Many spawning aggregations have been fished beyond the point of sustainability, leading to increased calls for protection through seasonal and/or site-specific fishery closures. Once a closure has been put in place, monitoring the aggregation is imperative in order to learn whether protections lead to the recovery of populations. Current methods for monitoring the status of spawning aggregations rely on simple counts, usually combined with capturing a subset of the fish to collect data such as length and weight. Handling fish during the spawning aggregation can be stressful for the fish, which could ultimately lead to decreased spawning success, increased susceptibility to predators, or increased mortality through capture trauma or infection. Here we present a novel method for monitoring fish on a spawning aggregation that does not require the capture and handling of the fish. We show that reliable length-distribution data can be collected by divers using a video-based system with parallel lasers calibrated to a specific distance apart. Annual changes in size distribution can be used as one measure of the health of the population, because the technique can detect recruitment of new individuals into the spawning population. In addition to tracking size distribution trends over time, the length distribution information can be combined with a length-weight regression and an estimate of total number present in order to accurately estimate spawning biomass. We discuss the validation and application of this method with a spawning aggregation of Nassau grouper, Epinephelus striatus, in the Cayman Islands.