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: Body see-through, backbone or gut visibleAt a glance It separates Silversides from Anchovies 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
Anchovies
Dorsal fin layoutdecisiveClose look
Two dorsal fins with a gap between
None of these
Which way the mouth pointsdecisiveClose look
Mouth angled up, lower jaw leading
Snout overhangs the mouth
One soft dorsal fin, no spinesdecisiveClose look
No
Yes
Spiny front dorsal fin (perch, sunfish)decisiveIn hand
Not diagnostic
No
Reported size
25–445 mm
20–178 mm
How it moves
schooling, close to the surface; moves offshore in winter — one assessment records up to about ninety miles
dense schools; spawning takes place in the evening
Reference seasonality
Estuary spawn: Apr, May, Jun, Jul
Warm-season spawn: May, Jun, Jul, Aug, Sep
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 Anchovies
One dorsal fin, a see-through body and a narrow stripe that fades toward the front is an anchovy. Two dorsals and a thick bright band is a silverside.
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.
Can you see light through the body? One dorsal fin, a large mouth and a narrow faint stripe with it, and you are on an anchovy.
What it is
Hold a bay anchovy up against the sky and light comes through it. The body is described as slender, silvery-green and translucent, grey-shaded over the back, with a narrow silver line down the middle of the side and a large mouth.
Stages
First year (No consulted source gives a first-year length; bay anchovy can mature at about 37 mm within a few months of hatching) · Full-grown (51–102 mm typically; the Pacific northern anchovy to about 178 mm)
Water
surf, inshore, nearshore
Separates from Silversides
Two dorsal fins and a thick bright band is a silverside. One dorsal and a see-through body is an anchovy.
Typical windows
Warm-season spawn (adult)
Patterns (correspondence)
None mapped yet.
Limits
The pronounced overhanging snout belongs to the Pacific northern anchovy in the sources, not to the bay anchovy, whose overhang is explicitly described as slight.