Oliver et al (2004) Monitoring bleaching

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of satellite images, aerial photos and nautical charts. Further details can be found in English et al. (1997) and Devantier et al. (1998). Sample size Never take just one sample from any area which you hope to draw conclusions about, or which you wish to compare with another area. If you have no idea of how variable the area is (how different one sample is likely to be from another in the same area), then 3 samples should be the absolute minimum, and 5 is likely to yield much more useful results. Areas which exhibit substantial differences between sampling sites, in the measured parameter, require larger numbers of samples. In general, more samples will yield a more precise result and a less ambiguous resutls . The optimal sampling size will depend both on how variable the area is, and on what magnitude of difference between areas you are hoping to detect (or how precise you wish your description of a single area to be). If your questions involve detecting differences at different spatial scales, or if you believe that the variability changes at different scales, then you should consider nested sampling, where replicate measurement are taken at replicate sites within replicate broader locations. A more comprehensive review of this topic can be found in Underwood (1997) and Andrew & Mapstone (1987). Fixed vs Random vs Haphazard sampling In order to ensure that the samples you monitor are representative, they should be randomly placed within the area of interest. Truly random sampling requires the use of random number tables (or a calculator which produces random numbers). Because this can be time consuming researchers often resort to “haphazard” or “arbitrary” sampling. Haphazard sampling means that the researcher places (or selects) samples without regard to any features on the reef, and without favouring any area or coral types. Haphazard sampling is not truly random, and can result in significant biases if an inexperienced person is involved in sampling corals for a conspicuous phenomenon such as bleaching. There is a strong temptation to over-sample bleached colonies in most situations. Efforts to reduce this type of bias should be used, either by using random number tables and allocation schemes, or by choosing a method whereby the researcher cannot see what the area which is to be selected looks like until after is has been selected. For instance, choosing sites from a boat without looking down at the reef is advisable. In placing quadrats or laying out lines, the diver should keep his eyes closed. Monitoring sites can also be established prior to a bleaching event to limit some sampling bias. If a series of measurement of the same area are to be made in order to track changes over time (increasing bleaching, increasing mortality, recovery) then consideration should be given to the use of fixed units (quadrats or transects) which are re-measured each time. These units are initially placed randomly but are then fixed for all other sampling times. Monitoring sites which are established prior to a bleaching event to limit some sampling bias. The alternative is to measure from a new set of randomly selected transects each time. There is some debate amongst experts regarding the use of fixed vs random samples for long-term monitoring. Here is a quick outline of the advantages and disadvantages of both. Table 2 Comparison of Fixed vs Random Transects Advantages Disadvantages Fixed • Can detect changes over time more precisely and thus • Requires fewer replicates to detect a specific level of change • Takes longer to set up each one with permanent markers • Takes time to relocated markers (especially in low visibility areas) • Markers need to be maintained and replaced (requires time and money) • Permanent markers may not be acceptable in MPAs or tourist sites Random • Much faster to set up and thus • Can achieve larger sample size for the same time in the water • No maintenance costs • No time needed to relocate markers • No unsightly markers left underwater • Much less precise estimates of changes over time 14
• The initially random selection of units can become less representative over time In general, if one of your principle objectives is to detect reasonably small changes over time then fixed samples are a good choice. However if the time required to relocated and resurvey fixed transects is more than 2-3 times greater than the time it takes to do survey a random site, then the advantage may lay with random transects (especially if the area is not highly variable). A good compromise might be to select a minimum number of fixed transects and then to supplement this with additional random transects. Fixed quadrats can be particularly useful if a primary objective requires the tracking of individual coral colonies over time. Pilot Studies An invaluable step in the development of a well resourced and rigorous monitoring program is to conduct a pilot study, where a small number of samples (at each spatial level if it is a nested design) is collected and used to calculate the variability of the area (variance) for each major variable to be monitored. This is then used to calculate with much more accuracy the number of samples needed to draw specific conclusions with a specified level of certainty. This last procedure, called Power Analysis, requires advanced statistical skills. Pilot studies may not be feasible if the decision to monitor is made after the onset of a bleaching event, or if it is likely be so short-lived that full sampling is needed immediately. However some estimate of variance can usually be used, either from previous bleaching events in the same area or other from other studies in other areas. Monitoring Procedures In theory, if we wish to know the percentage of living coral cover that is bleached in an area, then we should measure levels of bleaching and the total living surface area for every single coral colony within the area of general interest. Because this is not possible, a subset of these colonies (a sample) must instead be measured, with the intent that this subset is representative of the whole set of all corals, and thus that any conclusions drawn regarding this subset an accurate estimate of the whole set. Choosing a sample which adequately represents the total population for which you wish to draw conclusions about is a crucial step in the design of any monitoring program. It can be a complicated and sometimes highly technical process. If the monitoring program in question involves substantial investments of time and resources and/or if the conclusions which will be drawn form the program have important implications for reef management or socio-economic status of communities dependent on the reef, then it is worth seeking the assistance of someone with biostatistical training to help design the program. Poor allocation of samples can lead to ambiguous or erroneous results. We present here some general rules that can help to avoid the worst problems. However if at all possible, getting the final monitoring design reviewed by a third party with some formal training in sampling and monitoring design will be highly beneficial. In general it is recommended that any monitoring program include a hierarchy of special scales starting with broad regional assessments (questionnaires), then synoptic surveys and detailed rapid site assessments (manta tows, timed swims), followed by fine scale monitoring within specific areas (transects, quadrats) within these sites. Table 3 Attributes of various monitoring procedures. Procedure Advantages Disadvantages Questionnaires Inexpensive; can be widely circulate Can include retrospective assessments Can include a wide range of variables Data are usually based on subjective assessments, and subject to recollection error Reliability of information providers can be highly variable Considerable follow up needed to obtained completed forms Aerial Survey Covers huge areas in a short time Expensive; planes may not be available Requires low tides, fairly clear water and moderate to high bleaching Only overall bleaching data can be recorded and with fairly low precision; cannot reliably distinguish between some areas of dead coral and live coral, and high coral cover from low cover Manta Tow Covers large areas in short time Provides more reliable estimates of bleaching severity Best suited to reef edge and upper slope zones Can be dangerous in rough weather 15
Page 1 and 2: Draft A Global Protocol for Assessm
Page 3 and 4: Introduction Coral bleaching occurs
Page 5 and 6: This protocol is designed to assist
Page 7 and 8: and easily, even if only part of so
Page 9 and 10: which has very poor power to answer
Page 11 and 12: Monitoring for different scenarios
Page 13: Sites within a Location Locations w
Page 17 and 18: Recommended Manta Tows The manta to
Page 19 and 20: Anchor damage: Pictures: Comments:
Page 21 and 22: Buddy: Vessel: Depth: Replicate: Be
Page 23 and 24: Colonies can be tagged using large
Page 25 and 26: Appendices Appendix 1. Data Forms S
Page 27 and 28: WWF Bleaching Monitoring Program Ti
Page 29 and 30: major effect not only on coral cove
Page 31 and 32: the event. Electronic salinity logg
Page 33 and 34: mainly for landscape/seascape conse
Page 35 and 36: Table 10. Benthic codes Level 1 Lev

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