Comparison of soundscape contributors between two neighboring southern right whale nursing areas along the South African coast

Abstract

Southern right whales (Eubalaena australis) were absent along the Eastern Cape coast, South Africa, for more than a century after overexploitation by land-based whalers in the early 1800s. Currently, mother-calf pairs in particular are again frequently sighted in this area, raising concerns about the effect of recent port developments on their occurrence in nursing areas. Soundscape contributors and sound levels of two neighboring nursing bays, St. Francis Bay (one recreational port) and Algoa Bay (two commercial ports), were compared to address this concern. Wind, southern right whales, fish, snapping shrimp, and close vessels were contributing to noise levels in both bays. Additional sound sources in Algoa Bay were surf-zone noise, dolphins, and distant vessels. The overall sound levels in Algoa Bay were 5-25 dB re 1 μPa2/Hz higher. However, mother-calf pair sightings per unit effort was 0.99 (St. Francis Bay) against 1.11 (Algoa Bay), resulting in a lack of evidence that mothers prefer quieter bays or bays with less frequent anthropogenic sound sources to nurse their calves. Further acoustical data is required to assess the importance and range of each sound source within the soundscape, whereas behavioral observations may assist to assess differences in mother-calf behavior within different acoustic environments.