Submersible or ROV technology, although expensive, may be the only way to approach direct observations of aggregation and spawning in water below depths for SCUBA diving.
The only study that has used a submersible for observing spawning aggregations is that of Gilmore and Jones (1992) for deep-water groupers. They found their most productive observations were made when the submersible remained stationary on the bottom for at least an hour with unnecessary systems, such as external and internal lights and hydraulics, turned off. They remained on station for up to 7.5 hr and obtained unique observations in this manner. Low light level black and white video, which was more sensitive than their eyes, was used to watch behavioral interactions.
One of us (PLC) previously used a ROV to attempt to extend the duration of observations of a jewfish aggregation on a wooden boat wreck in relatively deep water (33 m, Colin 1994), but found the device was not especially useful. Despite the presence of up to 20 large groupers on a low profile debris field only 30 m in length, few fish were ever seen with the ROV. The ROV observations were unproductive, compared to diver observations, and on their own would have lead to a very distorted impression of fish abundance and activity. There may be some instances where a ROV is useful for observations, but, in general, any information obtained with such remote technology should be interpreted cautiously.