Two further potential sources of error relate to consistency of dive length and diver vigilance. For the former an overview of log-book records suggests that these were generally comparable between the three years. Similarly, it is the authors opinion that diver vigilance would have been broadly comparable in all three years. All observations were made by experienced divers, all of whom had specific tasks to perform during most dives, but who nevertheless had sufficient time and interest to scan the surrounding waters at regular intervals. Although only semi-quantitative, this brief study does show the potential value of selected diver logbook records as a means of gathering historical data.
Divers regularly record sightings of large pelagics and other interesting species.
With strict assessment and control, this method could be used more widely to assess changes in abundance of some species where no other quantitative records are available. The great decrease in shark sightings by divers between the 1970s and 1996 is believed to reflect a real decrease in shark numbers. This is almost certainly due to fishing. These fishermen visit the Chagos during the rough season around Mauritius, i.e. in the middle of the year. They apparently target finfish, but must also catch some sharks.