Identification of coral spawn source areas around Sekisei Lagoon for recovery and poleward habitat migration by using a particle-tracking model

A massive coral bleaching event occurred in 2016 in the interior of Japan’s largest coral lagoon, the Sekisei Lagoon, located in the Kuroshio upstream region in southwestern Japan. Recovery of the coral lagoon will require the influx of coral spawn and larvae; therefore, it is important to identify and conserve source sites. A surface-particle-tracking simulation of coral spawn and larvae was used to identify source areas of coral spawn outside of the Sekisei Lagoon for potential recovery of the interior lagoon. The northern coastal zone of Iriomote Island, including Hatoma Island, was identified as a major source area. Hatoma Island was also identified as a key source for the Kuroshio downstream region and for aiding the poleward migration of coral habitat under ongoing global climate change, making it one of the most important source areas in the Nansei Archipelago.


Methods
We used submesoscale eddy-resolving synoptic ocean models based on the Regional Oceanic Modeling System (ROMS) in a double-nested configuration coupled with a 3-D Lagrangian particle-tracking model embedded in the Japan Coastal Ocean Predictability Experiment 2 (JCOPE2) 4 . JCOPE2 is a numerical reanalysis product for the northwestern Pacific Ocean assimilated with a vast amount of satellite and in situ data 7 . ROMS-L1 (Fig. 1, red rectangle) has a spatial resolution of 3 km and a calculation period of 1 January 2005 to 2 November 2015. ROMS-L2 (Fig. 1, blue rectangle) has a higher spatial resolution of 1 km and a shorter calculation period of 27 December 2010 to 2 November 2015 4 ( Table 1).
To identify major source areas and common transport routes of coral spawn and larvae, about 29,000 virtual surface Lagrangian particles were tracked between 145 areas that served as both potential source areas and destination areas on land grids (Fig. 2) or on the boundaries of the ROMS-L2 model domain for at least 21 days with a time step of 200 s 8,9 . Connectivity was calculated by using probability density functions (PDFs) 10 ; areas with the highest probability density were identified as major sources. Lagrangian particles were discharged at 20:00 on the first night after the full moon during a spring tide in May, which is when mass coral spawning is Coastal areas around the Sekisei Lagoon (Iriomote Island, the Sekisei Lagoon, and Ishigaki Island) were divided into 18 groups (Fig. 2). Groups 3, 10, and 14-18 are located in the interior and the remainder are outside of the lagoon. All areas were considered as potential sources and as potential destinations. By using PDFs, we identified major source areas outside of the lagoon that could contribute to recovery of the interior lagoon.

Results
Identification of major source areas outside of the lagoon that could supply the interior lagoon. Group 16, on the southern coast of Kohama Island (Fig. 2), was the highest-probability destination area in the interior lagoon in each examined year (Figs. 3,4). Relatively more particles drifted from the northern coastal zone of Iriomote island, especially from the northern and northeastern coasts of Iriomote Island (Groups 1 and 2, respectively) and the coast of Hatoma Island (Group 6) (Fig. 4).
Two major routes toward Group 16 from Groups 1, 2, and 6 were revealed from the particle drift trajectories: one passes southward through the Yonara Channel on the west side of Kohama Island, and the second passes through the channel on the east side (Figs. 5, 6, 7). No particles drifted around Ishigaki or Iriomote islands to reach the interior lagoon.

Identification of major source areas around the Sekisei Lagoon for the Kuroshio downstream region and for poleward coral habitat migration.
We also sought to identify source areas around the Sekisei Lagoon for reefs in the Kuroshio downstream region and hence for aiding poleward coral habitat migration due to the influence of global climate change. To identify simulated particles that drifted from around the Sekisei Lagoon to the Kuroshio downstream region (including Okinawa Island) or otherwise drifted in a poleward direction, we focused on particles that were advected north of 26.5° N (hereafter referred to as longdistance particles [LDPs]). Many particles from the northern coasts of Iriomote and Ishigaki islands drifted to the Kuroshio downstream region (Fig. 8), as did particles from the northern coast of Hatoma Island (Fig. 9). This was not true for particles from southern coasts, such as the Urabishi reef (which suffered extensive damage from the 2016 bleaching event) (Figs. 9, 10). www.nature.com/scientificreports/

Discussion and concluding remarks
Our results reveal the location of source areas outside of the lagoon that could export coral spawn and larvae to the lagoon interior. This is an important process in the potential recovery of coral reefs in this area. Major source areas were located around Hatoma Island and the northern and northeastern coasts of Iriomote Island. Hatoma Island was also a major source area for simulated coral spawn and larvae arriving in the Kuroshio downstream region. In fact, it was the most important source area for both the interior Sekisei Lagoon and the Kuroshio downstream region. These results can be used to aid recovery of coral reefs in the entire coastal zone of the Sekisei Lagoon and the Kuroshio downstream region by informing the design of effective coral reef conservation areas in this region and directing efforts to remove Acanthaster planci (crown-of-thorns starfish) and transplant coral seedlings. Previous research has highlighted the difficulty of tracking simulated coral spawn and larvae in complex generalized zonal current systems such as the eastern equatorial Pacific, where the North Equatorial Current, South Equatorial Current, and Northern Equatorial Countercurrent interact with prevailing easterlies under the influence of ENSO variability 12 . In contrast, our study region features a relatively simple and strong poleward western boundary current, the Kuroshio, which maintains a distinct surface current and never directs equatorward. Furthermore, the Kuroshio passes near the Coral Triangle in the further upstream region, which is known as the center of global coral reef biodiversity 13 . Accordingly, poleward coral mass migrations can occur relatively easily and can be tracked with relative simplicity in this region. The areas investigated in this study (the Sekisei Lagoon and Nansei Archipelago) play a key role as stepping stone relay stations in poleward migration mostly by the Kuroshio, thus the identification of coral spawn and larval sources in these areas is of crucial importance.
Given that global climate change is causing corals to migrate poleward, it is particularly important to conserve coral populations in upstream regions, especially in the areas that serve as sources of coral spawn and larvae in western boundary current systems. It is not a coincidence that upstream source areas in the western boundaries