### BITB49 — Three Brothers
**CRD page:** [Three Brothers/Central Chagos Coral Surveys BITB38-BITB49 (Mar 2015)](https://coralreefdiagnostics.com/three-brotherscentral-chagos-20150321-to-0324)

**24 Mar 2015**
**GPS:** 6.11030°S, 71.65470°E
**Setting:** intermediate-exposure submerged lagoonal patch reef, inside the lagoon, 25 m
**Coral cover:** 26.8% (13 formal transects, D3–D4; **algal cover 40.6%**) · **published image bleaching:** the `Mean(%bleaching)` cell is blank for this site — no value exists, and none may be inferred
**Live hard coral cover across methods:** formal transects 26.8% · CoralNet points — none · Coralscapes masks — none. **No imagery exists for BITB49, so neither computer-vision product was ever generated and no multi-method comparison is possible here.** Where those products do exist elsewhere in Chagos they carry different sampling footprints and denominators from the transects and should not be averaged with them; at this site the transect figure stands alone.
**Temperature:** 30.2 °C · **Salinity:** 34.5
**Photo review:** **none — no photographs exist for this site anywhere.** **Anderson did not dive BITB49**; the Living Oceans Foundation expedition team completed the formal survey without him. No folder under `edited site photos for website` resolves to site number 49 under any prefix or casing, and the site returns zero photo directories and zero frames — no habitat-photo folder or image-derived product should be expected for it. **Everything below therefore rests on transect point counts alone, and reef condition at BITB49 cannot be assessed** — bleaching, tissue loss, disease, predation scarring, COTS, fish and macrofauna are all simply *unavailable*, and must never be read as zero or inferred from the neighbouring Three Brothers sites.

BITB49 is an algae-dominated, spatially heterogeneous lagoonal patch reef about 5 km south of BITB48, at 25 m inside the Three Brothers lagoon. Thirteen formal transects across two depth strata give **26.8% live coral against 40.6% algae — 83rd of 109 Chagos sites for coral cover and 55th of 109 for algal cover**: a low-coral, mid-algae site, well under the archipelago's coral mean of 37.9% and eighth of the ten Three Brothers sites (island mean 35.3%). Bare substrate is 18.8% and non-coral invertebrates 13.8%. The raw thirteen-transect mean is 25.51% ± 10.86 SD over a **10–46% range**, so as at its northern neighbour the site figure averages very unequal ground rather than describing it.

What separates BITB49 from BITB48 is diversity rather than abundance. Coral is spread across **26 nonzero genera — 46th of 109 sites for genus count**, above the archipelago's middle and four genera more than BITB48 on 20% less coral. All values below are **percent of total benthos, not percent of coral**, and the 26 genus columns sum to 26.81%, an exact match to the published cover figure. *Acropora* leads at **7.60% of the benthos (28.4% of the live coral present)**, then *Porites* **6.57% (24.5%)** and *Pocillopora* **3.83% (14.3%)**; those three hold about two-thirds of the coral between them. Behind them come *Montastrea* 1.81%, *Montipora* 1.29%, *Platygyra* 1.01%, *Favites* 0.82%, *Favia* 0.81%, *Echinopora* 0.76%, *Astreopora* 0.56% and *Leptastrea* 0.43%, with a fifteen-genus tail below 0.4% — *Goniastrea*, *Fungia*, *Cyphastrea*, *Pavona*, *Cantharellus*, *Stylophora*, *Psammocora*, *Acanthastrea*, *Echinophyllia*, *Mycedium*, *Symphyllia*, *Millepora*, *Gardineroseris*, *Lobophyllia* and *Leptoseris*. **No single genus exceeds 30% of summed coral cover**, and the indices bear that out: H′ = 2.16, Simpson = 0.83, evenness J′ = 0.66, effective diversity 8.63. This is a comparatively balanced assemblage sitting on comparatively little coral.

**The depth profile runs the opposite way to BITB48's, and it is the more interesting finding at this site: the deeper stratum carries more coral, more genera and less sediment.** D3 (9 transects) averages 23.42% coral against 43.73% algae; D4 (4 transects) averages **30.20% coral against 37.55% algae** — seven points more coral and six points less algae on the deeper ground, with genus count rising from 20 at D3 to 23 at D4. The compositional turnover is sharp. ***Acropora* collapses from 11.96% of the benthos at D3 to 3.25% at D4** — from half the shallow coral to a tenth of the deep — while ***Porites* doubles (4.24% → 8.90%), *Pocillopora* more than doubles (2.11% → 5.55%) and *Montastrea* rises thirty-fold, 0.11% → 3.50%**, from a trace to the third-ranked genus. *Platygyra* (0.31% → 1.70%), *Echinopora* (0.22% → 1.30%) and *Astreopora* (0.11% → 1.00%) follow the same direction. The algal budget flips with it: **sediment-laden turf falls steeply from 15.18% at D3 to 3.85% at D4**, and crustose coralline algae from 22.33% to 13.70%, while **macroalgae rises from 1.49% to 6.70% and erect algae from exactly zero to 7.20%**. Bare substrate climbs from 15.31% to 22.25% and non-coral invertebrates fall from 17.53% to 10.00%. **The shallow stratum is an *Acropora*-led patch reef under a CCA-and-sediment-turf floor; the deeper one is a mixed *Porites*/*Pocillopora*/*Montastrea* assemblage on cleaner, barer, more macroalgal ground.** The site-level genus order — *Acropora* first — describes only D3.

Site-wide, the 40.6% algal budget breaks down as **crustose coralline algae 18.02% (35th of 109 sites, upper third), sediment-laden turf 9.51%, plain turf 5.18% (93rd of 109, low quartile), macroalgae 4.09%, erect algae 3.60% and cyanobacteria 0.24%.** CCA is therefore 44% of all algae and plain turf only 13% — a reasonably well-grazed floor by Chagos standards, with the sediment signal concentrated at D3. Across the thirteen transects dead coral averages 2.95% (range 0–15%, the maximum on `BITB49_d3_T4`), and **recently dead coral reads exactly 0.00% on every transect**: whatever mortality is standing here is not recent. With no imagery, that cannot be independently checked.

<b>Assemblage composition.</b> Thirteen transects at two strata give 26.8% coral against 40.6% algae across 26 nonzero genera, led by *Acropora* (7.60% of benthos, 28.4% of coral), *Porites* (6.57%, 24.5%) and *Pocillopora* (3.83%, 14.3%), with no genus above 30% of the coral and an evenness of J′ = 0.66. The depth profile inverts the usual shallow-water pattern: the deeper D4 stratum carries more coral (30.2% vs 23.4%), more genera (23 vs 20) and far less sediment-laden turf (3.9% vs 15.2%) than D3, with *Acropora* giving way to *Porites*, *Pocillopora* and a thirty-fold rise in *Montastrea*. **There is no photographic record whatsoever, so none of this can be corroborated visually and no statement about the reef's condition — bleaching, mortality, disease, predation or fauna — is possible at BITB49.**

**Flags:** (1) **NO PHOTOGRAPHS EXIST FOR THIS SITE, AND ANDERSON DID NOT DIVE IT.** The LOF team ran the transects; zero photo directories resolve to site number 49 under any prefix or casing. There are consequently no CoralNet points, no Coralscapes masks, no multi-method comparison, and **no basis for any condition assessment**. Bleaching, disease, predation, COTS and fauna are unavailable — not zero — and must not be inferred from formal cover or from neighbouring Three Brothers sites. (2) **THE PUBLISHED `Mean(%bleaching)` CELL IS BLANK** for BITB49; the site carries no value in either CoralNet training run and appears in the `formal vs. informal comparison` sheet only as a placeholder. Corpus-wide the published bleaching figure is a **computer-vision product**, not an in-situ observation, with per-image human-vs-model agreement of r = 0.40 for bleached against r = 0.72 for live hard coral (`CHAGOS_MULTIMETHOD_README.md`) — but at this site there is no figure at all to caveat. (3) **THE SITE FIGURE IS THE UNWEIGHTED MEAN OF TWO STRATUM MEANS, NOT THE MEAN OF THIRTEEN TRANSECTS** — (23.42 + 30.20) / 2 = 26.81, whereas the raw thirteen-transect mean is 25.51%. **D4 rests on only four transects yet carries a full 50% of the headline number**, against nine transects at D3. The deep-stratum result — which is where this site's most interesting finding sits — therefore has thin replication behind it, and the D3→D4 contrasts described above should be read as indicative rather than established. (4) **THE SITE MEAN HIDES A NEARLY FIVEFOLD TRANSECT RANGE** (10–46%, SD 10.86 ≈ 43% of the mean); 26.8% is an average, not a uniform condition. (5) **POINT-COUNTS ARE MIXED AND UNWEIGHTED** — eight 100-point and five 500-point transects, 3,300 points total, pooled without effort weighting. (6) **Integrity checks pass:** the 26 genus columns sum to 26.81% against a published 26.8111%; the six algal categories sum to 40.64% against a published 40.6417%. Both reconcile within rounding, with no unexplained shortfall. (7) **Genus count of 26 sits 46th of 109** — this site's real strength — but corpus-wide, recorded richness scales with survey effort, and at 3,300 points BITB49 is mid-distribution; the count is a floor rather than a ceiling. (8) **Depth is given as 25 m at site level but only as strata D3–D4 in the depth table** — no per-stratum metre values exist, so the D3/D4 contrast cannot be tied to an absolute depth range. (9) **One transect (`BITB49_d3_T4`) carries 15% dead coral** against a site mean of 2.95%; **recently dead reads 0.00% everywhere.** Both are point-count facts with no imagery to interpret them.

**Benthic data:** [Download BITB49 benthic survey data (CSV)](../benthic_data/BITB49_benthic.csv) — scleractinian cover by genus with full names, plus substrate and algal categories (bare substrate, CCA, turf, sediment-laden turf, macroalgae, erect algae, non-coral invertebrates), from the LOF BIOT-1 survey (Mayfield & Dempsey 2026, *Oceans*, supplemental file S1).