This paper aims at studying how the fresh fish supply impacts on the spatial organization in 18th Century Venice. The mobility and agency of fishermen and fishmongers inside and outside Venice contributed indeed to redefine the external boundaries of a city which had neither walls nor gates. To this aim, the economic practices observed in Venetian peripheral areas are compared with Republican rules promoting spatial organization around central Rialto and San Marco. This paper focuses in particular on two court trials presented to the Giustizia Vecchia which involved a family of fishermen in 1706 and 1715 : these case surveys shed light on two ways of using urban space coming into conflict with each other, allowing to reflect on the definition of Venetian boundaries and on the various uses of those frontier areas.