Featured Image Credit: Marine Mammal Center
By Jessica Kittel
As one of San Francisco Bay’s most common shark species, one or two leopard sharks washing up on the beaches isn’t likely to make scientists and wildlife officials too concerned. Hundreds to thousands of dead leopard sharks (Triakis semifasciata) is a completely different story.
Scientists and Fisheries and Wildlife officials began to feel concerned when the shorelines near Redwood City, Foster City, Alameda, Hayward, Oakland, and San Francisco started to turn into makeshift leopard shark graveyards. Numerous sharks were found ashore and even more are believed to have perished and sank.
Sharks are fish and therefore have no air-filled lungs. However, unlike other fish, sharks do not even have a swim bladder to help them float. They’re actually slightly negatively buoyant, so they tend to sink soon after they stop actively swimming. This fact suggests that we’re only seeing a fraction of the actual deaths. Despite the lack of these sunken individuals, scientists estimate that there could be over a thousand dead during this die-off.
The silver lining to these sharks washing ashore was that it gave scientists the opportunity to collect samples in order to run tests to get some idea of what could be creating such a massive death toll.
Unfortunately, this isn’t the first leopard shark die-off in the San Francisco area. This is actually the third to occur in the past six years. However, die-offs in San Francisco first gained attention in 1967 when over 725 sharks and rays washed ashore over a period of two months. This event was followed by die-offs in 2002, 2006, 2011, and 2016. 2011 had the highest death toll.
So what did those test results show? The culprit turns out to be a viral and fungal infection that enters through the ears and nose. This infection attacks the shark’s brain and disorients them. Scientists believe the root cause is stagnant water in the surrounding areas as well as the huge amount of rain they’ve had this year. The stagnant water acts as the perfect breeding ground for fungus. Large amounts of rain then wash this cesspool into the bay.
Timing was not working in the leopard sharks’ favor either since this all happened right when they were coming into the shallow water for their breeding season.
Fortunately, leopard shark populations weren’t in dire condition before they lost this large chunk of individuals. As of right now, leopard sharks are listed on the IUCN Red List as being of least concern.
Hopefully this species, known to be particularly docile towards humans, will be able to fight of this fungus and flourish!