SEAPLEX Expedition:

mapping seabirds and plastic

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Description

In August 2009, our lab participated in an inter-disciplinary research cruises to survey oceanography, marine debris and seabirds in the North Pacific Ocean. This visual survey of concurrent seabird and marine debris distributions, funded by the National Fish and Wildlife Foundation, lead standardized survey methods for floating marine debris.

Cruise track of the SEAPLEX expedition (Aug. 2 - 21, 2009), from San Diego (CA) to Newport (OR), onboard the RV New Horizon.

 

Approach

Cruise Track: We conducted our work as part of a research cruise aiming to seek out and sample marine debris accumulations within the North Pacific Subtropical Gyre. The vessel left from San Diego, CA (32°42’N; 117°09’W), traveled west, reaching a maximum westerly extent of 141°W, and heading north east to Newport, OR (44°36’N; 124°3’W).

Seabird Surveys: A single observer (AJT) surveyed seabirds from the flying bridge of the R/V New Horizon, at a 10 m eye height above sea level, while the vessel was transiting between stations. The observer recorded all birds sighted within a 300 m range following standardized strip transect methods (Tasker et al., 1984), and identified the birds to the lowest possible taxonomic level.

To prevent observer fatigue, survey effort was restricted to a maximum of 8 hours per day , and split into approximately 1 hour transects (length = 17.1-20.6 km) spread throughout the day to maximize survey coverage. The observer recorded the Beaufort sea state (from 0 to 7) and cloud cover (from 0 to 100%) at the beginning and the end of each transect. The observer also recorded seabird prey (flying fish and squid), sighted within the 300-m strip width.

Marine Debris Surveys: The same observer carried out a marine debris surveys following standardized line distance sampling (Buckland et al., 1993). The observer counted all marine debris items sighted out to the horizon, and recorded their perpendicular distance from the ship’s track-line using one of seven pre-determined distance bins: 0-10 m, 10-50 m, 50-100 m, 100-200 m, 200-300 m, 300-600 m and >600 m.

The perpendicular distance from the ship was determined using a hand-held range finder when the debris was directly abeam of the ship (Heinemann, 1981), and these  distances were used to determine the effectivestrip width (ESW) (Buckland et al., 1993).

In addition, each marine debris item was assigned to one of three pre-determined size classes, based on its larger dimension: Small (0-10 cm), Medium (10-30 cm) and Large (>30 cm). The color of each item and a description were also recorded.

Graduate student Andrew Titmus surveying seabirds and floating marine debris distributions at-sea.

 

Marine debris varies widely in size and color, inhibiting the standardization of surveys and abundance estimates.

 

Results

Cruise Track: We surveyed concurrent distributions of marine birds and debris along 1,343 km of cruise track, during 74 hourly transects and 15 daily surveys, separated by ‘‘off effort’’ night-time periods.

Marine Debris Surveys: We sighted a total of 3868 marine debris items during the surveys, 95.5% of which were plastic. The remainder was comprised of glass, wood, cardboard and burlap. While some intact objects were seen, fragments were the most dominant plastic (90% of total, n = 3,464).

The marine debris density was highly variable on both the hourly transect scale, ranging from 0 – 15,222 pieces / sq km, and the daily survey scale, ranging from 0 to 6334 / sq km. Small pieces (2 – 10 cm) were the most abundant, accounting for 81% of the total, with medium (10 – 30 cm) and large (>30 cm) pieces accounting for 14% and 5%, respectively.

Despite observing a wide range of marine debris colors, white was the dominant color, accounting for 89% of all items. White, low visibility and high visibility colors were analyzed separately for detectability corrections.

Seabird Surveys: We sighted 235 birds comprising 22 species over the extent of the cruise, and identified 92.7% to species. Of these, 69% were tubenoses (order Procellariiformes), 26% were terns, phalaropes, skuas, gulls and alcids (order Charadriiformes) and 5% were tropicbirds and boobies (order Pelecaniformes).

The avifauna changed over the cruise, and was dominated numerically by Black-footed Albatross (Phoebastria nigripes) and Red-tailed Tropicbird (Phaethon rubricauda) in the southern warm-water areas, and by Leach’s Storm-petrel (Oceanodroma leucorhoa) and Sooty Shearwater (Puffinus griseus) in the cooler more productive waters of the California Current System.

Overall bird densities (birds / sq km) varied widely across hourly transects (range = 0.00 – 9.01) and across daily surveys (range = 0.04 – 2.93).

Perpendicular sighting distances for marine debris according to size class, demonstrate that larger items are detected farther away

Despite documenting a variety of colors, 89% of sighted debris was white

Conclusions

Analysis of debris and bird distributions revealed associations with oceanographic and weather variables at two spatial scales: hourly transects and daily surveys.

 

Marine Debris Distributions - Hourly Transects: Debris abundance (densities up to 15,222 pieces / sq km) higher in are as of lower wind, higher sea-level atmospheric pressure and deeper water.

 

Seabird Distributiuons- Hourly Transects:: Seabird abundance (densities up to 9.01 birds / sq km) higher in areas of lower wind and shallower water.

 

Marine Debris and Seabird Communities - Daily Surveys:

Analyzed 15 daily transects using NMDS ordination, which produced a two axis solution that explained 80.2% of the observed variance in the concurrent bird and debris community composition. These two axes were not perfectly orthogonal (76.3%), but were statistically significant (p = 0.019). Overall, the NMDS stress was 6.63, suggesting that it is a good ordination with no risk of drawing false inferences

 

These results suggest that the distribution and abundance of marine debris and seabird are influenced by similar environmental processes, but in opposing ways, with only three far-ranging seabird species (Black-footed Albatross, Cook’s Petrel and Red-tailed Tropicbird) overlapping with high debris concentrations over meso-scales. Note: the different types of marine debris (by size / color) used for the overall abundance estimation are plotted separately.

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