Entanglement of marine species in marine debris is a global problem affecting at least 200 species. Based on the literature reviewed in the United States alone, at least 115 species of marine mammals, sea turtles, sea birds, fish, and invertebrates are affected. This review of the literature focused primarily on marine debris entanglement specific to the U.S., incorporating over 170 reports dating as far back as 1928. Most reports of entanglement in marine debris involved pinnipeds, particularly northern fur seals (Callorhinus ursinus) and Hawaiian monk seals (Monachus schauinslandi), as well as sea turtles. Inconsistencies in defining and distinguishing marine debris from actively fished gear significantly limits assessments of marine debris entanglement rates. Further, marine species databases rarely list marine debris data as a separate field, they are not easily searchable and would be more effective if centralized into one database. While entanglement in marine debris is a source of morbidity and mortality for individuals of many species, the impacts of greatest concern are those that affect whole populations of organisms, particularly small populations that are threatened or endangered. For at least some endangered species, such as Hawaiian monk seals, available data suggest that entanglement in marine debris can produce significant adverse effects at the population level, and can contribute to declines in the total numbers of these already endangered animals. For other species, with seemingly large populations or those populations that are difficult to count, population-level effects are more uncertain. However, despite the difficulties in detecting population-level effects, marine debris clearly poses a threat to animal welfare for those individuals that become entangled. Future work should: 1) collect information on the various sources (e.g., ocean-based v. land-based debris) and types (e.g., fishing gear (active v. derelict) v. boating gear) of marine debris that negatively affect organisms, especially those animals that are critically endangered, 2) assess the relative impacts of marine debris amid other potential factors stressing these organisms/populations, such as changing weather patterns linked to climate change, climatological events (e.g., El Niño/La Niña Southern Oscillation), losses in food availability, and disease, and 3) accurately assess the proximate mechanisms driving the observed spatial and temporal patterns of entanglement of organisms in marine debris, such as existing overlap in animal active habitat with areas of higher debris concentrations (i.e., oceanic convergence zones), and increases in the relative contribution of land-based and ocean-based debris with growing human population and activity along coastlines. These patterns are not well established and require further attention. Understanding the sources and types of marine debris that affects organisms as well as the patterns of impact in a natural setting will be crucial to guide appropriate mitigation policies and practices.
Some of the major findings from this review of the literature include:
- 44 sea bird species, 9 cetacean species, 11 pinniped species, 31 invertebrate species/taxa, 6 sea turtle species reported entangled in marine debris in the United States.
- Entanglement rates varied across different species/taxa, but rates seemed to be greater in areas of overlap between high population densities and either human fishing intensity or areas of high debris accumulation (e.g., convergence zones). Often, the source of fishing gear remnants or other marine debris is unknown.
- Some of the highest known marine debris entanglement rates occur with Hawaiian monk seals (0.7% as of 2004) where population numbers are taken into consideration.
- For several species of marine mammals, juveniles and sub-adults have been found to be more susceptible to becoming entangled in debris when compared to more agile, developed adults.
- Overall, the reported entanglement rates of certain species should be used with caution since these rates can be biased based on the sampling method and the difficulty in distinguishing between actively fished gear and marine debris.
- Thus, there is a real need for records of wildlife entanglement to distinguish between entanglements in marine debris as opposed to entanglements in actively fished gear.
- In certain regions in the U.S. and likely elsewhere in the world, numerous species are underrepresented in the literature, implying that reported marine debris entanglement rates are inherently conservative.