Discussion

Techniques used to isolate and enumerate microplastics in fish and invertebrates have largely been adapted from studies focused on large vertebrates, or have derived from traditional biology methods; as these fields advance, it is vital that we continue to observe their progress and incorporate relevant methods. Owing to the challenges in sampling, isolating and identifying microplastics, and the diverse physiology of taxonomic groups under investigation, a degree of flexibility, innovation and ingenuity on the part of the researcher is clearly required.

Of the numerous studies investigating microplastic uptake in fish and invertebrates, it is analysis of wild-caught animals that presents the most issues. In these studies, potential sources of error are numerous, including microplastic losses or contamination during sampling; furthermore, there are substantial challenges in drawing links between exposure and effect. These studies, however, are essential for establishing ecologically relevant data, which ultimately provides researchers with a clear view of the quantity and types of plastic (and associated contaminants) experienced by biota in the natural environment. In this section, we address the need for standardizing protocols for microplastic quantification

[166], outlining preferred methods, best practice and steps for mitigating contamination. It is anticipated that compiling standardized methodologies will provide researchers with a grounding in developing future experimental design.

Controlling Sampling Bias

Throughout the sampling process, utmost care must be taken to prevent the artificial inflation or loss of microplastics. In Table 8.3, the commonly used methods for sampling and isolating microplastics across a range of taxa were outlined. In all cases, the least damaging sampling gear is preferable, and sampling periods should be kept as short as practically realistic. Organisms that spend longer in nets are subject to additional stress that increases the likelihood of regurgitation or stomach inversion and artificially increases contact time between microplastics and biota; this could facilitate microplastic ingestion and adherence to external appendages. Individuals should be rinsed following capture to remove adhered particles, and samples of fishing gear should be taken to

TABLE 8.3 Standard Sampling and Plastic Isolation Strategies Employed across a Variety of Subphyla

Ecosystem

Life

Strategy

Subphylum

Size

Range

Sampling Method

Initial Plastic Separation

Aquatic

Benthic

Annelida

Grabs

Digestion

Aquatic

Benthic

Crustacea

>50 mm

Trawls/creels

Dissection

Aquatic

Benthic

Crustacea

<50 mm

Otter-/beam-trawls

Digestion

Aquatic

Benthic

Echindodermata

Grab/trawls

Dissection

Aquatic

Benthic

Mollusca

>30 mm

Grabs

Dissection

Aquatic

Benthic

Mollusca

<30 mm

Grabs

Digestion

Aquatic

Benthic

Flatfish

Otter-/beam-trawls

Dissection

Aquatic

Nektonic

Crustacea (juv.)

<50 mm

Midwater trawls

Digestion

Aquatic

Nektonic

Gadids

Otter-/midwater

trawls

Dissection

Aquatic

Nektonic

Echindodermata

(juv.)

<2 cm

Trawls

Digestion

Aquatic

Nektonic

Mollusca (juv.)

<2 cm

Trawls

Digestion

Aquatic

Nektonic

Fish

<10 cm

Midwater trawls

Dissection

Aquatic

Planktonic

Annelida

<2 cm

Trawls

Digestion

Aquatic

Planktonic

Cnidaria

<10 cm

Trawls

Digestion

Aquatic

Planktonic

Crustacea

<2 cm

Trawls

Dissection

Terrestrial

Annelida

Sediment collection

Digestion

Terrestrial

Arachnida

Trapping/hand

gathering

Digestion

Terrestrial

Crustacea

Trapping/hand

gathering

Digestion

Terrestrial

Insecta

Trapping/hand

gathering

Digestion

Terrestrial

Mollusca

Trapping/hand

gathering

Digestion

exclude material ingested as a result of capture. Researchers are recommended to oversample where practical, so that individuals with recently emptied stomachs, or otherwise damaged during sampling, should be omitted from the dataset; this will help reduce the bias caused by regurgitation and enable more robust comparisons among animals sampled from different sites or collected using alternate sampling methods. Specimens should be rapidly transported to the laboratory or preserved promptly to avoid microplastic egestion in transit. The collection from commercial fish markets or artisanal fishers is not ideal, as the researcher will have less, if any, control on the method of capture and the handling conditions on transport. Where applicable, researchers are suggested to work closely with fishers to ensure animals are sampled appropriately and sufficient information on the capture procedure is collected.

 
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