For "key" species in marine sanctuaries (e.g., keystone or foundation species, indicator species and focal species) measures of condition and health can be important for determining the likelihood that these species will persist or recover and continue to provide vital ecosystem functions and services. Measures of condition may include growth rates, fecundity, recruitment, age-specific survival, tissue contaminant levels, pathologies (disease incidence tumors, deformities), injuries, and the presence and abundance of critical symbionts or parasite loads.
Key species that influence the health and structure of their ecosystem are often referred to as "keystone" and "foundation" species. "Keystone" species are those on which the vitality of a large number of other species depends; they are pillars of community stability. Their contribution to ecosystem function is disproportionate to their numerical abundance or biomass (e.g., corals, sea otters, top level predators). "Foundation" species such as krill, kelp and forage fish (e.g., anchovies and sardines) also control much of the structure of a community and influence the abundance and biomass of many other species. They may exhibit similar control over ecosystems as keystone species, but their high abundance distinguishes them and determines their level of influence.
Changes in the condition or abundance of keystone or foundation species due to health-related issues can transform ecosystem structure through dramatic increases or decreases in production, or changes in abundance of dependent species. While the condition of most of these important species has been reported to be relatively stable in sanctuaries, some noted historical decreases have led to broad ecosystem-level changes, including decreases of cod and river herring at Stellwagen Bank, krill along the West Coast, monk seals in the Hawaiian Islands, and Diadema in Florida Keys. Some sanctuaries have indicated significant successes relative to keystone and foundation species, including increasing populations of humpback whales at Hawaiian Islands Humpback Whale National Marine Sanctuary, giant sea bass and lobsters at Channel Islands, and predatory fish and corals at Papahānaumokuākea Marine National Monument and Florida Keys National Marine Sanctuart.
Monitoring data to evaluate the impact of animal health and protection vary in quantity and quality across the system. In most cases, sites rely heavily on support and information from partnering agencies to address essential monitoring questions regarding animal health, as well as for management actions.