Microplastics, or particles <5mm in diameter, comprise a significant portion of marine plastic pollution. These particles can be directly or indirectly taken up by fish and have been detected in multiple fish tissues. Ingestion of plastic particles was recently reported for free-ranging bottlenose dolphins (Tursiops truncatus) inhabiting Sarasota Bay, FL, USA, a community that also has prevalent exposure to plasticizers (i.e., phthalates) at concentrations higher than human reference populations. The sources of these exposures for Sarasota Bay bottlenose dolphins are currently unknown, but plastic-contaminated prey could be a possible vector. To test this hypothesis, muscle and the contents and tissues of the gastrointestinal tract (GIT) from prey fish collected in Sarasota Bay were screened for suspected plastic particles, and particle properties (e.g., color, shape, surface texture) were compared to those observed in gastric samples from bottlenose dolphins.  In total, 29 fish across four species (hardhead catfish, Ariopsis felis (n=2); pigfish, Orthopristis chrysoptera (n=12); pinfish, Lagodon rhomboides (n=10); and Gulf toadfish, Opsanus beta (n=5)) were collected from two sampling stations in Sarasota Bay, FL during September 2022. Suspected plastic particles were observed in 97% of fish (n=28), and particle abundance was higher for GIT tissue than muscle. Fish and dolphin samples contained fibers and films; however, foams were most abundant in dolphin samples and not observed in fish. Fragments, including tire wear particles (TWP), were not observed in dolphin samples, TWP fragments were not observed in bottlenose dolphin gastric samples, but 23.1% and 32.0% of fish muscle and GIT samples, respectively, contained at least one TWP fragment. While some similarities in particle properties were shared between dolphins and fish, small sample sizes and incongruent findings for foams and TWP particles suggest that further investigation is warranted to understand the potential for trophic transfer.

Microplastic screening of fish tissues followed methods used in Lusher et al., (2017) and Hart et al., (2022). Briefly, muscle and gastrointestinal (i.e., stomach and intestine) tissue from each fish were placed into a glass beaker, and organic (non-plastic) material in the samples was digested by adding a 10% potassium hydroxide (KOH) solution and incubated at 60°C for 24–72 hours (Karami et al., 2017). Following digestion, samples were vacuum filtered onto GF/A 1.6 µm glass fiber filters in a fume hood and left to dry in covered glass petri dishes. 

Particles of at least 35 µm were characterized visually using a dissection microscope (Leica EZ4, magnification 8-35x) according to physical attributes including shape (e.g., fiber, film, fragment, foam), surface texture (e.g., smooth, rough, rubber), and color (e.g., transparent, blue, black; Shim et al., 2017). Various parameters were used to identify potential plastic material. For example, suspected plastic fibers were indicated by a smooth, uniform surface with a length that exceeded the width (Lusher et al., 2020). Suspected plastic fragments were characterized by smooth or angular edges that appeared to be broken from a larger piece of debris (Lusher et al., 2020). Fragments were further identified as tire wear particles (TWP) if they were black, cylindrical, had a rubbery surface texture, and maintained their shape when manipulated with forceps (Leads and Weinstein, 2019).  Suspected foam particles were characterized by a round shape and honeycomb-like porosity (Manikanda Bharath et al., 2023). Suspected fiber bundles were characterized by 20 or more fibers tangled together in a way that prevents them from being separated (Unsworth et al. 2022; Rochman et al. 2019). All particles at least 100 µm in size and with characteristics previously described were tested with a hot needle (250°C) and suspected to be of plastic origin if the needle left a mark on or melted the particle surface (De Witte et al., 2014; Devriese et al., 2015; Leads and Weinstein, 2019). Fourier Transform Infrared (FTIR) spectroscopy was available for polymer identification of particles ranging from 500 µm to 5 mm.