Plastics and the hidden majority - microplastics and the conservation implications for a threatened UK freshwater fish

Accumulation and integration of microplastics (plastic pieces < 5mm, hereafter referred to as MPs) in freshwater environments are relatively less studied in comparison to marine systems. Recent studies have indicated that MPs are diverse in nature, widely distributed, and can have local hot-spots of accumulation. A recent study has indicated that densities of over 500,000 particles per m2 are present in some sediments in urban rivers.

It is becoming clear that MPs are not only distributed widely throughout riverine habitats, but are being ingested by many organisms, including some larval fish species. Concern is growing regarding the potential toxicity of leachates from the plastics themselves and chemicals that can become bound to the surfaces of MPs (e.g. fire retardants and antimicrobial agents), on organisms that directly or indirectly ingest them. Studies in North America have reported dangerously high levels of contaminants (e.g. pesticides, flame retardants and mercury) in tissues of some native lampreys.


The UK contains three native species of lamprey: the River lamprey (Lampetra fluviatilis), Brook lamprey (Lampetra planeri) and Sea lamprey (Petromyzon marinus). All three species have a protected status, with many examples of sites supporting lamprey populations being designated as Special Areas of Conservation (SACs). However, despite their ecological importance, we still have limited knowledge of their ecology and secretive life cycle.

Larval lampreys are primary consumers, and UK species spend an average of five years in freshwater environments filter-feeding in burrows made in fine sand and deposited river sediment.These are the same types of river habitat that MPs reach their highest recorded densities. High larval densities are often found in the vicinity of spawning sites with older, larger animals occupying habitats towards the lower reaches of rivers.

Temperate freshwater ecosystems rely heavily upon the activity of organisms that process organic detritus, releasing nutrients via their processing, or through them becoming prey for larger organisms. The action of burrowing by lamprey larvae enables bioturbation and facilitates nutrient processing. Returning adult lampreys can also redistribute considerable volumes of streambed substrates during the preparation of spawning pits. Accordingly, lampreys are considered to be important ecosystem engineers of freshwater systems.

Burrowing provides larval lampreys with a concentrated solution of food in the form of organic particles, algae and bacteria. Particles are pumped into the mouth and particle selection is likely made based on size by the oral cillae. Small particles subsequently get trapped by mucus in the pharynx prior to being passed to the midgut. Research suggests that organic detritus and sand particles make up the majority of gut contents in larvae, along with a much smaller proportion of algae and bacteria.

There does not appear to be a relationship between the sizes of the particles ingested and body length, suggesting that young or older larvae are equally vulnerable to ingestion of MPs in river sediments. The rate of larval lamprey feeding and growth is maximised during the Spring and Summer months. During these periods flows are generally low and consistent in UK rivers, and the deposition and accumulation of small MP particles may elevate their potential for ingestion by larval lampreys.With submicron or even nanometre size MPs likely to be present in sediments occupied by larvae, their role in facilitating direct release or surface transfer of toxins, and subsequent impacts on threatened UK lampreys, requires investigation.