By Jody Sackett
Colorful foliage, chillier days, pumpkin lattes – the signs of autumn. But the season brings significant changes underwater, too. Terrestrial inhabitants may not notice, but sea creatures are busy preparing for the coming cold weather.
What Happens To The Ocean?
Autumn brings strong winds, storms and cooler weather. Nor’easters are most common in October and April. These storms erode the beaches and create “cliffs” in the off-season, as fine-grain sand gets pulled offshore, leaving behind narrowed pebbly beaches. No worries, though – the gentle summer waves will return that sand and restore the beach. And that washed-off sand forms sandbars in the ocean that will protect the beach from erosion.
Who’s On The Run?
To figure out the aquatic migration schedule, let’s start lower on the food chain. Menhaden, also known as bunker, are baitfish beloved by predators like bluefish and stripers. Come cooler weather, huge schools of menhaden along the Mid-Atlantic coast head south to North Carolina’s deeper waters. As cold-blooded creatures, it’s easier to stay warm in temperate waters; plus spawning is more successful when everyone’s at the same place.
Striped bass and bluefish migrate south to follow their food source and also to keep warm in winter. Whales will be heading south too, traveling thousands of miles to the warm tropics after fattening up on plentiful plankton in northern waters. Whales migrate to calve in warmer waters and give the babies an easier start in life. NOAA researchers also theorize that since whales don’t molt in cold waters (to avoid the risk of hypothermia), another migration goal is to enable whales to safely shed their skin cells in warm tropical waters.
In fall, summer flounder migrate out of their feeding grounds and homes in the estuaries, creeks and inlets and head for the ocean where they’ll spawn. Autumnal plankton populations peak in October and November, which gives the newly hatched larval flounder a better survival rate. Winter flounder have a reversed cycle; in fall, they migrate from deeper ocean waters to shallower inshore waters, where they’ll spawn during winter and spring. Flounder, which spend their lives lying flat and camouflaged on the bottom, are born with one eye on each side of their head like most fish; but as they mature, one eye migrates to the other side of their head so adults will eventually have both eyes on the same side.
Who’s Staying Put?
Mollusks can’t easily swim away when cool autumn water arrives, so most clams will dig themselves under the sand and stay put. Mussels and oysters remain attached to surface substrates, though. Mollusks are filter feeders that eat plankton and detritus and they’re pigging out now to store up enough glycogen to get them through the winter. The extra glycogen is what makes fall oysters so sweet. Mollusks are cold-blooded, so staying warm in chilly water takes a lot of extra energy; a better alternative is to go dormant in the winter, curtailing growth to save bucketloads of energy. Scallops, which can move around by rapidly opening and shutting their shells, will wrap up spawning by November and then go dormant, too.
Who’s Moving to Deeper Waters?
Shallow water becomes colder in autumn, but deeper waters remain warm. As the water cools, blue claws and hermit crabs will start to leave their summer homes in shallow inshore waters and move toward deeper warmer water where they’ll dig in. Horseshoe crabs will continue to move into deeper waters to feed; they have been found at depths of 600 feet. Some fish, like the silverside min- now that usually lives in the shallows, will also move to deeper water to conserve energy.
Who’s Getting Buried?
Like hibernating bears, some marine critters shut down as winter approaches. Come cold weather, crabs and snails will bury themselves in the sand to slow their metabolism until they’re practically dormant. They are in a state of torpor, which is like a mild hibernation; they may emerge occasionally to feed, but with so little food they won’t grow or shed shells. The mummichog minnow also buries itself in sand as far down as seven inches to overwinter as dormant.
How Do Fish Live Under Ice?
Cold water holds more oxygen than warm water, so sub-zero water still has plenty of air. And when ice forms on top of the water, it acts like a lid that traps all that oxygen underwater. Fish need less oxygen in winter because, as cold-blooded creatures, their metabolism and breathing slow down considerably at low temperatures.
WHO ELSE IS MIGRATING FROM THE BEACH?
Most folks know it’s autumn when they see ducks migrating south to seek warmer nesting grounds and food. But other birds migrate. too. Osprey, those majestic hunting birds, have already left for Central and South America; a pair will mate for life but migrate to different winter places. In March they’ll both eventually return to their same nest to raise young. Also migrating are the incredible monarch butterflies that are headed to Mexico or California; and the endangered piping plover, those adorable little birds that fly south along the coast toward the Gulf of Mexico and the Caribbean.
What About Beach Plants?
Like maples and oaks, seaside plants also show their colors. Seaside goldenrod blooms in the fall, providing essential food for hungry monarchs. Common beach grass will turn autumnal yellows and browns as it heads into dormancy for the winter. Poison ivy is a showstopper with its gorgeous red leaves, but watch out for the urushiol on leaves that can still make you itch. And pickleweed, the small beach plant that resembles skinny rubbery fingers reaching out of the sand, will turn beautiful rainbow colors in autumn.
Now that you know what to look for, take a quiet walk along the ocean and bays this fall to observe the signs of autumn. Marvel at the seasonal changes on land and in the water. And maybe pick up a few pieces of plastic trash from the beach while walking, to help protect the wonderful marine miracles happening offshore.
The article originally appeared in the October 19 – 25, 2023 print edition of The Two River Times.
