Winter in Hawai'i has begun, and with it comes humpback whales returning to their local breeding grounds. Each year, thousands of whales leave their summer feeding grounds in Alaska and head to the warm waters of Hawai'i to mate and calf.

Over the past few decades, humpbacks have increasingly drawn the attention of residents and tourists, creating a major source of revenue to the state. Based on a 1999 survey, whale watching accounted for $16 million in total revenues, a figure that likely has grown since. However, despite this growing appreciation, the humpback whale population faces a new threat — collisions with ocean-going vessels.

The number of reports of vessels hitting whales has increased dramatically. From 1995 to 2003, the number of collisions averaged 1.6 per year (with 67 percent in Maui waters). However, during 2006 alone there were six documented incidents of ship-strikes. This increasing trend has understandably caused some concern among whale enthusiasts and government agencies charged with protecting this endangered species.

There are at least several reasons why the number of incidents is increasing.

Since their international protection from whaling in 1966, the North Pacific population of humpbacks has increased substantially. Beginning in 1993, I began a series of aerial surveys of humpback whales and determined that the size of the whale population in Hawai'i was increasing. From 1993 to 2000, the average rate of increase was 7 percent per year. At this rate of increase, the number of whales will double approximately every 13 years. With this increase in population comes an increasing risk of interactions with vessels. Simply put, the more whales there are in the ocean, the more likely someone will inadvertently bump into one.

There is a direct relationship between the number of collisions with whales and the average speed of boats. A recent scientific article showed an increase in collisions after the 1950s, when the number of power vessels capable of going over 14 knots rose rapidly. Increasing speed also corresponds with greater lethality. Recently, vessel speeds have increased further as modified hull designs and propulsion systems allow boats to travel 50 knots or more (for example, the proposed Hawai'i Superferry). When compared to the average rate of travel of a humpback whale, which is around three knots, it is easy to see that the potential for collisions exists.

Habituation of whales to vessels puts whales at increased danger. Habituation is a term used by behavioral scientists to describe how people and animals become less responsive to repetitive events. During the early years of whale research in Hawai'i in the late 1970s, it was fairly difficult to get close to humpback whales (Approaching inside the 100-yard legal limit requires a scientific permit). Over the years, humpbacks have become the object of considerable attention from researchers and the whale-watching public. In the waters off West Maui, it is not uncommon to see whales with several whale-watching vessels nearby. Unlike their counterparts several decades ago, Maui humpbacks are far less timid and may even appear aggressive at times. Reports of whales "mugging" whale-watching boats (approaching boats and remaining close by) have become commonplace. While this process of habituation has produced exciting close encounters for the whale-watching industry, it also means that whales are increasingly coming into harm's way when vessels are motoring by.

These converging trends indicate that vessel strikes involving humpbacks will continue to increase unless new "whale detection" technology emerges. Since forward-scanning sonar and surface radar are currently of limited utility, the best method of avoiding collisions is diligent visual scanning to detect whales surfacing near a vessel's path.

The people of Hawai'i love their whales, and we all can do our part to continue to protect them. Those of us who operate ocean-going vessels can be more vigilant in watching for the presence of whales during the winter breeding season. We can also continue to pressure operators of large high-speed vessels to maintain adequate safeguards. For example, public concern led to the Hawai'i Superferry developers' agreement to alter their proposed routing during humpback season. By being aware of the factors that cause boat collisions with whales, we can help to protect this majestic endangered species.

Joe Mobley is executive director of research at the Island Marine Institute in Lahaina, Maui. He is also professor and associate director of the Office of Research at the School of Nursing and Dental Hygiene at University of Hawai'i-Manoa.