₂) and the malformation rate increased significantly, with the larval shell length being smaller 75 h after hatching. These results suggest that ocean acidification will potentially impact the marine population of Ezo abalone as a human food source in the future.