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5. Using coral size-frequency data
to estimate the strength and magnitude of recent hard coral recruitment
The methodologies outlined below are applied to quantitatively assess recent rates of hard coral recruitment – specifically the levels of recruitment that occurred some 12 and 24 months prior to sampling. For the purposes of assessing the magnitude of recent coral recruitment events, corals in the 2-5 cm diameter range are assumed to be at least 2 years of age while 1 year-old corals are considered to be in the 1-2 cm size range. The assumptions about the average growth rates of a variety of scleractinian corals are based on recent field (Wallace 1985, Wallace et. al. 1986, van Moorsel 1988) and laboratory studies (Baird, pers. com.) of coral settlement and growth rates that have provided some useful estimates of actual in situ growth rates (namely for members of the fast-growing Genus Acropora). Wallace (1985, 1999) showed that, on the Great Barrier Reef (GBR), juvenile Acropora corals typically reached a maximum size of 1.0 - 1.5 cm diameter during their first year of growth with average sizes of between 2.0 and 5.0 centimetres being reached during the second year following settlement. There is some information to suggest that branching acroporids may, under highly favourable conditions, be able to achieve slightly faster growth rates (Tomascik et al. 1996). However, as there are no reliable measurements of growth rates available for corals in the Seychelles region, the growth estimates for the GBR are considered the most useful for the purposes of coral recruitment studies. Reliable estimates of natural growth rates are largely unavailable for members of other coral families and genera. However, it is generally assumed that massive corals, such as the members of the Families Faviidae and Mussidae, show significantly slower growth rates at all ages. |
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Table 1 provides a useful overview of
size-at-probable age estimates that may be applied to the formal
evaluation of recent rates of hard coral recruitment. However, it should
be noted that these growth estimates apply mostly to branching coral
species classified as 'broadcast spawners' - that is corals that release
egg and sperm into the water column for external fertilisation. 'Broadcast
spawners' typically participate in the annual mass coral spawning event
which, in the southern hemisphere, occurs around November and December
each year. Depending on larval life span, the subsequent settlement period
for coral larvae is likely to last from about November through to
February. In contrast, 'brooders' - corals that are able to release viable
larvae throughout much of the year have a significantly longer potential
recruitment period. As a result of this prolonged reproductive activity,
size-at-age relationships are much more difficult to determine.
Consequently, age estimates for brooding corals are likely to be far less
accurate than those calculated for 'broadcast spawners'.
Table 1: Size-at-probable age estimates (‘pseudo-cohorts’) for branching scleractinian hard corals on tropical Indo-Pacific reefs.
5.1 Estimating recruitment rates of 1 year-old hard corals (1-2 cm size class) using benthic quadrate sampling Coral recruit count as well as damage category data are collected using haphazardly-placed 1 m2 benthic quadrates. Individual coral recruits located within the quadrates are assigned to one of two size classes (1-2 or 2-5 cm size class) as well as to one of three categorical classes – branching corals of the Genus Acropora (ACB), branching corals of the Genus Pocillopora (PCB) whilst all massive, encrusting and fungid corals (MEF) are pooled into a single category of other corals. For quantifying the effects of grazing organisms on newly settled coral recruits, all small corals are further assigned to one of three possible damage categories – NO apparent skeletal damage (OK), skeletal damage consistent with sea urchin grazing activities (SU) or skeletal damage consistent with feeding by grazing reef fishes (F). Advanced mixed-model sampling designs may be used to simultaneously investigate patterns of scleractinian hard coral recruitment at a variety of spatial scales and / or across a range of environmental gradients. For example, as part of the Seychelles GEF Coral Monitoring Program, coral recruits are sampled in two distinct and locally predominant reef habitat types – a carbonate reef habitat (typically fringing reefs) and a granitic reef habitat characterised by extensive granitic outcrops and boulders. Within each of these two main habitat types the survey samples equal numbers of 1 m2 benthic quadrates across two specified substratum types – a structurally complex and three-dimensional (3D) substratum typically made up of remnant stands of dead standing corals and an essentially flat, two-dimensional (2D) type composed of either carbonate or granitic rocks and boulders. Furthermore, quadrates are sampled across two distinct depth zones – a shallow zone between 1.5 and 5 m depth and a deeper zone between 5 and 10 m depth. At each individual reef site, a minimum of 20 quadrates are sampled – 10 within each of the two depth zones. |
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| Last updated September 21, 2003 |
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