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Project: Trawl Debris Baseline Survey

Background and Objective

Debris in the ocean is a focal point of public concern and a visible sign of human impact on the marine environment. In the last few decades, marine debris has been recognized as a pollutant that poses risks to marine organisms via entanglement and ingestion. Many organizations, such as the Center for Marine Conservation, are currently collecting and analyzing beach debris data as a means to inform the public about this growing problem. Although marine debris has been increasingly studied over the past few decades, most of these address only the types and distribution of anthropogenic debris found on coastal beaches. A limited number of studies have focused on floating debris surveyed from ships, but very few have examined the types and distribution of marine debris on the seafloor.

This was the first study of debris on the seafloor of the Southern California Bight (SCB). The objectives of this study, part of a regional trawl survey of demersal fishes and megabenthic invertebrates in the 1994 SCB Pilot Project, were to assess the distribution, types, and amounts of both anthropogenic and natural debris on the seafloor of the mainland shelf of the SCB in 1994, in order to provide a baseline for future comparisons. 


This project was conducted in 1994.


Seafloor debris was surveyed in July-August 1994, where 113 trawl stations on the mainland shelf of the SCB were sampled at depths of 10 to 200 m. Trawl sites were selected using a stratified random design, with strata defined by three subpopulation categories: depth — inner shelf (10-25 m), middle shelf (26-100 m), and outer shelf (101-200 m); location — northern region (Point Conception to Point Dume), central region (Point Dume to Dana Point), and southern region (Dana Point to the United States-Mexico international border); and proximity to POTW monitoring areas.

Trawl sampling was conducted using standardized methods described in the field operations manual prepared by the Southern California Bight Pilot Project. Trawl samples were collected with 7.6-m head-rope and 8.8-m foot-rope semiballoon otter trawls with a 1.25-cm cod-end mesh. Otter boards were 76.2 cm wide and 40.8 cm tall, and bridles were 22.9m long. Deviations within 10% of these dimensions were acceptable. Trawls were towed along isobaths for 10 min at 0.8-1.2 m/sec (1.5-2.4 kn).

Debris was categorized into 13 predetermined types of anthropogenic and natural debris: plastic, metal, paper, medical waste, cans, glass bottles, fishing gear, tires, modified wood, terrestrial vegetation (twigs, branches, leaves, and uprooted plants), marine vegetation (drift algae, kelp, and seagrasses), rocks, and other debris. Debris in each category was then counted and placed into one of four categories by amount: trace (1 item); low (2-10 items); moderate (11-100 items); and high (>100 items). Debris was also weighed and placed into four categories by weight: trace (0.0-0.1 kg); low (0.2-1.0 kg); moderate (1.1-10.0 kg); and high (>10.0 kg).

Data analysis was focused upon determining the spatial coverage (percentage of area) of the different debris types on the mainland shelf, both as a whole and by subpopulation. 


Anthropogenic debris was not widespread on the mainland shelf, but was relatively common in the central region, outer shelf zone, and POTW areas. The higher occurrence in the central region (and southern region) can be attributed to the proximity of large population centers in these areas (e.g., Los Angeles and San Diego). The higher frequency of anthropogenic debris on the outer shelf — as well as the types of debris found on the outer shelf (fishing gear and plastic) — suggest that the source of this debris is the disposal of trash and other items from boats.

Spatial extent of different types of debris by percent area of the SCB

The types of anthropogenic debris (glass bottles, cans, and plastic) found in areas near POTWs offer additional evidence that marine vessels are a primary source of offshore anthropogenic debris. Glass bottles and cans, which were prevalent near POTW outfalls, are too large to pass through the screens covering POTW outfall pipes and thus could not be discharged from this source. However, because these outfall pipes are essentially artificial reefs, they are popular fishing spots for recreational anglers.

In contrast to anthropogenic debris, natural debris was most common in the northern region, on the middle and inner shelf zones. The more frequent occurrence in the northern (rural) region may be due to the increased marine and terrestrial vegetation in this area. Although the primary reason for describing the occurrence of natural debris in this study was to provide a marker for nearshore sources (e.g., stormwater runoff and nearshore reefs), an additional purpose was served because natural debris (particularly of marine vegetation) forms an important microhabitat for juvenile fishes. This study is the first to estimate the potential spatial coverage (73%) of natural debris, on the mainland shelf of southern California. Because no historical data are available from the mainland shelf of the SCB for assessing trends in anthropogenic or natural debris, this study provides baseline information for future comparisons.


This project was conducted in collaboration with many partners, including SCCWRP member agencies.

For more information on Trawl Debris Baseline Survey, contact Shelly Moore at (714) 755-3207.
This page was last updated on: 7/2/2014