Factors affecting marine debris deposition at French Frigate Shoals, Northwestern Hawaiian Islands Marine National Monument, 1990–2006

Abstract

Data on the amount and type of small debris items deposited on the beaches of the Hawaiian Islands National Wildlife Refuge Tern Island station, French Frigate Shoals were collected over 16 years. We calculated deposition rates and investigated the relationship among deposition and year, season, El Niño and La Niña events from 1990 to 2006. In total 52,442 debris items were collected with plastic comprising 71% of all items collected. Annual debris deposition varied significantly (range 1116–5195 items) but was not influenced by season. Debris deposition was significantly greater during El Niño events as compared to La Niña events. Although often deduced to influence floating marine pollution, this study provides the first quantitative evidence of the influence of El Niño/La Niña cycles on marine debris deposition.

Introduction

Marine debris from land-based and ocean sources continues to threaten wildlife and marine ecosystems around the world. The effects of this dangerous form of marine pollution are apparent throughout the islands of Hawaii, including the remote Northwestern Hawaiian Islands Marine National Monument (NWHI). The NWHI are home to endangered Hawaiian monk seals (Monachus schauinslandi), sea turtles, numerous species of seabirds, and an abundance of coral reefs (Miller and Crosby, 1998). Because of the NWHI’s unique and distinct bathymetry and relative geography, together with ocean currents aggregating derelict fishing gear and other forms of marine pollution (Kubota, 1994, Ingraham and Ebbesmeyer, 2001, Matsumura and Nasu, 1997, Henderson, 2001), marine debris continues to threaten coral reef ecosystems and associated wildlife of the NWHI (Donohue et al., 2001, Donohue, 2005, Donohue and Foley, 2007).

Ecosystem damage as a result of both large and small marine debris occurs at multiple trophic levels from coral reefs to marine mammals and other megafauna. In lower trophic levels, marine debris such as monofilament fishing line and derelict fishing gear, or ghost nets, damage coral reefs (Asoh et al., 2004). Active or derelict fishing gear, many times in large conglomerations, becomes entangled on a reef, causing the breakage of coral heads with wave action (Donohue et al., 2001, Donohue, 2005). Among pinnipeds (seals and sea lions), Hawaiian monk seals have one of the highest entanglement rates with all main breeding subpopulations of this species residing in the NWHI (Henderson, 2001). Endangered hawksbill, olive ridley, and leatherback sea turtles, which occur in the NWHI, have been documented entangled in marine pollution along with threatened green sea turtles (Henderson, 1984).

The majority of research and large-scale removal efforts have historically focused on large debris items, such as derelict fishing gear, due to the severe and highly visible effects of this form of debris. However, smaller debris items such as bottle caps, lighters, and plastic pieces, are also hazardous to wildlife. Ingestion of these smaller debris items is a threat to many animals, particularly seabirds. Seabirds mistake plastics, Styrofoam, fibers, bags, bottle caps, and small toys for prey and ingest them (Harrison et al., 1983, Dickerman and Goelet, 1987). Ingestion of these debris items may cause intestinal blockage and injury, loss of nutrition, starvation and death (Redford et al., 1997, Derraik, 2002). Though the effects of smaller debris items are great, just one study to date has investigated the deposition of these smaller items in the NWHI at Midway Atoll National Wildlife Refuge and Hawaiian Islands National Wildlife Refuge Tern Island station, French Frigate Shoals, both known for their large populations of seabirds (McDermid and McMullen, 2004).

In past years, significant efforts have been conducted to remove derelict fishing gear and other large marine debris from the NWHI (see Donohue, 2003 for a detailed review of these efforts). In 1999, a multi-agency effort removed derelict fishing gear from Lisianksi Island and Pearl and Hermes Atoll in the NWHI. Fourteen tons of fishing gear were removed and critical data were collected on the source and magnitude of this problem (Donohue et al., 2001). Efforts addressing the problem of marine debris in Hawaii have continued. In 2005, the US Department of Commerce, National Oceanic and Atmospheric Administration (NOAA) funded several marine debris projects including an aerial survey and debris removal project in the main Hawaiian Islands and an additional year of marine debris removal in the NWHI (NOAA, 2006).

While proximate mechanisms for the accumulation of marine debris on the NWHI remain under investigation, prior studies have suggested that circulation of the sea surface layer and effects of El Niño/Southern Oscillation (ENSO) events may play roles in the quantity and location of debris deposition in the Hawaiian archipelago (Harrison and Craig, 1993, Matsumura and Nasu, 1997, Ingraham and Ebbesmeyer, 2001). More recently, Donohue and Foley (2007) have documented increased Hawaiian monk seal entanglement during El Niño periods and attribute this to increased amounts of marine debris pushed further south by El Niño altered sea surface circulation.

A review and analysis of marine debris deposition rates were calculated from data collected between 1990 through 2006 at the Hawaiian Islands National Wildlife Refuge, Tern Island station, French Frigate Shoals, Northwestern Hawaiian Islands Marine National Monument. Specific objectives of the study were to describe the amount and type of marine debris deposited on Tern Island, and test whether deposition is correlated with year, climatological seasons, and El Niño/Southern Oscillation (ENSO) including both El Niño and La Niña periods. For a subset of years, 1990–2000, we test if local sea surface temperature is a suitable indicator of relative marine debris deposition and ENSO events.