Abstract

1. Variation in life-history characteristics is evident within and across animal pop- ulations. Such variation is mediated by environmental gradients and reflects metabolic constraints or trade-offs that enhance reproductive outputs. While generalizations of life-history relationships across species provide a framework for predicting vulnerability to overexploitation, deciphering patterns of intraspe- cific variation may also enable recognition of peculiar features of populations that facilitate ecological resilience.
2. This study combines age-based biological data from geographically disparate pop- ulations of bluespine unicornfish (Naso unicornis)—the most commercially valuable reef-associated species in the insular Indo-Pacific—to explore the magnitude and drivers of variation in life span and examine the mechanisms enabling peculiar mortality schedules.
3. Longevity and mortality schedules were investigated across eleven locations en- compassing a range of latitudes and exploitation levels. The presence of different growth types was examined using back-calculated growth histories from otoliths. Growth-type-dependent mortality (mortality rates associated with particular growth trajectories) was corroborated using population models that incorporated size-dependent competition.
4. We found a threefold geographic variation in life span that was strongly linked to temperature, but not to anthropogenic pressure or ocean productivity. All populations consistently displayed a two-phase mortality schedule, with higher than expected natural mortality rates in earlier stages of post-settlement life. Reconstructed growth histories and population models demonstrated that vari- able growth types within populations can yield this peculiar biphasic mortality schedule, where fast growers enjoy early reproductive outputs at the expense of greater mortality, and benefits for slow growers derive from extended reproduc- tive outputs over a greater number of annual This promotes population resilience because individuals can take advantage of cycles of environmental change operating at both short- and long-term scales.
5. Our results highlight a prevailing, fundamental misperception when comparing the life histories of long-lived tropical ectotherms: the seemingly incongruent combi- nation of extended life spans with high mortality rates was enabled by coexistence of variable growth types in a population. Thus, a demographic profile incorporat- ing contrasting growth and mortality strategies obscures the demographic effects of harvest across space or time in N. unicornis and possibly other ectotherms with the combination of longevity and asymptotic growth.

Brett M. Taylor, John Howard Choat, Edward E. DeMartini, Andrew S. Hoey, Alyssa Marshell, Mark A. Priest, Kevin L. Rhodes, Mark G. Meekan