The comparison scores every group at once. This page is for the moment you already have a pair of names in mind — mayfly or stonefly, scud or sowbug, shad or alewife — and want the structures that keep them apart, in one look. Nothing here is a current hatch.
The differences, and nothing else
Best separating check: Which way the mouth pointsClose look It separates Alewife / river herring from Shad (gizzard and threadfin) cleanly, and every pair of groups still in play differs on it (look closely with it still).
What to look at
Alewife / river herring
Shad (gizzard and threadfin)
Long thread trailing from the back findecisiveIn hand
No
Yes
Eye wider than the snout is longdecisiveClose look
Yes
Not diagnostic
Blunt flat head with a small mouthdecisiveClose look
No
Not diagnostic
Which way the mouth pointsdecisiveClose look
Varies
Snout overhangs the mouth
Reported size
4–250 mm
30–520 mm
How it moves
schooling in open water; moves shallow at night to spawn once the water passes about 15 degrees Celsius
constantly moving schools; leaping or skittering across the surface when pressed
Reference seasonality
Spawning run: May, Jun · Young move seaward: Jul, Aug, Sep, Oct, Nov
Spring spawning: Apr, May
The same for both: water, life stages.
Rows come from the reviewed diagnostic weights the comparison scores with. “Decisive” means one answer puts them at least four points apart — the carrier's own rule for a structure that separates a pair.
The two entries, side by side
Clupeidae (Alosa pseudoharengus)
Alewife / river herring
The sawbelly with the oversized eye.
First check
Is the eye wider than the snout is long, with no thread on the back fin? That is alewife rather than shad.
What it is
Alewife is the herring you meet without the thread. Same deep slab-sided silver body as shad, same sawtooth belly — sharper, in fact, than most of the family — but the back fin ends cleanly and the eye is conspicuously large.
Stages
Young of year (About 4 mm at hatching; in the Great Lakes, about 60–100 mm by autumn)
Water
lake-open, lake-margin, river, reservoir
Separates from Shad (gizzard and threadfin)
Shad trails a long thread from the last dorsal ray. Alewife's dorsal fin has no long projection.
Typical windows
Spawning run (adult) · Young move seaward (young-of-year)
Patterns (correspondence)
None mapped yet.
Limits
Landlocked Great Lakes alewife are much smaller than the anadromous Atlantic form; the sizes here follow Great Lakes survey figures.
Is there a long thread trailing off the back fin? That is the one character in this group that nothing else shares.
What it is
A deep, slab-sided, silver fish in a school near the surface is a common big-water forage sight. Two animals here fit that description before you look closer: shad and alewife. Cisco, the other silver schooling fish, is long and slender instead.
Stages
Young of year (About 30 mm when the adult shape forms; 100–180 mm by the end of the first year)
Water
reservoir, lake-open, river, lake-margin
Separates from Alewife / river herring
The thread on the back fin is the whole answer. Alewife has none, and has an eye wider than its snout is long.
Typical windows
Spring spawning (adult)
Patterns (correspondence)
None mapped yet.
Limits
This entry covers gizzard and threadfin shad together. Anal-ray and scale counts on a fish in hand separate them reliably; yellow fins and a black-speckled chin on a live threadfin are field hints.