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 Sunfish, young of year 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
Sunfish, young of year
Yellow perch, young of year
Dark ear flap on the gill coverdecisiveClose look
Yes
No
Dorsal fin layoutdecisiveClose look
Varies
Two dorsal fins with a gap between
Deep, slab-sided bodydecisiveAt a glance
Yes
Not diagnostic
Rounded, cigar-shaped bodyAt a glance
No
Not diagnostic
Reported size
21–50 mm
5–80 mm
Water it is recorded in
lake margin, reservoir, river, stream
lake margin, lake open, river, reservoir
How it moves
holding in and around weed and cover by day; juveniles may move into open water at night
schooling by size and age; feeding in daylight and largely inactive at night
Reference seasonality
Colonial nesting: May, Jun, Jul, Aug
Spring spawn, then shallows: Mar, Apr, May, Jun, Jul, Aug
The same for both: 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
Centrarchidae (Lepomis)
Sunfish, young of year
Deep as a coin, with a dark ear flap.
First check
One joined or barely divided dorsal fin on a very deep body, and no spines on the gill cover. That is a sunfish, not a perch.
What it is
A young sunfish is spiny-rayed like a perch, so the first cut puts them in the same half of the key. What separates them is the shape and the flap.
Stages
Young of year (21–50 mm in the first year)
Water
lake-margin, reservoir, river, stream
Separates from Yellow perch, young of year
Two separate dorsal fins with a gap, and spines on the gill-cover tip, is a perch. Sunfish has one joined fin, an ear flap, and no opercular spines.
Typical windows
Colonial nesting (young-of-year)
Patterns (correspondence)
None mapped yet.
Limits
In Iowa, first-year bluegill average roughly 25–50 mm; in one Indiana lake, fry returned from open water to cover at 21–25 mm.
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 Sunfish, young of year
A dark ear flap at the corner of the gill cover, and a much deeper rounder body, is a young sunfish. Perch has neither, and has gill-cover spines that sunfish lack.
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.