Low energy expenditure and resting behaviour of humpback whale mother-calf pairs highlights conservation importance of sheltered breeding areas

Abstract

Understanding the behaviour of humpback whale mother-calf pairs and the acoustic environment on their breeding grounds is fundamental to assessing the biological and ecological requirements needed to ensure a successful migration and survival of calves. Therefore, on a breeding/resting ground, Exmouth Gulf, Western Australia, we used animal-borne DTAGs to quantify the fine-scale behaviour and energetic expenditure of humpback whale mothers and calves, while sound recorders measured the acoustic environment. We show that: (i) lactating humpback whales keep their energy expenditure low by devoting a significant amount of time to rest, and their use of energy, inferred from respiration rates, is ~half than that of adults on their foraging grounds; (ii) lactating females mainly rest while stationary at shallow depths within reach of the hull of commercial ships, thus increasing the potential for ship strike collisions; (iii) the soundscape is dominated by biological sources; and (iv) even moderate increases of noise from vessels will decrease the communication range of humpback whales. Planned commercial infrastructure in Exmouth Gulf will cause a substantial increase in shipping traffic with the risk of ship strikes and acoustic disturbance potentially compromising energy reserves for the southern migration of humpback whales.

Figure 1

Study sites. (A) Exmouth Gulf, Western Australia: a known humpback whale breeding/resting area. (B) Godthaabsfjord and Disko Bay, Greenland part of the West Greenland humpback whale feeding ground. The location of whale tagging events and acoustics logger deployments in Exmouth Gulf (Stations 1–4) are depicted.

Figure 2

Behavioural budget of 25 whales: 16 lactating females (black); one adult male (red); and eight neonate calves (light green) on the Exmouth breeding/resting ground; and 17 adult whales (blue) on the West Greenland foraging ground. Behaviour was inferred based on DTAG accelerometers, focal follows and Minimum Specific Acceleration (MSA).

Figure 3

Typical dive profile of (A) a foraging humpback whale (ID: mn07_203a; Greenland), foraging lunges indicated by black dots, and (B) a resting lactating female (ID: mn13_265a) in Exmouth Gulf (B). Note lack of movement at depth as indicated by the MSA (in red); surface active behaviour (e.g. breach and tail slaps) indicated by blue stars.

Figure 4

(A–C) Percentage of time that humpback whales spent at <1 m, <5 m and <10 m during night and daytime for all whales (16 lactating females, one adult male, 8 neonate calves) on the breeding/resting ground (n = 25), all behavioural categories included, N indicates the number of whales included in the analysis, in the Total category depths from both day and night periods are included. (D) Depth at which adult humpback whales rest on breeding/resting grounds (n = 16 lactating females, n = 1 adult male), N indicates amount of minutes spent resting.

Figure 5

Respiration rates (respirations per min) of 25 whales (8 neonates, 16 lactating females, 1 adult male) on a breeding/resting ground in Exmouth, WA, and 17 adult whales on foraging grounds in Godthaabsfjord and Disko Bay, West Greenland. Red lines indicate standard deviation.

Figure 6

Ambient noise continuously recorded at four recording stations (station 1–4) for three days on Exmouth resting/breeding ground, quantified as third-octave sound pressure levels (TOLs) in dB re 1 μPa RMS (1-minute integration window). Water depth at each station is indicated in right bottom corners on upper panel plots. Upper panel (1–4a): recorded TOLs are shown as percentiles (5^th, median and 95^th) across 37 third-octave band frequencies (25 Hz–100 kHz). Self-noise of sound recorders is also plotted. Lower panel (1–4b): three third-octave bands (0.4, 1.25 and 10 kHz) with high ambient noise levels are shown in the time domain. Peak events above the threshold (the 95^th percentile (p95) +6 dB) at each station have been audited and marked according to the dominating sound source; ‘w’ = humpback whale, ‘d’ = delphinid, ‘v’ = vessel, ‘ss’ = snapping shrimp, ‘a’ = other anthropogenic noise. Two events (‘whale’ and ‘vessel’) are also shown as spectrograms (5b,c) and were further analysed into percentiles (5a; 1-second integration window).