To date, science has not identified a single issue that would explain that result. There are a number of things that come into play.
There is continued harvest, so there are continued removals both in the commercial fishery and in the bait fishery.
Then there's a second aspect, and that's the productivity aspect of the population itself, the ability of herring stocks to rebound on their own. We have seen relatively low recruitment of new individuals coming into the population. We've seen low individual growth. In fact, in southwest Nova Scotia and the Scotian Shelf, the Gulf of Maine and the southern Gulf of St. Lawrence, we've seen some of the lowest growth rates in our time series.
If you think about it from first principles, if animals are growing more slowly, they're taking longer to age, and they're not producing as many young. It's possible that the overall ability of the populations to replace themselves is lower than it has been in the past.