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: Dark spot behind the gill coverAt a glance It separates Shad (gizzard and threadfin) from Yellow perch, young of year 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)
Yellow perch, young of year
Spiny front dorsal fin (perch, sunfish)decisiveIn hand
No
Yes
Long thread trailing from the back findecisiveIn hand
Yes
No
Dark vertical bars down the flankdecisiveAt a glance
No
Yes
Sawtooth belly keel you can feeldecisiveIn hand
Yes
No
Dorsal fin layoutdecisiveClose look
None of these
Two dorsal fins with a gap between
One soft dorsal fin, no spinesdecisiveClose look
Yes
No
Dark spot behind the gill coverdecisiveAt a glance
Yes
Not diagnostic
Blunt flat head with a small mouthdecisiveClose look
Not diagnostic
No
Reported size
30–520 mm
5–80 mm
Life stages covered
young-of-year, adult
young-of-year
How it moves
constantly moving schools; leaping or skittering across the surface when pressed
schooling by size and age; feeding in daylight and largely inactive at night
Reference seasonality
Spring spawning: Apr, May
Spring spawn, then shallows: Mar, Apr, May, Jun, Jul, Aug
The same for both: water.
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 Yellow perch, young of year
A spiny front dorsal fin rules out both herrings here. Shad and alewife each carry a single soft-rayed dorsal fin.
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.
Two separate dorsal fins with a gap between them, a deep body flattened side to side, and bars rather than a stripe. That is a young perch; a darter is slender and nearly round.
What it is
The spiny-rayed cut is the most useful single division in freshwater forage. Minnows and herrings have one soft dorsal fin and no spines. Perch and sunfish have a spiny fin at the front. Everything else follows from that.
Stages
Young of year (About 5 mm at hatching; roughly 60–80 mm by late summer in Lakes Michigan and Erie)
Water
lake-margin, lake-open, river, reservoir
Separates from Shad (gizzard and threadfin)
A sawtooth belly and one soft dorsal is a herring. Perch has spines and a smooth belly.
Typical windows
Spring spawn, then shallows (young-of-year)
Patterns (correspondence)
None mapped yet.
Limits
Agency surveys in Lakes Michigan and Erie put age-0 yellow perch at roughly 60–80 mm total length by late summer and autumn. Earlier in the season they are far smaller.