The season for halibut fishing in Maine closes for the year at the end of this month. Commercial fishermen with a state permit may catch up to 25 halibut during the course of the two-month season. Those fish must be greater than 41 inches in length.

That restriction might have amazed Henry Bigelow, the Woods Hole Oceanographic Institution founder and co-author of “Fishes of the Gulf of Maine.” In his description of Atlantic halibut, a member of the flounder family, he notes that a halibut weighing 408 pounds was caught with a handline (emphasis mine) in the deep water between Brown and Georges banks in 1908. In June 1917 a halibut estimated to weigh 700 pounds was taken by Captain A. S. Ree off Cape Ann, Massachusetts.

Bigelow emphasized, however, that halibut greater than 300 pounds “always were rarities anywhere in the North Atlantic.” Halibut still grow large, however. Josh Lawson of Bernard, Maine, was longlining for halibut in 2012. Much to his surprise, a 257-pound, 7.3-foot-long halibut took the bait offered on a circle hook. Lawson held the fish from sinking back to the bottom long enough for a friend to get out to his boat and lend a hand in successfully landing it.

How can a flat flounder grow to such size? How can a whale shark, the world’s largest fish, reach 40 feet in length and 20 tons in weight? Why are so many of the world’s really big animals found at sea?

The basic answer is that all those animals exist in a supportive environment. Think about walking around on the Earth’s surface. Gravity pulls at your bones. Miles and miles of oxygen, nitrogen and other gas molecules found in the atmosphere put pressure on your body. Friction tears at your skin as you move through the air. It’s tough on the system to be out here in the dry world. Now imagine that you are a sleek, round object full of fat bobbing about in the ocean. Not only can you slip through the water with a minimum amount of drag, but the environment in which you live makes such movement much less of an effort.

Big bodies on land must have big bones and big muscles to move that whole contraption around. A land animal’s ability to reach a truly large size is kept in check by mechanical limits: at a certain point in growth, the bones won’t support the weight of the body.

One would think that the amount of food needed to keep truly giant bodies functioning would also place a limit on size. Not in the ocean. Take the blue whale.

Blue whales are the largest animals thought to ever exist on this planet. They are baleen whales, which means they use huge plates in their jaws to filter phytoplankton and zooplankton from the water for their food. A diet of such microscopic organisms staggers the mind when you consider that blue whales grow to more than 100 feet in length and weigh in at 191 tons. An adult blue whale needs about 1.5 million calories of energy to keep the lights on, so to speak, in that huge body (an adult human male needs 2,000 calories). But blue whales can take in up to 90 times as much energy as they need from plankton, thus enabling them to build up reserves of energy, commonly called fat.

Yet blue whales were not always the world’s largest living creatures. The species has existed for 30 million years and for much of that time blue whales were approximately 30 feet in length, pipsqueaks relatively speaking. A paper published in the Proceedings of the Royal Society B in May by researchers at the University of Chicago and the Smithsonian National Museum of Natural History argues that blue whales became colossal just 3 million years ago and that the trigger for that growth came from climate change.

The world was in one of its regular ice ages 3 million years ago. Great sheets of ice expanded from the two poles. The circulation patterns of the world’s oceans changed. Cold water sank and slid toward the equator. Winds pushed surface waters away from the coasts, which allowed the cold, nutrient-rich water to move toward the surface, a mechanism called upwelling. Upwelling in turn caused great concentrations of phytoplankton and zooplankton to bloom.

The researchers believe that the blue whales grew in size because they could access massive amounts of high-calorie food. It’s less work, theoretically, to eat seven thousand pounds of plankton in one giant gulp than it is to eat a thousand pounds in each of seven gulps. Furthermore, once the whales had tons of stored fat available to fuel their bodies, they could make long voyages to find the plankton blooms at different times of the year. Growing gargantuan had another benefit — it made them larger than any single predator.

It’s something to ponder: the increase in the abundance of microscopic plankton that allowed blue whales to change from average-sized to mammoth. If the oceans’ currents change as the Earth warms, if cold polar water ceases to sink to the depths, if upwelling peters out, the whales’ giant size will become a mortal liability.