Before revision:  We conjecture that the \mathsf{Fish}-number }  H_0/I_0 is a universal lower bound for any such composable sketch .
After revision:  We conjecture that the \mathsf{Fish}-number } We give circumstantial evidence that H_0/I_0 is the optimum Fish-number of mergeable sketches for Cardinality Estimation. We define a class of linearizable sketches and prove that no member of this class can beat H_0/I_0 is a universal lower bound for any such composable sketch .