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 last 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 mature females, 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 Lotze, a study author who is also at Dalhousie.
The study is scheduled for publication in the journal Conservation Biology and was posted online on Wednesday at www.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 experienced bigger losses of sharks and other large predatory fish, including tuna.
The long-term decline in the region was revealed by sifting decades of catch records and other scattered sources of data, which showed that over time the Mediterranean ecosystem has been utterly transformed. With top-tier predators removed, the populations of other fish and invertebrates shift in unpredictable ways.
In November, the World Conservation Union warned that more than 40 percent of shark and ray species in the Mediterranean were threatened with extinction because of intense fishing pressure, including the continued use of drift nets. The nets kill many sharks and rays even when they are not the target of the fishing effort.
A ban on fishing in deep waters (more than about 3,200 feet) and on cutting off shark fins, a delicacy in China, could help, but much more enforcement of laws is needed, the conservation union said.