Before intensive human hunting, monk seals were found in dense colonies throughout the Caribbean, both on mainland coasts and offshore islands and atolls. Historical data describe 13 breeding colonies, but patterns in the data suggest that at least one colony escaped mention in the historical record. Three locations emerge as candidates for this unrecorded colony: the Little Bahama Bank; eastern Honduras; and eastern Venezuela. Large numbers of descriptions of seals in the water in the Little Bahama Bank and several distinct place names in eastern Honduras suggest the presence of colonies, and the distance between the colonies in Klein Curacao and Guyana make it probable that an intermediate colony existed in eastern Venezuela.
The most persistent monk seal populations were found on offshore atolls, far from human disturbance and in the centre of the range in the central–western Caribbean. The extinction of colonies followed a predictable pattern in time and space, with colonies far from the species' population centre having a significantly higher probability of extinction earlier in time (). This pattern can be explained by two factors. First, these colonies were on the edge of the species' range and therefore less likely to be repopulated if reduced by hunting. Second, the reef area in the eastern Caribbean is less than that in Central America and the western Caribbean islands, so colonies in the Lesser Antilles probably supported fewer individuals. Although colonies on the edge of the range were eliminated quickly, the persistence of those in the centre and on offshore atolls indicates that monk seals are resistant to moderate to intense levels of human disturbance, which suggests that proper protection has the potential to save the remaining Hawaiian and Mediterranean monk seal colonies from extinction.
The widespread presence of dense monk seal colonies and their prominent role in pristine reefs provide evidence that the entire reef community has suffered major declines in overall animal biomass. In the Caribbean, colonies were found on mainland coasts, islands and atolls close to productive coral reef communities, where high densities of fishes are known to have existed based on the historical data (e.g. Wallace 1955
). Estimates of the historical fish biomass suggest that historical reefs were several times more productive than those in the Caribbean today. These results support hypotheses of total ecosystem effects of historical overfishing (Jackson et al. 2001
; Pandolfi et al. 2003
), and suggest that fishing and hunting has reduced animal biomass so that a once abundant predator such as the monk seals could not survive on the fish resources that remain in depleted Caribbean reefs. Values derived from Ecopath
must be interpreted cautiously as the model does not respond dynamically and therefore cannot account for changes such as prey switching, which is a characteristic of omnivorous monk seals. Nevertheless, these results provide yet further evidence on the magnitude of decline that has occurred in fish populations. Observations of emaciation in the Hawaiian monk seal have caused speculation that intensive fishing has reduced the prey base for this species as well (Craig & Ragen 1999
). Hawaiian monk seal recovery may be limited by food availability, even at severely depleted population levels, which suggests that intensive overfishing has lowered the carrying capacity for large predators across coral reef ecosystems. Thus, successful recovery plans must include efforts to reduce pressure on overexploited fish stocks used by these marine predators.
The reef fish and invertebrate biomass required to support Caribbean monk seal populations exceeds that observed in the most pristine and protected reef ecosystems in the Caribbean, and the average Caribbean reef has less than 25% of the fishes found on historical reefs (; Newman et al. 2006
). Estimates of historical fish densities are more similar to those observed in the most remote Pacific coral reef atolls, a result which indicates that reefs in the Caribbean were as productive as pristine Pacific reefs today. The close agreement between historical analyses and modern empirical data from remote reefs (Friedlander & DeMartini 2002
, Sandin et al. 2008
, NOAA Fisheries, Pacific Islands Fisheries Science Center, Coral Reef Ecosystem 2002, 2004, unpublished data) suggests that pristine reef systems contain similar biomasses of reef fishes across ocean basins. The loss of productivity and dramatic change in overall biomass in Caribbean reef ecosystems underscores the continued need to rebuild fish populations, even in areas with the most protected and effective management regimes.