Again from the Beach Seining we did in the Fraser to look at Species composition and Biomass. In the April sets in the south and North Arm we did find good numbers of Herring....but then we had a grad student from the Stated and he swore up and down they were Shad..Anyways we took a few samples back and they all were shad. Never very big - 8 to 12 cm. So my guess is you are seeing shad and not Herring
Interesting with the small body size in your seined samples, that's probably why they were mistakened as pacific herring (not that pacific herring do not grow bigger, they can in fact reach much bigger size as most know). Pacific shad's maximum length is over 2ft long. The two species look very similar and can easily be mixed up with one glance. The small differences include dorsal and anal soft ray counts. I think the most distinct difference is the row of spots across the lateral line found on American shad, but not on Pacific herring.
Both have very different life histories that may suggest their presence or absence in the Fraser River. American shads spawnin rivers, while Pacific herring spawn on kelp. There are no reasons for pacific herring to be in the Tidal Fraser River, especially right now when spawning are taking place in kelp forests (see commercial herring fishery that took place this past week on ECVI).
On the other hand, it is hard to explain why american shad are already in the Tidal Fraser River. They typically spawn in spring/early summer, which explains why they were showing up in April during the seine sampling. In the winter months they tend to stay in deeper water.
This is an introduced species by the way. Not a newly introduced species, but one that was transplanted to the West Coast over a century ago. Also known as Atlantic shad, their natural distribution is the Atlantic coast of North America.
I've by-caught them several times in Steveston.
Not exactly a great fighter at this size, but I can imagine why they are targeted when they reach this size:
http://globalflyfisher.com/pix/display.php?code=30654d8fe8bd8&p=