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 Shad (gizzard and threadfin) 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
Shad (gizzard and threadfin)
Cisco / lake herring
Long thread trailing from the back findecisiveIn hand
Yes
No
Small fleshy fin before the taildecisiveIn hand
No
Yes
Sawtooth belly keel you can feeldecisiveIn hand
Yes
No
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
Blunt flat head with a small mouthdecisiveClose look
Not diagnostic
No
Which way the mouth pointsdecisiveClose look
Snout overhangs the mouth
Varies
Reported size
30–520 mm
229–597 mm
Water it is recorded in
reservoir, lake open, river, lake margin
lake open, lake margin, river
Life stages covered
young-of-year, adult
adult
How it moves
constantly moving schools; leaping or skittering across the surface when pressed
schooling in open water
Reference seasonality
Spring spawning: Apr, May
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 (Dorosoma)
Shad (gizzard and threadfin)
Look for the thread, then feel the belly.
First check
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 Cisco / lake herring
A small fleshy adipose fin before the tail is cisco. Shad has none, and shad is deep and slab-sided where cisco is long and slender.
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.
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 Shad (gizzard and threadfin)
Shad is deep, slab-sided, keeled and threaded. Cisco is round-bodied with none of those.
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.