Unveiling the Mysteries of Basking Sharks: A Journey of Discovery

Basking sharks, the gentle giants of our oceans, remain shrouded in mystery. As the second-largest fish in the world, these plankton-feeding creatures play a vital role in marine ecosystems. Yet, their populations face threats, and our understanding of their lives is limited. This exploration delves into the world of basking sharks, revealing fascinating insights into their biology, behavior, and conservation.

The Enigmatic Basking Shark

Basking sharks are truly magnificent animals. Reaching lengths of up to 10 meters and weighing several tons, they are an awe-inspiring sight. Despite their size, they are harmless filter feeders, consuming vast quantities of zooplankton. These sharks are thought to filter an incredible amount of water every hour, playing a crucial role in marine food webs.

Historical Significance

For centuries, basking sharks have been important to coastal communities. In regions like Connemara, Ireland, subsistence farmers once ventured offshore to hunt these sharks. The primary target was their liver oil, which constituted a third of the shark's size. This oil was highly valued for lighting, wound dressing, and other practical uses. In fact, the streetlights of major cities like Galway, Dublin, and Waterford were once illuminated by basking shark oil.

A Boom-Bust Fishery

The most well-documented basking shark fishery existed off Achill Island, Ireland. Fishermen would use nets to capture the sharks, often leading to their drowning. Alternatively, they would kill the sharks with lances and tow them back to shore for processing. The oil was extracted, and the flesh was used as fertilizer. Shark finning, a major threat to sharks worldwide, also contributed to the decline of basking shark populations.

During the 1950s, the fishery peaked, with as many as 1,500 sharks killed annually. This unsustainable practice led to a rapid decline in the population, a classic example of a boom-bust fishery. Over a period, approximately 12,000 sharks were killed off Achill Island alone. Even into the mid-1980s, Norwegian vessels continued to hunt basking sharks, further impacting their numbers.

Conservation Status and Challenges

Basking sharks are now listed as vulnerable by the IUCN, highlighting the need for conservation efforts. While there are bans on catching and landing them in many European regions, they lack protected status in certain areas. One of the biggest challenges in protecting basking sharks is the limited knowledge we have about them. Much of what we know is based on surface observations, leaving their underwater behavior a mystery.

Tracking and Tagging

To gain a better understanding of basking shark movements and behavior, scientists have begun tagging them. Using simple, low-tech methods, researchers attach tags to the sharks to track their movements. Satellite tags provide valuable data on their underwater behavior, including location, water temperature, and depth. These tags detach after a set period, transmitting data to researchers.

Tagging studies have revealed that basking sharks can travel vast distances. One shark tagged off the Isle of Man traveled to Nova Scotia in just 90 days, covering thousands of kilometers. Others have been tracked to the Caribbean and Brazil, challenging previous assumptions about their habitat preferences.

Genetic Diversity: A Cause for Concern

Genetic studies have revealed surprisingly low genetic diversity among basking sharks. This lack of diversity suggests a possible genetic bottleneck in the past, potentially making them more vulnerable to environmental changes and diseases. Despite their global distribution, there is little genetic differentiation between basking sharks from different oceans. This is unexpected, as one would assume that populations separated by vast distances would exhibit greater genetic differences.

Population Size Estimates

Based on genetic data, scientists have estimated the effective population size of basking sharks to be quite small. Some studies suggest as few as 5,000 individuals, while others estimate around 20,000. Regardless of the exact number, these figures highlight the risk of extinction for this species, especially considering the historical exploitation they have faced.

The Quest for Samples

Genetic studies are limited by the availability of samples. Researchers rely on dead sharks washed ashore, fisheries bycatch, and other sources to obtain tissue samples. However, the total number of samples collected worldwide remains relatively small. To overcome this limitation, innovative methods are needed to collect samples from living sharks.