2009 10 8 Some statistics and observations of humpback whales around Bermuda the last three years Print E-mail
Written by Andrew Stevenson   

The mid-ocean behaviour of humpback whales on the Bermuda seamounts

In 2007 we spent about 30 days and some 200 hours looking for whales mostly along the South Shore. We spotted whales one out of three days and obtained 15 individual fluke ids.

In 2008 we spent more of our time heading out by South West Breaker to Sally Tuckers and Challenger Bank. We spent 23 days on the water, over 200 hours and estimated that we spotted 339 whales of which we obtained 62 individual fluke ids. We had one resighting over two days. We obtained plenty of whale song, mostly on the crown of Challenger Bank, but also once on the shallows of the Bermuda platform where we saw the singer and saw four whales agggregate around the singer in about 60 feet of water before they moved off.

In 2009 we again spent more of our time heading out from shore directly to Sally Tuckers and then across the canyon to Challenger Bank. We spent 20 days on the water, over 200 hours and estimated that we saw 311 whales of which we managed to obtain 168 fluke ids. We used the hydrophone extensively and by trail and error discovered that the loudest singing was on the crown of Challenger. We spent more time on Challenger than previously and confirmed that we would always find whales around the crown, often milling around a singer. We had 15 resightings, some of which were over a period of four days. We did not often get the weather window to go out two days in a row, or multiple days in a week, so the fact that we had so many resightings is remarkable and demonstrates that the humpbacks are not simply migrating past Bermuda.

For each of the days we went out we determined whether the whales were in singles, doubles, threes or four and more, and whether they were mothers with calves. On each day we went out we noted wind direction and speed, cloud cover and hours of sunshine, sea surface temperature, moon phase and high tide measurements. We also divided the perceived behaviour of the whales into different categories: feeding, aggregating, singing, breaching and a group of whales moving in the direction of their migration at a constant speed and heading. All of this data is now on Excel spreadsheets to better perceive any patterns in the behaviour of the whales. We also have a map with the GPS waypoints of each whale encountered within a hundred yards of the boat. One waypoint was used for one or more whales and was not entered again for the same group of whales. Whales breaching in the distance were not logged on the GPS.

Summary: Mid-way on the North Atlantic humpback whales’ migratory route between their breeding grounds in the Antilles and their feeding grounds in Eastern Canada, the Bermuda seamounts rise some 14,000 feet from the ocean floor. After three years (2007-2009) studying the humpback whales as they migrate past Bermuda in the spring, we have observed humpback behaviour that can be divided into several categories: feeding, singing, aggregating, mothers with calves, migrating, breeding, birthing and over-wintering.

 

Methods: The first year 2007, efforts at locating whales were concentrated on the South Shore using a boat based out of Devonshire Bay in the middle of the South Shore. Beginning early March to mid-May, the boat was used extensively to cruise about a mile to two miles along the South Shore. On average a whale was sighted one out of every three days with the most sightings at the south-western limit of the boat’s range towards Sally Tuckers or Challenger Bank.

 

In 2008 the same boat was kept at Devonshire Bay but efforts were concentrated using a bigger trawler to survey the south-western edge of the Bermuda Platform to Sally Tuckers and to the Challenger Platform. There were few days of survey where whales were not observed. In 2008 a concerted effort was made to recruit shore observers to report their sightings.

 

In 2009 efforts were focussed to the south-west again. Whales were seen on every outing from the beginning of March to the middle of May. If no whales were reported from land-based observers on South Shore, then the survey efforts followed the south-western edge of the Bermuda Platform to Sally Tuckers, the point at the extreme south-west of the Bermuda Platform. If no whales were observed on the edges or shallows of the Bermuda Platform, the survey vessel continued across the deep canyon to Challenger Bank. Usually with a crew of four on board, each crewmember on the upper deck was assigned a quadrant for the duration of the day. Top speed on the trawler is 6 knots, which allowed for thorough observation. If whales were not observed on the edges of Challenger, the vessel proceeded to the crown where either a singer or aggregating whales were always seen from the start to the end of the season. Hydrophone recordings were made en route before Sally Tuckers, at Sally Tuckers, mid-way across the canyon, and on the north-eastern edge before continuing to the crown where the singing was found to be the loudest and the presence of whales most likely.

 

  1. Feeding. Humpbacks are seen migrating by Bermuda in the spring after they have been fasting on the Antillean wintering grounds for five months. They are not seen in the fall when they migrate back south from the feeding grounds.  Humpback ehaviour consistent with foraging in the spring takes place primarily in 90’-140’ waters on the windward south-west edges of the shallows of the Bermuda Platform (Sally Tuckers- 6 miles offshore) and in 170’-250’ deep waters on the windward edges of the 15-mile diameter Challenger Platform, which itself lies some 12 miles to the south-west of Bermuda. Humpback diving patterns on the edges of the seamounts are 8-18 minutes, resurfacing close to where they dived. Whales demonstrating feeding behaviour on the edges are single or doubles. After diving and feeding along the edge of one platform whales sometimes cross the deep-water canyon between Challenger and the Bermuda platform (or vice versa) at five knots while surface breathing. They turn at a 90-degree angle once they reach the opposing platform and revert to 8-18 minute dives while moving slowly along the edge. Contents of a plankton tow on Sally Tuckers (see photographs) revealed high plankton productivity, which concurs with previous studies (Deevey, 1971; Moore, 1949) showing high levels of plankton in Bermuda during March and April.  Bermuda is surprisingly productive for a sub-tropical marine ecosystem with total zooplankton standing crop estimated to be about 30% of Georges Bank and 79% that of the inshore waters of Block Island- both areas are known feeding areas for summering humpbacks (Deevey, 1971). Related to the relatively high productivity of Bermuda waters is the presence of sound-scattering layers (Moore, 1949; Fisch, 1977), which occur at layers ranging from near the surface to over 600 metres. Humpbacks are found in the vicinity of these deep scattering layers. There is a direct correlation between commercial fishing effort on the windward edge of Challenger Bank for larger pelagic game fish and the presence of whales. Contents of a tuna fish stomach caught in the same area as humpbacks contained Xantichthys ringens (Sargassum triggerfish), Myctophum sp. (Lantern fish), Gonostoma sp. (Bristlemouth), teuthoid squid, and red decapod shrimps belonging to the same species.  On calm days balls of pilchards can be seen on the surface in the presence of feeding humpback whales. (See photos). Humpback defecations have been observed underwater but video footage has not been possible because of the rapid rate of dissipation. One surface photograph of a lobtailing whale reveals brown-green excrement from its anus. In each of the three years of dedicated observation of humpback whales from February to mid-May, the numbers of whales were greater leading up to and during the full moon. The numbers diminish within a day or two after the full moon. Cuvier beaked whales, sperm whales and dolphins are heard on hydrophones and observed on the sea surface over the 4,000’ deep canyon separating the Bermuda Platform from the Challenger Platform. On two occasions the remains of deepwater squids, Histioteuthis (see photo) have been observed near Cuvier beaked whales.

  1. Singing. Singing in Bermuda takes place from January to May and occurs predominantly on top of the dome-shaped Challenger crown, which rises 30’ in the centre of the 170’ deep 15-mile diameter Challenger Platform. The hard rock surface of the flat table-like 170’ Challenger Platform is scoured clean of sediment by winter storms. Although singers have been located by using hydrophones, they have not yet been observed singing underwater, presumably because they are deep enough to be below sound-reflecting thermoclines. The decibel level of humpback singing seems higher in Bermuda by comparison to whales singing in the breeding grounds of the Antilles. It appears that there is only one singer on the crown at any time. A singer was recorded at low volume on another smaller crown on the south-east side of Challenger. Singing has been recorded once in the middle of the south-west Bermuda Platform (60’ of water) where two groups of two and three whales aggregated around the singer and then moved off as a unit to feed on Sally Tuckers amidst eleven commercial fishing boats. Spectrogram analysis and comparisons of Bermuda recordings to previous years’ and to the same year’s song in the breeding grounds of the Antilles have not yet been made.

  1. Aggregating. As many as 30 whales may aggregate at a time within several hundred yards of the singer although sometimes these social groups will careen for hours or days over the Challenger Platform. Whales aggregating around the crown breathe relatively frequently with coordinated surface breathing. No aggressive behaviour has been observed within these groups. On one day 28 individual fluke ids were obtained on Challenger around a singer. Using fluke ids we have had re-sightings of whales on Challenger on successive days on twenty occasions. Of these, a group of four were re-sighted the next day, a group of nine were re-sighted the next day, three in a group were re-sighted the next day and twice a pair were re-sighted the next day. Two singles have been re-sighted the next day. We’ve had two pair re-sightings two days later, one single re-sighted three days later, and once we had a re-sighting eight days later. Given the fact that inclement weather prevents excursions to Challenger on a daily basis (we average excursions out to Challenger twice a week and it is rare that we are able to get out two days in a row), these re-sightings are all the more remarkable and indicate that the humpbacks are not simply using Bermuda as a navigational waypoint on their migration north. From 1968 to 2006 some 140 individual whales were identified in Bermuda waters. In 2007 and 2008 another 15 and 63 individual whale flukes were identified. Of a total of some 215 individual fluke ids taken in Bermuda 1968 to 2008, there are 87 matches to sightings in the feeding and breeding grounds. Whales have been re-sighted in Bermuda from year to year on #### occasions. (This proportion of multi-year re-matched Bermuda fluke ids indicate the whales may maintain a certain degree of site fidelity to their migratory routes and their stopover on the Bermuda seamounts.) In 2009 150 new Bermuda whale fluke ids were made but these have yet to be processed through the Allied Whale North Atlantic Humpback Whale Catalogue. This year NOAA tagged six humpback whales on the Silver Bank, Dominican Republic, towards the end of the breeding season. One female whale with a calf migrated to the west of Bermuda, stayed for some days 125 miles to the north-west before heading south-east to remain for some days 35 miles to the north-east of Bermuda, bypassing the Bowditch seamount. From the 20-24th of April we had large numbers of whales on Challenger, which coincided with the tagged mother and calf moving towards Bermuda and then waiting to the north-west. Were the tagged mother and calf waiting on the migratory route for this large group of whales to continue northwards from the Bermuda Platform? 33% of North Atlantic humpback whales photographed (Katona et al. 1980) have orca bite scars incurred mostly when they were calves on their first migration north (Clapham 2000).  Are the whales singing on mid-Atlantic seamounts to aggregate into a protective convoy before continuing the migration up north to run the gauntlet of orcas that apparently lie in wait for them? Anecdotal reports from sailors indicate that large groups of humpbacks move northwards from Bermuda in the spring. In the final paragraph of Dr Roger Payne’s article ‘Song of Humpback Whales’ on 13th August 1971 in Science Volume 173, Number 3997, he states, “In the North Atlantic, this time period (February to May) also corresponds to northward migrations. Thus one might imagine that the songs serve as a sort of flock call to hold a loose cluster of individuals together during their long migration. Until there is further evidence, we can only guess what function this remarkable series of vocalizations serves.”

  1. Mother and calves. Mothers and fifteen-foot calves are more often observed on the shallower Bermuda Platform than the Challenger Platform. Fifteen-foot calves have been observed alone on the shallow 50-70’ deep Bermuda Platform while the mother and sometimes an escort feed on the edge of the platform. On one occasion, a lone calf in 50 feet of water lobtailed multiple times. The mother feeding on the edge breached twice and arrived within a few minutes to fetch the calf. The mother breached several more times and was joined a few minutes later by an escort. The three whales returned to the deeper waters of the edge of the platform. Underwater footage of a chubby 26’ yearling and its emaciated mother indicates that some calves are still being weaned during the second migration north. Archival records of whaling in Bermuda reveal whalers targeted 15’ and 26’ calves in order to harpoon the protective mothers. Land-based observers off the South Shore have seen mothers and calves in shallow waters, sometimes just off the breakers, and often in January and February, an indication that these are not whales that have migrated up here from the Caribbean. The mothers and calves seem to be resting. One mother and calf were observed for an hour and a half just off the breakers within 100 yards of shore, diving for 30 minutes and resurfacing in almost the same place in 70’ of water. Darkness prevented further observation. In another instance a social unit of seven whales was followed 20 miles from Challenger to Sally Tuckers to Devonshire Bay in the middle of South Shore. At dusk the boat left the whales to moor for the night in Devonshire Bay. Early the next morning the same seven whales were still just off Devonshire Bay. Re-sightings of whales over successive days on Challenger is unlikely to have anything to do with a rest stop as the Challenger Platform is as exposed to wind and waves as the open ocean.

  1. Migrating. On four occasions a group of seven to twelve whales have been observed on Challenger moving in a convoy-like group at a constant speed of around five knots, and on a steady heading past Sally Tuckers and along the South Shore of the Bermuda Platform in the direction of their migratory route. Underwater film footage of one of these groups estimated on the surface to be seven whales clearly shows ten whales. The whales coordinate their surface blows, moving as a close-knit unit within a body-length of each other and show no aggressive displays or surface-active behaviour typical on their breeding grounds in the Antilles. Judging from their size and the dorsal fins, these seem to be a mixed group of males and females and younger whales.

  1. Breeding. On only one instance have we witnessed a mother and calf with an escort and two or more males repeatedly breaching and displaying surface-active behaviour. No overtly aggressive behaviour was witnessed. The breaching whales with the mother and escort appeared to be small in size and may have been juveniles. Underwater footage of a rowdy group of six juvenile (male?) whales exhibited typical breeding-type behaviour including bubble blowing, tail slashing, breaching, and high-energy displaying.

  1. Birthing. This year a young 15-foot humpback calf was seen on Challenger Platform mid-January. This was most likely a whale born in or around Bermuda as it would have been too early for a mother and calf to migrate from the Anitilles past Bermuda. This might be indicative of a healthy humpback population as archival records indicate whales over-wintered in Bermuda. We also have numerous shore sightings of what appears to be mother and calf pairs in shallow water.

  1. Over-wintering. We have consistently observed humpback whales from late December through January and February into the main migration season beginning late March into May. Many of these whales are observed from shore. This year fishermen reported more whales on Challenger in late December/January than they could ever remember. The behaviour of the whales was consistent with feeding. These appear to be whales over-wintering on the mid-Atlantic seamounts. This year’s numerous whale sightings coincided with the most productive winters commercial fishermen have experienced in recent memory. In early March, one long sequence of underwater footage revealed a tight-knit group of as many as six curious juvenile female whales remaining within a body-length of each other.

 

 
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