From Alan Trimble
The cohort of Pacific Oyster larvae we have been tracking for the past week is shrinking enough to be difficult to measure. While some larvae
are already reaching 200 microns, most have disappeared. There seems
to be continuous new additions of small (<90 micron) larvae, but these
aren’t growing or surviving enough to be likely to support additional
spatfall. Conditions appear now to be unfavorable for further succesful
reproduction in significant quantities.
This isn’t true for clam larvae; most samples this week contain numerous
setting size clams and often they are actively searching for substrate
crawling on our sorting dishes. It is pure speculation but competition
between these numerous clam larvae and the rarer Pacific Larvae seems
likely as a mechanism for recent reduced oyster larval growth and survival.
Here are the counts of small umbo – umbo larvae from samples taken yesterday afternoon at high water.
(per 20 gallons, consistent with historical measurements/records)
Site Low Count High Count Largest Size
Lewis Slough 0 1 180 microns
Long Island Slough 0 3 180
Cougar Bend 11 40 200
Naselle Bridge 0 2 220
Mill Channel 1 15 190
Peterson Station 0 5 150
Smokey Hollow 5 14 190
Bear River 0 4 150
I have also attached a PDF file of the spatfall data sent out last
bulletin with an additional column listing the highest per-shell
count for each site.
At this time we don’t anticipate any further significant spawning
or spatfall for this season. We will continue to sample larvae for
a few more weeks and we will count the next batch of shellsticks
for spatfall after we collect them in a week or so. Any interesting
developments will be included in the final Bulletin of the year which
should be generated by the end of September.