I took a quick look through the harbor seal stomach and scat data (there have been many, many more scats collected over the years compared to harvested stomachs that have been analyzed) and I see that herring are commonly found in harbor seal scats in Southeast during: Sept, Oct, December, March, and April. Note that we didn't collect scats during all months of the year - likely didn't collect May through July so as not to disturb animals during pupping or when moms are nursing pups. If I remember correctly, we didn't do much scat collecting in Jan-Feb either due to short days and challenging weather. As for stomachs, I see that from a few seals harvested around the Sitka area herring were found in stomachs in October and January (just 3 stomachs). Perhaps also of interest: we commonly found herring bones in scats collected in northern Bristol Bay during late July - September (scats not collected outside these months in Bristol Bay) and also in scats around Kodiak in August, September, October, and March.
I just spoke with John Moran, who works for NOAA at the Ted Stevens Lab in Juneau - he is very knowledgeable about herring. Following are a few points that he passed along that also may be of interest:
In the fall (September-October) herring move into 'Inside' waters in Southeast to overwinter. There are big aggregations of herring in Lynn Canal, Tenakee, near Amalga (Juneau area), likely Port Frederick and Seymour Canal. It is the older fish (~3-4 yr olds) that form these large aggregations and general they are in deeper waters ~50-100 meters. In February-March the aggregations begin to break up prior to spawning. In Lynn Canal, for example, the herring disperse in late winter, but then return in ~later April/May to spawn.
John said there are lots of herring around Sitka right now, and also a lot of small herring in Auke Bay where seals have been foraging on them. The small herring (first year) overwinter in the heads of bays. We were curious whether the seal stomach contained a single herring or several? Were the herring large or small? We were also interested in where the seal was harvested, if the hunter cares to share that information.