One Fish, Two Fish: Evaluating an in Situ Visual Mark-Resighting Method to Assess the Abundance of Spawners at Fish Spawning Aggregation

I will present an in situ visual mark-resighting study design that can be used to estimate total abundance of spawners at an aggregation site. I demonstrate the methods using recent findings from the Grouper Moon program, a collaborative research program between Reef Environmental Education Foundation (REEF), Cayman Islands Department of Environment, aimed at documenting the success of management actions established in order to protect Nassau grouper, and endangered Caribbean reef fish. The mark-recapture method takes advantage of the high density and approachability of aggregating grouper by SCUBA divers in order to tag a subset of aggregating Nassau grouper, and subsequently generate surveys of the proportion of tagged individuals in discrete counts. These proportions are subsequently used to estimate total population size. For a variety of reasons, including minimizing harm to the animals (i.e., short and long term tag induced mortality or tagging related changes to reproductive success), we were required balance the trade-off between number of tagged individuals and accuracy of population estimates based on resightings data. I will present simulation methods used to identify trade-offs between number of individuals tagged and number of subsequent surveys required in order to meet an acceptable level of uncertainty in population estimates.