Variations in life-history traits in exploited and unexploited populations of Perth herring (Nematalosa vlaminghi): evidence for density-dependent growth?

Abstract

Context Fishing can reduce population biomass, allowing remaining individuals to grow faster and mature earlier because of greater resource availability, which potentially compensates for lost reproductive output over a shortened lifespan resulting from fishing mortality. Aims To compare life-history traits of Nematalosa vlaminghi, a long-lived (20 years), semi-anadromous fish endemic to south-western Australia, in exploited and unexploited populations living in similar environments. Methods Populations were sampled in 2016–2018. Total mortality (Z) was estimated using catch-curve analyses, allowing for annual recruitment variability. Maturation age and length was estimated by logistic regression analysis. Growth curves were fitted to length-at-age data. Key results Compared to the unexploited population, Z was 2.6 times greater, juvenile growth was more than twice as fast, and maturity was attained at least 3 years earlier in the unexploited population. Conclusions Results supported the hypothesis of faster growth in exploited populations because of density-dependent processes. Environmental factors may also have contributed to trait differences between populations. Implications Faster growth and earlier maturation may partly compensate for lost reproductive output in exploited populations, although severe age truncation owing to fishing is still likely to decrease population resilience in this species, irrespective of any compensatory density-dependent processes.