Mass cetacean strandings - a plea for empiricism

The recent series of mass cetacean strandings in the stranding-prone regions of Australia and New Zealand (BBC News 2004) and the ensuing speculation regarding their cause demonstrate that debates surrounding this phenomenon continue to be of issue. As most interested in these debates are aware, hypotheses addressing their causes range from the more biologically plausible— including climate and oceanographic variation (Mignucci- Giannoni et al. 2000; Evans et al. 2005), navigational errors arising from particular magnetic configurations (Walker et al. 1992; Brabyn & Frew 1994) or bathymetric and ocean current features (Brabyn & McLean 1992), and anthropogenic noise and sonar interference (Balcomb & Claridge 2001; Madsen et al. 2002)—to the less-supported, including sensory stimulation (Mawson 1978), distraction (Wood 1979), regression to instinctive behaviors (Cordes 1982), and even the suggestion that earthquakes are responsible (ABC News 2004).

Rather than supplying more fuel for conjecture in light of severe shortages of convincing data, we believe it is more constructive to provide a scientific framework for testing hypotheses that seek to explain the patterns observed and mechanisms responsible for strandings. The most reliable and long-term data are the time series of stranding events. Recently, our research group, in collaboration with a large multidisciplinary team, examined the periodicity of these events in the southeastern region of Australia, including one of the world’s stranding hotspots—the southern island state of Tasmania (Evans et al. 2005). Substantial records of strandings are available in this region from the 1920s, and in some years there were >20 mass-stranding events recorded involving sometimes >300 individuals per event. Our analyses showed that not only were there clear cycles in the number of stranding events of approximately 12–14 years’ periodicity, but that these pulses were related to measurable changes in climate patterns. We found that increases in zonal and meridional winds resulting in colder and presumably nutrient-rich waters moving closer to southern Australian landmasses were good predictors of increases in stranding frequency.