Research Areas > Marine Debris > Trawl Debris in 2003 Survey

Project: Trawl Debris in 2003 Survey

Background and Objective

Marine debris aesthetically impairs coastal recreation areas and also threatens marine organisms through ingestion and entanglement. Many studies have documented the types and amounts of marine debris, but these have often focused on large debris found on coastal beaches. In Southern California, the Los Angeles Regional Water Control Board has set a total maximum daily load of zero trash for several area watersheds; however, few studies have documented the amount of trash that ends up in the ocean. The 1994 Southern California Bight (SCB) Pilot Project  and 1998 Bight Regional Monitoring Program initially documented the types and amounts of benthic debris on the ocean floor in southern California.

This was the third regional study of its kind in the SCB. The objectives of this study were to 1) assess the distribution, type, and amount of anthropogenic and natural marine debris on the seafloor of the mainland shelf of the SCB in 2003, and 2) compare these findings to those of a 1994 regional baseline survey and a 1998 regional survey of the SCB. 

Status

This project was conducted in 2003.

Methods

As in previous regional trawl surveys of the SCB, the Bight '03 regional trawl survey was based on a stratified random sampling design. In summary, stratification consisted of identification of strata or subpopulations of interest. Subpopulations were defined for region, shelf zone/habitat, and human influence categories. The following subpopulation categories were defined within this area:

1) Regions – The mainland shelf (Point Conception to United States-Mexico international border) was divided into three subregions: northern (Point Conception to Point Dume), central (Point Dume to Dana Point), and southern (Dana Point to United States-Mexico International Border). The island shelf consisted of San Miguel Island, Santa Rosa Island, Santa Cruz Island, Anacapa Island, and Santa Barbara Island.
2) Shelf (Depth) Zones – Bays/Harbors (5-30 m); Inner Shelf (5-30 m); Middle Shelf (31-120 m); Outer Shelf (121-200 m); and Upper Slope (200-500 m)
3) Human Influence Areas – large POTW; small POTW; and non-POTW areas

Trawl sampling was conducted using standardized methods described in the field operations manual prepared by the Bight’03 Field Methods Committee. 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 22.9 m 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 collected in a trawl was classified into 11 type categories: rocks, terrestrial vegetation, marine vegetation, lumber, plastic, metal debris, cans, glass bottles, fishing gear, tires, and “other” anthropogenic debris. The amount of debris in each category was reported as abundance and weight classes. Abundance classes were Present (1 item), Low (2-10 items), Moderate (11-100 items), and High (>100 items). Weight classes included Trace (<0.1 kg), Low (>0.1-1.0 kg), Moderate (1.1-10.0 kg), and High (>10.0 kg).

Findings

Anthropogenic and natural debris were found throughout the mainland shelf of southern California, generally in trace amounts. Anthropogenic debris was highest in the central region due to the proximity of large populations to these areas (i.e., Los Angeles). Prevalence of anthropogenic debris was similar for all shelf zones (about 30%) except for the inner shelf zone, which was about 1%.

The high occurrence of anthropogenic debris in bays and harbors is likely from land-based and marine vessel sources. The next highest occurrence of anthropogenic debris found on the middle shelf, outer shelf, and upper slope is likely from a combination of recreational fishing and boating sources, and from historical runoff events. The lack of anthropogenic debris in the inner shelf may be due to wave and tide transport of sand and debris downcoast to submarine canyons, as well as the lack of recent storms prior to the 2003 survey season. Plastic was the most common debris item across all subpopulations, except for the outer shelf island areas, where metal and cans were the most common.

Natural terrestrial debris, a potential indicator for the path of anthropogenic debris from land-based sources, increased in areal extent from north (30%) to south (76%). Bathymetrically, terrestrial debris was most common in the deeper waters of the upper slope, followed by the middle and outer shelves. It was almost absent in the inner shelf except for in bays and harbors. The lack of terrestrial debris on the inner shelf was similar to that of anthropogenic debris and may be due to the lack of wave and tide action and lack of recent storms as well.

The Bight 2003 debris study was part of the third region-wide monitoring effort of the SCB. The first study, in 1994, provided a baseline for comparing the amounts of debris to those found in 1998 and 2003. In 1994 about 14% of the SCB had anthropogenic debris, whereas in 1998 this number increased to 23%, and in 2003 remained about the same at 25%. This is likely due to the fact that more extensive surveys were done in 1998 and 2003 (314 stations in 1998 and 210 in 2003 compared to 114 for 1994), including bays, harbors, and islands. 

Partners

This project was conducted in collaboration with more than sixty agencies, including SCCWRP’s member agencies.