Some shark populations in the Mediterranean Sea have completely collapsed, according to a new study, with numbers of five species declining by more than 96 percent over the past two centuries.
“This loss of top predators could hold serious implications for the entire marine ecosystem, greatly affecting food webs throughout this region,” said the lead author of the study, Francesco Ferretti, a doctoral student in marine biology at Dalhousie University in Nova Scotia.
Particularly troubling, the researchers said, were patterns indicating a lack of females of breeding age, which are essential if populations are to recover even with new conservation measures.
“Because sharks are long-lived and slow to mature, they need fully grown females to keep their populations reproductively healthy,” said Heike K. Lotze, a study author who is at Dalhousie.
The study is scheduled for publication in the journal Conservation Biology and was posted on Wednesday at lenfestocean.org by the Lenfest Ocean Program, a private group in Washington that paid for the research.
The study focused on five species for which there were sufficient records to chart a long-term trend — hammerhead, blue and thresher sharks and two types of mackerel sharks. The Mediterranean is home to some 47 shark species, and similar declines are presumed to have occurred in many of them.
Sharks take years to reach sexual maturity and, unlike most other fishes, produce small numbers of young, making them particularly vulnerable to overfishing. Populations have declined worldwide, but experts say the Mediterranean — bordered by many countries with diverse rules and fished intensively for centuries — has had bigger losses of sharks and other large predatory fish, including tuna.
The region’s long-term decline was revealed by sifting decades of catch records and other scattered sources of data, which showed that over time the Mediterranean ecosystem had been utterly transformed. With top-tier predators removed, the populations of other fish and invertebrates have shifted drastically.
In November, the International Union for Conservation of Nature warned that more than 40 percent of shark and ray species in the Mediterranean were threatened with extinction because of intense fishing pressure.