Fish

The incidence of accidental entanglement of fish species is difficult to estimate, because certain fish are “intended” to become entangled in nets. Therefore, research emphasizes by-catch of endangered species. For example, between 1978 and 2000, 28,687 sharks were caught in nets that protected people at popular swimming beaches in Kwazulu, South Africa [4]. Microplastics resemble phytoplankters, which are eaten by fish and cetaceans [78]; see Figure 2.4. Ingested plastic debris has been found to reduce stomach capacity, hinder growth, cause internal injuries and create intestinal blockage [79].

Ghost fishing can lead to economic losses for fisheries [21]. For example, an experimental study on ghost fishing of monkfish from lost nets in the Cantabrian Sea in northern Spain, estimated that 18.1 tons of monkfish are captured annually by abandoned nets. This represented 1.46% of the commercial landings of monkfish in the Cantabrian Sea [80]. A study on ghost fishing by lost pots off the coast of Wales, UK, noted that potential losses to the brown crab fishery caused by ghost fishing could be large [81]. In the US, it is estimated that $250 million of marketable lobster is lost annually to ghost fishing [82].

Mammals

Many seal species are curious and playful and, especially young seals, are attracted to plastic debris and swim with it or poke their head through loops. Plastic rings, loops or lines easily glide onto the seal’s neck, but are difficult to remove owing to the backward direction of the seal’s hair (see Figure 2.5). As the seal grows, the plastic collar tightens and strangles the animal [4]. After entanglement in these nets, the animals are not able to reach the water surface, and thus, they drown. An estimated 58% of seal and sea lion species are known to have been affected by entanglement, including the Hawaiian monk seal, Australian sea lions, New Zealand fur seals and species in the Southern Ocean [21].

Whales also become entangled in marine debris. However, although some whale species are incapable of freeing themselves and consequently drown, the larger-sized whales often drag fishing gear away with them. This latter type of entanglement can cause strangulation and can affect the feeding ability of the whale in ways that causes starvation. At least 26 species of cetaceans have been documented to ingest plastic debris [83]. A young male pygmy sperm whale (Kogia breviceps) stranded alive in Texas died in a holding rank 11 days later. Stomach compartments were completely occluded by plastic debris, including a garbage can liner, a bread wrapper, a corn chip bag and two other pieces of plastic sheeting [83]. Entanglement is a particular problem for marine mammals, such as fur seals, which are curious when catching food and, to avoid predators, may incur wounds from abrasive or cutting action of attached debris [38,84]. According to Feldkamp et al. [85], entanglement can greatly reduce fitness, as it leads to a significant increase in energetic costs of travel for the northern fur seals (Callorhimts ursinus). For instance, they stated that net fragments over 200 g could result in a fourfold increase in the demand of food consumption to maintain body condition [85]. Most cetaceans live far from the shoreline which limits the amount of research on the ingestion of marine debris. If plastic causes unnatural death, cetaceans will most likely sink to the bottom of the ocean [83]. Occasionally, cetaceans will wash ashore, allowing for postmortem examinations. Due to cetacean echolocation capabilities, mistaken consumption of plastic is not probable [86].

Ingestion is most likely because the debris was mixed in with the desired food. Two sperm whales (Physeter macrocephalus) were found off the coast of northern California in 2008 with a large amount of fishing gear in their gastrointestinal tracts [87]. One of the sperm whales had a rupture in their compartment of the stomach caused by nylon netting. In the other whale, netting, fishing line and plastic bags were completely blocking the stomach from the intestines [87]. On the coast of Nova Scotia, Canada, a juvenile porpoise (Phocoenidae) was found dead with a balled-up piece of black plastic in the esophagus entangled with three spine stickleback fish [83]. In Brazil, the stomach analysis of Blainville’s beaked whale (Mesoplodon densirostris) showed the presence of a large bundle of blue plastic thread occupying a substantial part of the stomach chamber [86]. Ayalon et al. [88] argued that within the last decade, at least mass amounts of tangled nylon rope and other debris, including a crayfish pot and a buoy , have been found in the stomachs of whales. Currently, there have not been enough trends found in collected data that prove ingested plastics are the primary cause of death contributing to the decline of cetaceans [89-91]. Plastic entanglement with nets or other materials can result in strangulation, reduction of feeding efficiency and, in some cases, drowning [92]. Because of natural curiosity, pinnipeds often become entangled in marine debris of life [92].

 
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