Prey-size plastics are invading larval fish nurseries

Abstract

Life for many of the world's marine fish begins at the ocean surface. Ocean conditions dictate food availability and govern survivorship, yet little is known about the habitat preferences of larval fish during this highly vulnerable life-history stage. Here we show that surface slicks, a ubiquitous coastal ocean convergence feature, are important nurseries for larval fish from many ocean habitats at ecosystem scales. Slicks had higher densities of marine phytoplankton (1.7-fold), zooplankton (larval fish prey; 3.7-fold), and larval fish (8.1-fold) than nearby ambient waters across our study region in Hawai'i. Slicks contained larger, more well-developed individuals with competent swimming abilities compared to ambient waters, suggesting a physiological benefit to increased prey resources. Slicks also disproportionately accumulated prey-size plastics, resulting in a 60-fold higher ratio of plastics to larval fish prey than nearby waters. Dissections of hundreds of larval fish found that 8.6% of individuals in slicks had ingested plastics, a 2.3-fold higher occurrence than larval fish from ambient waters. Plastics were found in 7 of 8 families dissected, including swordfish (Xiphiidae), a commercially targeted species, and flying fish (Exocoetidae), a principal prey item for tuna and seabirds. Scaling up across an ∼1,000 km² coastal ecosystem in Hawai'i revealed slicks occupied only 8.3% of ocean surface habitat but contained 42.3% of all neustonic larval fish and 91.8% of all floating plastics. The ingestion of plastics by larval fish could reduce survivorship, compounding threats to fisheries productivity posed by overfishing, climate change, and habitat loss.

Keywords: larval fish; microplastics; nursery habitat; surface slicks.

Fig. 1.

Seafloor depths and surface slicks along the west coast of Hawai‘i Island, the southeastern most island in the Hawaiian Archipelago. (A) Seafloor depths. (B) Remotely sensed observations for September 23, 2018 revealed that surface slicks and ambient waters occupied 8.8% (90 km²/1,025 km²) and 91.2% (935 km²/1,025 km²) of all nearshore (≤6.5 km) ocean surface area, respectively. (C) Distance to nearest slick shown in B with 54.0% (505 km²/935 km²) of all nearshore ambient waters within 500 m of a surface slick. The spatial extent of remote sensing detection is the colored region shown in B and C. For additional survey time points, the area of surface slicks and ambient waters as a percentage of the study area and the percent area of ambient waters that are within 500 m of a surface slick are as follows: August 31, 2018, 8.8% (88 km²/998 km²), 91.2% (910 km²/998 km²), and 49.2% (448 km²/910 km²); October 3, 2018, 9.1% (94 km²/1,037 km²), 90.9% (943 km²/1,037 km²), and 47.3% (446 km²/943 km²); and October 11, 2018, 6.5% (67 km²/1037 km²), 93.5% (970 km²/1037 km²), and 47.0% (456 km²/970 km²) (SI Appendix, Fig. S1).

Fig. 2.

Accumulation densities, natal habitat composition of larval fish, and larval fish size in surface slicks compared to ambient waters. (A) Schematic of study system with indicative slick:ambient ratios for phytoplankton, plastics, zooplankton (i.e., larval fish prey), and larval fish. Note illustrations are not to scale. (B) Median (upper CI, lower CI) density of phytoplankton (i.e., chlorophyll-a, mg m⁻³), zooplankton (individuals m⁻³), larval fish (individuals m⁻³), and plastics (pieces m⁻³). (C) Median density (upper CI, lower CI) of larval fish by natal habitat. (D) Larval fish natal habitat composition, and (E) relative abundance (%) of larval fish size (n = 10,870 slick, n = 1,032 ambient). (B and C) Gray dots indicate individual neuston tow samples as follows: chlorophyll-a: n = 26 slick, n = 9 ambient; zooplankton, larval fish, and plastics: n = 63 slick, n = 37 ambient. Bootstrapped median densities (95% CI) and the probability that the median density is greater in surface slicks (light blue) compared with ambient waters (dark blue) [P(slick)] are: chlorophyll-a: 0.29 [0.37,0.23], 0.17 [0.22, 0.14], P(slick) = 0.98; zooplankton: 259.91 [382.53,164.98], 69.72 [100.71,43.25], P(slick) = 0.99; larval fish: 0.60 [0.99,0.34], 0.07 [0.12,0.04], P(slick) = 1; plastic: 3.92 [9.69, 0.95], 0.03 [0.04, 0.02], P(slick) = 1; pelagic 0.33 [0.62, 0.14], 0.01 [0.02, 0.006], P(slick) = 1; coral reef: 0.25 [0.36, 0.16], 0.05 [0.10, 0.02], P(slick) = 1; mesopelagic: 0.005 [0.007, 0.003], 0.002 [0.003, 0.001], P(slick) = 0.21.

Fig. 3.

Associations between larval fish and plastic, including prey size, in surface slicks compared to ambient waters and examples of larval fish plastic ingestion. (A) Linear fit (solid line) and 95% confidence intervals (CI, shaded region) of plastic (pieces m⁻³) and larval fish (individuals m⁻³) densities (dots) in surface slicks (n = 63) and ambient waters (n = 37). (B) Ratio of the median density of plastic to larval fish shown in Fig. 2B. (C) Relative abundance (%) of plastics by size in surface slicks (n = 107,656) and ambient waters (n = 480). (D) Median (upper CI, lower CI) densities of prey-size (≤1 mm) zooplankton (i.e., prey) and prey-size plastics (n = 60 slick, n = 33 ambient). Neuston plankton tow densities (grey dots) are overlaid with bootstrapped median densities (95% CI) as follows: surface slicks (light blue): prey-size zooplankton, 95.62 [129.51, 65.72] and prey-size plastic, 1.75 [4.49, 0.33]; ambient waters (dark blue): prey-size zooplankton, 39.52 [58.98, 23.66] and prey-size plastic, 0.012 [0.021, 0.006]. (E) Ratio of the median density of prey-size plastic to zooplankton prey shown in D. (F) Polymer composition of plastics sampled in surface slicks (n = 707 pieces) as follows: LDPE, low-density polyethylene; unknown PE, unknown polyethylene; HDPE, high-density polyethylene; PP, polypropylene; PP/PE, polypropylene/polyethylene mixture. (G–I) Flying fish (Exocoetidae; G), trigger fish (Balistidae; H), and a billfish (Istiophoridae; I) collected in surface slicks with example pieces of ingested plastics.