Side by side · same standard

What actually separates these two.

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.

Group AGroup B

The differences, and nothing else

Best separating check: Deep, slab-sided body At a glance It separates Cisco / lake herring 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	Cisco / lake herring	Sunfish, young of year
Spiny front dorsal fin (perch, sunfish)decisiveIn hand	No	Yes
Small fleshy fin before the taildecisiveIn hand	Yes	No
Rounded, cigar-shaped bodydecisiveAt a glance	Yes	No
Dark ear flap on the gill coverdecisiveClose look	No	Yes
Dark vertical bars down the flankdecisiveAt a glance	No	Yes
Deep, slab-sided bodydecisiveAt a glance	No	Yes
One soft dorsal fin, no spinesdecisiveClose look	Yes	No
Dorsal fin layoutClose look	None of these	Varies
Reported sizedecisive	229–597 mm	21–50 mm
Water it is recorded in	lake open, lake margin, river	lake margin, reservoir, river, stream
Life stages covered	adult	young-of-year
How it moves	schooling in open water	holding in and around weed and cover by day; juveniles may move into open water at night
Reference seasonality	Late-autumn spawning: Nov, Dec	Colonial nesting: May, Jun, Jul, Aug

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

Salmonidae (Coregonus artedi)

Cisco / lake herring

Called a herring, built like a trout.

First check

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 Sunfish, young of year

A spiny front dorsal fin on a very deep body is a young sunfish. Cisco has soft rays only.

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.

Full Cisco / lake herring entry →

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 Cisco / lake herring

This entry does not name Cisco / lake herring as a look-alike. That is a gap in the prose, not proof they cannot be confused.

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.

Full Sunfish, young of year entry →