Alameda Point’s harbor seal population fluctuates between single digits and 50 during most of the year on the specially-built harbor seal float. But when the Pacific herring arrive in the winter to lay their eggs, many more seals arrive to feast on the herring, causing a sudden spike. Last winter, a spike in seal numbers to a record 70 came on January 5, 2017, in the midst of the herring run. This winter, the herring arrived sooner, in December, and so did more harbor seals, causing a spike to a new record of 73 on Christmas Eve and Christmas Day.
In the brief time span since the new harbor seal float was set in place, local monitors have assumed that it was simply the colder water temperatures that enticed greater numbers of seals to use the float in the winter. But in fact, they discovered it’s not the full story.
It turns out that dropping water temperature indeed has an effect, but the effect is on the herring. Ideal water temperature for herring spawning is between 50 and 53.6 degrees Fahrenheit. The water temperature at Alameda Point dropped below 54 degrees the afternoon of December 16 and continued dropping another 2.3 degrees, according to data collected by the National Oceanic and Atmospheric Administration. This brought on the herring run and, in turn, the voracious seals.
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