What might be missing from the stories on leopard shark deaths in SF Bay?

Many news outlets are reporting on a spike in leopard shark deaths and bat rays in S.F. Bay.  Several theories related to pollution have been offered.  The San Francisco Estuary Institute (SFEI), and the US Geological Survey (USGS) have monitored the Bay’s water quality for several decades.  SFEI does not propose to have the answer to this complex problem.  But we want to offer an additional, under-reported factor that should be considered in the discussion: low salinity….in fact the lowest salinity in two decades.

According to UC Davis research scientist, James Hobbs, leopard sharks and bay rays are sensitive to salinity and can only tolerate levels down to 50% of seawater.  Below this level they begin to suffer significant stress.  In response to the Bay Area’s recent cycle of extreme drought followed by heavy rainstorms, the salinity in South Bay has swung from high to low in the past few months due to large inputs of fresh water.  This year, the salinity in the lower half of the bay is among the lowest in the last 40 years.  For the past few months the salinity has been below the 50% seawater level that leopard sharks and bat rays strongly prefer.

Poor circulation and slow flushing in the shallow waters of South Bay only compound the problem, contributing to lower salinity over a longer time. The last time the Bay experienced low salinity like this was in 2011, when there was also a spike in the leopard shark deaths. The 2017 low salinity anomaly may have increased the stress on leopard sharks, and should be considered as a factor for this year’s dramatic decline.

S.F. Bay Monitoring Results:  The color coded map below shows salinity levels in the Bay for late March as measured by the USGS as part of a SFEI study.  Salinity levels are shown in the bar on the right: Zero  parts/thousand (PPT) = Freshwater, 30 ppt = Ocean water.  15 ppt is approximately 50% salinity.  Leopard sharks like to breed in the shallow waters.  Note that the shallow shoreline areas are well below the 50% level and middle of the Bay is around the 50% level.

Further studies may provide a better answer to this problem and possible solutions.  But the impact of low salinity stress that may weaken the health of these creatures needs to be considered.