Since the introduction of plastics to the global market in the 1940’s, the amount of plastic waste in the ocean has increased exponentially (Thompson et al. 2009), and recent estimates suggest that over eight million tons of plastics enter the ocean annually (Jambeck et al. 2015). The same features that allow plastics to be durable and inexpensive make their disposal difficult, and once present in nature, plastics can become a persistent contaminant, especially in marine ecosystems (Rochman et al. 2013). Despite efforts to reuse and recycle plastics, large quantities of plastic end up in waterways, and eventually wind up in the ocean. Through prolonged exposure to ultraviolet radiation and ocean waves, plastics may breakdown into smaller fragments, and particles < 5 mm in diameter are generally referred to as microplastics (NOAA 2016). In addition, some microplastics are manufactured to be < 5 mm (e.g., microbeads from beauty products, textile fibers, plastic pellets, etc.) and some of these plastics end up in the marine environment because of improper disposal and accidental spillage (Fendall and Sewell 2009; Lestari and Trihadiningrum 2019). Although getting an accurate count of microplastics in the ocean is a challenge – especially for very small particles that are difficult to detect – microplastics are found throughout the world’s oceans, and can be very abundant in coastal waters near large cities (e.g., Browne et al. 2011; Sutton et al. 2016; Wiggin and Holland 2019).
The resemblance of microplastic particles to zooplankton, and the co-occurrence of microplastic particles and zooplankton in the ocean (Gove et al. 2019; van Sebille et al. 2020) may cause organisms to mistakenly ingest microplastics as food. For example, Gove et al. (2019) reported that the median density of microplastics in surface slicks – regions that larvae are routinely concentrated in (Shanks 1983; van Sebille et al. 2020) – was 126-fold higher compared to ambient waters. These results suggest larval fish may often feed in an environment where the concentration of microplastics is locally high. Indeed, microplastics have been found in the guts of many species (e.g., Browne et al. 2008; Lusher et al. 2013; Mazurais et al. 2015), and although studies of larval fishes are relatively rare (but see Rodrigues et al. 2019), fish larvae may be particularly susceptible to the presence of microplastics in their environment. During early development, most larval fishes experience a critical period for survival when they exhaust the energy supplied within the yolk sac and must begin feeding on their own (May 1974). Because larval fish need to grow quickly to avoid predation (Bailey and Houde 1989), they must consume volumes of food that are large relative to their body sizes. Larvae that have evolved to become voracious and relatively indiscriminate feeders can be at an advantage under natural circumstances when fast growth is favored, but these same characteristics may make fish larvae predisposed to consuming microplastics. If consuming microplastics has ill effects for the health of fish, then exposure to microplastics may pose a significant threat to fish populations.
Microplastics in the sea may affect the feeding rates of larval fishes in several ways. First, microplastics may affect the ability of larval fish to detect and consume prey. With floating plastics in the surrounding water, fish may find it difficult to isolate zooplankton and capture them effectively. For instance, if zooplankton and microplastics appear close together, fish may not have a clear approach at a zooplankter. Furthermore, if a fish mistakenly captures a microplastic particle, it may decide to stop feeding for a while and forego opportunities for additional feeding and growth. Conversely, it is also possible that microplastics in seawater actually stimulate feeding. For example, if larval fish actively inspect microplastic particles that are dispersed in seawater, then the effective foraging volume of a larva may increase, thus leading to more encounters with zooplankton and greater feeding rates. Very little is known about whether microplastics affect prey detention and capture, and more research is needed to understand these mechanisms.
Second, direct ingestion of microplastics may interfere with further feeding and digestion. For example, ingested microplastics may cause false satiation and decrease feeding rates (Welden and Cowie 2016). They may also accumulate in the gastrointestinal tract and interfere with digestion of food and assimilation of organic matter. If the ingested plastics are large enough, they may even block the digestive system entirely and be immediately fatal to the animals. Furthermore, once ingested, microplastics that have chemical additives or have adsorbed chemicals from the environment may leach these chemicals into the issues and bloodstream of the organism (Teuten et al. 2009). In addition to physiological harm, the stress of chemicals in the body may reduce feeding behavior and hunting performance.
Finally, there may be trophic transfer of microplastics up the food chain, and larval fish may ingest microplastics indirectly by consuming zooplankton that have ingested plastic particles. Accumulation of microplastics by organisms that are near the base of the marine food web may be especially important because many zooplankton feed by filtering particles from seawater and therefore, may be less able to discriminate plastic particles from organic particles of similar size (phytoplankton, etc.). The concentration of plastic particles may be magnified as plastics are transferred between trophic levels through ingestion. Zooplankton that feed at lower trophic levels are very abundant relative to larval fish, but a single fish larva can eat many zooplankton, and thus, accumulate the plastic ingested by hundreds to thousands of zooplankton. Larval fish that consume microplastics from this pathway may have some deleterious physiological effects. Athey et al. (2020) reported that short-term accumulation of microplastics had impeded growth and slowed the development of larval fish. While in other marine organisms, microplastic exposure has resulted in reduced growth rates (Besseling et al. 2014; Lo and Chan 2018), decreased reproductive output (Cole et al. 2015), and decreased survival (Mazurais et al. 2015).
Despite the growing interest in the study of microplastics, there is little information regarding the effects of microplastics on feeding rates of larval fishes. In this study, we tested whether microplastics in seawater affected feeding rates of larvae of California grunion, Leuresthes tenuis, a fish commonly found along the coast of Southern California. Our study consisted of several experiments. First, we tested whether exposure to microplastics in seawater had direct effects on feeding activity and food consumption rates. We also quantified the incidence of microplastic ingestion, and whether ingestion propensity varied with microplastic concentration. Finally, we tested whether trophic transfer of microplastics from zooplankton (brine shrimp nauplii) to larval fish occurred, and whether this indirect pathway had subsequent consequences for growth and survival of larval fish.