Philippine Journal of Science Vol. 145 No. 3, September 2016 This paper aims to review the possible effects of El Niño on marine fisheries resources in the Philippines as well as highlights key researchable areas that will enhance and support management strategies to mitigate its impacts. El Niño Phenomenon According to the National Oceanic and Atmospheric Administration (NOAA), El Niño refers to the three-month average positive sea surface temperature anomalies above the 0.5 °C threshold from the running means of multiple 30-year base periods in the Niño 3.4 region (5°N-5°S, 120°-170°W). It is the warm phase of El Niño Southern Oscillation (ENSO), which refers to the inter-annual climatic variability that causes changes in atmospheric and ocean conditions (Trenberth 1997; Lu et al. 1998). It brings heavy rainfall that causes flooding in the equatorial Eastern Pacific (EEP) and equatorial Central Pacific (ECP) regions while drought in the equatorial Western Pacific (EWP) region where the Philippines, Indonesia and Australia are located (Rasmusson & Wallace 1983). During normal conditions, a high atmospheric pressure exists in the EEP while a low atmospheric pressure exists in the EWP. The atmospheric gradient drives the easterly tradewinds westward pushing the warm surface water from the EEP and ECP towards the EWP (Figure 2a). The water stays confined causing higher water level and deeper thermocline in the EWP than in the ECP and EEP (Cane, 1983). At the same time, the low atmospheric pressure brings frequent tropical rainstorms in the EWP. As part of the EWP, the Philippine waters, as part of the EWP, are tropical in character, relatively warm and saline (Barut et al. 1997). Sea surface temperatures are generally above 28°C in summer and only a few degrees lower during the cold months. The thermocline depth is usually about 150 m and varies seasonally. Nutrient concentrations and biological productivity are highest over the shelves and decline rapidly with depth and distance Damatac II & Santos: Possible Effects of El Niño on Some Philippine Marine Fisheries Resources from the coast. When El Niño occurs, the atmospheric pressure falls in the EEP and rises in the EWP. This causes the easterly tradewinds to relax and the pool of warm water that normally resides in the EWP to spread across the equatorial plate (Figure 2b). The warm surface water distributes from the EWP to the ECP and EEP causing a shift in sea surface temperature, sea level and thermocline in the latter regions. The sea surface temperature becomes warmer in the EEP while the waters in the EWP become cooler (Rasmusson & Wallace 1983). The sea level decreases and the thermocline elevates in the EWP than in the ECP and EEP. There is also a reversal in the occurrence of tropical cyclones having more frequent events in the EEP and ECP. The lower atmospheric pressure in the latter regions brings excessive rains while the higher atmospheric pressure in the EWP brings drought. In the Philippines, monthly rainfall is reduced during an El Niño event, such that strong events suppress typhoons while weak to moderate ones shift the direction of typhoons towards the northeast (Delos Reyes & David 2006). Similarly, neighboring countries, such as Indonesia and Malaysia, experience the same trend where there is reduced rainfall during an El Niño year (Cheang 1993; Kirono et al. 1999). Studies also show that the bifurcation of North Equatorial Current (NEC) is affected by El Niño events (Zenimoto et al. 2009). In the Philippines, NEC bifurcation during El Niño shifts northward and weakens the Kuroshio Current going north while strengthens the Mindanao Current going south (Qu & Lukas 2003; Kashino et al. 2009; Yang et al. 2013; Gordon et al. 2014). Both currents affect the ocean circulation and transport processes in the country and neighboring regions, which in turn affects some marine organisms. Furthermore, since teleconnections distribute tropical climatic patterns towards the pole, El Niño affects climatic variability not only in the tropics but also in Figure 2. ENSO and normal events. a) normal b) El Niño. (Source: Trujillo & Thurman, 2001. Figure redrawn by Michael Mendiola). 285