For boaters in Hawai`i, it's not just Santa Claus that comes to town this time of year.

December is also the month when those mammoth aquatic visitors from the north - Pacific humpback whales - begin showing up around our island chain.

After a summer of feeding in the cooler, nutrient-rich waters of the Gulf of Alaska, the whales are now returning to our warm waters to breed, calf and nurse their young.

At its Web site, the Hawaiian Islands Humpback Whale National Marine Sanctuary points out that of the total North Pacific population of 6,000 to 10,000 humpbacks, approximately 2,000 to 5,000 individuals come here, and their numbers are growing by about 7 percent each year.

While they're here, anyone boating offshore must exercise caution, not only because humpbacks have been designated as an endangered species and are protected by the Endangered Species and Marine Mammal Protection acts, but because a collision with a 40- to 45-ton whale could be disastrous for most recreational vessels.

To help boaters understand the whales' activities, as well as what is expected of them by law, the sanctuary provides an Ocean-Users Handbook to guide their actions.

Perhaps the most important rule for anyone operating a vessel to remember is that at no time are they allowed to approach a whale closer than 100 yards (picture the length of a football field).

To do so is just one of several ways a boater may be cited for "disrupting a whale's normal behavior," which is considered to be detrimental to the health and welfare of its species.

Because humpbacks may approach within 100 yards of a vessel all on their own, most authorities agree that the best action for the boat's skipper is to stop and allow the whale the right of way.

While merely causing a whale to change its direction or speed can be interpreted as a disruption, still there are other actions boaters may take that can be more threatening.

One of these errors the handbook warns against is bringing a boat between a mother and her calf.

Causing a whale to use escape tactics such as prolonged diving, underwater course changes, underwater exhalations, evasive swimming patterns or abandoning frequented areas are violations for which a boat operator may also be cited.

Humpbacks have been observed swimming up to 20 mph for brief periods, however they usually cruise at a more leisurely 3 to 6 mph.

For this reason, boat operators need to be especially cautious when glare, darkness, or wind and sea conditions reduce visibility. Common sense should dictate keeping speeds under 20 knots at any time to avoid collisions.

Boaters who may witness any vessel being operated in violation to these rules are requested to contact NOAA at (808) 541-2727, or call its Fisheries Enforcement Hotline at 1-800-853-1964.

For more information about Hawaii's Humpback Whale National Marine Sanctuary, you can visit its Web site or call 1-800-831-4888.