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Saturday, August 2, 2014

Summer sailors -- Part 2

I've been meaning to follow up on a few comments submitted to the "Summer Sailors" post on 18 July 2014.  I wrote that it was somewhat unusual to see By-the-wind Sailors (Velella velella) during the summer because I typically observe them in the spring and their appearance on West Coast beaches is often mentioned as a springtime phenomenon (e.g., read description at The Jellies Zone).  But Emily wrote that she had seen Velella in Bodega Bay during July 2003, and Darris thought she recalled seeing them in the fall during some years.  The comments made me do some further research into the seasonality of By-the-wind Sailors. 

First, Velella has continued to wash up on local beaches, so I'll include a few more pictures.  Here's one from 30 July 2014:


In 1977, Bieri wrote a paper titled, "The Ecological Significance of Seasonal Occurrence and Growth Rate of Velella (Hydrozoa)."  He compiled observations from ships and beaches and plotted them by month, including their length measurements. (Note the October record from the Bodega Marine Lab in 1975.)


Bieri also proposed that Velella has two peaks in maximum size during the year one in the spring, and another in the late summer/fall (see graph below).


From this, Bieri suggested that it would be difficult to find large Velella at the surface between November-January and again between late May-early July. 

If Bieri's proposed cycle of growth is accurate, then it makes sense that Velella could appear on local beaches during mid-late summer and fall.  But the question still remains, are there more spring records, and if so, why?  

And when Velella is seen during mid-summer, as in 2014, what's different?  For example, Bieri proposed that it takes ~4 months for Velella to reach a length of 80 mm.  If that's true, are the large Velella that we're seeing now the tail end of the spring population?  If so, why are they here in late July?  Was the "spring population" delayed this year?  (Or was Bieri's proposal not quite right?  Or has something changed since Bieri collected his data?)

Because we don't know the answers , it shows that it's always worth recording whenever you see Velella (including their size and the numbers washing ashore), as well as the weather conditions leading up to their appearance.

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