Controlling factors of large-scale harmful algal blooms with Karenia selliformis after record-breaking marine heatwaves

Unprecedented, large-scale harmful algal blooms (HABs) dominated by Karenia selliformis occurred off the southeastern coast of Hokkaido, Japan, from late September to early November 2021, about a month after intense and extensive marine heatwaves (MHWs) had subsided. The aims of the present study were to understand the mechanism of development, maintenance, and decay of the HABs as well as to investigate the effect of the MHWs on the HABs. We developed a one-dimensional, lower trophic-level ecosystem model (NEMURO+) to simulate the HABs. The model successfully simulated the 2021 HABs and indicated that their development, maintenance, and decay were controlled primarily by changes of water temperature. Nitrate supply from subsurface layers by seasonal vertical diffusion in autumn also helped to maintain the HABs. Vertical diffusion following MHWs in 2021 contributed to the long duration of the preferred temperature for K. selliformis and the occurrence of pre-bloom of K. selliformis, resulting in preconditioning and accelerating the HABs. However, simulations for normal years (i.e., the climatological mean during 2003–2018) showed that HABs could have occurred, even in the absence of MHWs. The simulations indicated that massive blooms of other phytoplankton species (e.g., diatoms) would not have occurred in 2021, even in the absence of a K. selliformis bloom. The implication was that the HABs in 2021 were the species-specific responses of K. selliformis. The proposed mechanism of the HABs was peculiar to our study area and differed from that previously reported for other K. selliformis blooms. Specifically, the preferred temperature for the HABs of K. selliformis was clearly lower than the previously reported preferred temperature of K. selliformis; thus, the physiological characteristics of the K. selliformis that bloomed in our study area differed from those of other K. selliformis strains. These discoveries provide the first evidence to explain how MHWs affect HABs, and to understand how inter-regional dissimilarities of K. selliformis can lead to large-scale, devastating outbreaks under different oceanographic conditions

Unprecedented Outbreak of Harmful Algae in Pacific Coastal Waters off Southeast Hokkaido, Japan, during Late Summer 2021 after Record-Breaking Marine Heatwaves

Unprecedented large-scale harmful algae blooms (HABs) were reported in coastal waters off the south-eastern coast of Hokkaido, Japan, in mid-to-late September 2021, about a month after very intense and extensive marine heatwaves subsided. To understand the physical–biological processes associated with development of the HABs, we conducted analyses via a combination of realistic ocean circulation models, particle-tracking simulations, and satellite measurements. The satellite-derived chlorophyll concentrations (SCCs) and areal extent of the high SCCs associated with the HABs were the highest recorded since 1998. More specifically, the extent of SCCs exceeding 5 or 10 mg m−3 started to slowly increase after 20 August, when the marine heatwaves subsided, intermittently exceeded the climatological daily maximum after late August, and reached record-breaking extremes in mid-to-late September. About 70% of the SCCs that exceeded 10 mg m−3 occurred in places where water depths were <300 m, i.e., coastal shelf waters. The high SCCs were also tightly linked with low-salinity water (e.g., subarctic Oyashio and river-influenced waters). High-salinity subtropical water (e.g., Soya Warm Current water) appeared to suppress the occurrence of HABs. The expansion of the area of high SCCs seemed to be synchronized with the deepening of surface mixed layer depths in subarctic waters on the Pacific shelves. That deepening began around 10 August, when the marine heatwaves weakened abruptly. However, another mechanism was needed to explain the intensification of the SCCs in very nearshore waters off southeast Hokkaido. Particle-tracking simulations based on ocean circulation models identified three potential source areas of the HABs: the Pacific Ocean east of the Kamchatka Peninsula, the Sea of Japan, and the Sea of Okhotsk east of the Sakhalin Island. Different processes of HAB development were proposed because distance, time, and probability for transport of harmful algae from the potential source areas to the study region differed greatly between the three source areas

Expression of GnRH genes is elevated in discrete brain loci of chum salmon before initiation of homing behavior and during spawning migration

Our previous studies suggested the importance of gonadotropin-releasing hormones (GnRHs) for initiation of spawning migration of chum salmon, although supporting evidence had been not available from oceanic fish. In farmed masu salmon, the amounts of salmon GnRH (sGnRH) mRNAs in the forebrain increased in the pre-pubertal stage from winter through spring, followed by a decrease toward summer. We thus hypothesized that gene expression for GnRHs in oceanic chum salmon changes similarly, and examined this hypothesis using brain samples from winter chum salmon in the Gulf of Alaska and summer fish in the Bering Sea. They were classified into sexually immature and maturing adults, which had maturing gonads and left the Bering Sea for the natal river by the end of summer. The absolute amounts of GnRH mRNAs were determined by real-time PCRs. The amounts of sGnRH mRNA in the maturing winter adults were significantly larger than those in the maturing summer adults. The amounts of sGnRH and chicken cGnRH-II mRNAs then peaked during upstream migration from the coast to the natal hatchery. Such changes were observed in various brain loci including the olfactory bulb, terminal nerve, ventral telencephalon, nucleus preopticus parvocellularis anterioris, nucleus preopticus magnocellularis and midbrain tegmentum. These results suggest that sGnRH neurons change their activity for gonadal maturation prior to initiation of homing behavior from the Bering Sea. The present study provides the first evidence to support a possible involvement of neuropeptides in the onset of spawning migration

Changes in the plasma levels of insulin-like growth factor-I from the onset of spawning migration through upstream migration in chum salmon

An increase in activity of the pituitary-gonadal axis (PG-axis) and gonadal development are essential for the onset of spawning migration of chum salmon from the Bering Sea. In the Bering Sea, fish with larger body sizes initiated gonadal development and commenced spawning migration to the natal river by the end of summer. We thus hypothesized that insulin-like growth factor-I (IGF-I), a somatotropic signal that interacts with the PG-axis, can be one of such factors responsible for the onset of migration, and examined changes in plasma levels and hepatic expression of IGF-I gene in oceanic and homing chum salmon in 2001, 2002 and 2003. The plasma IGF-I levels and corresponding body sizes in maturing adults, which had developing gonads, were significantly higher than those in immature fish in all years examined. Such increase in the plasma IGF-I levels in maturing fish was observed even in the Gulf of Alaska during February 2006, while coincident increase was not observed in the hepatic amounts of IGF-I mRNA. In autumn, the plasma IGF-I levels in homing adults decreased during upstream migration in the Ishikari River-Ishikari bay water system in Hokkaido, Japan. In conclusion, the plasma IGF-I levels increased with gonadal development when chum salmon migrated from the winter Gulf of Alaska to the summer Bering Sea. Circulating IGF-I may interact with the PG-axis and promote gonadal development that is inseparable from the onset of spawning migration. Circulating IGF-I levels were thereafter lowered in accordance with final maturation during upstream migration in the breeding season