Feeding Biology and Diet Composition of the Freshwater Sardine, Sardinella Tawilis from Taal Lake (Batangas) in Southern Luzon, Philippines
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International Conference on Environmental and Biological Sciences (ICEBS'2012) December 21-22, 2012 Bangkok (Thailand) Feeding biology and diet composition of the freshwater sardine, Sardinella tawilis from Taal Lake (Batangas) in southern Luzon, Philippines Richard M. Magsino grounds in the country, it will always be important in Taal Abstract—The study investigated the feeding biology and diet Lake to study the feeding relationships of organisms inhabiting composition of the only freshwater sardine, Sardinella tawilis its freshwaters. sampled from Taal Lake in Batangas, Philippines. A total of 540 Fish feeding biology studies are important for many several samples, composed of two size-classes, were collected monthly from reasons. They provide better understanding of trophic January – June 2010, from three sampling points in Taal Lake in the relationship among different species in a specific aquatic morning, noon and afternoon periods. These were analyzed for gut ecosystem, thus they prove useful in the preliminary estimation content and zooplankton analyses in the laboratory. Results revealed that the major diet of S. tawilis in Taal Lake contained different types of trophic levels [3],[4]. These trophic models will then be of zooplankton including cladocerans, calanoid copepods, cyclopoid used as tools to understand complex coastal ecosystems. These copepods, nauplii, rotifers, ostracods, and various species of studies are also useful in defining predator-prey relationships, protozoans and small fishes. The current study showed that copepods where they may also result in the establishment and were the most abundant at ~45–57%, followed by the cladocerans identification of stable food preferences specific for a species ranging from 28–31% also in both size classes. Various copepod [4]. However, there is paucity of information on the dietary species (e.g., cyclopoida and calanoida) and protozoans were variably requirement of this species of Sardinella among Philippine chosen both fish size classes. Smaller frequencies (ranging from freshwaters, except for the one worked in Taal Lake [5]. 2.1% – 18.4%) of rotifers, ostracods and small fishes were also found The present study dealt with investigating the food habits of in the gut of S. tawilis. There were significant differences in terms of the seasonal consumption of prey items in both size classes of S. Sardinella tawilis in Taal Lake. It is considered one effective tawilis. Small-sized fish consumed most of its prey in May and June approach to be able to investigate the aquatic ecology of this and this was significantly different with their prey consumption freshwater ecosystem. S. tawilis sampled approximately less pattern from January to April. In terms of feeding activity and main than a decade ago, exhibited intense feeding on various food index (MFI), most S. tawilis fishes have empty stomachs during zooplankton species including calanoid and cyclopoid the morning (Er=21.2%) and afternoon (Er=30.1%), while at night, copepods, Bosmina, Moina, Ceriodaphnia cladocerans, and most of the fish have food in their stomachs and the mean empty ratio many others [5]. However, they failed to identify size- is lowest at Er = 16.7%. This is the first study to report differences in dependent patterns in the feeding biology of this freshwater the food diet of two size classes, the MFI and frequency of food species. Moreover, they did not estimate the vacuity and the occurrence, and feeding activity of S. tawilis in relation to vacuity index or % of empty stomachs. Main Food Item indices which will indicate the food habits of this species. S. tawilis is important in the food chain of aquatic Keywords—feeding biology, fish diet, Sardinella tawilis, Taal areas such as the Taal Lake, thus [6]-[7] noted that prospects Lake of possible culture development of various fish species necessitates a detailed study on their nutrition and feeding I. INTRODUCTION behavior. In addition, results are aimed at better understanding of the biology of predator and prey species, as ARDINELLA tawilis [1] is the only freshwater sardine well as being useful for stock and ecosystem-based analyses species in the Philippines and probably in the world. It is S [4]. endemically found in the country exclusively inhabiting The general objective of the study is to determine the Taal Lake in the province of Batangas in southern Luzon [2]. feeding biology and diet composition of the freshwater Sardinella fisheries in Taal Lake form one of the most sardine, Sardinella tawilis in Taal Lake in Batangas Province. important freshwater resources in the country. They are caught in commercial quantities and available all year round, Specifically, it aims to determine the differences in the food hence it is always affordable as common source of animal diet of two size classes of S. tawilis, to determine the main protein. Being one of the most important freshwater fishing food item and frequency of food occurrence, and to determine the feeding activity of the species in relation to vacuity index Richard M. Magsino is with the Biology Department, College of or % of empty stomachs. Education, Arts and Sciences of De La Salle Lipa, Lipa City, Batangas, Philippines (Phone: +06343-7565555; fax: 437565555; e-mail: [email protected]). 9 International Conference on Environmental and Biological Sciences (ICEBS'2012) December 21-22, 2012 Bangkok (Thailand) II. MATERIALS AND METHODS number of stomachs sampled [6]. Three primary indices (N, W and F) were calculated to determine the diet of S. tawilis: N is A. Sample Collection the number of individuals of an item, W is the weight of one Samples of the freshwater fish, Sardinella tawilis were item, and F is the number of stomachs containing each item. collected using a 0.3 mm fish net for the young, and 0.8 mm These indices were calculated for each prey item and for each haul net for the adults from three sites in Taal Lake: offshore fish size class. N1 = 100 x number of individuals of item 1 / Talisay, Balete and Laurel (Fig. 1). Collected fish samples total number of prey; W1 = 100 x weight of item 1 / total (~30 individuals) were grouped into two: small fishes from weight of prey; F1 = 100 x number of stomachs containing 70-140 mm and large fishes from 150-250 mm. Samples were item 1 / number of non-empty stomachs examined. randomly collected monthly from January to June 2010, with The Main Food Item (MFI) index was then computed morning, afternoon and evening samplings. These were then combining these three indices [8]. This index was improved by placed in ice-filled container to slow down any bacterial [9] where MFI was summed and for each prey item was digestion process in the fresh stomachs and for easier prey expressed as the ratio of the total. Prey items were classified identification. Fish samples were brought in the laboratory for by decreasing relative values and cumulated ratios are gut content analyses. calculated, following the classification: Main prey: first rank cumulated MFI > 50%; Secondary prey: MFI cumulated ratio between 10% and 50% and Accessory prey: cumulated ratio MFI < 10%. III. RESULTS AND DISCUSSIONS A. Diet Composition of S. tawilis A total of approximately 540 individuals of S. tawilis were sampled from the three sites in Taal Lake from the period spanning January to June 2010. Morphological measurements of the fish samples revealed that ~ 65% were small-sized (i.e., 70-140 mm) while the rest of 35% were large (i.e., 150-250 mm). No effort to determine the sex of the fish samples was Fig. 1 shows the sample collection sites for Sardinella tawilis in conducted. However, visual inspection of the samples revealed Taal Lake dominance of females based on the presence of gravid individuals especially in May and June collections. B. Laboratory Analyses Results of the present study revealed that the major diet of The procedures followed in [5] were conducted in the S. tawilis in Taal Lake contained predominantly of different laboratory analyses. Samples were measured by total length types of zooplankton (Fig. 2). These zooplankton include and body weight before dissection inside the laboratory. several species of cladocerans, calanoid, harpacticoid and Internal organs were removed and stomach contents flushed cyclopoid copepods, rotifers, ostracods, and various types of with distilled water before examination under a compound protozoans and small/larval fishes. A study found that the microscope. Gut contents were examined and all food items copepod species were the most common feeding items in their were identified up to order level of classification following S. tawilis samples collected in 2003-2004 [5]. The current standard protocols. Zooplankton samples were then separated study showed that indeed, copepods were the most abundant at and preserved in 5% formalin solution. Only entire organisms approximately 45 – 57% in both size classes, followed by the were counted, those that were partially digested but could still cladocerans ranging from 28 – 31% also in both size classes. be identified were considered for counting. This was significantly different with results reporting that around 90% of the zooplankton diet of S. tawilis in their C. Statistical Treatment of Data samples is composed primarily of copepods [5]. However, in Two methods were used to describe the diet composition of the present study, the various copepod species (e.g., S. tawilis: the numerical method and the frequency of cyclopoida, harpacticoida and calanoida) when combined, occurrence (FOC) method (adapted from [6]). All components were still the most preferred food items of S. tawilis. of each feeding item were recorded for the two length classes Protozoans were variably chosen by the two fish size classes. and results were expressed in percentages (numerical). On the Smaller frequencies (ranging from 1% – 8%) of rotifers, other hand, the number of specimens where feeding ostracods and small fishes were also found in the gut of the components will be found were recorded independently of size two size classes of S.