Long-term demographics of a coral-reef fish: growth, survival and abundance at several spatial scales

In marine organisms, pelagic larval dispersal often results in genetic panmixis among widely separated populations; however, local conditions may produce populations of marine organisms that differ in key life history traits. Here, we assess spatial differences in growth, body condition, survivorship and movement over a decade for lemon damselfish Pomacentrus moluccensis. Newly recruited fish were tagged on the southern (One Tree Island, 23 degrees S), and northern Great Barrier Reef (Lizard Island, 15 degrees S), 1200 km apart, and monitored for over a decade to provide direct data on growth and survivorship. New recruit physiological condition (lipid content) had a weak positive influence on early survival, but not on long-term persistence or growth, and the larval supply signal was lost after 2 years, suggesting post-settlement processes were key in long-term population persistence. Fish exhibited extreme site fidelity, with older/larger fish moving most (all less than 7 m). At Lizard Island, under 5% of individuals remained after 3 years, none after 4 years, while at One Tree Island 5% of individuals survived over 5 years, with 1% remaining after a decade, suggesting Lizard Island populations may require more frequent replenishment to persist. Among-site (within location) variation in mortality was high, and not related to density, suggesting local conditions such as wave exposure and predators were important. Surprisingly, mean growth at the two locations, which differ by around 2 degrees C on average in sea temperature, was practically identical, levelling off after 3 years, but with high variation among nearby sites within each location, which would affect size-based fecundity and time of first reproduction at these smaller spatial scales. Neither early post-settlement growth nor mortality were density-dependent, and early growth was not related to overall longevity. These small reef fishes are highly sedentary and some demographic parameters, but not others, differ at a range of scales, with post-settlement populations likely driven largely by mortality patterns at the larger spatial scale. Latitudinal and smaller-spatial scale differences in performance (growth, survival) of this species, leading to offsets in expected time to maturity an fecundity, are contrasted by evidence of stability of adult populations over two decades at One Tree Island, which suggests that resilience to environmental change/climate change and other externalities differ considerably across this species' distribution.

Automated summary

In this study, spatial differences in growth, body condition, survivorship, and movement of lemon damselfish Pomacentrus moluccensis were evaluated over a decade in the southern and northern Great Barrier Reef regions. The study found high within-site variation in mortality, unrelated to density, likely influenced by local conditions, and showed no clear relationship between sea temperature and growth. Additionally, at smaller scales of variation, individuals exhibited high site fidelity, indicating both population stability and dynamics.