Everyone has a date. Tax day. The third week of April. When the forsythia goes. Somebody's grandfather's rule, handed down, and it works often enough that nobody questions it.
The fish are not using a calendar. They are using a thermometer, and we know roughly what it says.
The number that starts it
River herring run initiation in southern New England sits at about 9.0 °C stream temperature. That is the threshold Ellis and Vokoun put on it, and later work has kept landing near the same place.
The other end is just as sharp. The literature puts the end of the run around 20–21 °C, and in Chesapeake Bay rivers the runs ended at average water temperatures of 15.4 °C for alewife and 18.9 °C for blueback herring (Legett et al. 2023) — two species, two different exit points, both temperature-keyed.
So the run is not an event on a date. It is a window that opens when the water crosses a threshold and closes when it crosses another, and the calendar is simply the thing that usually happens to be nearby.
Your date is wrong for somewhere an hour away
Here is the finding that should change how you use anybody else's information.
Legett and colleagues studied twelve Massachusetts streams and found that the northerly runs migrate later, at warmer temperatures, and in a narrower window than the southern ones.
Read that carefully, because it is not just "the north is a bit behind". The northern runs are not the southern runs shifted along by a fortnight — they run on a different temperature relationship, compressed into less time.
A single statewide date for "the herring run" is therefore not merely imprecise. It is wrong by construction: no one date can describe a set of runs that key off different temperatures in windows of different lengths. If your rule came from someone fishing a different part of the state, it was never going to transfer cleanly, and no amount of experience will make it.
Where the timing is moving
Where streams have warmed, runs have moved with them — about 12 days earlier in southern New England streams since the 1970s (Ellis and Vokoun 2009). In North Carolina, over four decades, alewife arrived 16 days earlier and left 27 days earlier (Lombardo et al. 2020).
Massachusetts does not move as one. Across twelve Massachusetts runs there is no single statewide trend, and three moved later (Dalton et al. 2022) — another reason a date fails.
Why a striper fisherman should care about alewife
Because the bass are not there for the scenery.
River herring stack at stream mouths for a few weeks, and bass gather there too — imaging sonar at the Herring River tide gate in Wellfleet logged thousands of striped bass sightings during the run (Rillahan et al. 2021). Getting the timing right puts you there while the run is on, and the timing is available to you in a way it is not for most things in this sport: you can go and measure it.
What to do with it
Watch the water, not the wall. A cheap stream thermometer at the mouth you fish, checked through the back half of March and into April, will tell you more than any date. You are looking for the water to come up through the high single digits Celsius — around the high forties Fahrenheit.
Keep your own dates, per stream. Not one date for spring, but a record for each specific run you fish. Given the north–south differences within one state, your streams are separate events and deserve separate notes.
Check early. A thermometer reading in late March costs nothing and tells you whether your stream is close.
And do not extrapolate other people's dates. A friend's reliable date from a run thirty miles away is real information about their stream and nearly nothing about yours.
How StripedLines uses this
StripedLines is built for striped bass. The water temperatures StripedLines shows are coastal readings, not the stream itself, so your own thermometer at the run is the better instrument.