Laudator Temporis Acti

Erasmus, Adages IV ii 97 (tr. Denis L. Drysdall with his note):

Piscis primum a capite foetet
The head of a fish is the first part to smell

Ἰχθὺς ἐκ τῆς κεφαλῆς ὄζειν ἄρχεται, The head of a fish begins to stink first. Used of bad rulers, whose contagion poisons the rest of the people. The expression seems to derive from the language of common people.

Apostolius 9.18. Tilley F 304 A fish begins first to smell at the head

PISCIS PRIMVM A CAPITE FOETET

Ἰχθὺς ἐκ τῆς κεφαλῆς ὄζειν ἄρχεται, id est Piscis a capite primum incipit putere. Dictum in malos principes, quorum contagione reliquum vulgus inficitur. Apparet ab idiotarum vulgo sumptum.

Jennifer Speake, ed., Oxford Dictionary of Proverbs, 6th ed. (Oxford: Oxford University Press, 2015), p. 114:

The FISH always stinks from the head downwards

The freshness of a dead fish can be judged from the condition of its head. Thus, when the responsible part (as the leaders of a country, etc.) is rotten, the rest will soon follow. ἰχθὺς ἐκ τῆς κεφαλῆς ὄζειν ἄρχεται, a fish begins to stink from the head.

1581 G. PETTIE tr. S. Guazzo's Civil Conversation III.51 If the prouerbe be true,...that a fishe beginneth first to smell at the head,...the faultes of our seruantes will be layed vppon vs. 1611 R. COTGRAVE Dict. French & English s.v. Teste, Fish euer begins to taint at the head; the first thing that's deprau'd in man's his wit. 1915 W.S. CHURCHILL Letter 3 Dec. in M. Gilbert Winston S. Churchill (1972) III. Compan. II.1309 The guilt of criminality attaches to those responsible. 'Well,' said the Aga Khan, 'fish goes rotten by the head.' 1981 Sunday Telegraph 3 May 16 'The fish', as the saying goes, 'always stinks from the head downwards.' Last Sunday we deplored Mr. Michael Foot's liking for the street politics of marches and 'demos'. Since then, a hundred Labour MPs...have followed their leader's example. 2012 Times 2 July 20 The fish rots from the head. The appetite of employee for dishonesty is largely governed by the signals from the top floor.