Plastic pollution has become one of the most visible and persistent problems in marine environments. As larger plastic materials are exposed to sunlight, wave action, mechanical abrasion, and other natural processes, they may break down into smaller particles known as microplastics. Microplastics are commonly defined as plastic particles smaller than 5 mm (Law & Thompson, 2014; Andrady, 2017; Auta et al., 2017). Because of their small size, these particles can be widely distributed in coastal waters, sediments, and biological organisms (Barnes et al., 2009; Gewert et al., 2015; Peng et al., 2020). Marine organisms may ingest microplastics directly when the particles resemble food, or indirectly through contaminated prey and feeding substrates (Rochman et al., 2013; Lusher et al., 2017; Roch et al., 2020). In fish, ingestion may depend on habitat, feeding behavior, food availability, particle color, and particle shape (Santos et al., 2016; Mizraji et al., 2017; Liang et al., 2023). Microplastics may remain in the gastrointestinal tract and may cause digestive stress, reduced feeding efficiency, or exposure to chemicals associated with plastic particles (Wright et al., 2013; Hirt & Body-Malapel, 2020; Hossain & Olden, 2022). The Philippines is highly exposed to plastic leakage into marine environments due to its long coastline, growing coastal population, and ongoing challenges in solid waste management (Jambeck et al., 2015; Galarpe et al., 2021). Local studies have reported microplastics in Philippine coastal waters, sediments, and fish, showing that the issue is not only global but also relevant to local fisheries and seafood safety (Abreo et al., 2016; Espiritu et al., 2019; Galicia et al., 2023; Go et al., 2024; Alburo et al., 2024). Since many coastal communities depend on fish for food and income, documenting microplastic occurrence in commonly consumed species is important. Rabbitfish (Siganus spp.) are economically and ecologically important fishes in tropical coastal ecosystems. They are herbivorous fishes commonly associated with seagrass beds, reef flats, and coastal habitats where they graze on algae and seagrass-associated materials (Hoey et al., 2013; Fox & Bellwood, 2014; Parawansa et al., 2020; Latuconsina et al., 2021). Because seagrass and algal surfaces can trap suspended particles, rabbitfish may accidentally ingest microplastics while feeding. This makes rabbitfish a relevant species for assessing microplastic exposure in coastal habitats. Pujada Bay in Davao Oriental is a biologically rich coastal area that supports fisheries, aquaculture, and nearby coastal communities. Tamisan is one of the areas within Pujada Bay where wild rabbitfish are collected. Despite the ecological and economic importance of rabbitfish, there is still limited information on microplastic ingestion in wild rabbitfish from this area. This study therefore, focused on the gastrointestinal tract of wild rabbitfish from Tamisan and aimed to determine the occurrence, abundance, and visual characteristics of suspected microplastics. The study was limited to visual identification because the recovered particles were too small for FTIR confirmation.
The study of Microplastics in marine ecosystems is supported by theoretical frameworks that explain their sources, transport, and biological uptake. Microplastics are derived from both primary sources, such as industrial microbeads, and secondary sources resulting from the fragmentation of larger plastic debris, which are then dispersed through oceanic and coastal processes (Andrady, 2017; GESAMP, 2019). Once in the marine environment, these particles are transported by currents and tend to accumulate in coastal and benthic zones, increasing their availability to marine organisms (Lebreton et al., 2019; van Sebille et al., 2015). The concept of exposure pathways explains how organisms ingest microplastics either directly from the water column or indirectly through contaminated food and sediments, with feeding behavior influencing susceptibility (Wright et al., 2020). In particular, benthic and herbivorous species are more prone to ingestion due to their interaction with sediment-associated particles. The theory of trophic transfer further suggests that microplastics can move through the food web via prey-predator interactions, potentially increasing exposure across trophic levels, although evidence for biomagnification remains inconclusive (Provencher et al., 2019; Savoca et al., 2021). Additionally, the physical characteristics of microplastics, including size, shape, and color, influence ingestion patterns, with fibers frequently reported as dominant due to their widespread origin from synthetic textiles and fishing-related activities (Barboza et al., 2018; Covernton et al., 2021). Methodologically, microplastic research follows standardized procedures involving digestion, filtration, and microscopic examination, while advanced techniques such as Fourier Transform Infrared Spectroscopy (FTIR) and Raman spectroscopy are recommended for polymer confirmation; however, limitations in detection and accessibility often result in reliance on visual identification, particularly in developing regions (Primpke et al., 2020; Cowger et al., 2021). These theoretical perspectives provide a comprehensive framework for understanding the occurrence, ingestion, and potential impacts of microplastics in marine organisms and coastal ecosystems.
The study was conducted in Tamisan, a coastal area within Pujada Bay, Davao Oriental, Philippines. Pujada Bay is located on the Pacific coast of the southern Philippine island of Mindanao. The bay was declared as a protected area under the National Integrated Protected Areas System (NIPAS) through Presidential Proclamation No. 431 in 1994. Pujada Bay is also considered as one of the most beautiful bays in the world (Ybañez, 2024). The bay is recognized for its productive marine ecosystems and supports local fisheries, including wild rabbitfish. Tamisan was selected as the sampling site for wild rabbitfish because it is associated with nearshore habitats, and it has a wide seagrass area where rabbitfish commonly feed and caught (Figure 1). This study used a descriptive-quantitative design to determine the occurrence and characteristics of suspected microplastics in the GIT of wild rabbitfish. A total of 60 wild rabbitfish (Siganus guttatus) samples were collected from Tamisan. After collection, the fish samples were wrapped in aluminum foil to reduce contamination, placed in an ice box during transport, and stored in a freezer until laboratory processing. To minimize possible contamination, clean laboratory materials were used and samples were handled carefully during processing. Each fish was dissected and the whole gastrointestinal tract was removed using clean dissection materials. The GIT was placed in a clean glass container and digested using 10% potassium hydroxide (KOH). The samples were left until the organic tissues were sufficiently digested. After digestion, the solution was filtered using a vacuum filtration set-up. The filter papers were then dried and stored in covered petri dishes until microscopic examination. The dried filter papers were examined under a stereomicroscope. Suspected microplastics were recorded based on visual characteristics such as morphology and color. Morphology was categorized as fiber, fragment, film, pellet/granule, or foam when applicable. Color was recorded according to the visible color of each suspected particle. In this study, polymer confirmation through FTIR was not completed because the recovered suspected particles were too small for the available FTIR reading. For this reason, the results are reported as visually identified suspected microplastics rather than confirmed microplastics. The occurrence of suspected microplastics was calculated as the percentage of fish samples containing at least one visually identified suspected microplastic particle relative to the total number of fish examined. Abundance was expressed as the mean number of suspected microplastic particles per fish and presented as mean ± standard deviation.
Figure 1. Map of the study area.
This study used a descriptive-quantitative design to determine the occurrence and characteristics of suspected microplastics in the GIT of wild rabbitfish. A total of 60 wild rabbitfish (Siganus guttatus) samples were collected from Tamisan. After collection, the fish samples were wrapped in aluminum foil to reduce contamination, placed in an ice box during transport, and stored in a freezer until laboratory processing. Only the wild rabbitfish GIT data from Tamisan were included in this article.
To minimize possible contamination, clean laboratory materials were used and samples were handled carefully during processing. Containers and materials were covered when not in use. Filtered distilled water was used for rinsing, and the samples were handled in a way that avoided unnecessary exposure to airborne particles. Natural-fiber clothing and non-plastic handling materials were preferred during field and laboratory work whenever possible.
Each fish was dissected and the whole gastrointestinal tract was removed using clean dissection materials. The GIT was placed in a clean glass container and digested using 10% potassium hydroxide (KOH). The samples were left until the organic tissues were sufficiently digested. After digestion, the solution was filtered using a vacuum filtration set-up. The filter papers were then dried and stored in covered petri dishes until microscopic examination
The dried filter papers were examined under a stereomicroscope. Suspected microplastics were recorded based on visual characteristics such as morphology and color. Morphology was categorized as fiber, fragment, film, pellet/granule, or foam when applicable. Color was recorded according to the visible color of each suspected particle. In this study, polymer confirmation through FTIR was not completed because the recovered suspected particles were too small for the available FTIR reading. For this reason, the results are reported as visually identified suspected microplastics rather than confirmed microplastics.
The occurrence of suspected microplastics was calculated as the percentage of fish samples containing at least one visually identified suspected microplastic particle relative to the total number of fish examined. Abundance was expressed as the mean number of suspected microplastic particles per fish and presented as mean ± standard deviation. Descriptive statistical analyses were used to summarize the data. The morphology and color of suspected microplastics were categorized and quantified using frequency counts and percentage distribution. Morphological categories included fibers, fragments, and films, while color categories included blue, red, black, and transparent particles. The frequency of each category was computed relative to the total number of suspected particles observed. Data tabulation and descriptive statistical computations were performed using Microsoft Excel. Results were presented in tables to facilitate comparison of occurrence, abundance, morphology, and color distribution of suspected microplastics in wild rabbitfish collected from Tamisan, Pujada Bay.
Out of the 60 wild rabbitfish examined from Tamisan, 16 samples contained suspected microplastics, while 44 samples had no observed suspected particles. This corresponds to an occurrence rate of 26.7%. Since each positive sample contained one visually recorded suspected particle, the total number of suspected particles was 16. The mean abundance was 0.27±0.45 suspected microplastic particle per fish (Table 1).
| Parameter | Value |
|---|---|
| Number of fish examined | 60 |
| Fish with suspected microplastics | 16 |
| Fish without suspected microplastics | 44 |
| Occurrence (%) | 26.7 |
| Total suspected particles recorded | 16 |
| Mean abundance (particles/fish) | 0.27 ± 0.45 |
The suspected particles observed in the GIT of wild rabbitfish were mainly fibers. Fibers accounted for 14 out of 16 particles, or 87.5% of the total particles recorded. Only one fragment and one film were observed, each representing 6.3% of the total suspected particles. The suspected microplastics showed four observed colors: blue, red, black, and transparent. Blue was the most common color, representing half of the total particles recorded.
Figure 2. Morphological characteristics of suspected microplastics
The suspected microplastics showed four observed colors: blue, red, black, and transparent. Blue was the most common color, representing half of the total particles recorded. Red particles were the second most common, followed by black and transparent particles (Figure 3).
Figure 3. Color distribution of suspected microplastics.
Suspected microplastics were recorded in the following Tamisan fish samples no. (TF): TF3, TF4, TF11, TF12, TF13, TF15, TF16, TF17, TF18, TF22, TF32, TF37, TF41, TF44, TF55, TF60.
The present study revealed that 26.7% of wild Siganus guttatus collected from Tamisan contained visually identified suspected microplastics, with a mean abundance of 0.27±0.45 particles per fish. The occurrence of suspected microplastics in more than one-fourth of the examined fish indicates that microplastic contamination is already present within the coastal environment of Pujada Bay. Although the abundance observed in this study was relatively lower compared to reports from highly urbanized or industrialized marine systems, the findings remain ecologically significant because rabbitfish are widely consumed and play an important role in tropical coastal food webs. Similar ingestion frequencies ranging from 10% to 40% have been documented in marine fishes from tropical and coastal environments, where contamination levels vary depending on habitat characteristics, anthropogenic pressures, feeding ecology, and proximity to pollution sources (Lusher et al., 2017; Savoca et al., 2021; Hossain & Olden, 2022). In the Philippine setting, the occurrence observed in this study is comparable to previous reports documenting microplastic contamination in marine organisms from coastal ecosystems. The relatively moderate abundance recorded in Tamisan may reflect the semi-rural condition of the area, which may experience lower plastic input compared to densely populated urban coastlines. However, continuous coastal activities such as fishing, domestic waste disposal, and shoreline human settlements may still contribute to the gradual accumulation of plastic-derived particles in the marine environment. The ingestion of suspected microplastics by S. guttatus may be strongly associated with the species' herbivorous feeding behavior and habitat preference. Rabbitfish commonly graze on algae, seagrass surfaces, and benthic-associated materials where suspended particles may accumulate (Mizraji et al., 2017; Latuconsina et al., 2021). Seagrass ecosystems and benthic habitats are recognized as sinks for fine suspended particles because reduced water movement promotes sedimentation and particle retention (Peng et al., 2020). Consequently, rabbitfish feeding within these habitats may accidentally ingest microplastics together with algal matter, detritus, or sediment-associated food materials. This suggests that ingestion in wild rabbitfish is likely incidental rather than selective and occurs during normal feeding activities within contaminated coastal habitats. A notable finding of this study is the dominance of fiber morphology, which accounted for 87.5% of the total suspected particles observed. The predominance of fibers is consistent with findings from numerous global and regional microplastic studies involving marine organisms (Lusher et al., 2017; Savoca et al., 2021).
In the Philippine setting, the occurrence observed in this study is comparable to previous reports documenting microplastic contamination in marine organisms from coastal ecosystems. Espiritu et al. (2019) reported the presence of microplastics in selected fish species from Batangas, Philippines, although differences in abundance were influenced by species-specific feeding behavior and environmental conditions. Similarly, Go et al. (2024) documented microplastics in fish, seawater, and beach sediments from tourist coastal destinations, suggesting that microplastic contamination has become widespread even in areas that are not heavily industrialized. The relatively moderate abundance recorded in Tamisan may reflect the semi-rural condition of the area, which may experience lower plastic input compared to densely populated urban coastlines. However, continuous coastal activities such as fishing, domestic waste disposal, and shoreline human settlements may still contribute to the gradual accumulation of plastic-derived particles in the marine environment.
The ingestion of suspected microplastics by S. guttatus may be strongly associated with the species’ herbivorous feeding behavior and habitat preference. Rabbitfish commonly graze on algae, seagrass surfaces, and benthic-associated materials where suspended particles may accumulate (Mizraji et al., 2017; Latuconsina et al., 2021). Seagrass ecosystems and benthic habitats are recognized as sinks for fine suspended particles because reduced water movement promotes sedimentation and particle retention (Peng et al., 2020). Consequently, rabbitfish feeding within these habitats may accidentally ingest microplastics together with algal matter, detritus, or sediment-associated food materials. This suggests that ingestion in wild rabbitfish is likely incidental rather than selective and occurs during normal feeding activities within contaminated coastal habitats. A notable finding of this study is the dominance of fiber morphology, which accounted for 87.5% of the total suspected particles observed. The predominance of fibers is consistent with findings from numerous global and regional microplastic studies involving marine organisms (Lusher et al., 2017; Savoca et al., 2021). Fibers are commonly recognized as the most abundant microplastic type in aquatic environments because they are continuously released from synthetic textiles, domestic laundry wastewater, ropes, fishing nets, and other fishing- related materials (De Falco et al., 2020). Their elongated and lightweight structure also allows them to remain suspended in seawater for longer periods, increasing the likelihood of ingestion by marine organisms. In addition, microfiber contamination has been reported to persist even after wastewater treatment because many treatment systems are unable to completely remove very fine synthetic fibers before discharge into coastal waters (Auta et al., 2017; De Falco et al., 2020).
The dominance of fibers observed in this study may therefore reflect both land-based and marine-based sources of plastic pollution within the study area. Similar observations were reported in Philippine coastal waters, where fibers were frequently associated with household wastewater, improper solid waste disposal, fishing activities, and degradation of synthetic fishing materials (Galarpe et al., 2021; Galicia et al., 2023). Since Tamisan supports active fishing activities and nearby coastal communities, the presence of fibers in wild rabbitfish may indicate continuous exposure to locally generated plastic debris and fishing-related materials entering the marine environment. Color distribution analysis further showed that blue particles were the most dominant, followed by red, black, and transparent particles. The predominance of blue particles may be linked to the widespread use of blue-colored fishing ropes, synthetic fishing lines, and netting materials in coastal fisheries (Santos et al., 2016; Lusher et al., 2017). Meanwhile, red and black particles may originate from degraded packaging materials, painted surfaces, domestic plastic waste, or fragments of consumer products transported into the marine environment. Previous studies have suggested that fish may unintentionally ingest colored particles because some plastics visually resemble prey organisms or suspended organic materials in the water column (Santos et al., 2016). However, the interpretation of color origin should be approached cautiously because environmental weathering processes such as ultraviolet radiation, oxidation, abrasion, and biofouling can alter the original appearance of plastic particles over time (Andrady et al., 2022).
Although the suspected microplastics detected in this study were confined to the gastrointestinal tract, their occurrence remains ecologically relevant. Microplastics may not always accumulate permanently within fish because particles can be egested after ingestion (Roch et al., 2020). Nevertheless, continuous exposure may lead to repeated ingestion and prolonged interaction with digestive tissues, potentially causing physical stress, reduced feeding efficiency, gut irritation, and exposure to toxic chemicals adsorbed onto plastic surfaces (Wright et al., 2013; Hirt & Body-Malapel, 2020). Microplastics may also act as carriers of contaminants such as heavy metals and persistent organic pollutants, which could further increase ecological risks to marine organisms (Teuten et al., 2009). From a food safety perspective, the occurrence of microplastics in edible fish species raises concerns regarding possible human exposure, particularly in coastal communities where whole fish or internal organs are occasionally consumed (Smith et al., 2018; Cox et al., 2019; Prata et al., 2020). Compared with studies reporting substantially higher microplastic abundance in fish, the relatively lower abundance observed in this study may be attributed to differences in environmental contamination levels, species ecology, sampling strategies, and analytical methodologies. Studies utilizing more advanced analytical techniques capable of detecting smaller particles often report higher microplastic abundance because microscopic particles that are not visible under conventional stereomicroscopy may also be identified (Prata et al., 2020; Liang et al., 2023). Consequently, the actual abundance of microplastics in the examined rabbitfish may have been underestimated in the present study.
One important limitation of this study is the absence of polymer confirmation using Fourier Transform Infrared Spectroscopy (FTIR). While visual identification remains widely used in preliminary microplastic assessments, it may result in possible misidentification of non-plastic materials such as natural fibers, organic debris, or degraded biological particles (Hidalgo-Ruz et al., 2012; Lusher et al., 2017). For this reason, the particles identified in this study were cautiously classified as “suspected microplastics.” Similar methodological limitations have also been recognized in previous studies involving very small particle sizes and limited analytical resources (Prata et al., 2020). Despite this limitation, visual identification still provides valuable baseline information for areas where advanced spectroscopic instruments are not readily accessible. The findings of this study contribute baseline information on microplastic contamination in wild rabbitfish from Pujada Bay and provide additional evidence that economically important marine fish species are already exposed to plastic- derived particles in Philippine coastal ecosystems. The consistent detection of fibers and the measurable occurrence of suspected microplastics emphasize the need for continued environmental monitoring, improved coastal waste management practices, and further studies incorporating polymer confirmation techniques and environmental sampling to better understand the sources, transport pathways, and ecological implications of microplastic contamination in the region.
This study documented the occurrence of visually identified suspected microplastics in the gastrointestinal tract of wild Siganus guttatus collected from Tamisan, Pujada Bay, where 16 out of 60 individuals (26.7%) were found to contain suspected microplastics. Fibers were the predominant morphology, with blue particles being the most frequently observed, indicating possible links to common anthropogenic sources such as fishing gear, textiles, and domestic waste. These findings suggest that wild rabbitfish in the study area are exposed to microplastic contamination, reflecting the broader environmental condition of coastal ecosystems in Pujada Bay. However, due to the absence of polymer confirmation through Fourier Transform Infrared Spectroscopy (FTIR), as the recovered particles were too small, the results should be interpreted with caution and considered as visually identified suspected microplastics. In light of these findings, it is recommended that future studies incorporate confirmatory analytical techniques such as FTIR or Raman spectroscopy to improve identification accuracy. Expanding sampling efforts to include multiple sites, larger sample sizes, seasonal variation, and environmental matrices such as seawater and sediments is also essential to better understand the sources, distribution, and pathways of microplastics. Furthermore, strengthening local waste management practices and promoting community awareness in coastal areas are critical steps toward reducing plastic inputs into the marine environment. Continuous monitoring and research are therefore encouraged to support evidence-based management and conservation strategies for the sustainable protection of marine resources in Pujada Bay.
Despite providing baseline information on the occurrence of suspected microplastics in the gastrointestinal tract of wild Siganus guttatus, this study has several limitations that open opportunities for further investigation. Future research is recommended to employ advanced analytical techniques such as Fourier Transform Infrared Spectroscopy (FTIR) or Raman spectroscopy to confirm polymer composition and improve identification accuracy. Expanding the study to include multiple sampling sites, larger sample sizes, and seasonal variation would provide a more comprehensive understanding of microplastic distribution in Pujada Bay. Comparative analyses between wild and cultured populations of Siganus guttatus, as well as the inclusion of environmental matrices such as seawater, sediments, and potential food sources, are also suggested to better trace the sources and pathways of microplastics. Furthermore, studies exploring the biological and ecological effects of microplastic ingestion, including potential impacts on fish health and trophic transfer, would contribute to a deeper understanding of the implications of microplastic pollution in coastal ecosystems.
The authors would like to express sincere gratitude to all individuals who contributed to the completion of this study. Special appreciation is extended to the faculty and mentors from the BS and MS Environmental Science program for their guidance, support, and valuable insights throughout the research process. The author is also grateful to the Davao Oriental State University, Integrated Coastal Resource Management- RIC XI, University Marine Science and Research Laboratory and M2P2 project for granting permission to utilize the laboratories and equipment needed for the conduct of the study. Heartfelt thanks are given to the Barangay Officials and the coastal community of Tamisan, Pujada Bay, for their cooperation and support during the sampling and data collection. The assistance of colleagues, research aides, and field companions is also deeply acknowledged for their help in laboratory processing and fieldwork activities. Finally, the authors express deepest gratitude to family and loved ones for their unwavering encouragement, understanding, and moral support, which made the completion of this research possible.