combined was 2-6 times higher in shallow (-10m to -20m) compared to deep water 

 (-35m) (Table 1). This reflects, in part, depth-related changes in the proportion 

 of small colonies in the populations, since large corals are much less likely to 

 be killed than small (e.g. Connell 1973, Loya 1976, Hughes and Jackson 1980). How- 

 ever, within-size class mortality rates were also invariably higher in shallow 

 water, particularly for the largest corals. Not a single colony out of 32 greater 

 than 200cm 2 was killed at -35m in three years, compared to the loss of 8 out of 39 

 colonies of the same size at -10m to -20m. 



Further more, total tissue losses (from both injuries and whole colony mortal- 

 ity) were greater in shallow water (Table 1). The relative importance of injuries 

 and whole mortality also varied with depth. At -35m fully 80% of the coral tissue 

 destroyed was due to injuries, particularly to large colonies, while in shallow 

 populations losses from injuries and whole mortality were almost equal. 



The higher rates of colony and tissue losses in shallow water were compensated 

 somewhat by enhanced amounts of larval recruitment, which was more than twice the 

 levels of the deep (-35m) quadrats (Table 1). 



Table 1. Various aspects of coral population dynamics as a function of depth 

 for the three species common at all depths. (A) Percentage mortality of coral 

 colonies monitored photographically from 1977 to 1980. The total number of 

 1977 colonies was 425 shallow (-10m to -20m), and 301 deep (-35m). (B) Total 

 percentage area of coral tissue destroyed by injuries and whole colony mortality. 

 (C) Number of new recruits observed settling within the permanent quadrats (equal 

 areas shallow and deep) from 1977 to 1980. 



colonies of corals at -35m and -55m exceeded the extension rates of specimens in 

 shallow water (Fig. 1), Deeper foliaceous corals were less likely to have their 

 growth interrupted by partial mortality, and also had thinner skeletons (Hughes 

 1982), i.e., they could increase their surface area with a smaller addition of cal- 

 cium carbonate, so that they often grew surprisingly fast relative to shallow 

 foliaceous corals. 



DISCUSSION 



Several previous studies have shown only a very weak relationship between 

 lateral coral growth and depth because of considerable within-depth variation 

 between colonies (see Highsmith 1979 for review). This variability is often 

 artificially eliminated by collecting biased samples of undamaged colonies, which 

 probably grow faster than colonies that have recently suffered injuries. Coral 

 injuries were found in the present study to be common events; of 883 colonies 



18 



