The feasibility of at-sea detection

The feasibility of at-sea detection and removal of abandoned, lost or otherwise errant nets, lines, and buoys that comprise derelict fishing gear (DFG) in the North Pacific Ocean has been investigated for over a decade (Brainard et al., 2001). When DFG enters shallow-water areas around the Hawaiian Islands, particularly the protected areas of the Papahānaumokuākea Marine National Monument, it abrades and breaks coral reefs and entangles the critically endangered Hawaiian monk seal (Monachus schauinslandi), threatened green sea turtle (Chelonia mydas), and other fauna ( Balazs, 1985, Donohue et al., 2001 and Boland and Donohue, 2003). Reducing entanglement in DFG is identified as a crucial step in the recovery of the Hawaiian monk seal ( NMFS, 2007).

The removal of DFG after it has entered shallow waters and become snarled on coral reefs is a labor-intensive, dangerous, and expensive endeavor. At-sea removal of DFG minimizes the dangers of working in shallow waters, prevents damage to coral reefs, and minimizes the nearshore entanglement hazard to the Hawaiian monk seal. Due to these benefits, at-sea removal of DFG has been identified as a priority in the Papahānaumokuākea Marine National Monument Management Plan (PMNM, 2008).

While the impacts of DFG in shallow waters are better known and documented, deep water environments have also been found to experience DFG impacts (Matsuoka et al., 2005 and Gilardi et al., 2010). Prevention of observed and potential impacts of DFG on deeper-water environments is an equally compelling justification for at-sea detection and retrieval of marine debris.

At-sea detection is a primary challenge to removing DFG before it impacts marine environments and species. The great distances involved, coupled with the logistics and expense of operating vessels and aircraft, underline the importance of the ability to locate DFG at sea on very refined spatial scales. The following illustrates this challenge. To survey less than one percent of the North Pacific Ocean, e.g., a 3-degree swath within 30° to 35°N and from 150° to 180°W, requires covering approximately 1 × 106 km2. To cover such an area performing visual observations for DFG within 100 m off each side of a ship (as described in Mio et al., 1990a), traveling at 11 knots (approximately 20 km/h), and surveying during daylight hours (8 am to 6 pm, or 10 h/day), would require 68 ships 1 year, or one ship 68 years. Clearly, to efficiently locate DFG at sea, a combination of direct and indirect detection methods and technologies are needed.

The vastness of the potential search area favors the initial use of indirect methods to refine the detection capability for DFG. Brainard et al. (2001) suggested using satellite observations to identify general DFG concentration zones, deploying aircraft and ships to those zones, and working at progressively finer scales of indirect and direct detection until actual removal of DFG could be accomplished. Indirect methods use proxies for debris such as identifying oceanographic conditions that will likely aggregate or concentrate debris spatially or temporally. Once a putative debris area is identified indirectly, locating DFG items or aggregates for removal is expected to require direct detection. Direct detection involves identifying and locating specific pieces of debris through remote sensing technologies or direct human observation.

A small number of focused efforts to advance the direct detection of pelagic DFG have occurred in the North Pacific Ocean (Pichel et al., 2007) and the Gulf of Alaska (Pichel et al., 2012) integrating modeling, analysis of satellite remote sensing data, and airborne visual observations. In 2008, a research cruise led by the National Oceanic and Atmospheric Administration (NOAA) tested the feasibility and use of a prototype ship-launched unmanned aircraft system (UAS) for the short-range detection of pelagic DFG with modest but encouraging results (R. Brainard and T. Veenstra, personal communication).

While these efforts and others have contributed information needed to develop the capability to detect and remove DFG at sea, the slow pace of progress toward the goal led participants in the 2008 NOAA cruise noted above to propose that a cohesive, multidisciplinary approach and strategy for at-sea detection of DFG were needed. Here, we employ such an approach integrating knowledge from physical oceanography, remote sensing technology, marine policy, and marine debris studies to outline a strategy toward efficient and reliable at-sea detection of DFG, with the ultimate goal of locating and removing pelagic DFG.