Sympatric species evolve mechanisms to avoid competition and coexist. In the Humboldt Current System (HCS), populations of South American sea lions (SASL, Otaria byronia) and South American fur seals (SAFS, Arctocephalus australis) fluctuate mostly due to ENSO events and prey availability. We evaluate population trajectories of Peruvian sympatric otariids and discuss mechanisms for competition and/or resource limitation. For this purpose, we analyzed population trajectories of SASL and SAFS in a sympatric breeding site in Punta San Juan, Peru between 2001 and 2019. Wavelet analysis was used to extract trends and derivatives to estimate rates and turning points. Age-class proportions and biomass times series were constructed from weekly counts and evaluated. Both populations show a growth phase and subsequent decline. SAFS started to decline similar to 2.25 years before and at a rate 1.5 times faster than SASL. Decrease in juvenile age-class suggests that resource limitation is the main contributing factor for current population decline.