Because species that belong under genus Sardinella is morphologically difficult to distinguish from each other, they are often prone to misidentification (Willette et al., 2011). This was the case in Sardinella lemuru and Sardinella longiceps. For a long time, S. longiceps has been included in the list of the Philippine Sardinella species but was inconsistent with the geographic range it belongs. Sardinella longiceps (known as Indian oil sardine) is endemic to Indian Ocean and extends within a range from eastern Africa, north to the Gulf of Oman and Gulf of Aden, along the Indian coastline to Sri Lanka and possibly as far east as the Andaman Sea. Its range clearly does not include the Philippines unlike in S. lemuru, a morphologically similar species of S.
longiceps, but is found along the country’s coast. Tracing its history, the origin of Clupeoid fishes was found to be marine and tropical. It thrived in the Indo-West Pacific (IWP) region, the largest tropical region in the world, which currently shelters 4,000 species of fishes, most of them endemic (Lavoue et al., 2013). 119 Million Years Ago, the most recent common ancestor of Clupeoidei lived in the proto-IWP region and underwent speciation during the rest of Cretaceous period, with six out of the nine current main lineages originating from the IWP region.
Afterwards, these diversified fishes dispersed around the world in three batches: the first batch northward to the Northeastern Atlantic, the second batch southward to South Australia, and the last eastward towards the East Atlantic. The lineage where the Sardinella subfamily (Clupeinae) belongs to is part of the first batch of dispersion, and is characterized to be the only tropical subfamily with a synapomorphy of gill-rakers that do not overlap (Lavoue et al., 2013).
Once they have dispersed, geographic isolation played a major role in the high degree of variation in the current phenotype of these fishes (Luceño et al., 2014). Currently, Sardinella lemuru is located in southern Japan, Taiwan, Philippines, and Indonesia to the southern limit in Western Australia (Willette & Santos, 2012).
In the Philippines particularly, it is most abundant in Sulu Celebes Sea in Mindanao, although it could be found in other Philippine coastal waters as well. Besides S. lemuru, seven other species found to be thriving locally are S.
albella, S. brachysoma, S. fimbriata, S. gibbosa, S.
tawilis, S. longiceps (although it might be misidentified), and S. melanura (Quilang et al., 2011). Based from Lavoue et al. (2013), the three possible factors for the migration of these fishes are the vast expanses of open ocean, water temperature, and salinity. Of these, salinity seemed the least likely, as several lineages of Clupeiformes developed diadromy as a way to cope with varying levels of salt concentration (Bloom & Lovejoy, 2013). On the contrary, the one most likely to cause the migration is temperature.
As stated by Pedrosa-Gerasmio et al. (2014), temperature greatly affects the physiological needs of these fishes, namely, growth, reproduction, and survival. Other causes of migration can be food resources, body mass recovery, survival against predators, and movement to areas less accessible to fishermen, where they can regain their population (Sambah et al., 2012). The changing sea temperature that drives S. lemuru from Bali Strait to the Sulu-Celebes Sea is brought about by the Asian monsoon and its effects on the sea.
According to Villanoy et al. (2011), the northeastern blow of Asian Monsoon (locally termed as Amihan) produces cooler temperatures that enhance convective mixing, giving rise to upwellings, which are signalled by elevated concentration of chlorophyll-a produced by algal bloom or algae considered as zooplankton and brought up from the sea floor to be used for detecting the formation of upwellings (Sartimbul et al., 2010). Common upwelling species (which are sardines and anchovies) migrate to these locations to feed on zooplankton and reproduce.
Because Asian Monsoon create upwellings between December and March (northeastern monsoon), it is expected for Sardinella lemuru to be abundant in those months, although that is not always the case. A case report on sardine catch in Northern Zamboanga states that the glut period of tuloy occurs between March and May (RFLP, 2013), while another study asserts that its major spawning season is from November to March, with minor spawning in July-September (Bagarinao & Campos, 2015).