Physiological measurements provide evidence of stressors that impact the health of wildlife. Baleen, the feeding apparatus of whales, accumulates hormones as it grows allowing retrospective assessment of long-term trends in a whale’s physiology. In Patagonia, Argentina, Kelp Gulls, Larus dominicanus (KG), have learned to parasite on living southern right whales, Eubalaena australis (SRW) which results in constant harassment and sizable wounds. With the increasing frequency of gull attacks, and a marked preference of KG to harass calves, the SRW population off Península Valdés (PV) experienced a period of elevated calf mortality that peaked in 2012. We quantified lifetime patterns of glucocorticoids (GCs, an index of general physiological stress) and thyroid hormone (T₃, an index of metabolic stress) in baleen recovered at necropsies of 36 SRW calves (~ 1 and 4 months old) with varying severity of KG lesions. GC levels correlated positively with the degree of wounding, while T3 remained stable irrespective of the severity of KG lesions. Our results suggest no evidence of malnutrition in mildly vs. severely wounded calves. However, the positive correlation of GCs with lesion severity indicates that heavily wounded calves suffered high levels of physiological stress throughout their short lifespans. Thus, KG wounding and harassment may have contributed to the high SRW calf mortality observed at the PV calving ground.