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 Sunfish, young of year from Alewife / river 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
Sunfish, young of year
Alewife / river herring
Spiny front dorsal fin (perch, sunfish)decisiveIn hand
Yes
No
Sawtooth belly keel you can feeldecisiveIn hand
No
Yes
Dark ear flap on the gill coverdecisiveClose look
Yes
No
Dark vertical bars down the flankdecisiveAt a glance
Yes
No
One soft dorsal fin, no spinesdecisiveClose look
No
Yes
Dark spot behind the gill coverdecisiveAt a glance
Not diagnostic
Yes
Eye wider than the snout is longdecisiveClose look
Not diagnostic
Yes
Dorsal fin layoutClose look
Varies
None of these
Reported size
21–50 mm
4–250 mm
Water it is recorded in
lake margin, reservoir, river, stream
lake open, lake margin, river, reservoir
Life stages covered
young-of-year
young-of-year, adult
How it moves
holding in and around weed and cover by day; juveniles may move into open water at night
schooling in open water; moves shallow at night to spawn once the water passes about 15 degrees Celsius
Reference seasonality
Colonial nesting: May, Jun, Jul, Aug
Spawning run: May, Jun · Young move seaward: Jul, Aug, Sep, Oct, Nov
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 Alewife / river herring
A sawtooth belly and a big eye on a soft-rayed fish is alewife. Sunfish is spiny and smooth-bellied.
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.
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 Sunfish, young of year
This entry does not name Sunfish, young of year as a look-alike. That is a gap in the prose, not proof they cannot be confused.
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.