Six known fishing grounds for Naujan White Goby(NWG) were chosen as sampling sites for the study. These sites are located around Naujan Lake and are within the four municipalities surrounding the lake. Butas (Site 1) in Barangay Bayani, is located on the northern part of the lake, at the river outlet where Butas River drains towards Tablas Strait. Site 2 was directly at the mouth of Borbocolon River, where water drained into the lake. Together with Malayas (Site 4) and Pasi (Site 5), these sites were positioned on the lake’s western side. Site 3 was in Barangay Tigbao on the eastern coast of the lake, along with Site 6 (Tagbakin). All sites were associated with river mouths except Site 1, which was an outlet. Sites 1 and 3 are within the municipality of Naujan, while Sites 2 and 4 are within the Municipality of Victoria. Site 5 is under the jurisdiction of the Municipality of Socorro, while Site 6 is within the Municipality of Pola Figure 1. Only Site 1 had a salinity of 3 - 5 ppt. The climate in all sites was Type III, with no pronounced wet or dry season and maximum rainfall from June to September. The warmest month was May, the wettest was December, the coldest was February, and the driest was April. The average annual temperature ranged from 28 °C to 32 °C (Weather & Climate n.d.).

Fish sampling was done experimentally on all sites using a 1,000-m gill net number 14 with a mesh size of approximately 1.5 cm. The gill nets were used to capture a minimum of 50 adult fish and set for 8 hours to standardize fishing efforts in all sampling sites. Samples were collected 2 or 3 days before a full moon every month from February 2020 to January 2021. Collected samples were stored in an iced-styrofoam box and transported to the Fisheries Biology Laboratory of the Institute of Fisheries, Mindoro State University Bongabong Campus for further examination. The fish were identified morphologically based on the arrangement of pit organs and sensory papillae (Akihito & Meguro, 1975);(Abdulmalik-Labe et al., 2022).

The gut content analysis was conducted using the methods described by (Dinh et al., 2022). In the laboratory, sex, body weight (BW, g), and total length (TL, mm) of individual adult fish were determined. The whole gut was removed from each fish, weighed (gut-weight, g), and measured (gut length, mm). Gut contents were taken by dissecting the stomach and examined under an LB 220 compound microscope. The stomach was categorized as containing food (non-empty) or without food (empty).

The wet weight of the prey item was recorded to the nearest 0.01 g using an analytical balance and classified based on the following prey types: shrimp, fish, phytoplankton, zooplankton, mollusks, insect larvae, and other food items. Prey identification involved analyzing both partially and fully digested remains. Partially digested prey items are characterized by degraded soft tissues and varying degrees of fragmentation, while fully digested prey items have unrecognizable structure and the gut appears homogenous.

For the identification of prey items, presence of hard structures was used, specifically head fragments, otoliths, teeth, vertebrae, carapaces, shells (exoskeleton) and legs. For shrimp, a single individual is equivalent to 1 carapace, 5 pairs of walking legs, and 1 abdominal region. Separately, vertebrae, head pieces, and otoliths were identified among the fish remains. To estimate the individuals for both fish and shrimp, the most commonly occurring remains were identified. Miscellaneous food items, such as prey bones, plant detritus, sand, mud, and other unidentifiable gut materials were not included as they constitute only smaller proportions of the diet.

The following indices were determined from 1,646 G. aureus with stomach contents to characterize itsdietary patterns and food preferences:

The seasonal variation in the feeding activity of NWG was assessed by determining the Gastrosomatic Index (GaSI). The GaSI values were obtained by dividing the gut weight by fish weight, then multiply by 100 (Biswas, 1993). Relative Gut Length (RGL) was also measured to determine the feeding habits of NWG by calculating the ratio of gut length with the total length of the fish. A Relative Gut Length (RGL) of < 1 indicates a carnivorous diet, 1 < RGL < 3 and RGL > 3 signifies omnivorous and herbivorous diets, respectively (Borlongan et al., 2002);(Ward-Campbell et al., 2005).

Alternative Kruskal-Wallis Test was performed to assess the seasonal and site differences in the feeding intensity of NWG at P< 0.05. The test was used since the normality and homoscedasticity assumptions were unmet. Pairwise comparisons were performed following significant results to highlight specific differences among groups. Significant values have been adjusted by the Bonferroni correction for multiple tests. Differences in feeding intensity between male and female NWG were evaluated using a t-test. Data analyses were performed using SPSS software v26 at P < 0.05 significance level.

A total of 1,938 NWG (G. aureus) (1,094 male and 844 female), ranging from 75 mm (3.33 g) to 280 mm (148 g), were collected from February 2020 to January 2021. Site 6 (Tagbakin) had the highest number of fish samples collected, followed by Site 3 (Tigbao). Site 1 (Butas), Site 2 (Borbocolon), Site 4 (Malayas), and Site 5 (Pasi) had sample counts of 292, 307, 260, and 216, respectively.

The G. aureus has been seen foraging on a variety of food throughout the year. The five prey items were shrimp, fish, phytoplankton, zooplankton, and insect larvae. Shrimp constituted the bulk of the food consumed (40.52%), while mollusks showed the lowest occurrence frequency (0.26%). Fish and phytoplankton were observed to have relatively high occurrence in the gut. The remaining food items, such as zooplankton, insect larvae, and mollusks, were the rarely consumed diets. The Index of Relative Importance (IRI) also indicated that shrimp is the primary food item NWG consumes Table 1. Fish comprised the next essential prey item, followed by phytoplankton.

The rest of the food items were occasionally consumed. Similar findings were reported for G. aureus in the Mekong Delta, where shrimp, particularly Acetes spp., were identified as the primary food source; additionally, diets were entirely of animal origin, consisting predominantly of zoobenthos (Phan et al., 2023). Differences in the diets within the same fish species have also been observed in G. giuris captured from distinct water bodies. In the Philippines, adult G. giuris in Lake Mainit predominantly consumes fish, followed by shrimp (Uy et al., 2019).

In contrast, those in Lake Taal primarily feed on crustacean eggs, with shrimp absent from the diet (Bejer, 2015). These dietary differences may be influenced by food availability, environmental conditions, and habitat-specific resources (Heng et al., 2018). Therefore, discriminating the diet composition of G. aureus from its natural habitat is necessary to have a basis for its nutritional requirements when cultured under captive conditions.

The seasonal fluctuation in the diet composition of G. aureus in Naujan Lake was clearly shown in Figure 2. Pairwise comparison showed a significantly high percentage occurrence of shrimp observed during the dry months (January to April) and rainy months (September to November) compared to the lowest ingestion recorded in June and July (P < 0.05).

Teleost fish was the most preferred food during the onset of the cool season in December and the end of the hot, dry season in May. Seasonal variations in the diet components were also found in G. giuris but were not observed in G. sparsipapillus (Dinh et al., 2024);(Tran et al., 2021). This pattern may reflect seasonal variations in shrimp availability, environmental conditions, or changes in the feeding activity of the fish. The decline in shrimp consumption during June and July may coincide with shifts in prey abundance or other ecological factors influencing prey-predator dynamics (Heng et al., 2018).

Phytoplankton was the preferred diet, recorded only in June, July, and August. Glossogobius aureus may have exhibited seasonal opportunism in feeding during this period, which occasionally consumes readily available and abundant phytoplankton without being classified as omnivorous. (Beaudoin et al., 1999) reported opportunistic feeding of carnivorous North pike when the usual prey is scarce. In Naujan Lake, shrimp were recorded in a minimal proportion of the NWG diet from June to August, which may likely lead to occasional opportunistic feeding behavior. The high occurrence of phytoplankton in the gut samples of adult NWG collected during the rainy months might be triggered by an increase in the concentration of nutrients carried to the lake by runoff, particularly in Butas and Tagbakin. Additionally, the consumption of phytoplankton may indicate a trophic transfer of plant material within the food chain, as the fish could ingest small amounts of phytoplankton found in the stomachs of its prey (Horn & Ferry-Graham, 2006).

Shrimp dominated the diet composition of NWG across the six sampling sites; however, a significant variation in the proportion of other dietary items was recorded Figure 3. Dietary components of samples from Site 6 (Tagbakin) significantly varied with that of Sites 3, 4, and 5, respectively (P < 0.05). Site 1 (Butas) had significantly lower proportions of fish in the diet than Site 3 (Tigbao) and Site 4 (Malayas). However, Site 4 (Malayas) samples exhibited a more diverse diet with the occurrence of all the six food items identified in the gut.

The findings may be attributed to the diversity and availability of food sources across the sampling sites. Additionally, competition for food, manifested in diet overlaps, was observed among fish species (Das et al., 2018);(Matern et al., 2021). Insect larvae were absent from the diets of fish sampled at Site 1 (Butas) and Site 2 (Borbocolon) but were present in varying proportions at the other sampling sites. The limited occurrence of insect larvae in deep waters may explain their absence in the stomachs of the samples from Sites 1 and 2, which are both deep sites. Meanwhile, mollusks were observed only in Site 4 (Malayas). Their distribution may be influenced by substrate type, water depth, and hydrochemical parameters (Bespalaya et al., 2021).

Phytoplankton abundance in the stomach of G. aureus was relatively high in Site 1 (Butas), Site 2 (Borbocolon), and Site 6 (Tagbakin), but relatively lower occurrence in Site 4 (Malayas). All sites are associated with rivers with varying flows. Among the rivers, Malayas (Site 2) has the biggest flow. Phytoplankton growth is always associated with nutrient inputs from runoff but also results from complex dynamics of water flow, light penetration, and temperature (Lu et al., 2023). The presence of phytoplankton in the stomach of all samples, irrespective of sites, indicates they are part of the regular diet of the species. Specifically, the consumption of benthic forms, as gobies are benthic feeders.

With a preference for shrimp and fish and the ability to exploit various prey types, NWGis potentially classified as a carnivorous generalist feeder. It showed a highly carnivorous diet correlated with the computed relative gut length of 0.631. Because meat is easier to digest, carnivorous fishes often have short, more or less straight intestines (Shukla, 2009). The burrowing goby Parapocryptes serperaster also showed a similar trophic strategy (Dinh et al., 2017), differing from the specialist feeding behavior of goby Economidichthys pygmaeus (Gkenas et al., 2012).

From 1,938 gut samples, 1,646 (85%) contained food and 292 (15%) were empty. This percentage of empty stomachs is lower relative to the 65.36% reported for G. sparsipapillus (Tran et al., 2021). Variations in the rate of empty stomachs may be linked to differences in spawning events or reproductive cycles. The food intensity of G. aureus was observed based on the gastrosomatic index (GaSI) and gut food volume taken monthly. (Sanchez-Escalona et al., n.d.). (in press) identified March, June, and December as the peak spawning months for male G. aureus and June and November for females. The highest percentage of empty stomach of NWG was recorded in March in males and June in females, coinciding with the spawning season of G. aureus in Naujan Lake. The lowest percentage of empty stomachs of NWG was observed from November to January for both sexes. There was no significant variation (t-test, P = 0.60) in the mean GaSI values in both sexes; however, males generally feed more actively than females. The mean GaSI estimates showed that females forage intensively in March, April, and October, while in males, a high feeding activity was observed in April, September, and November Figure 4. The GaSI values were observed to be high before spawning months and decreased during spawning season in both sexes.

A similar result was observed in the feeding intensity of G. guiris in Bangladesh, which showed maximum feeding during the off-breeding season while minimum during the spawning period (Hossain et al., 2016). The findings may suggest possible biological or behavioral differences that are not strongly linked to the GaSI values. Halting food intake during most of the spawning period is likely associated with previous build-up and stored energy reserves from high feeding intensity during pre-spawning periods. The suppression of feeding activity may also have been due to less space in the body cavity for food (Hoar et al., 1983). However, as males do not bulge as much as females, the male's body cavity was likely able to provide more space for food than the female's (Maklad et al., 2016). Male NWG grazed more actively than females during the breeding season. In naked goby (Gobiosoma bosc), males forage less intensively during the breeding season (D’Aguillo et al., 2014). Males of the species are in charge of egg guarding and territory defense, reducing their opportunity for foraging activity and lowering their feeding intensity. Differences in feeding intensity between sexes of a species, thus, may be the reproductive status (Rideout, 1999) and the member's investment in parental care (D’Aguillo et al., 2014).