English translation: All is fish that comes to the net
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If it looks like a duck and it walks like a duck...
Explanation: I'm not familiar with the Portuguese expression so this may be way off base, but it seems to imply the same as this English phrase. If the thing is exhibing the behavior of a duck (or a fish) then it probably is one. We usually leave the second half of the phrse (then it probably is one) unspoken.