3.9 Measuring plastics accumulation

Probably not relevant for Palau at this point.

Stephanie B. Borrelle, BirdLife Pacific, Suva, Fiji

Seabirds feed at or near the ocean surface, along convergence zones and eddies that are rich in prey species. These zones are also where marine plastics occur in high densities, making seabirds particularly vulnerable to plastics ingestion. Reports of plastic ingestion in seabirds started in the early 1960s, and a relatively comprehensive body of work documenting the number and type of plastic ingestion for over 200 seabird species now exists.76

Although these studies help to demonstrate the magnitude of plastic ingestion in marine species, until recently, a lack of standardised methods for collecting, analysing, and reporting data for multiple taxa have undermined comparisons for populations of the same species,77 and limited population level impact analysis.78 Standardised approaches to reporting plastic ingestion, or lack thereof, are important to better understand spatial, temporal, and taxonomic trends in plastic ingestion, and shed light on population level effects using correlative studies (e.g. plastic load, types, and demographic parameters).

To assess whether you think there may be a plastic ingestion issue with your target species, it is a good idea to search the area around the nests or burrows. Adults often loose portions of meals they regurgitate to their chicks and the presence of plastics around the nest site is a good first indicator of a potential issue. Collect information on size and colour of plastic.

There are three main techniques used to evaluate the ingestion of plastic in seabirds: necropsies, regurgitations, and pellet analysis. Each of these have advantages and disadvantages to collecting data on plastic ingestion (Table 3). Regardless of the methods, there are key metrics that are recommended to be collected in any plastic ingestion study. These include the species, location of sample collection (including colony if known), age (adult, juvenile, chick at the minimum), sex, body condition metrics (body mass, subcutaneous fat, pectoral muscle size), and the cause of mortality with necropsy studies when possible. It is also important to report zeros as these are important for understanding demographic drivers of plastics ingestion.

	Pros	Cons
Necropsy— carcass examination (dissection)	Can determine entire plastic burden Tissues can also be easily taken for plastics-associated contaminants analysis Sex and age, health status, and cause of death can be determined In case of beach-cast birds: non-invasive	Does not always allow for consistent repeated sampling Is usually opportunistic without options for preplanned sampling in relation to season, sex or age In the case of hunting specifically for research samples this technique is invasive
Regurgitations	Can be repeated on the same individuals to compare plastic load between seasons and years In comparison to hunting birds for research, regurgitations are relatively less invasive	Not all species can be made to regurgitate Does not guarantee a complete sample Low but real possibility of injury occurring to the bird The individual may lose a meal that was costly to acquire
Pellet collection	Easily collected Non-invasive collections Can be repeated between seasons and across years Can be regularly sampled Can be implemented easily and at low cost	Cannot be used to assess microplastics Depending on the species, may be difficult to attribute pellets to individuals Does not guarantee a complete sample Typically only useful during the breeding season when birds are occupying nesting sites on land

A significant challenge with plastic ingestion studies is that samples and sources of mortality may not represent a true sample of the population for a species and its interactions with plastics. Because sampling is generally opportunistic, e.g., beach cast and bycaught birds, the plastics data may be biased towards certain locations or events that lead to reduced body condition where plastic may have contributed to the death of the animal or not. For some species it is possible to get regurgitations from birds in the field, which can serve to take a more targeted approach to sampling a species, including focusing on specific age classes and annual trends in ingestion.

However, there are limitations as studies have found there are considerable differences in the load of plastics when comparing regurgitation data to necropsy data79 due to differences in the gastrointestinal tracts among species. For example, the gizzard may not be fully washed out in Procellariforms leading to an underestimate of plastics load. Pellets or boluses are another opportunistic but non-destructive sampling method for seabirds such as shags, terns, and skuas but the key limitation to this approach is collecting data on biometrics (as above) and the consideration of species-specific differences in ejected versus retained plastics. Other species, such as Procellariforms that have been reported with some of the highest loads of plastics generally do not produce pellets (although it is possible to find regurgitates outside of burrow entrances with plastics in them). Albatross chicks do a pellet regurgitation before fledging to offload squid beaks. It is possible adults do the same thing at sea and certainly adults feeding chicks are offloading their plastic loads into their chicks.

Consideration of the species being assessed, sampling opportunities and study question being posed, standardised methods for sampling and reporting are recommended to improve our understanding of the impacts of plastics ingestion on seabirds. Refer to Reference 78 for a comprehensive guide on performing necropsies and data collection for plastic ingestion including separating micro-plastics from macro-plastics.