Field measurements reveal exposure risk to microplastic ingestion by filter-feeding megafauna

The study addresses how much microplastic filter-feeding baleen whales ingest and through which pathways (direct filtration from seawater versus trophic transfer via prey). Plastic production and environmental loading have increased dramatically, and at least 1500 marine species ingest plastics. While baleen whales are suspected to be at high risk due to filter-feeding on krill and forage fish, the extent and mechanisms of microplastic ingestion remain poorly quantified, particularly at the depths where whales feed. Prior work often uses surface microplastic data or coarse spatial overlap, which may not reflect depth-resolved exposure. The purpose here is to integrate depth-specific plastic distributions with empirical whale foraging behavior and prey contamination to estimate exposure risk and inform ecological and health risk assessments for recovering whale populations.

Previous studies show widespread plastic ingestion across marine taxa, with top predators often exposed primarily via trophic transfer. Baleen whales have been found with microplastics in gastrointestinal contents and plastic-derived contaminants in tissues, but quantitative ingestion rates vary widely (10^2–10^6 pieces day−1 in earlier estimates) and often rely on surface plastic concentrations or limited sample sizes. Microplastics are frequently detected in zooplankton and forage fish, suggesting prey-mediated exposure. Spatial overlap models for whales and plastic pollution have used coarse metrics and often emphasize low-latitude breeding areas rather than high-latitude/temperate feeding grounds. Depth-resolved plastic surveys reveal concentrations at 200–600 m can be an order of magnitude higher than surface waters, relevant because rorquals typically feed at 50–300 m. Body size affects engulfment capacity and feeding rate in rorquals, potentially modulating exposure. There is limited, region-specific information on microplastic frequency of occurrence in key prey (krill, anchovy) within the California Current Ecosystem, and long-term monitoring data are scarce.

Empirical field data and modeling were combined to estimate daily microplastic ingestion by blue (Balaenoptera musculus), fin (B. physalus), and humpback (Megaptera novaeangliae) whales feeding on krill and forage fish in the California Current Ecosystem (CCE). Data included: 191 high-resolution tag deployments (2010–2019) across Monterey Bay, Channel Islands, Gulf of the Farallones, and Cordell Bank; 36,487 identified lunge-feeding events; prey swarm density from fisheries acoustics; and depth-integrated microplastic concentrations from Monterey Bay (surface 0–0.5 m and 5–1000 m profiles). Whale engulfment capacities were estimated via published allometries using total body length distributions derived from drones (68 humpbacks, 178 fins, 40 blues) and historical length records. Lunge depth distributions were binned (surface, sub-surface, shallow, moderate, deep) and partitioned by diel period to estimate daily lunge rates per species and prey type. Microplastic ingestion P (pp day−1) was modeled as P = V r f_a C_a + (V_p / m_p) C_p, where the first term represents plastic filtered from seawater (volume engulfed per lunge V, baleen retention r, lunge frequency by depth f_a, plastic concentration by depth C_a) and the second term represents plastic consumed in prey (prey volume V_p, individual prey mass m_p, prey contamination C_p). Three exposure scenarios (low, medium, high) were simulated to capture uncertainty in unknown parameters, including baleen retention (25%, 50%, 75%) and prey plastic frequency of occurrence (FO). Prey biomass density (krill and anchovy) followed established methods. Prey contamination (C_p) used literature-derived FO for krill: low 0.01 (remote North Atlantic), medium 0.06 (Northeast Pacific zooplankton), high 0.50 (South China Sea); and for anchovy: low 0.02 (Pacific herring), medium 0.30 (Pacific anchovy, California), high 0.77 (Japanese anchovy, Tokyo Bay). Each prey item with plastic was assumed to contain one particle (conservative). Depth-resolved seawater microplastic concentrations used surface data (0–0.5 m) and profiles from 5–1000 m. Preliminary flume tests on baleen indicated retention of 1–5 mm particles, motivating retention fractions. Monte Carlo simulations (MCMCglmm in R) ran 1000 iterations per scenario to generate posterior distributions; GLMMs tested differences in lunge rates by prey type. Tag data were processed at 10 Hz; lunges were manually verified from kinematic signatures (speed peak and rapid deceleration).

• Baleen whales predominantly forage at 50–250 m, overlapping with depth strata having the highest microplastic concentrations; 99.6% of feeding occurred below 1 m depth, with 83.75% of blue whale lunges deeper than 50 m.
• Nearly all microplastic ingestion (>98–99%) is via trophic transfer from prey rather than direct filtration from seawater across species and prey types.
• Under the most likely (medium) scenario, krill-feeding rorquals ingest a median 5.7 × 10^6 (Q1–Q3: 4.0 × 10^6–1.1 × 10^7) particles per intensive feeding day; a large blue whale may exceed one billion pieces over a 90–120 day feeding season.
• Estimated microplastic mass ingestion for a blue whale is 2.51–43.6 kg per day (based on 0.23–4 mg per particle).
• High-risk scenario (e.g., near human population centers): blue whales projected to ingest 4.62 × 10^7–1.52 × 10^8 particles day−1 (Q1–Q3). Low-risk scenario: 9.66 × 10^−3–1.3 × 10^5 particles day−1 (Q1–Q3).
• Prey-type effect: fish-feeding humpbacks ingest ~1.03–3.12 × 10^5 particles day−1 versus 2.12–6.37 × 10^6 for krill-feeding humpbacks (order of magnitude higher for krill-feeding). Fish-feeding rorquals ingest ~98.5% of plastic via prey; krill-feeding rorquals >99% via prey.
• Lunge-feeding rates are ~3-fold lower for fish-feeding individuals than krill-feeding (63–158 vs. 169–442 lunges day−1; MCMCglmm p < 0.0001). Engulfment capacity increases with body size while feeding rate decreases, leading to similar mass-specific ingestion across krill-feeding species.
• Data integrated: 36,487 lunges across 191 deployments; eight vertical water column microplastic samples used for depth profiles.
• The study’s estimates are at or above the upper end of prior reported ranges (10^2–10^6 particles day−1), suggesting previous assessments likely underestimated ingestion by not accounting for depth-resolved plastic and actual foraging behavior.

The results indicate that baleen whales’ foraging depths overlap with elevated microplastic concentrations, and that trophic transfer is the dominant exposure route. Consequently, using surface microplastic concentrations to assess risk for deep-feeding predators can substantially underestimate exposure. Krill-feeding whales face higher ingestion rates than fish-feeding whales due to higher feeding rates, greater prey consumption, and prey contamination profiles. Despite larger engulfment capacities in larger rorquals, lower feeding rates lead to similar mass-specific exposure among krill-feeding species, implying that exposure risk depends more on prey contamination and depth-resolved plastic distributions than body size alone. The magnitude of ingestion suggests potential physiological and toxicological burdens, particularly given evidence that small particles (<100 μm) can translocate into tissues and that microplastics can be fragmented into nanoplastics by prey (e.g., krill) and possibly by whales. The findings refine exposure estimates by coupling depth-specific plastic profiles with quantified foraging behavior and prey fields, providing a more realistic basis for ecological risk assessments and for informing mitigation strategies in regions where baleen whales feed.

This study provides quantitative, depth-resolved estimates of daily microplastic ingestion by blue, fin, and humpback whales, demonstrating that the vast majority of exposure arises through trophic transfer from prey and that krill-feeding whales face substantially higher ingestion than fish-feeding conspecifics. By integrating high-resolution foraging kinematics, prey density, and water column plastic profiles, the work advances understanding of exposure pathways and magnitudes for filter-feeding megafauna. Future research should: empirically measure elimination and retention times for micro- and nanoplastics in whales; improve prey contamination estimates (FO and particles per prey) through larger, long-term sampling in feeding habitats; characterize baleen retention across particle sizes and morphologies; assess exposure in other feeding strategies (surface skim, benthic suction) and taxa (balaenids, mobulid rays, basking and megamouth sharks); and evaluate potential energetic consequences of plastic-laden prey on whale fitness and population recovery.

• Prey contamination (FO) values for krill and fish in the CCE rely on limited or dated datasets and were borrowed from other regions for scenario construction; actual current FO may differ.
• Assumed one microplastic per prey item with positive FO, likely underestimating true ingestion, especially for fish where multiple particles per individual are common.
• Baleen retention fraction is unknown; retention rates (25–75%) were assumed based on preliminary tests and expert input.
• Elimination (egestion) and potential tissue incorporation rates of ingested plastics were not modeled due to lack of empirical data.
• Depth-resolved microplastic data were limited to available profiles and may not capture spatial and temporal variability across all feeding habitats.
• Microplastic mass estimates rely on broad particle mass ranges, introducing uncertainty in mass-based ingestion estimates.
• The study did not measure plastics directly in whale scat for validation in the primary scenario.