Growth and survival of larval fishes is highly variable and unpredictable. Our limited understanding of this variation constrains our ability to forecast population dynamics and effectively manage fisheries. Here we show that daily growth rates of a coral reef fish (the sixbar wrasse, Thalassoma hardwicke) are strongly lunar-periodic and predicted by the timing of nocturnal brightness: growth was maximized when the first half of the night was dark and the second half of the night was bright. Cloud cover that obscured moonlight facilitated a ‘natural experiment’, and confirmed the effect of moonlight on growth. We suggest that lunar-periodic growth may be attributable to light-mediated suppression of diel vertical migrations of predators and prey. Accounting for such effects will improve our capacity to predict the future dynamics of marine populations, especially in response to climate-driven changes in nocturnal cloud cover and intensification of artificial light, which could lead to population declines by reducing larval survival and growth.
EndDryadContent

Methods of data collection and processing are described in the following publications:

• "Reproductive phenology across the lunar cycle: parental decisions, offspring responses, and consequences for reef fish" (Ecology, https://doi.org/10.1002/ecy.3086)
• "Lunar rhythms in growth of larval fish growth" (Proceedings B, doi:10.1098/rspb.2020.2609)

Data

• Full data set (thalassoma-hardwicke-otolith-growth increments.csv) contains 46 variables and 40517 rows.  Each row gives information relating to a given otolith growth increment of a given individual.  Some fields contain missing values because data do not exist or fields are not applicable for some observations (these are denoted by ‘NA’).
  • fish.id is a unique identifier for each fish; bag.id is an identifier of sample grouping ("batch number") for otolith reading; stage is the ontogenetic stage of each fish (settler, juvenile, or adult); increment identifies otolith growth increment (1=first visible growth increment near the otolith core); larval.age is an estimate of age (days since egg fertilisation) of a fish at increment formation (= increment + 2); pld is the pelagic larval duration (number of increments before the settlement mark);  post.set.age is the number of increments between the settlement mark and the otolith outer edge; total.age is the sum of pld and post.set.age; days.prior.to.set is the number of increments remaining until settlement; date is the calendar date of increment formation;  birth.date is the calendar date corresponding to egg fertilisation; set.date is the calendar date corresponding to the night of settlement; incr.width is the measured width of a growth increment (in microns); radius.width is the measured distance from the otolith core to the focal increment (in microns); radius.min is the measured width of the first growth increment (in microns); total.otolith.radius is the width of the otolith from the core to the outer edge of the otolith (measured along the postrostral axis, in microns); post.set.tot.incr  is the cumulative width of post-settlement growth increments (microns); ave.post.set.GR is estimated as: post.set.tot.incr/post.set.age; site is an acronym defining the location the site (within the lagoon of Moorea, French Polynesia) where the fish was collected; reef is a numerical identifier of the focal reef where the fish was collected (missing values indicate the fish was not collected focal reefs); fresh.tl is the measured total length of the fish at capture (in mm); etoh.tl.est is the estimated total length of the fish at capture, accounting for shrinkage following preservation in ethanol (estimated based on a relationship derived from a subset of samples; units = mm); etoh.tl is the total length of the fish at capture (measured after preservation in ethanol, in mm); fresh.sl is the measured standard length of the fish at capture (in mm); etoh.sl is the standard length of the fish at capture (measured after preservation in ethanol, in mm); fresh.mass is the measured weight of the fish (settlers and juveniles only) after capture (in g); etoh.mass is the weight of the fish (settlers and juveniles only) at capture (measured after preservation in ethanol, in g); date.collected is the calendar date of collection (adults only); weight is the measured weight of the fish (adults only) after capture (in g); liver is the measured weight of the dissected liver tissue (adults only) after capture (in g); gonads is the measured weight of the dissected gonadal tissue (adults only) after capture (in g); gender is the identified sex (adults only); lunar.month is the lunar month of increment formation (Brown lunation number); lunar.day is the lunar day of increment formation (a 29.53d cycle; 0 = ‘new moon’); lunar.day.bin is binned lunar days (bin size = 2); lunar.phase.precise identifies the precise dates of four lunar phases (new moon, first quarter, full moon, last quarter;  NA appears for dates not falling precisely on these phases); lunar.phase.category.centered estimates the lunar phase of increment formation where phase is centered on lunar.phase.precise; lunar.phase.category.centered.month adjusts Brown lunation number corresponding to lunar.phase.category.centered; lunar.phase.category.start estimates the lunar phase of increment formation where phase is starts on lunar.phase.precise; time.moonrise indicates local time of moonrise; time.moonset indicates local time of moonset; time.moonrise.1 indicates local time of moonrise; time.meridian.passing indicates local time of moon passing the meridian; distance.from.earth.km indicates the distance from the moon to Earth (km); illumination indicates the proportion of the lunar disc that is illuminated (1 = ’full moon’); cloud.cover.airs.nighttime.3x3 indicates the proportion of the night sky (in a 3degx3deg box centered on Moorea, French Polynesia) that is obscured by cloud cover

• Ancillary data sets required to replicate Fig 2a and Fig 3:
  • moonlight_ancillary_fig_2a.csv contains 3 variables and 390 observations (no missing values).  lunar.day is the lunar day within the lunar cycle (a 29.53d cycle; 0 = ‘new moon’); order represents an hour of the night to facilitate plotting of the y-axis in Fig 2a (12 = 18:00, 11 = 19:00, 10 = 20:00 ... 6 = midnight ... 0 = 06:00 the following morning); illum indicates the proportion of the lunar disc that is illuminated (1 = ’full moon’)
  • moonlight_ancillary_fig_3.csv contains 5 variables and 209 observations (no missing values).  date is the calendar date; lunar.month is the lunar month (Brown lunation number); lunar.day is the lunar day within the lunar cycle (a 29.53d cycle; 0 = ‘new moon’); proportion.first.half is the proportion of the first half of the night (from 18:00-23:59) when the moon is above the horizon; proportion.second.half is the proportion of the second half of the night (from 0:00-06:00) when the moon is above the horizon.

R Code

• R_code_for_analyses_and_figures_for_lunar_rhythms_paper.txt gives all R code to replicate analyses and figures presented in "'Lunar rhythms in larval fish growth", with annotations.