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 Yellow perch, young of year from Sunfish, 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
Yellow perch, young of year
Sunfish, young of year
Dark ear flap on the gill coverdecisiveClose look
No
Yes
Dorsal fin layoutdecisiveClose look
Two dorsal fins with a gap between
Varies
Deep, slab-sided bodydecisiveAt a glance
Not diagnostic
Yes
Rounded, cigar-shaped bodyAt a glance
Not diagnostic
No
Reported size
5–80 mm
21–50 mm
Water it is recorded in
lake margin, lake open, river, reservoir
lake margin, reservoir, river, stream
How it moves
schooling by size and age; feeding in daylight and largely inactive at night
holding in and around weed and cover by day; juveniles may move into open water at night
Reference seasonality
Spring spawn, then shallows: Mar, Apr, May, Jun, Jul, Aug
Colonial nesting: 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
Percidae (Perca flavescens)
Yellow perch, young of year
Spines at the front, bars down the side.
First check
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.