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: Deep, slab-sided bodyAt a glance It separates Alewife / river herring from Cisco / lake herring cleanly, and every pair of groups still in play differs on it and you can check it at a glance.
What to look at
Alewife / river herring
Cisco / lake herring
Sawtooth belly keel you can feeldecisiveIn hand
Yes
No
Small fleshy fin before the taildecisiveIn hand
No
Yes
Rounded, cigar-shaped bodydecisiveAt a glance
No
Yes
Deep, slab-sided bodydecisiveAt a glance
Yes
No
Dark spot behind the gill coverdecisiveAt a glance
Yes
Not diagnostic
Eye wider than the snout is longdecisiveClose look
Yes
Not diagnostic
Reported size
4–250 mm
229–597 mm
Water it is recorded in
lake open, lake margin, river, reservoir
lake open, lake margin, river
Life stages covered
young-of-year, adult
adult
How it moves
schooling in open water; moves shallow at night to spawn once the water passes about 15 degrees Celsius
schooling in open water
Reference seasonality
Spawning run: May, Jun · Young move seaward: Jul, Aug, Sep, Oct, Nov
Late-autumn spawning: Nov, Dec
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 Cisco / lake herring
Cisco carries an adipose fin and is long and round-bodied; alewife has a sharp keel and no adipose fin. This is the cleanest pair in the group.
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 small fleshy fin between the back fin and the tail? That is an adipose fin. A cisco has no teeth; a rainbow smelt, which also carries the fin, has obvious teeth on the jaws and tongue.
What it is
The name is misleading and the body settles it. Cisco is a member of the trout and salmon family, and it carries the family's badge: a small fleshy adipose fin between the back fin and the tail. The look-alike that shares that fin is the rainbow smelt, not a herring.
Stages
Adult (Adults average about 230–460 mm; the largest reach about 600 mm)
Water
lake-open, lake-margin, river
Separates from Alewife / river herring
Adipose fin present and a round body is cisco; keel present and no adipose fin is alewife.
Typical windows
Late-autumn spawning (adult)
Patterns (correspondence)
None mapped yet.
Limits
Sources disagree on whether the cisco's lower jaw projects, so jaw position is not offered as a field character.