Rachel Cartwright teaches biology at California State University Channel Islands and has been studying trends in habitat use in humpback whale mother-calf pairs in Hawaiian waters. This is her final post.
Tuesday, April 2
We head out into the Au?au Channel on a bright, warm morning and notice clear declines in the numbers of whales. Lingering blows dust the horizon, but they are few and far between. It is early April, and another whale season is drawing to a close.
Humpback whales, like most other baleen whales, seek out tropical breeding waters for just the winter months. They arrive in Hawaii through December and January. By February, the channels and coastal waters echo with whale song, and adults compete for mates while mothers raise their young. But around mid-March, we see the first signs of numbers falling, and by the end of March, the exodus is in full swing. Hawaii?s humpback whales head back north across the open waters of the Pacific to feeding grounds from Alaska to the Russian peninsula, across the North Pacific Basin rim.
I?ve spent the last three months here in Hawaii, idyllically immersed in warm waters and whale song, and there is some real trepidation as the clock ticks down. I?m on sabbatical, and my time is drawing to an unavoidable close. I look at my data files and hope I have enough.
It?s been something of a bumpy season. Fair weather at the start of the season soon gave way to steady winds. Trade winds from the northeast are blocked by the West Maui Mountains, but the wind has shifted frequently to the east, which gives much less protection, and then back due north, which means we have no protection at all. We?ve had some pretty wet and windy days on our little 20-foot research boat, and I wonder if this has affected my sampling protocols and data collection.
I had two key questions at the start of the season. First, do patterns of habitat use change over the course of the season. Second, if so, does this matter? Is there any indication that changes in habitat use are biologically significant?
Using some nifty apps on my iPad allows me an early view of the data set. It does seem that seasonal shifts are apparent; in mid-February, at the height of the season, when adult numbers peak and rowdy males joust in the midchannel waters, mother-calf pairs were to be found back along the shoreline of West Maui, in regions that they avoid at other times of the year.
Looking at fine-scale patterns, our hourlong follows of mother-calf pairs suggest that there may be quite pronounced differences in patterns of travel in different areas. Out in midchannel waters, resting pairs frequently stay in one basic spot, tracing out a slow, steady pretzel-shaped path of travel that returns repeatedly to a central core location. In near-shore waters, mother-calf pairs seem to travel more consistently, punctuating this with only brief resting periods.
Is this biologically significant? My plan is to compare energetic costs in terms of calories used, based on speed and time spent in travel and at rest.
But it may well be that the real threat to whales in this region is far more tangible than my concern with calories, energetics and statistics. Early counts suggest that whale numbers were up this year, but it?s also been a record year for whale strikes. The final tally as of this column: 10 known strikes.
The Pacific Islands Stranding Network has reports of four additional calves stranded around the islands, and of course these are just the incidents we find out about. Many more go unseen and unreported.
How do the subtle changes in behavior that I am studying stack up against the tangible threats of vessel strikes and entanglement? I worry that we may be mopping up the galley as the ship sinks around us.
Certainly, Hawaii is a region steeped in some of the very best environmental practices. From the earliest times of human occupation, systems of sustainable fishing and land use were embedded into the culture. The tradition of malama ?aina, care for the land, permeates island life today. Can we somehow marry these ancient traditions with the realities of modern life and manage the overlap between our use of the marine environment and the vast array of life that dwells there?
On our last day of research we fight our way south through choppy waters. Charter boat captains have told us of an unusual group of whales forming in the area.
As we approach, we spot three or four small groups of whales, and interspersed among them are about a dozen bottlenose dolphins. The dolphins buzz the boats in the area and then buzz the whales too, bow-riding on the whales? snouts and leaping alongside their giant playmates.
Interspecies groups like these are quite common in Hawaiian waters, but today it?s a really loose affiliation. Movements are erratic, and the whales seem unsettled, restless, ready to head north.
The migration takes around four weeks, maybe longer for mother-calf pairs. It?s not unusual to see whales in Hawaiian waters through the end of April, but these are just the stragglers; for most whales, the lure of the feeding grounds propels them into motion, a journey of 2,500 miles across mostly open water.
We don?t really know how they navigate, and once in Alaskan waters, they fan out across a wide range of different feeding areas; maternal site fidelity seems to be the key factor in choosing their destination.
As the whales leave, so too do the whale-watching captains, the naturalists, the boat crews and the research teams. We trade in board shorts and sun lotion for warm fleeces and a good waterproof layer. We?re headed north for the cool, ethereal fjords of Southeast Alaska, the stark beauty of Glacier Bay and the deep emerald green forests of the Tongass.
The scenery changes, but we?ll see some of the same whales, and hopefully the calves we have been watching in the warm waters of Hawaii will make it too, safely running the gantlet of transient orca that wait for their seasonal snack of young baleen whales.
The orca will target the youngest and smallest of the calves. It?s hard to know how many succumb to predation, but 15 percent to 30 percent of calves will arrive in Alaska sporting tooth rake marks ? evidence of skirmishes with orca along the way.
With upper age estimates around 60 to 70 years, many adult whales have made this migration countless times. Over the years the challenges have no doubt changed, from the days of whaling to these days of vessel traffic, ocean noise and a changing climate. But we should still celebrate the victories, and humpback whales are a conservation success story.
Numbers in the North Pacific dropped so low in the 1960s that the population seemed to be bound for extinction. But since then, through protective measures, not least the worldwide ban on commercial whaling, humpback whales have made a comeback.
The North Pacific population now numbers around 20,000 and the trajectory is good, with population increases in the range of 5 percent a year.
Long ago, John Muir described humpback whales as the warm, pulsing heartbeat of the ocean. As the last fluke of the season flies high in the air in Hawaii, it seems like an invitation. Time to head north.
All images taken during permitted research activities conducted under National Marine Fisheries Service Permit No 10018-1. Additional photos by Dan Cesere can be found at c3submerged.com.
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