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The spatial distributions and feeding habits of Pacific cod (Gadus macrocephalus) larvae, and the spatial distributions of copepod nauplii and copepodites, their main prey, were examined in Mutsu Bay from February to March during 1989–92. Yolk‐sac larvae were caught at 30–45 m depth at the bay mouth. Larvae without yolk were collected at 8–45 m depth at the bay mouth and the inner part of the bay, and large larvae were chiefly found in the bay. This geographical pattern in larval size may have been because of transport to the inner part of Mutsu Bay by the Tsugaru Warm Current. The dominant taxa of copepod nauplii and copepodites in the diet and the environment changed each year. Larvae fed mainly on abundant taxa in the environment, suggesting that larvae are opportunistic feeders. Nauplii and copepodites were abundant in the bay, especially in 1992. Copepodites were slightly more abundant in the diet of cod larvae in 1992 than in 1991, but this difference was smaller than in the environment. In addition, larvae with empty digestive tracts were scarce in 1991 and 1992. Prey concentrations in the bay in 1991 and 1992 seem to have been high enough to sustain most Pacific cod larvae.

We carried out shipboard observations in Funka Bay, Hokkaido, Japan, monthly or bimonthly from December 2015 to November 2016. We measured vertical profiles of isoprene, chlorophyll-a (chl-a), and other parameters from surface to bottom layer (about 95 m) near the center of the bay. We found substantial increases in isoprene concentration in the surface mixed layer from February to March during the peak of the spring diatom bloom, in the bottom layer from March to April after the peak of the bloom, and in the subsurface layer (below the surface mixed layer) in summer from July to August, where there were also substantial chl-a concentration maxima. We attribute the isoprene increases in the surface and subsurface layers to photosynthetic production of isoprene by the dominant phytoplankton in the spring bloom and in summer, and that in the bottom layer to dark production of isoprene by diatom aggregates that settled from the surface euphotic zone. We also measured isoprene production in laboratory incubation experiments. The in-situ production rates of isoprene per unit chl-a in the surface mixed layer in the spring bloom, in the dark bottom layer during the bloom, and in the subsurface layer in summer (0.82, 0.03-0.13, and 7.38 pmol (μg chl-a)-1 d-1 , respectively) were consistent with our incubation results. We believe that this is the first report focused on dark production of isoprene by diatoms; the production rate of isoprene under the dark condition ranged from 4% up to 16% of that by photosynthesis.

To examine predation on larval and juvenile Pleuronectes yokohamaeby Crangon uritai, the spatial distribution and feeding habits of C. uritai were studied.C. uritai fed on various prey, including P. yokohamae. Density-dependent predation on juvenile Crangon spp., larval and juvenile gobiid fish Chaenogobius heptacanthus, mysids, and gammarids was observed. The abundance of alternative prey might therefore play an important role in reducing the pressure of predation on P.yokohamae. In each year, C. uritai migrated from depths of 10m in March to depths of 3m in May. There was a significant negative correlation between the weighted mean depth (WMD) of C. uritai and the weighted mean bottom water temperature (WMBWT). The migration was therefore delayed in the cold spring, during which C. uritai occupied depths of approximately 10m. In contrast, no significant correlation was found between the WMD of P. yokohamae and WMBWT.Settlement of P. yokohamae began at depths of 15m, mainly during early April.The spatial distributions of these two organisms show greater overlap during the cold spring, resulting in increased opportunities for predation.