Coral growth, survivorship and return-on-effort within nurseries at high-value sites on the Great Barrier Reef

Abstract

Coral reefs are deteriorating worldwide prompting reef managers and stakeholders to increasingly explore new management tools. Following back-to-back bleaching in 2016/2017, multi-taxa coral nurseries were established in 2018 for the first time on the Great Barrier Reef (GBR) to aid reef maintenance and restoration at a "high-value" location-Opal Reef-frequented by the tourism industry. Various coral species (n = 11) were propagated within shallow water (ca. 4-7m) platforms installed across two sites characterised by differing environmental exposure-one adjacent to a deep-water channel (Blue Lagoon) and one that was relatively sheltered (RayBan). Growth rates of coral fragments placed onto nurseries were highly variable across taxa but generally higher at Blue Lagoon (2.1-10.8 cm2 month-1 over 12 months) compared to RayBan (0.6-6.6 cm2 month-1 over 9 months). Growth at Blue Lagoon was largely independent of season, except for Acropora tenuis and Acropora hyacinthus, where growth rates were 15-20% higher for December 2018-July 2019 ("warm season") compared to August-December 2018 ("cool season"). Survivorship across all 2,536 nursery fragments was ca. 80-100%, with some species exhibiting higher survivorship at Blue Lagoon (Acropora loripes, Porites cylindrica) and others at RayBan (A. hyacinthus, Montipora hispida). Parallel measurements of growth and survivorship were used to determine relative return-on-effort (RRE) scores as an integrated metric of "success" accounting for life history trade-offs, complementing the mutually exclusive assessment of growth or survivorship. RRE scores within sites (across species) were largely driven by growth, whereas RRE scores between sites were largely driven by survivorship. The initial nursery phase of coral propagation therefore appears useful to supplement coral material naturally available for stewardship of frequently visited Great Barrier Reef tourism (high-value) sites, but further assessment is needed to evaluate how well the growth rates and survival for nursery grown corals translate once material is outplanted.

Fig 1

(A) Location of sites on Opal Reef, Great Barrier Reef (Australia), including nursery sites RayBan (RB; 16°13'27"S 145°53'22"E) and Blue Lagoon (BL; 16°12'18"S 145°53'54"E) in relation to their location within the region and country. Delineation of the management zones for Opal Reef (see main text) is indicated by the dashed line. (B) The remotely sensed mean (± standard error, n = 3–4 per month) Sea Surface Temperatures (SST) for Opal Reef. Imposed onto the SST are points showing the time when measurements were taken for both RayBan and Blue Lagoon nurseries.

Fig 2

Comparisons of (A) Growth rates (areal extension; ΔG, cm² month^-1) measured over the full year (August 2018-July 2019) for the 9 coral taxa maintained at Blue Lagoon (BL) nursery: Acropora humilis (A.hum), Acropora hyacinthus (A.hya), Acropora loripes (A.lor), Acropora millepora (A.mill), Acropora intermedia (A.int), Acropora tenuis (A.ten), Montipora hispida (M.his), Porites cylindrica (P.cyl) Pocillopora cf. verrucosa (P.cf.ver). Box plots for ΔG show the interquartile range (representing 50% of data) and mean (horizontal line)–whiskers extend to the maximum and minimum values of data and excluding outliers (lie outside the 10th and 90th percentiles) (B) Mean (± standard error) ΔG of comparative species grown over a full year at BL versus at RayBan (RB; May 2018-February 2019) nurseries; (C) Mean (± standard error) ΔG of species grown at BL for data collection periods August-December 2018, “cool season” (Fig 1), versus December 2018-July 2019, “warm season” (as per S4 Table); (D) Mean (± standard error) for the RRE score of comparative species grown over a full year at BL versus RB (Table 1). Diagonal line in plots (b-d) signify 1:1.

Fig 3

Visual examples of coral growth (Acropora hyacinthus) on the Blue Lagoon coral nursery from (A) 18^th October 2018; and then (B) 26^th August 2019. Examples of coral growth taken from RayBan nurseries from (C) 22^nd October 2018; versus (D) 9^th September 2019. Photographs taken by John Edmonsdon.