Minimizing Adverse Impacts On Cetaceans
Cetaceans are carnivorous and finned marine mammals including whales and dolphins. It is important to track changes in their numbers, density, and distribution over time to understand the potential impacts of humans and environmental changes on their populations. Understanding these impacts could allow for various interventions to protect marine mammals, especially endangered species.
In this study, published in Deep-Sea Research II, the authors evaluated the density and distribution of cetaceans off the coast of southern California. They used sighting data collected by the California Cooperative Oceanic Fisheries Investigations on thirty-seven cruises between 2004 and 2013. Two trained observers used 7×50 binoculars to systematically record data on all cetaceans encountered in daylight, including species, group size, behavior, relative angle from the bow, ship’s position, and sighting cue. The authors’ aim was to estimate seasonal and yearly trends in the six species most often sighted. A statistical model was then developed to predict population trends.
The analysis may be useful for researchers and marine conservationists for several reasons. First, the study shows that the density of short-beaked common dolphins has not changed overall in the last ten years, unlike previous studies that suggested a decrease. The authors thus establish the long-term stability in density of this species, and show that annual variations in density can sometimes be misinterpreted without long-term monitoring. Second, the study suggests that the ten-year decrease in the density of Pacific white-sided dolphins is probably not due only to variations in climate, but rather due to multiple factors. These findings indicate that continuous, long-term monitoring can provide useful information that inconsistent or short-term monitoring cannot.
The authors suggest that future studies examine the relationship between cetacean density and various habitat characteristics to better understand the variety of factors likely affecting changes in density. They indicate that their data are currently being combined with data from other studies eventually to create models that can forecast “near real-time marine mammal distributions off southern California.” They propose that, moving forward, such work be used to inform operations of the U.S. Navy in an attempt to minimize any adverse effects on marine animals in the region.
[Contributed by Stella Bang]