The New Zealand sea lion is the country's only native pinniped species and is listed as "nationally critical."
The species breeds in the sub-Antarctic on islands south of New Zealand's mainland, but breeding has declined by 40 percent in the Auckland Islands since 1998, a study published in the journal Mammal Review said.
"The most plausible hypotheses based on available evidence is that the decline of breeding females in the Auckland Island population is caused by fisheries-induced resource competition and by-catch incidents," study author Bruce Robertson from the University of Otago said.
By-catch is defined as marine creatures inadvertently caught in nets during fishing for another species.
While non-human factors such as disease, predation, migration or environmental change are affecting the species, human factors such as environmental pollution and resource competition are having significant impacts, the study said.