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The Anthropogenic Effects on Basking Sharks

By Naomi Grace-Decker

Photograph By Doug Perrine

Basking sharks (Cetorhinus maximus) are the second largest sharks in the world, only outsized by the whale shark (Rhincodon typus). Basking sharks are versatile creatures. They inhabit various locations within the Atlantic and Pacific oceans, ranging from Cape Cod to the Mediterranean Sea, and extending along the Australian Coast (Government of Canada). 

Basking sharks' large size may appear threatening, however, they are virtually harmless to humans and most sea life. Basking sharks are planktonic filter feeders. They swim through the ocean with their mouths extended wide, filtering thousands of gallons of water through their gills. In their feeding process, they use their gill slits to trap food and ultimately can consume millions of plankton, fish eggs, and crustaceans a day (WWF)

Unfortunately, the future of these gentle giants is not guaranteed. The basking sharks’ conservation status is set as endangered, meaning their population is declining (Britannica). Since the 1960s, their population has diminished by roughly 50%, and during this time there were not many laws to protect marine life (McInturf et al., 2022). In molecular ecology, such a significant population decline is termed a bottleneck event. These events often result in a huge loss of genetic diversity, and in many cases can lead to total extinction.  Many factors contribute to the decline of basking shark populations including anthropogenic influences, environmental destruction, and their low reproductive rates. 

We are now aware that human activities have inflicted significant harm upon nearly every ecosystem on Earth. However, in what ways have humans specifically impacted the ecosystems where basking sharks reside? Basking sharks are known to dive to deeper depths, around 250 meters, in the winter months. During the summer months, they tend to stay in the epipelagic zone. This seasonal distribution is likely a result of the species adapting to changing prey distributions. Unfortunately, their distribution in the summer months at the ocean's surface puts them at increased risk to ocean plastics, also known to gather on the ocean's surface. 

Basking sharks seem to get the shorthand of the stick. As they push thousands of gallons of water through their gills to feed, they pick up tons of micro and macro plastics. Basking sharks consume plastic both directly through ingestion and indirectly through their food. Their food supply, including crustaceans and shrimp, also consumes plastic, resulting in the bioaccumulation of plastic in their systems through various avenues.

The physical and chemical characteristics of plastics have negative effects on the health of marine organisms. These effects can be dire. Plastics can lead to physical blockages, strangulation, and tears in organs externally and internally. While these are terrible side effects of plastic pollution, it is not nearly the worst of it. Plastics can break down into nano-plastics, and these nano-plastics release many toxic chemicals. Plastics are known to contain chemicals classified as endocrine disruptors. Endocrine disruptors work by modifying the hormonal cycles of organisms, ultimately throwing their reproductive systems out of order (Gallo et al. 2018).  

Basking sharks are known for their low reproductive efficiency and are considered to have low fecundity. Females of this species produce a shark pup once every 2 to 3 years at most, and this is a generous estimate. Basking sharks also take around 12-16 years to reach sexual maturity (Florida Museum). The chemicals leached from plastic directly interfere with their already slow reproductive cycles, leading to grave implications on their overall population.

The sharks' historically low fecundity rates indicate that their life history cannot be the sole reason for the population decline. Fishing nets, boating interactions, and poachers all contribute to a decrease in the number of sharks reaching sexual maturity. This reduction in sexually mature individuals within the population can have significant genetic consequences. Studies have shown that compared to their close relatives like the great white shark (Carcharodon carcharias), basking sharks have very low genetic diversity. In one particular study, great white sharks were shown to have 29 haplotypes and 77 variable sites within their DNA sequencing while basking sharks had a mere 5 haplotypes and 6 variable sites (Hoelzel et al. 2006). In simple terms, a haplotype is a grouping of amino acids in a DNA sequence that is usually passed along from one generation to the next (NHI).

Low genetic diversity can lead to a whole host of issues. Reduced genetic diversity increased the risk that a single illness could wipe out the entire species. Species with dwindling genetic diversity struggle to successfully adapt to an ever-changing environment. In our current situation, with warming temperatures and ocean changes, this is extremely dangerous. While there is hope that basking shark populations may someday rebound to a non-endangered level, the loss of genetic variation the species once had will be irreversible (UC Museum of Paleontology).

All hope is not lost. Through conservation and advancements in molecular ecology, there is a chance this amazing species can recover. Genetics is a field exponentially growing, with new advancements every day. Public opinion supports conservation efforts for these vulnerable species as governments and agencies have implemented full protection of basking sharks from poaching and hunting. Organizations are working tirelessly to help curb the levels of plastic pollution in the ocean, employing technological advancements to manage existing plastic pollution and lobbying to help prevent future pollution. Numerous steps can be taken by individuals to aid in the preservation of the basking shark species. Proper disposal of plastic and trash, particularly in coastal areas, is of paramount importance. Boat owners should exercise heightened caution when navigating the open ocean to minimize boating-related incidents that endanger basking sharks, practicing vigilance and mindfulness while fishing is also essential, ensuring that fishing gear is responsibly managed and avoiding any disruption to wild basking sharks during fishing activities. With dedication and perseverance, these magnificent basking sharks’ endangerment can come to an end. With your help, these sharks will be able to be observed and loved for generations to come.

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