Much of humanity has a love-hate relationship with sharks, a fascination and respect for their grace and power and an instinctive satisfaction when a shark is reported dead. Like a snake or a spider, the shark evokes something primal and fearful in many people. The vast majority of human contact with sharks is through a screen, a virtual world that is not actual contact at all—merely visual, imaginary. The Hollywood movie Jaws (1975) no doubt contributed to today’s disregard for their natural place in the wild kingdom—humanity’s impact on this planet has dethroned the ocean’s apex predator and the implications are far-reaching. Sharks are key in maintaining a healthy marine ecosystem; their very presence creates a “fear landscape” that molds and fine-tunes the fitness of myriad species that fall under the predator’s pyramid of influence. Even the human being, a terrestrial air-breathing mammal, is struck with fear similar to the nervous systems of the other sensing creatures who encounter the awe-inspiring shark.
Many reports indicate that the global shark population, along with many other marine species, has declined at an unprecedented rate and to a drastic extent. It is estimated that ~30% of the hundreds of shark species are threatened (vulnerable to endangerment in the near future). Reports estimate global shark stock has declined (in the last ~50 years) somewhere between 50% and as much as 99%. Many studies favor the ninety percentile estimate and few—if any—studies offer more positive data that would indicate global shark populations are healthy or even stable. There is also consensus about the leading cause of the decline, overfishing. This human act is so far-reaching in its effects that the global population numbers of many shark species has reached dangerously low levels. The iconic great hammerhead shark is listed as endangered. Sharks regardless of their endangered status continue to be intentionally plucked out of the sea via nets and hooks, ~100 million sharks are killed by humans each year.
It turns out that shark populations are not easy to study. The claim that ~90% of the world’s shark stock has vanished in the last few decades should raise suspicion. Sharks are indeed a diverse class consisting of hundreds of species (new species are actively being discovered). They are elusive for much of their life cycle; their reproductive and migratory patterns are not well understood. This is an area of study ripe for scientific exploration, a class of animals ideal for continued conservation efforts.
Challenges estimating the baseline
A major problem facing shark conservation is estimating the baseline of shark populations in the past, prior to human influence. What were their numbers in the past and where were they distributed? These details are not well known. Historic records of fishing logs, going back in some cases a couple hundred years, and modern reports from divers, as well as technical methods such as retrieved telemetry and even innovative DNA testing (measuring trace genetic fingerprints of sharks) are some of the best data used to piece together their ranges and population numbers.
The decimation of the global shark population is largely ignored and misunderstood, its relevance underestimated like so many ecological factors. In some cases, the ignorance is so great that the decline is seen simply as a triumph over a beast. Unlike cuddly koalas or even the regal but deadly tiger, sharks do not for the most part illicit compassion nor public support for their conservation.
Sharks are the apex predators of the marine environment. This “apex” means they are key, or to use another ecological term, sharks often represent the “keystone species” in a marine ecosystem, the top species in the hierarchy of natural ecological function. Their role is crucial in maintaining a healthy balanced marine environment. Keep in mind that sharks have patrolled Earth’s oceans for more than 400 million years—well before Pangea began to converge into a great supercontinent and prior to the presence of any reptile or trees on the planet. The imbalance created by weakening the apex species in an ecosystem triggers a myriad of known and unknown factors. The one thing that is apparent is that shark populations have declined and overfishing and outright intentional slaughter of sharks worldwide has dropped the global population to ~10% of what it was in the mid-1900s. That would be equivalent to the global human population of ~7.8 billion (2020) declining suddenly within a few decades to 780 million people.
Measuring the big picture trends of the current global shark population is tricky for several reasons. Namely, there are relatively few baseline estimates of what “healthy” pre-industrial shark populations actually looked like; it is unclear what their numbers were, it is unclear even how shark populations migrate and spread today. There are many mysteries sharks keep, areas of study left to explore. In the research article published in Science Advances, computational marine ecologist Francesco Ferretti from the Hopkins Marine Station, Stanford University, USA, and fellow authors, used fishery and scientific survey data (including scientific scuba diving records from 1975, incidentally the same year Hollywood’s Jaws was released) to model shark populations in the “pristine” Chagos Archipelago in the Indian Ocean, south of the Maldives. Even in this pristine setting many of the studied shark species showed a decline. From the scuba data (1975 to 2012) a marked 75% decline in overall detected sharks occurred, though some species of sharks showed an increase. The remote island atolls of Chagos represent one of the last untouched marine environments where some of the shark species appear to be thriving, a vestige of a lost world from which a balanced marine ecosystem baseline could be modelled.
There are endless possibilities open to marine ecologists seeking to understand ocean ecosystems and the role sharks play, present and past. Marine scientists acknowledge that the omission of relevant historical information (fishing quotas, dive logs, etc.) typically results in an overly optimistic conservation plan, resulting in population recovery targets being set too low compared to the reality of their thriving numbers in the not so distant, but undocumented, past. If shark populations were already declining by 1975, to goal back to that baseline is to accept an already disrupted equilibrium. Shark decimation seems to be yet another example of how humans have drastically altered the planet.
5 March 2020