Estimating the extent of urchin barrens and kelp forest loss in northeastern Aotearoa, New Zealand

Figures & data

Figure 1. Study locations on the northeastern coast of the North Island, Aotearoa New Zealand (A). The seven mapping locations are shown in pink and labelled. The thin grey outline is the extent of the two regional scale base maps used to estimate the extent of subtidal reef <30 m depth. An aerial photo taken at Maitai Bay showing typical zonation of dominant shallow subtidal reef habitats in the region (B), with shallow mixed forests from 0–2 m depth (C), urchin barrens from 2 to ∼10 m (D) and kelp forest from ∼10 to 15 m in this location (E), with patches of sand below.

Table 1. Management status and mapping information for locations mapped in northeastern New Zealand. The maximum depth of barrens in each location was used to delineate ‘shallow’ and ‘deep’ reef areas. Aerial imagery sources: Kerr and Grace collection contact the author, Oceans 2020, National Institute of Water and Atmospheric Research 2009. Oceans 20/20 Bay of Islands aerial photo colour mosaic. NIWA GIS data. https://www.linz.govt.nz/hydro/projects-programmes/ocean-survey-2020/about-ocean-survey-2020 *A Traditional Management Rahui (full protection) has been established since mapping was completed.

Location	Management status at time of mapping	Ground truthing surveys	Reef area mapped	Max. Depth of barrens (m)	Primary source and date of Aerial Imagery	Map reference
Maitai Bay	Fished*	2017–19	218 ha	16	Grace and Kerr 2003, 2009, Google Earth 2018	Supplementary material figure S1
Doubtless Bay	Fished	2005	2,766 ha	12	Grace and Kerr 2005	Supplementary material figure S2
Waewaetorea, Bay of Islands	Fished	2011–15	182 ha	12	BOI Ocean’s 20/20, 2010	Supplementary material figure S3
Maunganui Bay, Bay of Islands	Fished*	2012–16	37 ha	12	BOI Ocean’s 20/20, 2010	Supplementary material figure S4
Mimiwhangata	MPA allows recreational fishing	2003–4	1,141 ha	12	Grace and Kerr 2003	(Kerr and Grace 2005) & (SM figure S5)
Leigh	No-take MPA and adjacent fished area	2005–6	133 ha	10	Grace and Kerr, 2006	(Leleu et al. 2012) & (SM figure S6)
Tawharanui	No-take MPA and adjacent fished area	2005–6	78 ha	10	Grace and Kerr, 2006	Supplementary material figure S7

Figure 2. Simplified habitat maps showing dominant reef habitat types (<30 m depth) at each of the study locations: Doubtless Bay (A), Maitai Bay (B), Waewaetoria, Bay of Islands (C), Maunganui Bay, Bay of Islands (D), Mimiwhangata (E), Leigh (F) and Tāwharanui (G). Algal turfs and C. flexuosum forest habitats were relatively rare so were combined as ‘Minor reef habitats’ for these simplified maps (see Supplementary material for higher resolution detailed maps of each location). Dashed lines indicate marine protected area (MPA) boundaries. The MPA at Mimiwhangata is partially protected, whereas at Leigh and Tāwharanui the MPA’s are no-take marine reserves. The locations of historical imagery comparisons (Figure 3) are outlined by black squares in (B) and (E).

Table 2. Area of reef habitat types mapped across the seven study locations (A) and regional estimates of the extent of reef habitats (<30 m depth) on the mainland east coast of Northland, New Zealand (B). Shallow reef is defined as the area of reef within the depth limit that urchin barrens occur in each location (Table 1). The cover of urchin barrens is given for each location in (A) relative to the overall (Total) extent of reef (<30 m) and relative to the extent of shallow reef encompassing the maximum depth barrens occur. The regional extent of each habitat is estimated in two ways, based on the overall cover in mapped locations over the total reef area and when constrained to the shallow reef area. *Mimiwhangata is a marine park but allows recreational fishing.

A – Mapped Locations	Reef habitat types (ha)							Reef area (ha)		Urchin barren coverage (%)
Fished locations	Shallow mixed forest	Urchin barrens	Algal turfs	C. flexuosum forest	Shallow Kelp forest	Deep Kelpforest	Sponge Garden	Shallow	Total	Shallow	Total
Maitai Bay	15.2	49.4	0.0	0.0	74.0	79.7	0.0	138.7	218.3	35.7	22.6
Doubtless Bay	138.0	107.4	0.0	6.0	390.5	2,086.6	0.0	641.9	2,728.5	16.7	3.9
Waewaetorea	15.3	17.2	0.0	0.0	60.6	89.3	0.0	93.1	182.4	18.5	9.5
Maunganui Bay	4.2	1.7	0.0	0.0	20.2	11.2	0.0	26.1	37.3	6.5	4.5
Mimiwhangata*	71.2	242.6	0.0	6.0	177.1	644.7	0.0	496.9	1,141.6	48.8	21.2
Leigh	2.7	3.3	0.9	0.0	1.7	4.9	13.4	8.6	27.0	38.5	24.5
Tawharanui	8.6	10.2	0.0	0.0	7.6	2.2	0.0	26.4	28.6	38.4	35.5
Total area/Percentage	255.3	431.8	0.9	12.0	731.6	2,918.6	13.4	1,431.7	4,363.7	30.2	9.9
Reserve Locations
Leigh	27.5	1.2	16.2	0.0	37.2	52.4	1.2	82.0	135.6	1.4	0.9
Tawharanui	23.9	1.0	1.0	0.0	26.0	7.9	0	51.9	59.7	1.9	1.7
Total area/Percentage	51.4	2.2	17.1	0.0	63.2	60.2	1.2	133.9	195.3	1.6	1.1
B – Regional estimate (ha)	Reef Habitat types (ha)
Percentage of shallow reef	17.8%	30.2%	0.1%	0.8%	51.1%	–	–	Estimated reef area
Percentage of total reef	5.9%	9.9%	0.0%	0.3%	16.8%	66.9%	0.3%	Shallow	Total
Reef habitat area (<30 m)	1,805	3,053	7	85	5,172	20,633	95	10,122	30,849

Figure 3. Historical comparison of the extent of macroalgal forest and urchin barrens on shallow reef at a study site at Maitai Bay (A) and Mimiwhangata (B). In both study sites the mapped reefs extend to a maximum depth of ∼15 m. Note: the boundaries between the different macroalgal dominated habitats could not be distinguished in the historical imagery, so are combined into ‘Macroalgal forest’ for this comparison. Original images are converted to black and white for consistency (left) and comparison with associated habitat map (right).

Kerr VC, Grace RV. 2005. Intertidal and subtidal habitats of Mimiwhangata Marine Park and adjacent shelf. Department of Conservation Research and Development Series. 201:55.

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Leleu K, Remy-Zephir B, Grace R, Costello MJ. 2012. Mapping habitats in a marine reserve showed how a 30-year trophic cascade altered ecosystem structure. Biological Conservation. 155:193–201. doi:10.1016/j.biocon.2012.05.009.

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Data availability statement

An archive of the GIS resources involved in this research have been loaded into a DRYAD archive, https://doi.org/10.5061/dryad.8gtht76w3.