all have returned to the south. For both north and south-bound migrations the major portions of each curve for the two areas are approximately parallel and therefore follow similar sequences, but with different commencing dates.

The actual dates for either north or south-bound migrations cannot be compared closely as the data for the Whangamumu apply to different seasons from those recorded at Centre Island, and it has already been shown from Cook Strait records (Fig. 4) that the dates for the passage of different proportions of the humpback group vary considerably from season to season. However, this should not affect the validity of a comparison between the periods required for the overall cycle of south to north and north to south migrations past each of the two localities. From Fig. 6 it can be seen that the cycle from commencement to termination takes approximately eight weeks longer at Centre Island than at Whangamumu, but the interval between the mid-points of each of the curves is rather less. The actual dates of peaks calculated from the daily returns (as previously described for Cook Strait seasons) are June 15 and November 8 for Centre Island, and July 12 and October 20 for Whangamumu. Hence this cycle takes approximately 20 weeks at Centre Island and 14 weeks at Whangamumu.

The difference of six weeks should represent the time required for the main peak of the humpback migration to pass through 22° latitude made up by the 11° latitude northwards from Centre Island to Whangamumu and later back southwards through the same distance on the return passage. This indicates a relatively slow rate of progression averaging less than four degrees latitude or about 220 nautical miles per week in a direct north and south line. The cruising rate of humpbacks on passage observed by Chittleborough (1953) averaged 4–5 knots, and the writer has had many experiences when whale marking in Tonga from a 5-knot launch, of maintaining the same relative distance from a humpback or gaining on it only very slowly, so that the whale's speed agreed with Chittleborough's data. On the other hand, some of the humpbacks in Tongan waters could be seen circling slowly or stopping in the same general area for hours on end, and so long as they were undisturbed the forward progression in any one direction was slight. Similar behaviour has already been described for humpbacks feeding in western Foveaux Strait, and movements of this type have usually been regarded as a feature of their behaviour only after the breeding or feeding areas had been reached. The great difference in rate between 4–5 knots and little more than 1 knot in the overall rate of advance described above, indicates that there are frequently periods of rest and random movements on quite a large scale in the course of their main migratory route.

An interesting capture of a humpback at Tangalooma only six days after it was marked about 500 miles away at St. George's Head, New South Wales (Robins, 1955) shows that humpbacks sometimes travel for considerable distances at relatively high speed. However, the present data indicate that comparable sustained speed is not typical of the main group of migrating humpbacks which passes New Zealand.

The period of peak concentration at the breeding grounds north of New Zealand can be inferred from the Centre Island and Whangamumu data. The latter shows that the last of the main northbound group leave New Zealand in late August, and in most seasons there is a period of nearly four weeks before the first of the returning south-bound humpbacks are seen. The peak of the concentration at the northern limits probably occurs about midway between the dates of departure and return or between the peaks of the north-bound and south-bound migrations past New Zealand. For the Whangamumu data the former date is September 7 and for Centre Island during seasons other than those used in the Whangamumu records, it is September 9, but as the dates midway between the peaks of the two migration streams past Whangamumu and Centre Island are August 30 and 26 respectively, the beginning of September can be taken as the approximate time of the peak concentration at the breeding grounds. At Point Cloates (lat. 20° 30′ S.), West Australia,

Chittleborough (1953) found almost equal numbers of whales moving northward and southward close to August 24. Presumably the peak concentration is then at the northern point of the migration with equal proportions coming to or leaving that point. There was some overlap of north-bound and south-bound whales at the latitude of Point Cloates for approximately 10 weeks, whereas it is rare although not unknown for south-bound whales to reach New Zealand latitudes before the last of the north-bound whales has departed.

The tropical regions north of New Zealand already known as centres of humpback concentrations are Tongan waters and the Coral Sea, which both extend north to at least 15° S., and there is evidence that some humpbacks travel considerably further north, although probably not so far in a compact group as do the South Atlantic humpbacks which form a breeding concentration near the equator in the Congo area. Most of those passing Centre Island probably travel between 30 and 40 degrees further north and then back again in a period of not more than the 20 weeks between the peaks of north-bound and south-bound migrations past Centre Island. Similarly those passing Whangamumu probably travel north between 20 and 30 degrees and then return the same distance within 14 weeks.

There is no direct evidence of the period spent in one place in the tropical waters, but the frequent observations of slow movement in apparently random directions, or breaching and other behaviour in the one spot in warm waters, does indicate at least some time spent in the general area before migration is resumed. If the migration rate during passage remains approximately the same as in the 11° of latitude traversed along the New Zealand coast in three weeks, the peak of the migrating group would be expected to travel between northern New Zealand and 15° S. and return in about six weeks, or even to latitude 5° S. and return in less than 12 weeks. Depending on the distance travelled, the average period spent by humpbacks at the breeding ground probably varies over a period of the order of two to six weeks and is certainly very much shorter than the period spent on the southern feeding grounds.

The greatest humpback concentrations on the feeding grounds in Area V have been encountered near the latitudes of the Balleny Islands, which are 20° S of Centre Island. From the dates of peaks at Centre Island and the rate of progression past New Zealand, the main stock would be expected to have left the southern concentrations by the beginning of May and to have arrived back between mid and late December, thus allowing a period of four to five months at the southern end of the migration route. These times relate only to the hypothetical mid-point of the migrating stock, so as there is a spread of approximately six to seven weeks on either side during the period that whales pass New Zealand, the time of departure and return for the beginning and end of the migrating stream at each locality would be correspondingly modified.

The seasonal duration of sighting records on north-bound whales at Centre Island, Whangamumu and also Tory Channel, average approximately 12 to 15 weeks, while that for south-bound whales at Whangamumu and Centre Island is only 8–10 weeks. This difference could be caused either by a difference of speed in the migration or in the relative spacing of the individual animals resulting from greater or less spread in the departure times. There is no New Zealand data to show which is the more probable alternative, but if there is little variation from the overall rate of progression past New Zealand, then the spread of the north-bound migrating stock at its maximum extension would include more than 40° latitude, and the south-bound stock approximately 30° latitude. A greater speed would increase the latitude spread or result in some humpbacks having arrived at one end of the migration route before others had departed from the opposite end.

A hypothetical reconstruction of the cycle followed by the central portion of the migrating stock, expressed in very general terms, is as follows: After departure fromx