A Systematic Review of Changes in Marine Mammal Health in North America, 1972-2012: The Need for a Novel Integrated Approach

Abstract

Marine mammals are often cited as "sentinels of ocean health" yet accessible, synthesized data on their health changes that could effectively warn of ocean health changes are rare. The objectives of this study were to 1) perform a systematic review of published cases of marine mammal disease to determine spatial and temporal trends in disease from 1972-2012, including changes in regions and taxa affected and specific causes; and 2) compare numbers of published cases of neoplasia with known, hospital-based neoplasia records to explore the causes of discrepancy between numbers of published cases and true disease trends. Peer-reviewed literature was compiled, and data were collected from The Marine Mammal Center database in Sausalito, California for comparison of numbers of neoplasia cases. Toxicoses from harmful algal blooms appear to be increasing. Viral epidemics are most common along the Atlantic U.S. coastline, while bacterial epidemics, especially leptospirosis, are most common along the Pacific coast. Certain protozoal and fungal zoonoses appear to be emerging, such as Toxoplasma gondii in southern sea otters in California, and Cryptococcus gattii in cetaceans in the Pacific Northwest. Disease reports were most common from California where pinniped populations are large, but increased effort also occurs. Anthropogenic trauma remains a large threat to marine mammal health, through direct mortality and indirect chronic disease. Neoplasia cases were under-reported from 2003-2012 when compared to true number of cases, and over-reported in several years due to case duplication. Peer-reviewed literature greatly underestimates the true magnitude of disease in marine mammals as it focuses on novel findings, fails to reflect etiology of multifactorial diseases, rarely reports prevalence rather than simple numbers of cases, and is typically presented years after a disease first occurs. Thus literature cannot guide management actions adequately, nor inform indices of ocean health. A real-time, nationally centralized system for reporting marine mammal disease data is needed to be able to understand how marine mammal diseases are changing with ecosystem changes, and before these animals can truly be considered 'sentinels of ocean health'.

Fig 1. Process for selection of publications included in the systematic review.

Fig 2. Proportion of calculated cases extracted from published reports on marine mammal disease for each disease category and eight coastal regions of the United States (U.S.) and Canada.

Symbol sizes represent the cumulative number of calculated cases per region that were associated with clinical disease, which ranged from 425 calculated cases for Hawaii to 16,275 calculated cases for California. Map created using ArcMap 9.3 (ESRI, Redlands, CA).

Fig 3. Annual number of (a) published reports on marine mammal disease and (b) calculated cases extracted from these reports for the United States and Canada, 1972–2012.

When more than one disease process contributed to the cause of morbidity for an individual animal, it was assigned to more than one category and counted multiple times as distinct calculated cases.

Fig 4. Annual number of calculated cases by disease category and region extracted from published reports on marine mammal disease.

For each disease category, regions representing less than 10% of the cumulative number of calculated cases were grouped (other). CA = California, GoM = Gulf of Mexico, NW = Pacific Northwest, AK = Alaska, NE = Northeast U.S., and SE = Southeast U.S.

Fig 5. Annual number of (a) calculated cases of neoplasia extracted from published reports on marine mammal disease and (b) individual California sea lions (Zalophus californianus) with urogenital carcinoma extracted from The Marine Mammal Center dataset.

All calculated cases for California from 1979–2003 were from California sea lions with urogenital carcinoma originating from TMMC’s dataset, except for one case per year in 1984, 2000, and 2001.

Fig 6. Annual number of calculated cases of trauma extracted from published reports on marine mammal disease.

Source of trauma was designated as anthropogenic or non-anthropogenic/unknown.