Locomotor behaviours and respiratory pattern of the Mediterranean fin whale (Balaenoptera physalus)

Abstract

Twenty-four Mediterranean fin whales were tracked in open sea with a method based on the assessment of the animal differential position in respect of the observer's absolute position aboard a vessel, with the concomitant recording of the respiratory activity. Short distance video recording was also performed in two whales, permitting the simultaneous determination of single breath expiratory (T _E) and inspiratory (T _I) durations. In the 24 whales swimming at an average velocity of 1.39 (0.47) m·s⁻¹ [mean (SD), range: 0.62–2.44 m·s⁻¹], 2068 breaths organized in 477 respiratory cycles were observed. Each cycle entailed a prolonged apnoea dive phase [225 (91) s, T _dive) followed by a period near the surface [62 (28) s, surfacing], during which a series of breaths [4.6 (1.8)] was performed at short intervals. On the basis of track length and swimming velocity, two groups of animals were devised differing for convolution of the course (p<0.001), extension of ranging territory (p<0.01) and horizontal swimming velocity (p<0.05), which may represent two distinct behaviours. A possibly general mechanism of control of breathing in cetaceans was found, consistent with a model of constant tidal volume and variable respiratory frequency. Coherently with this model, T _E was independent of T _I or T _dive, in line with a passive expiration, while T _I appeared to be negatively correlated with T _dive (p<0.05), otherwise suggesting, similarly with terrestrial mammals, a significant role of hypercapnic stimulation. The estimated O₂ consumption of about 150 l·min⁻¹ is in line with the general allometric regression for mammals and corresponds to an energetic expenditure of 85–95 kJ·kg⁻¹·day⁻¹.

Appendix

The net distance covered by the whale (W _t(0–1)) and its average swimming velocity (v _t(0–1)) during the time interval (t _(0–1)) of two consecutive measurements was calculated on the basis of the simultaneous determination of the ship's position in metric coordinates (by means of a GPS device) and of the whale distance (d) and azimuth (α) relative to the ship [by means of a compass integrated RADAR device or of a Laser Range Finder (LRF) binoculars], as it is also graphically expressed in Fig. 9. Thus, assuming a negligible stream:

Fig. 9.

Diagrammatic geometrical representation of the elements used by an observer situated on ship (S) for two consecutive determinations of whale (W) position (at time t ₀ and t ₁), based on whale distance (d) and azimuth (α). See Appendix for further details

$$ W_{{\rm{t}}\left( {0 - 1} \right)} = \surd \left[ {W\Delta {\rm{lat}}_{{\rm{t}}\left( {0 - 1} \right)}^{\rm{2}} + W\Delta {\rm{long}}_{{\rm{t}}\left( {0 - 1} \right)} ^2 } \right] $$

where WΔlat_t(0–1) and WΔlong_t(0–1) represent the metric difference in whale latitude and longitude, respectively, and occurred in the interval t _(0–1). Alternatively, these differences can be expressed as:

$$ {\eqalign{ & W\Delta {\rm{lat}}_{{{\rm{t}}{\left( {0 - 1} \right)}}} = {\left| {S\Delta {\rm{lat}}_{{{\rm{t}}(0 - 1)}} + SW\Delta {\rm{lat}}_{{{\rm{t}}(1)}} - SW\Delta {\rm{lat}}_{{{\rm{t}}{\left( 0 \right)}}} } \right|}{\rm{ and}} \cr & W\Delta {\rm{long}}_{{{\rm{t}}{\left( {0 - 1} \right)}}} = {\left| {S\Delta {\rm{long}}_{{{\rm{t}}{\left( {0 - 1} \right)}}} + SW\Delta {\rm{long}}_{{{\rm{t}}{\left( 1 \right)}}} - SW\Delta {\rm{long}}_{{{\rm{t}}{\left( 0 \right)}}} } \right|} \cr} } $$

where SΔlat_t(0–1) and SΔlong_t(0–1) are the metric variation of ship latitude and longitude, respectively, which occurred in the time interval t _(0–1) and are calculated from GPS data, while SWΔlat_t(0), SWΔlong_t(0), SWΔlat_t(1), and SWΔlong_t(1) represent the difference in latitude and longitude between the ship and the whale at the time t ₍₀₎ and t ₍₁₎, respectively. Furthermore, at the time n of any measurement, the difference in latitude and longitude between the ship and the whale is given by:

$$ {\eqalign{ & SW\Delta {\rm{lat}}_{{{\rm{t}}{\left( n \right)}}} = d_{{{\rm{t}}{\left( n \right)}}} \cos \alpha _{{{\rm{t}}{\left( n \right)}}} {\rm{ and}} \cr & SW\Delta {\rm{long}}_{{{\rm{t}}{\left( n \right)}}} = d_{{{\rm{t}}{\left( n \right)}}} {\rm{sen}}\;\alpha _{{{\rm{t}}{\left( n \right)}}} . \cr} } $$

Whale average swimming velocity during the interval t _(0–1) of two measurements was thus determined as:

$$ {v_{{{\rm{t}}{\left( {0 - 1} \right)}}} = {W_{{{\rm{t}}{\left( {0 - 1} \right)}}} } \mathord{\left/ {\vphantom {{W_{{{\rm{t}}{\left( {0 - 1} \right)}}} } {\Delta t_{{{\left( {0 - 1} \right)}}} }}} \right. \kern-\nulldelimiterspace} {\Delta t_{{{\left( {0 - 1} \right)}}} }}. $$

Since the height of the observers above the water surface was negligible in comparison with the distance of the animals, it was not taken into account in the algorithm.