The azure waters of the Gulf of California, home to a myriad of marine wonders, conceal a tale of ecological upheaval. Also known as the Sea of Cortez, it is a body of water that is teeming with an extraordinary array of life that has captivated scientists, divers, and nature enthusiasts for generations. The Gulf’s underwater landscapes are adorned with a kaleidoscope of marine species, each playing a unique role in this complex ecological symphony.
El Bajo seamount off Espíritu Santo Island and Las Animas seamount off San Jos ́e Island have long been revered as scuba-diving havens, offering enthusiasts a front-row seat to observe the large gatherings of scalloped hammerhead shark (Sphyrna lewini), a semi-pelagic species. Known to congregate in large schools at seamounts worldwide, the same tale rings true within this pristine ecosystem.
Or, rather, it did.
A recent study, fueled by the insights of local scuba divers, unveils a disconcerting decline in the abundance of these majestic creatures. Researchers devised a semi-quantitative questionnaire that they asked a myriad of people to take part in; from seasoned dive guides to photographers and researchers, the diverse pool of 50 participants provided invaluable insights.
The participants, having explored the seamounts over the decades, vividly described the changing tapestry of hammerhead abundance. From the glory days of ‘very abundant’ schools to the disheartening reality of ‘not many,’ the narrative unfolded. Yet, with fewer participants from the 1970s and 1980s, the team underscores the urgency of capturing historical data before the voices of older generations fall silent. Still, by utilizing the firsthand experiences of the participating divers, the study bridged the gap between scientific data and the nuanced observations of those intimately acquainted with the Gulf’s underwater wonders. Statistical analyses, employing non-parametric tests and generalized additive models, illuminated the intricate relationship between diver demographics, time of first dive, and perceptions of abundance.
“Most divers who first dived the seamounts in the 2010 s assessed the current S. lewini population to be recovering, whereas divers who had first dived in decades previously assessed the population to still be declining,” the authors expand. “This difference in views is in accord with the shifting baseline phenomenon, with newer divers having a different perception of what ‘natural’ abundance levels really are. The perception may be valid, however, given that in recent years (since 2018) there has been a rise in the number of videos of large schools of up to 100 S. lewini at the seamounts. However, most of these large schools are made up of juvenile hammerheads, indicating a false sense of recovery, since the large schools documented in the previous decades were composed primarily of adult females.”
The local ecological knowledge uncovered by the study revealed a staggering 97% decline in the average number of sharks encountered per dive at El Bajo, plummeting from 150 sharks in the 1970s to a mere 5 sharks in the 2010s. Similarly, Las Animas witnessed a devastating 100% decline, from an average of 100 sharks in the 1970s to a haunting absence in the 2010s.
The study scrutinized the factors contributing to the somber reality playing out in the Gulf as well. Overfishing emerged as the resounding theme, followed by concerns over fisheries management, changes in prey abundance, habitat degradation, and the omnipresent specter of climate change. Divers, eyewitnesses to the unfolding drama, provided poignant anecdotal evidence—fishing nets ensnaring hammerheads and the disruptive influence of increased human activity on these once-thriving seamounts. An examination of catches in Mexican shark fisheries uncovered a notable trend, with S. lewini emerging as the predominant hammerhead species, accounting for a substantial 83.8% of catches and constituting 11.3% of the overall shark species haul. This pattern extended to the Sinaloa fishery in the eastern reaches of the Gulf of California, where catches predominantly comprised juvenile S. lewini, including neonates. The intriguing life cycle of this shark adds depth to the narrative, as they typically utilize mangrove habitats as nursery areas before embarking on a migration to seamounts, where they assemble as larger individuals.
Yet, the intricate dance of climate change looms large on the stage of species distribution. In particular, alterations in sea water temperature, a hallmark of our changing climate, emerge as a significant player influencing the presence of S. lewini at seamounts and shaping their broader distribution. A poignant historical note from the 1970s underscores the impact, as S. lewini numbers experienced a decline at El Bajo seamount when a cold-water upwelling mass traversed the area. As climate change continues to shape our oceans, understanding and mitigating its effects on the delicate balance of marine ecosystems, especially for vulnerable species like S. lewini, becomes an imperative chapter in the ongoing saga of conservation.
This study leaves scientists at a critical juncture. Once a symbol of abundance, this species now stands on the brink of ‘Critically Endangered’ status. The presented work not only serves as a stark warning but also as a clarion call for immediate targeted conservation measures. It reinforces the indispensable role of local ecological knowledge in unraveling the mysteries of the deep and urges all to become stewards of the oceans before the echoes of decline become irreversible.
