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 Silversides from Menhaden (bunker, pogy) cleanly, and every pair of groups still in play differs on it and you can check it at a glance.
What to look at
Silversides
Menhaden (bunker, pogy)
Dorsal fin layoutdecisiveClose look
Two dorsal fins with a gap between
None of these
Rows of small spots behind the shoulder spotdecisiveClose look
No
Yes
Sawtooth belly keel you can feeldecisiveIn hand
No
Yes
Deep, slab-sided bodydecisiveAt a glance
No
Yes
Body see-through, backbone or gut visibledecisiveAt a glance
Yes
No
One soft dorsal fin, no spinesdecisiveClose look
No
Yes
Rounded, cigar-shaped bodydecisiveAt a glance
Yes
No
Spiny front dorsal fin (perch, sunfish)decisiveIn hand
Not diagnostic
No
Reported size
25–445 mm
30–500 mm
How it moves
schooling, close to the surface; moves offshore in winter — one assessment records up to about ninety miles
dense schools that ripple or splash at the water surface; young fish corralled at the surface or tight to the shoreline in a feeding blitz
Reference seasonality
Estuary spawn: Apr, May, Jun, Jul
Inshore season: May, Jun, Jul, Aug, Sep, Oct
The same for both: water, 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
Atherinopsidae
Silversides
The band down the side is the easy part. Count the back fins.
First check
Count the dorsal fins. Two, with a clear gap, on a slim fish with a bright band down the side — silverside. One dorsal — go and look at the anchovy and menhaden entries instead.
What it is
A silverside is the fish most people picture when they say bait: slim, translucent-green over the back, and carrying a bright metallic band the whole length of the flank. The band is what you notice from the bank, and it is also what gets people into trouble, because a bay anchovy has a stripe too. The difference is that a silverside's is thick and iridescent and runs all the way forward, and an anchovy's is narrow and often fades out toward the head — and fades after death.
Stages
First year (No Atlantic first-year length in the sources; Pacific topsmelt reach 64–102 mm and jacksmelt average 114 mm in their first year) · Full-grown (51–178 mm for the Atlantic silverside, commonly 76–102 mm; Pacific jacksmelt reach 445 mm and topsmelt 368 mm)
Water
surf, inshore, nearshore
Separates from Menhaden (bunker, pogy)
A deep slab-sided body with a sawtooth belly and a dark shoulder spot is menhaden. Silversides have no scutes and no shoulder spot in any source consulted.
Typical windows
Estuary spawn (adult)
Patterns (correspondence)
Epoxy Silverside
Limits
This is the family, not a species. Atlantic and inland silverside are described as very difficult to separate without magnification.
Everyone knows the shoulder spot. The shoulder spot is not the answer.
First check
Find the shoulder spot, then look BEHIND it. Rows of smaller spots trailing back is menhaden. A single spot on its own settles nothing.
What it is
A menhaden is a deep, slab-sided, oily fish with a head about a third of its body, swimming with its mouth open and straining plankton through a basket of gill rakers. In a school they ripple and splash at the surface, and when something is under them the young ones get corralled tight to the shoreline in what Massachusetts calls, in an official publication, a blitz. That is the animal most of the coast is actually fishing over.
Stages
First year (30–150 mm; the NOAA staging runs larva under 30, prejuvenile 30–38, juvenile above 38) · Full-grown (152–500 mm depending which source you read — see the limitations)
Water
surf, inshore, nearshore
Separates from Silversides
Two dorsals and a bright thick side band is a silverside. Menhaden have one dorsal at midbody and no side band in any source consulted.
Typical windows
Inshore season (adult)
Patterns (correspondence)
None mapped yet.
Limits
Adult size is stated five incompatible ways across five agencies, one of them exceeding another's stated maximum.