In response to concerns over wastage and negative impacts on stocks, the damage (antennae loss) and collateral mortality of artisanally trawled-and-discarded juvenile seabob shrimp (Xiphopenaeus kroyeri) and the key factors explaining variability were assessed in the South Brazil Bight. During 20 deployments across 7 d of fishing, 1562 X. kroyeri (14.5 ± 3.1 mm carapace length, CL) were discarded into replicate on-board tanks at both the start and end of sorting, with some individuals monitored immediately for antennae loss and mortalities, and the rest assessed after 3 h. Virtually, all X. kroyeri discarded towards the end of sorting (mean ± SD air exposure of 15.9 ± 3.9 min) died. By comparison, those discarded at the start (2.7 ± 2.8 min) had variable temporal mortalities (total of 52.3% immediately vs. 66.2% after 3 h) which, like antennae loss (total mean ± SE of 50.4 ± 34.2%) were positively associated with haul duration/catch weight and deck exposure, and more frequent among individuals with soft than hard exoskeletons. Antennae loss was also negatively correlated with CL. The results support (i) improving trawl selectivity to reduce the catches of small, unwanted X. kroyeri and other bycatch (and therefore total weights of catches) and/or (ii) sorting catches in water (to minimize air exposure). Such modifications might be promoted through awareness of the potential harvest benefits to fishers associated with reducing unaccounted fishing mortality of the targeted species.