Post-settlement dynamics of Japanese flounder Paralichthys olivaceus in Tango Bay: Seasonal patterns in growth, mortality, and recruitment potential.

Japanese flounder Paralichthys olivaceus is one of the most valuable coastal flatfish species in East Asia. To investigate post-settlement growth and mortality, juveniles were sampled in Tango Bay (Japan) weekly throughout the settlement period in 2007 and 2008. Otolith (lapillus) microstructure analysis enabled the categorization of juveniles into six biweekly cohorts each year. Later cohorts exhibited higher growth rates possibly because of higher water temperatures. A key observation was the direct relationship between high mortality and high density in mid-season cohorts in both years, pointing to density-dependent mortality. This increased mortality may be attributed to predation, including cannibalism by earlier cohorts. Furthermore, growth-selective mortality was evident soon after settlement, underscoring the vulnerability of slow growers to predation during the early juvenile stage. Although earlier and later cohorts were less abundant but showed promising recruitment potential, the prospective contribution of mid-season cohorts to the adult population remained uncertain. The results clearly highlight the importance of density-dependent mortality in population regulation in post-settlement Japanese flounder.

Seasonal alternation of the ontogenetic development of the moon jellyfish Aurelia coerulea in Maizuru Bay, Japan.

Outbreaks of moon jellyfish Aurelia spp. are frequently reported from many parts of the world's coastal areas. Aurelia spp. canonically show a metagenetic life cycle in which planulae transform into sessile polyps, which can drastically increase in number through asexual reproduction. Therefore, their asexual reproduction has been recognized as one of the major causes of the outbreaks. Aurelia spp. also show direct development that lacks asexual reproduction during the polyp stage, which prevents us from understanding the mechanisms of its outbreaks. To clarify the seasonality of the metagenetic and direct-development life cycles of Aurelia sp. in Maizuru Bay, Japan, we conducted field observations and laboratory experiments throughout the year. Additionally, the two life cycle types were genetically analyzed to confirm that they belong to the single species Aurelia coerulea, which dominates in coastal waters in Japan. From July until October, Aurelia coerulea produced smaller eggs and planulae all of which developed into polyps. However, from December until May, larger eggs and planulae were produced and 90% of the planulae developed into planktonic ephyrae bypassing the sessile polyp stage. Our results demonstrated that a single species, A. coerulea, seasonally shifts between their two life cycle types at a water temperature threshold of 20°C in Maizuru Bay. The higher energy storage of larger planulae was suggested to enable the planulae to develop into ephyrae without external energy input through feeding during the polyp stage. The adaptive significances of the two life cycle types were also discussed.

Environmental DNA as a 'Snapshot' of Fish Distribution: A Case Study of Japanese Jack Mackerel in Maizuru Bay, Sea of Japan.

Recent studies in streams and ponds have demonstrated that the distribution and biomass of aquatic organisms can be estimated by detection and quantification of environmental DNA (eDNA). In more open systems such as seas, it is not evident whether eDNA can represent the distribution and biomass of aquatic organisms because various environmental factors (e.g., water flow) are expected to affect eDNA distribution and concentration. To test the relationships between the distribution of fish and eDNA, we conducted a grid survey in Maizuru Bay, Sea of Japan, and sampled surface and bottom waters while monitoring biomass of the Japanese jack mackerel (Trachurus japonicus) using echo sounder technology. A linear model showed a high R(2) value (0.665) without outlier data points, and the association between estimated eDNA concentrations from the surface water samples and echo intensity was significantly positive, suggesting that the estimated spatial variation in eDNA concentration can reflect the local biomass of the jack mackerel. We also found that a best-fit model included echo intensity obtained within 10-150 m from water sampling sites, indicating that the estimated eDNA concentration most likely reflects fish biomass within 150 m in the bay. Although eDNA from a wholesale fish market partially affected eDNA concentration, we conclude that eDNA generally provides a 'snapshot' of fish distribution and biomass in a large area. Further studies in which dynamics of eDNA under field conditions (e.g., patterns of release, degradation, and diffusion of eDNA) are taken into account will provide a better estimate of fish distribution and biomass based on eDNA.