Abstract
Passive acoustic monitoring was used to document the presence of singing humpback whales off the coast of Northern Angola, and opportunistically test for the effect of seismic survey activity in the vicinity on the number of singing whales. Two Marine Autonomous Recording Units (MARUs) were deployed between March and December 2008 in the offshore environment. Song was first heard in mid June and continued through the remaining duration of the study. Seismic survey activity was heard regularly during two separate periods, consistently throughout July and intermittently in mid-October/November. Numbers of singers were counted during the first ten minutes of every hour for the period from 24 May to 1 December, and Generalized Additive Mixed Models (GAMMs) were used to assess the effect of survey day (seasonality), hour (diel variation), moon phase and received levels of seismic survey pulses (measured from a single pulse during each ten-minute sampled period) on singer number. Application of GAMMs indicated significant seasonal variation, which was the most pronounced effect when assessing the full dataset across the entire season (p<0.001); however seasonality almost entirely dropped out of top-ranked models when applied to a reduced dataset during the July period of seismic survey activity. Diel variation was significant in both the full and reduced datasets (from p<0.01 to p<0.05) and often included in the top-ranked models. The number of singers significantly decreased with increasing received level of seismic survey pulses (from p<0.01 to p<0.05); this explanatory variable was included among the top ranked models for one MARU in the full dataset and both MARUs in the reduced dataset. This suggests that the breeding display of humpback whales is disrupted by seismic survey activity, and thus merits further attention and study, and potentially conservation action in the case of sensitive breeding populations.
Highlights
The coasts and pelagic regions of Africa support a diverse assemblage of marine life, including populations of large whales in various states of recovery from commercial whaling
To investigate the potential impact of seismic survey pulse activity we considered two variables, namely Peak Power Received Level (RL), and Power Score, a categorical variable 0–5 where each category corresponds to intervals of increasing Peak Power received levels (0 = Seismic survey pulse not detected during the 10 min sample, 1 = 65–75 dB, 2 = 75–85 dB, 3 = 85–95 dB, 4 = 95–105 dB and 5 = .105 dB)
Humpback whale song was first detected on the Marine Autonomous Recording Units (MARUs) on 9 June 2008, increased steadily throughout June and into July, was constantly present but fluctuated in a non-linear manner until November, and steadily decreased in occurrence until recording ended in early December (Figure 1)
Summary
The coasts and pelagic regions of Africa support a diverse assemblage of marine life, including populations of large whales in various states of recovery from commercial whaling Cetaceans in these regions are among the most poorly understood and documented on the globe, with many open questions regarding species presence, distribution, timing of migrations and importance of habitat for critical life functions. The region is a focal area for new offshore development activities that will generate underwater noise, and includes important breeding, feeding and migratory habitats for several cetacean species [2,3]. It is a necessarily important research frontier in the effort to understand, plan for, and mitigate anthropogenic acoustic impact on cetaceans. Since song is an important breeding display, the impact of anthropogenic activities, either by disturbing singing males or acoustically masking song display, can have potentially negative effects on the reproductive success of individuals and populations
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