By McKenna Hardie, B.S. Graduate Student, Florida Institute of Technology & Shark Angels Intern
Scientific study: Large filter feeding marine organisms as indicators of microplastic in the pelagic environment: the case studies of the Mediterranean basking shark
Background
The basking shark, Cetorhinus maximus, is the second largest shark in the world. With the basking shark reaching lengths of 26 feet, most wonder how a shark of this size can survive off of the ocean’s tiniest inhabitants, zooplankton. Basking sharks will swim near the oceans’ surface with their mouths gaping open. Their mouth contains comb-like structures called gill rakers that are used to filter the zooplankton from the water. The zooplankton gets trapped by the gill rakers and the excess water is then filtered out of the shark through their gill slits. The whales then use their tongue to remove the plankton from the baleen before swallowing their food. Basking sharks can filter up to 2,000 tons of water per hour (Fossi et al., 2014). 2,000 tons of water is equivalent to the amount of water in eighteen water towers. That is a lot of water to filter every hour!
There has been recent global concern that human debris, or “litter”, has highly affected marine organisms and their environment. A large amount of the litter that contaminates marine environments are plastic materials that have been broken down by weathering (Fossi et al., 2014). Plastics can be classified as either macroplastics (> 25 mm), mesoplastics (5-25 mm), or microplastics (< 5 mm).
Microplastics come from; plastic bottles, beach toys, and any other large plastic items that are worn down, foam and small fibers that come off fleece clothing, cigarette butts, and diapers (Waterkeeper, 2016) and beauty products in the form of tiny exfoliating beads (microbeads). This article shines the spotlight on microplastics, which makes up 70-80% of the debris in the Mediterranean Sea (Fossi et al., 2014).
Microplastics can carry dangerous pollutants and toxins that can harm the health of marine organisms, their environment, and even people. These pollutants are typically called POPs (persistent organic pollutants). Microplastics can be classified as primary (pellets from plastic industry) or secondary (fragments from once larger items) (Fossi et al., 2014).
With microplastics taking up the surface of our oceans, it is important to consider how much plastic and debris gets ingested by large, filter feeding, marine animals like Basking sharks. Compared to small fish and invertebrates, little data has been collected on how these larger organisms are affected by plastic pollution (Fossi et al., 2014). It’s important to see how ingesting microplastic and debris could further harm basking sharks since they are considered “Endangered” on the IUCN Red List. Basking sharks may be good bioindicators, meaning their status in the ecosystem is used to represent the whole ecosystem’s health. So, if their health is suffering the entire ecosystems health could be suffering too. Basking sharks serve to indicate levels of microplastics in the Mediterranean. They accumulate large levels of microplastic contaminants which provides a reliable indication of environmental contamination at a given time. Since basking sharks are such a migratory species, they make great bioindicators for a wide range of ecosystems. (Fossi et al., 2014). It is important to note that even the smallest organisms can be great bioindicators (Fossi et al., 2014). If those small organisms show significant plastic intake, that means the larger organisms that feed on them are consuming that same plastic. Basking sharks may provide awareness and warning as to how hazardous microplastic pollution can be to other marine organisms and to humans.
Methods
This study focused on fin whales and basking sharks in the Mediterranean Sea. The Mediterranean Sea was chosen as the study site because there has been little research done on this area and its inhabitants relative to microplastic pollution (Fossi et al., 2014). The research took place from 2007 to 2013.
The researchers coordinated with small-scale local fisheries in order to acquire basking shark specimens. Every year basking sharks are accidentally caught by local fisheries (Fossi et al., 2014). Deceased basking sharks from these accidents were used in the study. *It is important to note that no basking sharks were purposefully killed for this study. Muscle samples were taken from six basking sharks. The area where they were found as well as their gender were also recorded. The researchers also collected 30-40 specimens of the plankton that they knew basking sharks were eating to see if there was evidence of microplastics in them (Fossi et al., 2014).
DEHP and MEHP are two chemicals typically found in plastics (Fossi et al., 2014). The researchers were looking to see if DEHP and MEHP appeared in any of the muscle and zooplankton samples. If DEHP and MEHP were present, that indicated to them that the basking sharks and/or zooplankton did have plastics in their body. Toxins like HCB, DDT, and PCBs were also toxin indicators that the researchers used to identify whether or not basking sharks and/or zooplankton have eaten any plastic (Fossi et al., 2014). HCB, DDT, and PCB are organochlorine compounds which are commonly found in plastics and pesticides. Once a basking shark has consumed plastic, these harmful compounds can make their way to the muscle tissue of the shark. These toxins are not only harmful to marine organisms but also to people who may consume those organisms.
Findings & Results
The researchers found large amounts of MEHP in both the basking shark muscle samples and the zooplankton samples. All of the three toxins previously talked about were found in all of the basking shark specimens except for one (Fossi et al., 2014). Large amounts of PCBs were found in basking sharks. PCB in the basking sharks provides strong evidence that POPs are present within these sharks (Fossi et al., 2014). The presence of POPs in these six basking sharks means that these sharks definitely consumed microplastics (Fossi et al., 2014). The sharks may have ingested the microplastics when they were feeding by either: 1) consuming microplastics around the large groups of zooplankton or 2) consuming zooplankton that have already eaten microplastics.
Conclusions
Researchers found that the Mediterranean Sea contains an overwhelming amount of microplastics. Dangerous chemicals and toxins that are typically found in microplastics have been found in specimens of basking sharks and zooplankton. The amount of chemicals and toxins found in the samples was extremely high and that raised much concern between the researchers. The researchers found high levels of MEHP and DEHP in zooplankton samples which is alarming because that is the only food source of the basking shark (Fossi et al., 2014).
Basking sharks are much more vulnerable to the harmful chemicals that come from microplastic because they are indirectly ingesting those chemicals when they feed on their prey. In a sense, basking sharks are getting double the impact just by eating zooplankton which contains microplastics as well as simply opening their mouths and ingesting the microplastic filled water with their food.
The basking sharks gill rakers not only allow them to filter the tiny organisms which they eat but also any microplastics or other debris that is sitting on the ocean surface. The researchers calculated that a hungry basking shark can swallow at least 13,110 microplastic items and other debris daily (Fossi et al., 2014). If each of those microplastics is around a gram, then altogether, those microplastics can weigh as much as 15 boxes of spaghetti! Or as much as 20 dozen eggs!
This study is the first to show evidence of plastic inside Mediterranean basking sharks. Researchers now look to figure out better ways to detect the presence of microplastics in other filter feeders. The most daunting question remains, how much plastic consumption is too much for these large filter feeding sharks?
Fossi, M. C., Coppola, D., Baini, M., Giannetti, M., Guerranti, C., Marsili, L., … & Clò, S. (2014). 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, 100, 17-24.