Managing the prey of large marine predators is essential to sustain viable populations of predatory fishes. As such, a lot of research effort is placed into deciphering the diet of ecologically and environmentally important marine predators. Two of the most used methods for filling these research gaps, stomach content analysis and DNA metabarcoding, are both limited by their lack of ability to discern between targeted prey and non-target prey. In particular, as prey dissolves in a predator’s stomach, the prey inside that prey also ends up in the predators stomach, termed secondary predation. Secondary predation can and does confound stomach content and metabarcoding results causing certain taxa to be over-represented. Despite this, few studies have looked at finding ways to better distinguish between primarily and secondarily consumed prey. We used a “Russian Doll approach”, undertaking stomach content and metabarcoding analysis of both blue marlin and any intact prey within their stomachs to better identify taxa likely to be secondarily predated. Once overlapping “suspect” taxa had been identified, we assessed the other prey within the stomach and the state of decomposition of those prey to better understand the occurrence of this secondary predation. This research will help ensure more accurate prey representation in dietary studies and better management for these marine predators.