To evaluate the effects of jellyfish on the anchovy fishery stock in Hiuchi-nada, central Seto Inland Sea, we created growth models for anchovy larvae and jellyfish linked with prey–predator dynamics models among nutrients, phytoplankton, copepods, and jellyfish. Fluctuations in copepod (main food items for jellyfish and anchovy larvae) biomass during spring–summer 2001–2005 were simulated with the model to investigate the relationship between the jellyfish abundance and the anchovy growth and survival. Anchovy growth and survival rate from 5.7 mm (first feeding stage) to 40 mm standard length were inversely proportional to jellyfish abundance at the time of occurrence. The simulation revealed that anchovy larvae could not survive by the shortage of food items at first feeding when abundance of the jellyfish of 100-mm bell diameter was >10 ind./100 m3. Mean jellyfish abundance during May–August 2007–2009 reached a maximum of 10 ind./100 m3 when mean bell diameter was 72 mm. These results suggest that jellyfish abundance affected anchovy fishery stock during the late 2000s.
Age, growth, maturity and food habits of spotted mackerel Scomber australasicus caught at the Emperor Seamounts from 2008 to 2015 by bottom trawl fishing vessels were examined. Fork length (FL) ranged from 211.1 to 443.5 mm FL. Ages were estimated from 0–9 years by scale reading. The von Bertalanffy growth formula based on the age-FL relationship was expressed as FLt=432.7×(1-exp(-0.24(t+2.59))), where FLt is the FL (mm) at t (year). Seasonal changes in the gonad-somatic index suggested that the spawning period extended from March to July. Stomach contents analysis showed that the major prey organisms were Euphausiidae, Decapodiformes, Thecosomata, Salpidae, Pyrosomatidae and Piscese. The year-class in 2008 was the most dominant and clear recruitments of other yearclasses were not found. Since previous surveys reported high egg production of spotted mackerel around southern waters off Hachijo-jima in the Izu islands in 2008, it is likely that individuals spawned in these waters were transported to the Emperor Seamounts by the Kuroshio and Kuroshio Extension.
We carried out aerial surveys to obtain information on the distribution and abundance of the finless porpoises Neophocaena asiaeorientalis in Ise Bay and Mikawa Bay, central Japan. We flew seven times in 2002, 2003 and 2014. Transects were set in an east–west direction. We covered 90 transects (1,869 km in total length) in 2002, 60 transects (1,244 km) in 2003 and 26 transects (614.4 km) in 2014. In 2002, a total of 225 individuals were seen along those transects. Porpoise abundance was estimated at 2,776 individuals (CV=24.9%) with the density of 1.09 individuals・km-2. In 2003, a total of 216 individuals were seen. Abundance was estimated at 3,176 individuals (CV=19.7%) with the density of 1.25 individuals・km-2. In 2014, a total of 178 individuals were seen. Abundance was estimated at 3,920 individuals (CV=21.9%) with the density of 1.41 individuals・km-2. No significant differences were detected in abundance among the three years.
Age and growth of the spear squid, Heterololigo bleekeri, from 288 squids caught in the coastal water off Miyagi Prefecture from July 2014 to June 2015 were examined using a statolith microstructure.
The logistic growth equation was well fitted to examine the relationship of age and mantle length. The growth equations were shown as follows, M.L.=312(/ 1+e4.87–0.0294t) for male and M.L.=225(/1+e4.68–0.0317t) for female, where M.L. is mantle length in mm and t is age in days. A significant difference in growth equations between males and females was detected, which confirmed that males grew bigger than females (F=44.2, p<0.01). The hatching periods of H. bleekeri were estimated to be from February to September, with the main periiods being from April to June.
This study calculated the economic effects on the fishers of medium-scale round-haul net fishery of JF Kitaura in Miyazaki Prefecture who use the oceanic condition information that the Prefectural Government provides. As a result, the total yearly economic benefit was approximately 219 million yen. The breakdown of this benefit included increased catch quantity for increased fishery opportunities due to fishing ground inference (approximately 181 million yen), reduced fuel consumption (approximately 24 million yen), and reduced labor time (approximately 14 million yen) for halting fishing operations to escape from strong current. The calculation of this study could be a useful simple method to evaluate the economic effects on fishers who use oceanic condition information.
The migration, growth, and exposed water temperature of immature giant octopuses (Enteroctopus dofleini) were investigated in the coastal Okhotsk Sea area of Hokkaido using the mark and recapture method. Disk tags were used to document the migration and growth patterns, and data loggers were used to measure exposed temperature. Data loggers were also placed on the sea bottom off Notoro Cape, Abashiri City, which is one of the main giant octopus fishing grounds in this area, to document the spatial and temporal changes in water temperature during the summer and winter. The daily growth of the immature giant octopuses changed seasonally, and the daily growth of marked individuals was faster during summer and autumn than during winter and spring. A major portion of the released individuals (86%) were recaptured from the area where they were initially released, and the migration distance of 76% of the octopuses was ≤ 10 km, suggesting a small scale of migration in the area. However, a seasonal pattern of migration between deep and shallow areas was evident, with more individuals migrating toward deep areas during July–September and toward shallow areas starting in October. In the summer of 2013, sea-bottom temperature was higher in shallower areas, particularly in August and September, when the water temperature of 20- to 40-m deep areas exceeded 20°C. This suggests that the driving factor of summer migration to deeper waters is related to the avoidance of overheating. Highest exposed water temperature was 20.1°C and mean temperature of 4 individuals (50%) was ≥ 15.0°C. The present study indicates the water temperature of the giant octopus fishing ground changes rapidly in response to both the sea currents and the passing of strong low pressure. It also reports, for the first time, information on the movement, growth, and exposed temperature of immature octopuses in the area.