Marine Debris: Entanglement

A trawl or seine net entangled on a cauliflower coral (Pocillopora meandrina).
A trawl or seine net entangled on a cauliflower coral (Pocillopora meandrina).

The effects of derelict fishing gear (DFG) on coral reef ecosystems in the Northwestern Hawaiian Islands are numerous. A primary effect is structural damage to the coral substrate that forms the physical habitat for reef biota. DFG floats into this region and becomes entangled on shallow-water coral reefs, and wave energy eventually transports DFG to the more protected, low-energy, environments inside barrier reefs of the NWHI. During this process, coral degradation and abrasion occur until the debris is removed, sinks because of fouling or accumulation of broken corals, or is fully incorporated into the reef structure.

In addition to its direct effect on coral substrate, DFG poses a serious and potentially lethal entanglement hazard to the highly endangered Hawaiian monk seal (Monachus schauinslandi), which is listed as endangered under the Endangered Species Act (ESA) and critically endangered on the International Union for Conservation of Nature and Natural Resources (IUCN) Red List, threatened and endangered sea turtles, and other marine life. Furthermore, DFG potentially may propagate the introduction of non-native species. Other non-biological effects include hazard to boat navigation and degradation of the aesthetic value of coastlines.

Two shearwaters found entangled in a net.
Two shearwaters found entangled in a net.
Green sea turtle (Chelonia mydas) entangled in a large trawl or seine net.
Green sea turtle (Chelonia mydas) entangled in a large trawl or seine net.
Two Hawaiian monk seals resting on a large pile of nets entangled on an exposed forereef of Midway Atoll. NOAA photo by Don Potts.
Two Hawaiian monk seal resting on a large pile of nets entangled on an exposed forereef of Midway Atoll. NOAA photo by Don Potts.

The total mortality caused by entanglement has not been reliably estimated because it is difficult to observe mortality events. However, survey data from the PIFSC Protected Species Division show that DFG entanglement is a serious mortality risk for monk seals. During the period of 1982-2010, 303 Hawaiian monk seals were found entangled in the NWHI of which 206 were rescued, 82 escaped unaided, 8 died, and the fate of 7 others is unknown. All six extant breeding subpopulations of this seal are located in the NWHI, and this species suffers the greatest reported entanglement rates (averaging 15 seals per year) of any pinniped (seal or sea lion). Until DFG can be stopped at its source, removal of accumulated debris from reefs may be the only effective way to reduce its effects.