What does vendace eat?

A vendace eats one thing, and the anatomy for it is visible from outside.

IT IS A SPECIALIST ZOOPLANKTON FEEDER. The diet is dominated by small planktonic crustaceans — copepods and cladocerans, of which the water flea Daphnia is the best-known — taken from open water rather than picked off a bottom or a plant.

THE MECHANISM IS THE GILL RAKERS. Vendace carry long, fine, closely-spaced projections on the gill arches. Water taken in at the mouth passes over them on its way out through the gills, and anything larger than the gaps between them is retained. That is filtration rather than hunting, and the spacing of the rakers is effectively the mesh size of the animal: it determines the smallest particle that can be captured and therefore what the fish can and cannot live on.

WHICH PLACES THE FISH IN OPEN WATER PERMANENTLY. A margin feeder can work a weed bed, a bottom feeder can work sediment, and a filter feeder of this kind can only work a volume of water containing suspended plankton. So vendace occupy the pelagic zone of a large lake — the middle, away from the shore and below the surface — and they are caught there rather than along an edge.

AND THE DIET MOVES VERTICALLY WITH ITS PREY. Zooplankton in a stratified lake migrate up the water column at night and down during the day, avoiding visual predators in the lit surface layer. A fish that eats them follows, which is why the depth a vendace occupies changes through a twenty-four-hour cycle as well as through the season.

THE COMPARISON WITH ITS NEAREST RELATIVE IS THE USEFUL ONE. Common whitefish, which shares lakes with vendace across northern Europe, carries a blunt snout overhanging a subterminal mouth and feeds mainly on bottom invertebrates. Vendace has a projecting lower jaw and an upturned mouth and feeds upward and outward in open water. The two mouths are a direct readout of two feeding strategies, and the pair is one of the clearest cases in this catalogue of anatomy predicting diet.

Everything an angler does about a vendace follows from a fish that filters plankton in open water, and most of it is about where rather than what.

THE FIRST INFERENCE IS LOCATION. A fish that cannot feed along a margin is not caught along one. Where the species is fished at all it is fished out over the deeper open water of a large lake rather than from the bank, and finding the depth band the plankton and the fish are occupying is the whole exercise. That band moves through the day and through the season.

THE SECOND IS SCALE. An animal whose entire food supply is a few millimetres across is not looking for a large item, and where vendace are targeted the approach is correspondingly small and fine — this is a practitioner inference from the anatomy rather than a claim the biology establishes.

THE THIRD IS THE ONE THAT MATTERS MOST IN BRITAIN. Where the species is protected it must not be targeted at all, and any fish caught incidentally should be returned in the water without being lifted. Vendace is also physiologically poor at surviving being brought up from depth, so an angler fishing deep water where it occurs should understand that the release decision is effectively made before the cast.

AND THE COMMERCIAL CONTEXT IS WORTH KNOWING because it explains why the same species appears in two completely different lights. In Finland and Sweden vendace supports a substantial net fishery and is an everyday food fish; in Britain it is among the rarest native freshwater fishes. Both are facts about populations rather than about the species.

This page describes a feeding strategy rather than a menu, and the single prey edge behind it names a taxon rather than a species — because copepods and cladocerans are not species records and never will be. Zooplankton communities differ substantially between lakes and through a season, so which taxa dominate the diet in a given water at a given time is a local question this catalogue cannot answer. Growth and condition vary enormously between populations for the same reason, which is why size figures for this species transfer badly. And the vertical distribution described here is a general pattern in stratified lakes rather than a prediction about any particular water on any particular day.