Federal officials will alter busy shipping lanes in and out of Boston Harbor next week in an effort to protect one of the world's most endangered whales.

The harbor is a busy area for both shipping and whales. More than half of the world's North Atlantic right whales congregate in the area each spring. And the port hosts about 3,500 trips each year by ships carrying everything from merchandise to natural gas.

Under the changes by the National Oceanic and Atmospheric Administration and the U.S. Coast Guard, large ships more than 300 gross tons will be required from April through July to avoid an area where the whales feed. And ships coming from the south will be required to enter Boston Harbor from a slightly different path. The shipping lane will give a wider berth of protection for the whales.

Agency scientists estimate the changes will reduce the risk of collisions between ships and North Atlantic right whales by 74 percent over the summer.

The alterations to the shipping lanes -- which take vessels from southeast of Cape Cod into the port of Boston -- are among several efforts by the federal government to ramp up protections for what has been a dwindling population of whales. Scientists estimate that fewer than 400 North Atlantic right whales remain in the world.

Slow-moving whales are highly vulnerable to ship strikes, since many of their feeding and migration areas overlap with shipping lanes. On average, there are two fatal whale collisions with ships per year. The incidents are the greatest source of known deaths of right whales.

The federal government has made some efforts to reduce those numbers. The Bush administration finalized rules last year that require vessels longer than 65 feet to slow down near ports in an effort to reduce deadly collisions with whales. And NOAA and the Coast Guard have been tweaking the shipping patterns around the busy Boston Harbor. Two years ago, the agencies narrowed and shifted the lanes northward -- the first time in U.S. history that shipping lanes were altered to protect wildlife.

The new alterations taking effect Monday will reroute ships but will not further alter the speed restrictions or reduce the number of ships allowed into the port. NOAA scientists proposed the new changes after assessing 20 years of sighting data to determine the greatest areas of risk to whales.

"Through years of study we have determined that these changes will likely provide a safer environment for whales and mariners," said Jim Balsinger, acting administrator of the National Marine Fisheries Service.

Environmental groups say the combination of speed restrictions and new shipping lanes should provide greater protection to the species.

"Two decades ago, it was literally a hit-or-miss proposition for ships moving in and out of the port of Boston," said Patrick Ramage, global whale program director for the International Fund for Animal Welfare. "The challenge in these things is always to develop some practical, real-world solutions that improve the situation for animals and people -- we think this is a good example of that."

Mariners have much better information on whales than they had 20 years ago. Buoys in the traffic lanes have acoustic devices that can detect whales. And Boston Harbor pilots get daily e-mails with updates on whale sightings.

Andy Hammond, executive director of the Boston Harbor Pilot Association, which guides ships in and out of Boston Harbor, said those improvements have helped his group. Alterations to shipping lanes are generally not a problem for ships, unless the traffic zones become so narrow that it is hard for them to navigate, he said.

"In the end, the mariners will adapt and will work around and work with the changes, and hopefully we'll meet everybody's goal of being able to facilitate commerce and protect the whales -- that's what we all want," Hammond said.

NOAA and the Coast Guard proposed the changes last year to the International Maritime Organization, the U.N. agency overseeing maritime safety. An IMO subcommittee approved the changes last July, and the maritime safety committee gave final approval last winter -- allowing the changes to go into place six months later, June 1.

Right whales were decimated during whaling at the turn of the last century. They got their name by being the "right whale" to harpoon and kill. NMFS has estimated the population is so critically low that it cannot afford another death.