Factors affecting coastal migration route.
The available data on the route followed by humpbacks in New Zealand waters shows that they seldom approach close to certain sectors of the coast, but that large numbers pass inshore at other regions such as the south-west corner of the South Island. The regions approached or avoided differ in a number of respects for the north-bound as compared with the south-bound humpbacks. These two facts strongly suggest that one or more, probably a number of local factors operate which affect the route followed by the migrating stock. The possible part played in this connection by the following will be considered separately:
(1) Direction of coastline.
The east coast of New Zealand shows a continuous north-easterly trend as far north as East Cape. North-bound whales approaching any portion of the coast to the south of this point are necessarily compelled to continue in a north-easterly direction except for those which pass through Cook Strait. Beyond East Cape the coastline has a general west or north-west trend, and whales in this region can move freely in any direction within the northerly quarter. Nevertheless the majority continue to follow the trend of the coastline closely through Bay of Plenty and off North Auckland until the northern tip of New Zealand is reached.
Those which pass through Cook Strait must necessarily move in a north-westerly direction until they reach the vicinity of Cape Egmont, from where they could travel in various directions within the northerly quarter. It is apparent from the scarcity of sightings along the remaining portion of the North Island coast that the majority continue to move in the north-westerly quarter, and few come close to the coastline again north of Cape Egmont. A similar north-westerly trend away from the coast-line is followed by the stocks which reach the Jackson's Bay area after following the west coast of the South Island as far north as that area.
In the New Zealand coastal waters considered as a whole the north-bound humpbacks show a definite trend towards the north-west in all those areas where there is freedom of movement to any part of the northerly quarter, and this occurs whether that trend is along or away from the coastline. Where the coastline has a northeasterly trend the humpbacks appear to desert it unless the coastline lies across their pathway.
Southbound humpbacks encountering any portion of the coast between North Cape and East Cape are deflected by the coastline towards the south-east before it is possible for them to continue steady migration in any other part of the southerly quarter. They are therefore observed frequently near the coastline as far as East Cape, but south of this point the coast trends towards the south-west and very few south-bound humpbacks are seen along it even though it is a coastline followed earlier in the season by many north-bound humpbacks. Apparently most of the whales continue due south or possibly south-east on a course which takes them away from the coastline south of East Cape, and the only record which possibly represents a part of their further course is the concentration described near the Bounty Islands.
Along the west coast of the North Island the sightings for the south-bound whales are more numerous for the section down to Cape Egmont than they are for those north-bound earlier in the season. Although the numbers are not large it is clear that there is a regular movement along the south-easterly trend of this coast suggesting that the whales came in from the north-west as any from the north-east or even due north would be largely obstructed by the presence of intervening land. However, there is very little evidence of humpbacks resuming the south-easterly direction beyond Cape Egmont, as this course would bring them through Cook Strait, and it is certain that very few south-bound whales pass through these waters. The reasons for the avoidance of Cook Strait during the southward migration are not yet apparent, but once it has been by-passed the direction followed by those moving past the South Island must be south-westerly as far as Puysegur Point, and even then the humpbacks travel well offshore until the Jacksons Bay area is reached. From Jacksons Bay southwards the humpbacks not only travel close to shore to Puysegur Point but continue to follow it closely in an almost due easterly direction towards Centre Island, and the steady augmentation of numbers between those localities strongly suggests that many south-bound whales first encounter the New Zealand coast in this area. At Centre Island all turn sharply to the south-west, thus avoiding Foveaux Strait, and so pass along the west coast of Stewart Island.
South-bound humpbacks in general, therefore, closely follow only those coastlines which are across the path of their approach from the north or north-west, and their course largely by-passes the whole east coast of New Zealand south of East Cape. In addition they avoid passing through either Cook or Foveaux Straits, although the westerly approaches of both are traversed.
The routes followed in New Zealand waters cannot be explained merely in terms of preference for moving close to any available coastlines. Most of the sectors which are followed closely are only those that lie across the path of their main south-north or north-south migration routes. The fact that there are indications of trends towards the north-west and south-east during these movements serves to increase this effect, as approaches from these directions are almost at right angles to the longest axis of New Zealand, and so increase the effective width of land across their migration pathway.
(2) Oceanographic factors.
Such properties of the individual water masses as temperature and salinity are unlikely to be major factors in modifying the migration route of humpbacks as they traverse the widely differing Antarctic, sub-Antarctic and sub-tropical water masses in differing proportions according to the route followed by each of the major whale groups in the southern hemisphere. The direction of current flow appears also to be unimportant as the known south-north movements of humpbacks in the southern hemisphere along the coasts of each of the main land masses shows that some groups move in the same direction as northward moving cool currents—e.g., off West Australia, West Africa and Western South America, in the West Australian, Benguella and Humboldt Currents respectively, while others at the same phase of their migration travel against southward moving warm currents—e.g., off East Australia, East Africa and Eastern South America in the East Australian, Agulhas and Brazil Currents respectively. The reverse applies in each case during the north to south migration. The general direction and season of humpback migrations are therefore much the same throughout the southern hemisphere regardless of the physical properties and direction of flow of the water masses traversed en route.
Townsend (1935), after plotting the distribution of nearly 54,000 whales of various species, including 2,883 humpbacks, referred to studies on the effects of ocean currents on whale movements and stated that “the present writer, after much study of oceanographic literature, abandons his attempt to set forth what is known
of their relationship”. Harmer (1931), when referring specifically to water temperature, concluded that the direct effects of variations of temperature are probably not important in the case of adults at least. He suggested that any apparent correlation between the location of whales and temperature is not solely due to their preference for water of a particular temperature.
Humpbacks under direct observation from Tory Channel lookout hill have been noted as retarded or accelerated by tidal streams in Cook Strait to such an extent that the lookout is often abandoned well before dusk on days in which the afternoon tide sets against the course of the north-bound humpbacks. Thus, although it is certain that there is no consistent relationship between ocean currents and the major movements of humpbacks, the possibility that some of the smaller deviations from certain regions of the coast are locally modified by currents and tidal streams must be considered.
The direction of flow and physical properties of New Zealand coastal waters depend primarily on the relative extent and intermixing of two main water masses—the cool west wind drift of the Southern Ocean and the warm East Australian Current (Barlow 1938, Dell 1952). A boundary of mixed water (the sub-tropical convergence) occurs along the New Zealand coast and differs in position between the west and east coast and also changes in latitude at various seasons (M.O. 516 charts 1949, Garner 1953, 1954). This should make it possible to note the local effects on migration, if any, as the humpbacks cross from cool to warmer waters and encounter a change in direction of current flow at differing points for the two coasts during both the northward and southward migrations.
Humpbacks approaching New Zealand from the south travel in the Southern Ocean drift which, during May to August, flows north-eastwards along the east coast of the South Island as far as Cook Strait. Up to this area the whales therefore move in the same general direction as the drift. Along the east coast of the North Island the current comes as a tongue from the East Australian Current which, on striking the west coast of New Zealand, flows northwards to Cape Reinga, where part of it comes down the east coast of the North Island and is therefore flowing against the general direction of whales migrating up this coast. At East Cape, where the whales could follow an unobstructed route northwards and at the same time be travelling with current partly side on, a large proportion of the local stock turn to follow the coastline and so remain orientated directly against the current. On the other hand those which pass through Cook Strait to travel up the west coast of the North Island are then travelling in the same direction as the East Australian current.
Up the west coast of the South Island the whales first meet a southward deflection of the East Australian Current which appears to fork into streams flowing in opposite directions (Garner 1954) from the Milford Sound Jacksons Bay region. Up to this region the whales are therefore moving against the current, but they then leave the coastline at just the region where the north flowing coastal fork originates. North-bound humpbacks therefore travel both with and against the main current drifts in differing proportions on the two sides of New Zealand, and there is no indication that the direction of current flow modifies the route of north-bound humpbacks.
During October-December south-bound humpbacks along the east coast of the North Island move in the same direction as the current when they follow the coastline to East Cape. South of East Cape they no longer follow the coastline closely, but probably continue temporarily in the same direction as the southward moving tongue of warm water from the East Australian Current. Russell (1950) and Garner (1954) have shown that this current varies in its southward extension, but at the above months it usually extends considerably further south than in the May-August period. By February, in 1951, there was evidence from temperature records
(Garner 1953) that this current continued southwards as a tongue of warm water a few miles offshore almost to the southern end of the South Island.
Close to shore and out beyond the eastern margin of this tongue the water was markedly colder, and Captain Turnbull, of the Port Waikato, which travels between Lyttelton and Chatham Islands, states that he sees whales only when crossing the warm tongue which is frequently partly enveloped by fog. To date this is the only local observation indicating any preference for particular water masses by humpbacks, and it may be purely fortuitous as the tongue extends south along much the same line that southbound humpbacks from East Cape would probably follow in any case.
Along the west coast of the North and South Islands as far as the Jacksons Bay area, humpbacks travel against the current, which also has an arm extending into the southern side of Cook Strait (N.Z. Pilot) and should therefore facilitate the passage of south-bound whales through the Strait. Instead, most of the humpbacks avoid the Strait, and current movements appear to give no solution to this behaviour. Garner (1953) found evidence of an inflow of sub-tropical water from the East Cape current coming into eastern Cook Strait during October. 1950, but this fact does not further an explanation for the humpbacks' route.
South of Jacksons Bay a tongue of the warm East Australian Current is deflected southwards towards Foveaux Strait, and it is along this stretch that the greatest concentrations of south-bound humpbacks accumulate. However, some of this current probably flows through Foveaux Strait as it has tidal streams of up to 3 knots, but Bary (1951) found that Foveaux Strait contained very few oceanic plankton organisms and suggested that the water exchange was not very effective and may be primarily oscillations of an almost stationary body of water. If this proves to be the case it may help to show reasons for the striking turn of the migrating humpbacks from west to south at Centre Island. Nevertheless it is clear from other parts of the New Zealand coast that even in the local movements of humpbacks the nature and direction of flow of the major water masses show no consistent relationship with the known changes of course in New Zealand waters.
The only effect so far recognised has been the retardation or acceleration of humpbacks encountering strong tidal streams, and there has been no evidence of deviations from course which could be attributed to direction or temperature of ocean currents.
After heavy rain has produced a large run-off of muddy water which is often distinguishable as a discoloured zone in coastal waters, especially near river mouths, humpbacks travel further offshore to skirt the turbid area. From the whaling lookout at the entrance to Tory Channel it is sometimes possible to observe humpbacks travelling around the edge of muddy water and even following the projections and indentations closely without entering any tongues of muddy water. The degree of avoidance under these somewhat extreme conditions is too slight for it to be likely that river outflows or coastal turbidity along exposed coasts is an important factor in affecting the migration routes. It does not appear to be related to the main deflections away from the coast, such as the way that north-bound humpbacks leave the coast from Jacksons Bay and Cape Egmont, but it may be a factor in the avoidance of Foveaux Strait. The average depth of the latter is only about 15–20 fathoms, and the Waiau, Jacobs, Oreti and Mataura rivers discharge into it from the South Island, and Patersons Inlet carries sediment from Stewart Island. Foveaux Strait is described by local observers as being frequently muddy, and it may be sufficiently turbid to account for the spectacular change of course in the migrating stocks that approach Centre Island.
(4) Depth of Water.
Many fishermen, especially in the Bay of Plenty to North Auckland area, have commented that they usually sighted humpbacks over waters of 20 to 60 fathoms, and have attached some special significance to these depths. However, there is a range in depth from several hundred fathoms along the south-west corner of the South Island to 20 or less in Bay of Plenty at points approximately the same distance from land, and both are traversed frequently by migrating humpbacks. At Tory Channel humpbacks occasionally enter the channel and have been known to go up as far as Picton, a distance of about 20 miles from the open sea; while Whangamumu whalers have described humpbacks coming in close enough to shore to rub themselves against the rocks in waters of 3–5 fathoms. The net whaling there was carried out for 20 years across a channel only 5 fathoms deep. Apparently even a considerable distance of shallow water is no deterrent to humpbacks, as there is a distance of some 30 miles or more in places of water less than 6 fathoms deep in the approaches to the Carnavon station of West Australia, yet the catches in these shallow waters are excellent. No part of the New Zealand coast avoided by humpbacks is less than this depth, and most are considerably deeper, so it is highly improbable that the depth of water can account for any part of the migration route in this area.
(5) Presence of Whale Feed.
During the northwards migration most of the humpbacks caught at Tory Channel or Whangamumu have had empty stomachs, and the whales are only rarely seen feeding along the New Zealand coast during winter months. There are no records of blooming by Munida gregaria at this season, but the whalers have described swarms of a small, transparent crustacean known to them as mana. On the occasions on which humpbacks have been seen feeding at this time of year, they have been moving through these swarms, and the crustacean remains in stomach contents have been identified by Dr. B. M. Bary as predominantly the euphausid Nyctiphanes australis which has been found compacted into masses of several bushels with the texture of moist chaff. Humpbacks which have been found with crustacea in the stomach were killed during May and the first half of June, and after that time it is exceptional to find food remains in the stomach or the intestine. During the northwards migration there has been no evidence of humpbacks deviating from course to follow possible food concentrations.
The return migration past the New Zealand coast coincides with the earliest swarms of Munida gregaria which occur in large concentrations from Cook Strait southwards between October and February, Young (1925), Marine Department Annual Reports (1928–1935), and from fishermen's descriptions it seems probable that it occurs along the eastern coast of the North Island at the same time of year. Whales have frequently been observed passing through concentrations of red crustacea along the North Auckland and Bay of Plenty coasts, and they appeared to be feeding en route. The swarms of crustacea vary considerably in their distribution per season and during each season, but local observers claim that south-bound humpbacks as well as various petrels follow the concentrations as they appear. For example, humpbacks penetrate much more deeply into the upper reaches of Hauraki Gulf when feed is present there than at other times, and in various areas small groups may remain in approximately the same place for a number of days as though feeding. Most observations of this kind have been made between North and East Capes, probably because south-bound humpbacks are seen much less frequently south of this area. Many of the humpbacks caught near Whangamumu during October to December contained large amounts of crustacea. After December most of the humpbacks are to the south of New Zealand, but the apparent peak of Munida swarming, at least along the east coast of the South Island, occurs between December and
February. Along the latter coast very few south-bound humpbacks are seen, as they appear to travel further offshore than when north-bound, and it is possible that they encounter Munida away from the coast, although there is no evidence to suggest that this is the reason for their route away from the east coast of the South Island.
Near Centre Island in November, large numbers of humpbacks were frequently observed feeding in small schools which circled slowly in the western side of Foveaux Strait. The food organisms were usually too far below the surface to be seen clearly, but there were a number of surface patches from which samples could be taken, and the main organism in each during November was the euphausid Nyctiphanes australis. The patches were discontinuous and all those noted had dense flocks of muttonbirds (Puffinus griseus) and prions feeding on the surface where they served as a valuable guide to the location of humpbacks, which were often feeding in the same plankton shoal. All descriptions for several seasons in the Foveaux Strait area, and my own observations agree that the great majority of south-bound humpbacks entering this region occur on the western side of the Strait, and the density of petrels and plankton shoals noted at the surface are much greater in this area than elsewhere in the strait. Dr. B. M. Bary informs me that there is evidence of upwelling and mixing of water in this region resulting in a much higher plankton production than occurs elsewhere in the vicinity. It therefore seems probable that the deviation of southbound humpbacks from Puysegur Point into western Foveaux Strait may be influenced largely by the high plankton production in the latter locality. After an unknown period of feeding they then continue southwards past the western side of Stewart Island.
In New Zealand sub-Antarctic waters there is evidence (Dawbin 1954) that Munida occurs in dense concentrations and is a major item of diet for the local marine vertebrates from November to April, but there have been only a few reports of humpbacks close to the shore of the Auckland or Campbell Islands. The Munida concentrations were not observed directly and it is possible that both they and the humpbacks occur sufficiently far from land to be seldom seen by shore parties and the crews of small craft in the coastal waters.
The localised humpback concentrations reported at months other than those in which the north and south migrations normally occur, were each associated with local concentrations of food organisms. Those off the Waitaki River mouth in March and April were apparently feeding on transparent crustacea (rather than Munida) and those near Whale Island (Bay of Plenty) in late April and early May were feeding on small fish. Both these groups are described more fully in the section on “Irregular Movements”.
In general, concentrations of feed occur infrequently during the period of northward migration by humpbacks and do not appear to play any part in modifying the routes followed at that time. During the period of their southward migration, humpbacks do feed in the waters between North and East Cape as well as in western Foveaux Strait, and at times they deviate towards local concentrations of feed or linger for a period before continuing south. The “out of season” sightings of humpbacks occur at times and places where whale feed is present, suggesting that some humpbacks remain for relatively long periods in such areas and possibly may not continue to Antarctic waters.