It is not news that the human impact has harmed marine biodiversity. A recent study that used fossilized shark scales found that Caribbean shark populations have experienced a sharp decline since the mid-Holocene. The Holocene is a period in Earth’s history that began 10,000 years ago.
Cartilaginous fish like sharks, sawfish, and stingrays have tiny, sharp, tooth-like scales on their bodies that help them move around in the water and also prevent microorganisms from lodging on the skin of the fish. sharks. These are called dermal denticles (dermal = skin, denticle = shaped like teeth). The study used dermal denticle deposits as indicators of ancient shark populations as well as their species composition. There are currently over 500 species of sharks.
In order to find assemblages of mid-Holocene sharks, researchers collected sediment containing denticles from mid-Holocene reefs and compared them to those from modern reefs.
The researchers identified five morphotypes of dermal denticles, each corresponding to a particular ecological group of sharks. The approach allows for an almost direct reconstruction of the past size of the shark population and species composition.
I am delighted to share our new paper, now available in @PNASNews. We used #fossil #shark scales (denticles) preserved in coral reef sediments to reconstruct the pre-exploitation baseline of a reef shark community in the Caribbean Panama. https://t.co/Sbl0sOm0J2 @odealab @mccauley_lab (1/18) pic.twitter.com/vvxAx1abkm
– Erin Dillon (@erinmdillon) July 6, 2021
Almost all shark species have experienced a decline from the mid-Holocene to the present day. The abundance of sharks in the mid-Holocene was almost three times that of present-day reefs.
Denticles corresponding to pelagic sharks recorded the greatest reduction in number. In contrast, demersal sharks have not shown as much decline. Pelagic organisms are those that are attached to the surface of the ocean, while demersal or benthic organisms live on the bottom. Nowadays, bottom sharks are more abundant than pelagic sharks like the fast swimming sharks and hammerhead sharks that live close to the shore.
The authors note the possible reasons for this change – it could be that the sharks of yesteryear may have been larger, leading to more denticle accumulation.
Role of overfishing
The decline in postindustrialization shark numbers in Caribbean reefs also closely resembles the decline observed in this study from the mid-Holocene to the present day.
Another recent study published in 2020 found, via video surveillance, that overfishing had almost completely wiped out sharks from several reefs. Indeed, historical records from the early 20th or 19th century speak of “shark-teeming seas,” and archaeological records from the early second millennium AD also show evidence of shark teeth.
Substantial degradation of populations of sharks, but also of marine carnivores in general, has taken place. long before coral disease and bleaching
The authors note how “modern Panamanian fisheries selectively capture pelagic sharks,” which “implicates” overfishing as a key factor in reducing the number of pelagic sharks, as noted above.
But what about the decline in the number of nurse sharks, which live near the ocean floor and have little monetary value? Could some reason other than overfishing be at play? Coastal development, land clearing and agriculture have all had their share of responsibility, resulting in low oxygen content of the water, disease, bleaching – a trend seen in the Caribbean. That, and overfishing targets not only sharks but also other fish that serve as food for sharks.
The ecological impacts of the reduction in populations of sharks and marine carnivores, such as that on food webs, are still under study.
The authors hope that delving into ancient fossil palimpsests that predate human impact could help establish more solid baselines for biodiversity restoration goals in these areas.
– The author is an independent scientific communicator. (mail[at]ritvikc[dot]with)
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