Recent scientific publication by PICRC explains larval self-recruitment and connectivity among fish populations in Micronesia

Area covered by the study

The Palau International Coral Reef Center (PICRC) just published an important scientific article in the peer-reviewed journal “Estuarine, Coastal and Shelf Science” entitled “Oceanographic chaos and its role in larval self-recruitment and connectivity among fish populations in Micronesia”. This study discusses the recruitment of rabbitfish in Guam, groupers in Palau, and the connectivity of groupers within Micronesia (i.e. which islands provide fish larvae to other islands). The study was a collaborative effort among Dr. Eric Wolanski, Professor at James Cook University in Australia, Dr. Robert H. Richmond, Research Professor and Director at the University of Hawaii-Manoa’s Kewalo Marine Lab and Dr. Yimnang Golbuu, CEO of PICRC.  The three authors of the study have collaborated on a variety of research projects in Micronesia for the last 21 years.

Throughout the years, patterns of recruitment of rabbitfish in Guam and groupers in Palau could not be explained based solely on the impacts of El Nino/La Nina current and ocean circulation conditions. The authors found that normal ocean circulation patterns (i.e. meso-scale turbulence – these are huge oceanic eddies typically 50-200 km in diameter) are also just as important as the El Nino Southern Oscillation (ENSO) in Micronesia.  They discovered this from mapping the large-scale oceanic currents in Micronesia from satellite altimetry over 30 years at a time scale of 5 days. They used those data in a high-resolution biophysical model both of Guam and Palau to map the fate of fish larvae after spawning. Their model included the consideration of ocean weather and ENSO, as well as the swimming behavior of mature fish larvae.

The authors found that in Guam and Palau, the success or failure of recruitment varies strongly from spawning event to spawning event and from year to year due to ocean current patterns and weather.  For Guam, the results explained the substantial change from year to year of the rabbitfish catch. Fish recruitment can be zero after one spawning event and can be extremely high at the next spawning event, depending on both the ocean weather and the ENSO conditions at that time. For Palau, the authors found that the three spawning grounds of groupers (Ebiil Passage, the Western Passage and the Southwest Passage) were connected, which historically made the Palau grouper population resilient. Unfortunately, overfishing has reduced the fish populations on two of the aggregation sites, with only Ebiil Passage having a healthy population of groupers.  Therefore, it is absolutely critical to continue to protect the fish in Ebiil Passage and surrounding marine protected areas in order to provide fish for the future.  

The grouper larval fish plume after spawning in Ebiil Passage for (a) normal year and (b) an ENSO year showing the connectivity between the three historical spawning grounds of the groupers.

Dr. Golbuu and colleagues also found that 99% of the fish larvae are exported from Guam and Palau, hence, these larvae provide key fish population connectivity among the islands of Micronesia. They mapped the patterns of connectivity of Grouper fish larvae in Micronesia. They discovered that there are two major stocks with high connectivity, one cluster being Irian Jaya-Palau-Yap-Mindanao, and the other cluster being the FSM and surrounding islands. Irian Jaya is a major source of fish larvae. The larvae from Guam and Saipan are mainly exported towards Luzon and the Kuroshio. There are some keystone transit reefs such as those in Kapingamarangi and Chuuk.  Pohnpei and surrounding islands are also transit areas, largely receiving larvae from Irian Jaya and the Marshall Islands and providing larvae to Chuuk and Guam. Papua New Guinea reefs provide little fish to Micronesia. Palau and Yap receive a net inflow of groupers from Irian Jaya, Yap has a net export to Palau and both Yap and Palau have a net export to Mindanao. Saipan is a net, but small, provider of fish larvae to Guam.

A snapshot of the 5-days averaged surface currents in Micronesia as measured by satellite altimetry

The likely connectivity of reef fish populations among Micronesian Islands demonstrated by this study supports government efforts at regional cooperation in implementing conservation initiatives and sustainable management of their fisheries.  Most notable of this regional, cooperative approach is the establishment of the Micronesia Challenge, with the different governments in Micronesia coming together and agreeing to effectively conserve their marine resources as a legacy for their future generations.