Marine Debris: A Major Threat to the World’s Sharks
One of the major issues facing the world’s oceans and marine species living within them is the presence of marine debris. According to the National Oceanic and Atmospheric Administration (NOAA), marine debris is defined “as any persistent solid material that is manufactured or processed and directly or indirectly, intentionally or unintentionally, disposed of or abandoned into the marine environment”. Marine debris can be anything as large as an abandoned vessel and fishing nets to as small as microplastics; however, despite its size, marine debris will always harm the marine wildlife. One species of marine life that has been greatly impacted by the presence of marine debris are sharks; with hundreds of different types of sharks in the world’s oceans today, all species face the threat of marine debris whether they are as small as the dwarf lantern shark or as large as a whale shark.
One major marine debris threat to some of the largest species of shark is as small as five millimeters. Microplastic (MP) is defined by NOAA as any type of plastic that is less than five millimeters in length and is a major threat to the few species of shark that are known to filter feed like the whale shark and the basking shark. Microplastics can be divided into two separate groups based on their usage and origin; the primary group consists of pellets used in the plastics industry, and certain applications such as abrasives, while the secondary group is fragments resulting from the degradation and breakdown of larger items (Fossi). A majority of microplastics have been noted at the sea surface as they are transported by ocean currents and are most noted in regions where water circulation is relatively stationary or on coastlines. Due to the filter-feeding behavior of three species of shark, these sharks are put at a greater risk for ingesting these microplastics at the ocean’s surface.
In 2014, a pilot study was done examining the toxicological effect of microplastics on basking sharks in the Mediterranean by measuring the levels of phthalates in these sharks (Fossi). The basking shark is one species of shark that are known to filter seawater for planktonic prey. To obtain its food, the basking shark practices filter-feeding, in which it forward swims with an open mouth, so that water passively flows across the gill-raker apparatus allowing it to ingest large amounts of plankton at a single time. While this method is very effective in gathering large quantities of plankton, it is just as effective for ingesting MPs. This study concluded that a “high presence of plastic particles have been detected in superficial neustonic/planktonic from the Pelagos Sanctuary areas investigated,” and with the rate at which a basking shark filters seawater it was estimated that the shark had a “total daily consumption of approximately 13,110 micro debris items” (Fossi). This high amount of microplastics being ingested by the basking shark is chronically exposing it to persistent and emerging contaminants for many endocrine disruptors such as MEHP (mono-(2-ethylhexyl) phthalate). This study was able to show the first set of evidence of phthalates from microplastics in basking sharks found within the Mediterranean and was able to set a precedent for “future research both on detecting the presence of and looking for toxicological impacts of microplastics in filter-feeders” (Fossi).
Following this study in 2014, another pilot study was done to perform the first ecotoxicological investigation on whale sharks found in the Gulf of California. Similar to the basking shark, the whale shark filter feeds on plankton found at the sea’s surface through a process known as surface ram filter feeding. The plastic micro debris these whale sharks may be ingesting can serve as carriers of persistent organic pollutants (POPs); therefore, this investigation used skin biopsies of whale sharks to determine the microplastic concentration and polymer composition for within these sharks (Baini). This study concluded that “higher contributions of PBDE-209 may result from the superficial filter-feeding activities of this species” which confirmed a “positive correlation between the size of the sharks and the level of congeners PBDE-209 in the tissues,” meaning the larger the whale shark the more they need to filter feed, thus the more microplastic they are ingesting. This study was the first of its kind to be done in the Gulf of California; therefore, further ecotoxicological investigations are recommended to fully understand the full effect the microplastics in the Gulf are having on the marine species, including whale sharks, that are inhabiting the area. While microplastics pose a large risk to many species of filter-feeding sharks, other forms of plastic debris also pose a risk to all sharks.
One form of larger plastic debris that has had an impact on sharks is plastic rings. In the southwest Atlantic, three juvenile Brazilian sharpnose sharks were caught in gillnets and found to have plastic ring around their gills and mouth region making it look as though they were wearing a collar. The rings found on these sharks were identified as the detachable rings found on plastic bottles, but sharks have also been known to get caught in six-pack yokes. The largest of the sharks was found with a ring covering the corners of its mouth and “was emaciated and probably was unable to open its mouth wide, which would hamper its normal feeding,” while the other two sharks were found with “severe erosion in the head tissue” due to the ring as it was covering regions of the gills causing a “hindrance both to its normal feeding and perhaps ventilation as well” (Sazima). Due to its abundance, coastal habitat and small size, the Brazilian sharpnose shark may be one shark most aﬀected by small, plastic rings discarded in the inner continental shelf of southeast Brazil. However, while only three sharks of this species were studied in this case, they can serve as an indicator for other sharks in the region as Carcharhinid sharks are the most affected by plastic collars (Sazima). The Carcharhinid sharks are not the only species of shark that are affected by entanglement as many can become entangled in much larger objects.
According to NOAA, “ghost fishing refers to lost or abandoned fishing gear…that continues to capture fish and other marine animals after the gear is no longer under the control of a fisherman,” and these nets have had a major negative impact on the survival of many sharks. Similar to the sharpnose sharks found with plastic rings that were studied in the southwest Atlantic, a mature shortfin mako was studied after it was found suffering from prolonged entanglement in a discarded fishing net off the coast of San Diego, California in 2011. Upon discovery, this mako shark was found to be suffering from “deep abrasions, scoliosis of the back, and undernourishment” due to the 1.9 cm thick, three-strand twisted natural fiber rope wrapped around its body. The scientists that discovered the shark removed the rope and discovered fifty-two pelagic pedunculate barnacles, which allowed them to use the larger of the barnacles to calculate the rope had been on the shark for at least 150 day though due to the severity of the wounds it was believed to have been present longer (Wegner). After releasing the shark from the rope, a tracker was attached to the base of its dorsal fin to study its recovery. In studying the data retrieved from the tag, it was determined that the shark’s “return to ‘typical’ behavior, in conjunction with 54 confirmed days of survivorship, suggest that this shark was able to recover from its entanglement-induced injuries;” however, this is not always the case as thousands of shark die every year due to ghost fishing. While ghost nets have a major effect on sharks throughout the world’s oceans, marine debris as a whole is a major threat world’s shark population.
Marine debris is a major issue that threatens the world’s shark population as it kills an estimated 100,000 marine creatures annually; with this number only taking into account those that are found. About 8 million metric tons of plastic are thrown into the ocean annually and are only a portion of what is considered marine debris, with derelict fishing gear, and derelict/abandoned vessels making up the rest. Currently, many studies being done around the world investigating the effects various marine debris has on different species of sharks. There are over 500 species of sharks, with more being discovered every year, but out of these almost sixteen percent are threatened with extinction (ranked as Vulnerable or higher by the IUCN). There are many efforts to clean up the world’s oceans, but it has been estimated that by 2050 there will be more plastic in the oceans than there are fish (by weight). Due to the amount of marine debris, it can also be assumed that many of the fish that humans consume, including brown trout, cisco, and perch, have at one time or another, ingested plastic microfibers. Therefore, marine debris is no longer a marine species problem, but a world problem that must be dealt with soon.
Rebecca Schurr, University of Delaware
Intern at Cape May Whale Watch and Research Center
Baini, Matteo, et al. “Are Whale Sharks Exposed to Persistent Organic Pollutants and Plastic Pollution in the Gulf of California (Mexico)? First Ecotoxicological Investigation Using Skin Biopsies.” Comparative Biochemistry and Physiology Part C: Toxicology & Pharmacology, Elsevier, 6 Mar. 2017, www.sciencedirect.com/science/article/pii/S1532045617300558.
Fossi, Maria Cristina, et al. “Large Filter Feeding Marine Organisms as Indicators of Microplastic in the Pelagic Environment: The Case Studies of the Mediterranean Basking Shark (Cetorhinus Maximus) and Fin Whale (Balaenoptera Physalus).” Marine Environmental Research, Elsevier, 24 Feb. 2014, www.sciencedirect.com/science/article/pii/S0141113614000373.
Sazima, Ivan, et al. “Plastic Debris Collars on Juvenile Carcharhinid Sharks (Rhizoprionodon Lalandii) in Southwest Atlantic.” Marine Pollution Bulletin, Pergamon, 16 Apr. 2002, www.sciencedirect.com/science/article/pii/S0025326X02001418.
Wegner, Nicholas C, and Daniel P Cartamil. “Effects of Prolonged Entanglement in Discarded Fishing Gear with Substantive Biofouling on the Health and Behavior of an Adult Shortfin Mako Shark, Isurus Oxyrinchus.” Marine Pollution Bulletin, Pergamon, 14 Dec. 2011, www.sciencedirect.com/science/article/pii/S0025326X11006096.