Philippine Journal of Science Vol. 145 No. 3, September 2016 the sub-tropics and some mid-latitude regions (Allan et al. 1996; Kumar & Hoerling 1997; Gergis & Fowler, 2009). Thus, this phenomenon acts on a global scale with strong influences on the functioning of marine as well as terrestrial ecosystems (Ulloa et al. 2001; Holmgren et al. 2001). Particularly in marine ecosystems, El Niño affects individual organisms such as members of the phytoplankton community (Morales-Ramirez &BrugnoliOlivera 2001; Iriarte & Gonzalez 2004), fishes (Cushing, 1981; Sharp & McLain 1993; Lehodey et al. 1997), corals (Glynn 1984; Brown & Suharsono 1990); and marine mammals (Benson et al. 2002; Crocker et al. 2006; Santos & Aquino 2012). Relation to Primary Productivity During non El Niño events, the shallow thermocline supports the natural upwelling in the EEP where nutrientrich cool water rises towards the surface (Meuser et al. 2013). However, the upwelling is decreased during El Niño due to the eastward movement of warm surface water that leads to the depression of thermocline. The upwelling persists but the deeper thermocline brings nutrient-less warm water resulting to the low primary productivity in the EEP (Barber & Chavez 1983). This is opposite in the EWP where the primary productivity increases as in the 1997/1998 El Niño (Murtugudde et al. 1999). The resulting shallow thermocline and the strong wind stress allowed water column mixing and brought the surface well-mixed layer below and deep nutrientrich layer above that favoured primary production in the region (Vialard & Delecluse 1998; Lehodey 2001). In the Philippines, Cabrera et al. (2011) revealed that barrier layer is one mechanism that inhibits upwelling in the Bohol Sea during an El Niño event. They showed that Damatac II & Santos: Possible Effects of El Niño on Some Philippine Marine Fisheries Resources barrier layer forms thinner during less precipitation, which results to weak water stratification. Stratification prevents water column mixing and its weakened condition during El Niño allows deep nutrient-rich water to rise necessary for the primary production (Cabrera et al. 2011). Satellite images from SeaWiFS revealed that the primary production in EWP was high and coastal upwelling was enhanced during 1997/1998 El Niño compared to the 1998/1999 La Niña event that followed after (Figure 3). Lehodey (2001) also showed the same trend when El Niño resulted to an increase in water productivity during1982/1983 and 1997/1998using composite satellite images. Maclean (1989) was able to link dinoflagellate, Pyrodinium bahamense var. compressum, blooms to El Nino in Papua New Guinea, Borneo, and the Philippines from 1970 to 1988. Simultaneous to the 1982/1983 event, a red tide was recorded on 1983 in Samar, Philippines (Hallegraeff, 1989). These were followed by blooms on 1987 and 1991-1994 in Zambales, Philippines when mild El Nino years were recorded (Caturao 2001). Effects on Fisheries One of the environmental factors that generally affects the biology and migration of many fishes is temperature (Magnuson et al. 1979; Gulland 1980; Binder et al. 2011). Magnuson et al. (1979) coined the concept of thermal niche to refer the preferred temperature of fishes. Skipjack tunas (Katsuwonus pelamis), for example, are found mostly in the EWP because they prefer the warm pool of water normally residing in the region (Lehodey et al. 1997; Sugimoto et al. 2001). Mostly for pelagics, the geographical distribution of fishes is influenced by fluctuating temperature that even small changes may allow the fish stock to extend it distribution further towards Figure 3. Primary production in the Pacific Ocean. SEAWIFS satellite images of chlorophyll concentration during 1997/1998El Niño and 1999 La Niña (Source: NASA GES DISC Giovanni; http://disc.sci.gsfc.nasa.gov/) 286