Thank you for the question. There isn't a simple answer to it. There are a number of things that could come into play.
One, of course, is continued fishing.
The second is that in the last decade or so we have seen general warming in ocean conditions from the Gulf of Maine into the Bay of Fundy, the Scotian Shelf and the Gulf of St. Lawrence.
In that same time, we've also seen a decrease in the weight at age of individual fish. They're skinnier than they have been in the past at the same age.
There has also been a general decrease in the length at age of herring. If they're skinny, they're also shorter at the same age. This can have a population-level effect, because the number of eggs or sperm produced by individual herring is a function of both their length and how fat they are. If you have the same number of herring and they are smaller in size, the maximum number of eggs or sperm that can be produced is less than it would have been if they were growing faster.
It's also possible, as noted in my “key points” slide, that natural mortality may have increased. It could be due to predation, or it may not be. This is an area of active research within the department, and the answer isn't settled.
Finally, the number of fish coming into the populations is fluctuating, but it has remained relatively low compared with the case in previous years. This may be a function of a number of things, some of which I've just mentioned.
There is not, however, a smoking gun, if you will.