NWHI Derelict Fishing Gear Accumulation
The Northwestern Hawaiian Island (NWHI) reefs have been reported to contain extraordinarily high densities of derelict fishing gear (Kubota, 1994; Donohue et al., 2001; Boland and Donohue, 2003; Pichel et al., 2003). Lost and abandoned fishing gear, primarily large trawl and drift nets, float into the region and come to rest within the more protected, low-energy, shallow water environments inside barrier reefs of the NWHI. During 1996-2006, an extensive multiagency debris removal program led by the Pacific Islands Fisheries Science Center removed 511 metric tons (MT) of derelict fishing gear (Friedlander et al., 2005) from various sites within the NWHI. With the completion of the 2005 marine debris field season, most shallow benthic habitats ( < 10 m) thought to contain high densities of debris were surveyed and cleaned. Beginning in 2006, NWHI marine debris removal efforts were reduced to a maintenance mode aimed at keeping pace with new accumulations. To develop an efficient and effective program to address ongoing debris accumulation, it is necessary to obtain some understanding of the rates and processes associated with the accumulation of derelict fishing gear in the NWHI.
To identify reef areas in the NWHI predisposed to high debris accumulation densities, we examine the relevant regional physical forcing mechanisms and outline the common properties of reefs that enhance the settlement of derelict fishing gear. These properties exist on a range of spatial (ocean-basin, atoll, sub-atoll) and temporal (interannual, seasonal, storm events) scales and must be considered accordingly. On the ocean-basin scale, a key physical forcing mechanism in the North Pacific is the convergence of surface waters in the Subtropical Convergence Zone (STCZ), a phenomenon resulting from westerly winds in mid-latitudes and easterly winds in the tropics that shifts on seasonal and interannual time-scales (Kubota, 1994; Brainard et al., 2000; Bograd et al., 2004).
North Pacific Mean Winds from QuikScat 1999 - Present
Kubota (1994) suggested this convergence zone acts as a mechanism for disproportionate accrual of marine debris in the area north of the Hawaiian Islands, a notion substantiated by aerial surveys (Veenstra et al., 2004). Models based on remotely-sensed observations of wind stress curl, sea surface temperature, and surface chlorophyll levels have successfully predicted these areas of marine debris accumulation (Polovina et al., 2001; Pichel et al., 2003). Theoretically, the mean position of the convergence zone in the winter months, as it reaches its most southerly latitudes, influences the annual accumulation rates in the NWHI, both between atolls/islands and within the archipelago as a whole. On the atoll scale, the primary geomorphological and physical characteristics of reefs that influence settlement of derelict fishing gear are shallow water depth, high-relief reef structure, and low exposure to significant wave energy (Donohue et al., 2002). These characteristics increase the likelihood that fishing gear will settle and determine a reef's suitability as potential 'net habitat'.
Two survey methods were used in the accumulation rate study as a result of their respective appropriateness in the two dominant survey habitats. Manta tow surveys were used in the relatively even-depth, contiguous backreef habitats, while non-linear swim surveys were used in the comparatively high-relief and patchy lagoonal reef habitats. To estimate yearly accumulation of marine debris for the reef ecosystems of the NWHI, we extrapolated observed sample marine debris densities across all NWHI reef areas with high net accumulation potential. Accumulation potential was based on three significant accumulation factors: bathymetry, benthic habitat, and energy regime.
Debris accumulates in higher densities (numerical and weight-based) in low-energy lagoonal areas than in backreef areas. The nearly three-fold difference in average weight of debris fragments between low-relief backreef areas and relatively high-relief lagoonal reefs suggests that large fragments of fishing gear are forced through areas of high wave-stress into lower energy environments where entanglement and long-term settlement occurs. Calculated by extrapolating our sample accumulation densities to all predicted accumulation areas in the NWHI, the annual estimated debris accumulation in the NWHI is 52.0 MT.
Calculation of annual accumulation by location shows that backreef habitat and lagoonal habitat at Pearl and Hermes Atoll potentially have the highest and second highest annual accumulation totals, respectively.
Annual Accumulation by Location
As seen in the pie chart, Pearl and Hermes Atoll is predicted to receive 54% of the yearly accumulation in areas of the NWHI considered by the current model. This suggests that Pearl and Hermes Atoll will require the most dedicated marine debris removal effort in the future. This prediction figure is supported by the fact that 294 of the total 492 MT (60%) of derelict fishing gear removed from the NWHI through 2005 have been removed from Pearl and Hermes Atoll. In addition to possessing considerably more net habitat than any other atoll/reef, Pearl and Hermes Atoll has an extensive barrier reef that surrounds a reticulated reef system, creating an expansive low-energy environment. The only other location with a similar low-energy interior environment is Maro Reef which, we estimate, has the third highest annual accumulation rate among the NWHI habitats. Though the other three locations (Midway, French Frigate Shoals, Kure Atoll) show considerably lower annual accumulation, likely as a result of their relative lack of reticulated reef areas and overall lack of potential net habitat, each location has been proven to accumulate substantial quantities of debris (114.3 MT removed during 1996-2005 period). While these locations may require a lower effort level to keep up with annual accumulation than will Pearl and Hermes Atoll and Maro Reef, removal efforts should remain proportional to the estimated share of the annual accumulation by location.
To make sound management decisions related to marine debris, more accurate estimations of NWHI debris accumulation are needed. Estimates should consider interannual and latitudinal settlement variability, two factors that may have a significant effect on debris accumulation. Aerial surveys of the main Hawaiian Islands coastline in early 2006 have demonstrated that numerous masses of derelict fishing gear can reach as far south as 18° N. Through continued monitoring of debris settlement onto NWHI reefs, a broader understanding of the sources of variability associated with NWHI marine debris accumulation will be obtained, improving our capability to effectively mitigate this significant threat.