Marine Habitats, Part IV: Life in the Open Sea

We loaded the boat in the first flickering light of dawn. Three hours later we were 20-plus miles/32 km out to sea, completely out...

We loaded the boat in the first flickering light of dawn. Three hours later we were 20-plus miles/32 km out to sea, completely out of sight of land, over water in excess of 2,000 feet/610 m deep. Someone facetiously said, “There’s the ‘x’ that marks the spot,” and the skipper stopped the boat. Actually, the “x” was a small, free-floating paddy of giant kelp that was drifting out here, in the middle of nowhere. Giant kelp only grows in near-shore waters up to 130 feet/40 m deep, but it is not uncommon for several entangled plants to get pulled free of the bottom during storms that produce big waves and heavy surf. Some of the kelp that gets torn free ends up on Southern California beaches. Other strands drift out to open sea, where they often form an oasis in the vast expanse of blue.

My dive buddy, Mark Thurlow, and I have dived out in open ocean more times than we can count, but every time, as I begin to suit up, I get a little knot in the pit of my stomach. I never know what to expect. A lot of times we don’t see much of anything other than empty blue water, but there are also those magic dives when we have seen blue whales, fin whales, humpback whales, pilot whales, schools of albacore tuna, dolphins, hammerhead sharks, ocean sunfish, pelagic stingrays, migrating turtles and so much more. What would this day bring? I didn’t have a clue. That is why we come, and why I always seem to get a little knot in my gut that is part excitement, part anxiety. It always strikes me as strange that I feel unsettled when gearing up and when I first enter the water, but as soon as I see anything, whether a whale, shark, school of mackerel or jellyfish, I instantly feel relaxed. The “boogie man” never is as big and bad in real life as he is in my imagination.

Among the first things you learn when you explore the open ocean is that it is an enormously vast expanse, and you are very small and slow. Those facts seem so obvious, but they are driven home more and more every time I dive in the places we call the “middle of nowhere.” I have been out there, in a location that we could identify only by coordinates on a Global Positioning Satellite (GPS) navigation system, with a half-dozen or more blue whales surrounding the boat. I slipped over the side as quietly as I could and never saw a single animal. Imagine that! An adult blue whale can be over 100 feet/30 m long and weigh in excess of 100,000 tons. Tough not to see, but it is a very big ocean, especially for a slow-swimming member of our species.

There have been other dives when I never had a clue that a pod of whales was in the neighborhood until I looked up and saw several of them looking at me. It happened with pilot whales in the open sea in the waters off San Diego when I was hoping to photograph an ocean sunfish. I have also seen a minke whale and a fin whale cruise past when I had no idea they were around. Admittedly, the presence of animals that big gives my heart a bit of a jolt when I first see them.

One of the ways of increasing your odds of encountering “somebody interesting” in the open sea is to dive the kelp paddies when they are present. Open-ocean animals often congregate under paddies and other flotsam. Today was no exception, as we were treated to the sight of several big yellowtail and an ocean sunfish that was hanging under the paddy. But the real surprise was an unexpected visit from a blue shark. Both Mark and I have done a lot of work with blue sharks. In fact, they were our photographic targets this day. But usually we bait them in. On this day, we were just trying to get in an early dive before we put the bait into the water.

Of course, a lot of open-ocean animals are not attracted by bait. Jellyfish and salp chains are not. Filter-feeding whales like blue whales, fin whales and humpback whales are not. But some animals, like blue sharks, oceanic whitetip sharks, mako sharks, dorado, yellowtail and schools of mackerel, certainly are. The odds for an encounter with these species are greatly increased by baiting. Many other creatures, such as ocean sunfish, are attracted by objects floating on the surface. These fish, like so many open-ocean species, have an amazing ability to find a boat, kelp paddy or other object that is floating on the surface.

This is the fourth article in a four-part series that examines various marine habitats. We have previously taken a look at nature’s nurseries, the realm of the sand and rubble, and life in a coral reef community, and we conclude by examining life in the open sea.

Here Today, Gone Tomorrow

With the exception of the beaches, where tidal changes occur within minutes, environmental conditions change faster and more dramatically in the open sea than in any other part of the world’s oceans. The open-sea setting always seems to be in a state of flux. You can go to a place one day and be overwhelmed by the amount of activity. Go back to the exact same place the following day, and the area appears to be a huge void with nothing but water in sight. The mantra “here today, gone tomorrow” truly describes life in this feast-or-famine setting, where it can be very difficult to accurately predict the presence of specific animals on a given day in a particular locale.

Most divers rarely explore the open sea. As is the case with most boaters and fishermen, we tend to be going from one place to another with a purpose in mind when we are out in the blue. While we might notice whales, dolphins, activity around kelp paddies and feeding frenzies that occur around big gatherings of fish called bait balls, as a rule we rarely stop to look at small life forms. Yet in many instances, the presence of these larger attention-grabbers is predicated upon the availability of tiny planktonic organisms.

Every food chain in the sea is built upon a foundation of tiny planktonic plants known as phytoplankton. Occurring in extraordinary numbers, these diatoms and other single-celled plants are readily preyed upon by animals ranging in size from the smallest single-celled animals, known as zooplankton, to the great filter-feeding whales. Planktonic life forms attract more animals than one can possibly list. In the end the presence of squid, mackerel, anchovies, herring and sardines, as well as apex predators such as dolphins and sharks, is predicated upon the presence of very small plants and animals.

In essence, the task of trying to predict where larger animals might be at any given time boils down to predicting when and where various plankton blooms will occur, and how those blooms will impact populations of other larger creatures. This task is easier said than done, even for the world’s experts.

The presence or absence of local concentrations of plankton is influenced by many factors such as currents, tides, wind, swell, water temperature, air temperature, the amount of sunlight, the time of year, salinity and more. Oceanographers spend their careers trying to learn about these factors, and the dynamics of trying to put them all together at any given instant has proven to be an enormous challenge. In the final analysis, concentrations of plankton may occupy only a few hundred square yards, or they can extend for thousands of square miles. When present, the concentrations might be near the surface, or they might suddenly descend and scatter.

Other animals, such as jellyfishes, Portuguese men-of-war, salp chains, comb jellies and by-the-wind sailors, have little control over where they end up, going wherever winds and currents take them. Sometimes the fates bring these creatures together in dense concentrations, but at other times they are nowhere to be found.

They Go Where They Want to Go, Do What They Want to Do

With the exception of the mysterious kingdom of the deep oceans, the vast expanse of the open sea is the least-known part of the marine world. Many of us simply know it as the realm of migratory species such as albacore, skipjack, dogtooth, bluefin and yellowfin tuna, swordfish and marlin. Other members of this community, like oceanic whitetip sharks and big schools of jacks, simply wander the great expanse, inhabiting broad areas without making well-defined annual migrations.

Over the course of a typical year, schools of albacore range from Clarion Island, several hundred miles south of Mexico’s Baja Peninsula, all the way to Japan and Alaska. When broken down, that journey translates to a trip of close to 6,200 miles/10,000 km made by covering an average of no less than 16 miles/26 km per day. And that is the straight-line course, without taking into account any turns and backtracking, which certainly occur. Tagged bluefin tuna have crossed the Atlantic Ocean in 119 days, a journey of more than 4,830 miles/7,700 km. Many bluefin tuna inhabit the waters off California until they reach an age of 3 to 6 years, then head off to the waters of the western Pacific, where they mate.

Blue sharks that have been tagged in the waters off New England have been recovered off the coast of Spain, while others tagged during the same study were recaptured off the west coast of Africa. Considerably less is known about the migrations of billfish, as far fewer have been tagged and released. Because sport fishermen rather than scientists have tagged most billfish, tagging studies have focused on marlin and sailfish. These studies have shown that many billfishes make transoceanic migrations.

The bodies of tunas and billfishes combine both aesthetic and functional characteristics. Their fusiform bodies are streamlined for speed. Scientists suggest that the corslet in tunas, a patch of modified scales behind the head, increases the turbulence of water around the widest part of the body and reduces drag. Their large, nearly symmetrical, lunate-shaped tail is designed to supply fast-swimming fishes with maximum thrust with high hydrodynamic efficiency. The bill of billfishes, known to be used to capture prey, is also believed to be an adaptation for speed. It lowers the resistance to water as they swim, much like a missile’s design helps it cut through the resistance of air. Unique among fishes, the circulatory and respiratory system of tunas is designed to conserve or dissipate heat as needed to help them maximize their use of energy. Variations in muscle structure allow for bursts of speed, as well as stamina.

As conditions change in the open sea, big migratory animals simply go where they please. There are no artificial boundaries, only those imposed by the prevailing conditions of the day.

However, scientists and fishermen are certainly aware of some predictable patterns among the creatures of the open sea. As examples, fast-swimming mako sharks are commonly encountered in the waters off Southern California in summer and fall. Like many sharks, blue sharks tend to be sexually segregated. In Southern California waters, we tend to encounter almost all males in late summer and fall, and all females in winter and early spring. Exactly when and where blue sharks get together to mate remains unknown to our species, but male and female blue sharks seem to manage to find each other quite well.

Anchovies, herring, sardines and other small schooling fishes are the feeder fishes of the open sea. These fishes prey upon plankton and, in turn, are heavily pursued by other fishes, squids, seabirds, sharks, dolphins, whales, seals, sea lions and pretty much any other animals that can catch them. These species are critical links in countless food chains. Numerous species of squids play vital roles in open-ocean food chains. They are commonly found in the stomach content analyses of swordfish, marlin, tuna, dorado, yellowtail, dolphins, toothed whales and sharks such as blue sharks, mako sharks and silky sharks. Other sharks, such as common threshers and bigeye threshers, prefer to feed on small schooling fishes, while basking sharks, the world’s second-largest fish, prey upon tiny planktonic life forms that occur in the open sea.

While the open sea often appears to be a great void, at times every member of the food chain seems to gather for brief but extremely intense feeding periods. Divers and fishermen often refer to these gatherings as bait balls, because tightly packed schools of small baitfish are often at the center of activity. Plankton and other smaller fishes attract the small fish, and the presence of all this food seems to attract the rest of the world. Dolphins, marlin, tuna, rainbow runners, pilot whales and sharks gather to rush the schooling fishes as seabirds dive bomb them from above. Instinct drives the individual fish in the bait ball to pack tighter as they seek safety in numbers, but the end result often seems to be an easier target for skillful predators, who gorge themselves until all that remains are a few very fortunate scattered survivors.

Open-Ocean Drifters

It is not just the prospect of encountering big animals that makes open-ocean exploration so fascinating. The open sea is filled with fascinating invertebrates and small larval fishes as well. Jellyfishes, salp, comb jellies, Portuguese men-of-war, by-the-wind sailors and so many more often-ignored invertebrates drift at sea, going where the fates of nature take them.

With bodies made up of more than 90-percent water, the graceful, pulsating creatures we know as jellyfishes seem to be more ocean than animal. The world’s 200-plus species of jellyfishes are described in the phylum Cnidaria, and all cnidarians possess stinging cells called nematocysts, which fire upon contact. With only a little control over their movement, jellyfishes often trail long, potent tentacles used to capture their prey.

Brushing up against the translucent tentacles of many jellyfishes is the last mistake many curious fishes will ever make. The toxins in the nematocysts, which are located in the tentacles and in a ring around the mouth, are both potent and fast-acting. In addition, when fired, the stinging cells ensnare their victims with small, harpoonlike structures. This allows the jellyfish to contract the tentacles and bring the food to the animal’s mouth.

Despite the deadly risk, a variety of crabs take refuge on the bell of jellyfishes, and many small fishes seek protection amongst the tentacles, which in some species can be in excess of 40 feet/12 m long. These daring animals must be extremely wary and agile. While they gain protection from potential predators that give the jellyfishes some room, the fishes and crabs are not immune to the jellyfishes’ potent toxins. A mistake can be deadly. Interestingly, some animals, such as a variety of sea turtles, readily prey upon jellyfishes.

The Portuguese man-of-war is another of the open-ocean drifters. Often thought of as a type of jellyfish, it is actually a hydroid. Like jellyfishes, the Portuguese man-of-war is described in the phylum Cnidaria, because its tentacles and other body parts are armed with potent stinging cells. Though their coloration varies to some degree, the surface float, or bell, is usually an indigo blue to deep purple. Truly, their beautiful appearance belies their potent nature.

Upon first glance, comb jellies look like a type of jellyfish, but they are described in the phylum Ctenophora. Comb jellies lack the stinging cells found in cnidarians, but they do have the distinguishing characteristic of eight rows of ciliated “combs,” which look like lines covered by tiny hairs, along the body. The cilia often reflect and refract light into shimmering rainbows of color that ripple along the sides of these small, semitranslucent creatures. Though capable of independent movement via the rowing motion of the cilia, comb jellies tend to drift where wind, currents and tides take them. For this reason, you often see them by the dozens, or you don’t see them at all.

Most animals described in the phylum Chordata have a backbone, and we commonly refer to them as vertebrates. However, this is not the case with salps, animals that are often mistaken as small jellyfish. While salps lack backbones and are not vertebrates, in their larval stage they possess the forerunner of the backbone, a structure known as a notochord.

Salps are free-swimming tunicates, and while capable of self-propulsion, these gelatinous-looking, translucent animals float in mid-water in the open sea, going wherever the prevailing current carries them. Salps have an incurrent opening on one end of the body and an excurrent opening on the other. Muscular contractions draw water in the incurrent opening and push it through the body in a primitive form of jet propulsion. Food and oxygen are extracted from the incoming water, and wastes are eliminated with the excurrent flow.

Individual animals are typically 1 to 6 inches/3 to 15 cm long. Salp reproduce by asexual budding. At times, in some species, a number of buds are attached to one another in linked chains, commonly called salp chains. The chains often form beautiful spirals, or they are curled in a captivating circular pattern. Salp chains are documented to reach lengths in excess of 40 feet.

Diving in the open sea is not every diver’s cup of tea, and certainly it is not the place to start one’s diving career. But knowing about the marine creatures that inhabit the open ocean and gaining a fundamental understanding of the conditions they must cope with provide all of us with a greater appreciation and understanding of King Neptune’s realm.

How Do They Get to There and Back?

Ever wonder how fishes like albacore, yellowfin tuna, bluefin tuna, marlin and swordfish make great migrations across vast, seemingly featureless expanses of the open sea and end up in virtually the same place at the same time year after year? The world’s most advanced navies make these trips, but they enjoy the benefit of satellite communication and the most sophisticated navigational equipment known to man. Tuna and other open-sea animals perform this rather remarkable task using their comparatively small brains, natural adaptations and instincts. Scientists and commercial fishermen have been interested in the answer to this question for years. In recent years specialists have learned that some fishes have particles in their heads called magnetites that are extremely sensitive to slight variations in the Earth’s magnetic field. It is believed that the ability to interpret this magnetic field information is the key to keeping these creatures on course.

Even the Big Boys Use Countershading

Countershading is a form of camouflage found in many open-ocean animals, including tunas, billfishes, manta rays and sharks. Animals that are countershaded are darkly hued along the top of their body and lightly hued below. This coloration helps them blend in with the surrounding water. If another animal looks down onto a countershaded shark, the dark upper portion of the shark’s body will help the shark blend in with the dark water below. If viewed from below, a lightly hued underbelly will help the shark blend in with lighter surface waters above. Of course, vision is only one of the senses animals use to keep track of the world around them, but camouflage can be a huge advantage to both prey and potential predator in the real-life game of survival.

Diving in the Blue

Blue-water diving is unpredictable. It can be exhilarating, or it can be boring. You will likely spend a lot of time in a state of suspense as you stare into the blue. But when something comes along, it is likely to be unusual, exciting and often large. On the days that you hit pay dirt, you are treated to sights rarely seen. On the days when nothing much swims by, you need to show a lot of patience and not let your disappointments ruin the experience.

No matter how much diving you have done, you are probably going to feel some anxiety the first time you step off a boat in the middle of nowhere. Diving in the open sea is not just a matter of motoring a boat out of sight of land and going for a dive. Anyone who wants to dive safely in the open sea must be prepared. Even in clear water on a sunny day, it is too easy to become disoriented in a sea of blue or green with no bottom features in sight. Often, your only points of reference are rays of sunlight or lighter water toward the surface and the blue-black color of the water below. With few visual clues, divers must rely closely on their depth gauges to know if they are ascending, descending or remaining neutrally buoyant.

Currents can be difficult to detect in many settings without a reference point, and it is all too easy to let a boat that is being pushed by wind get too far away. Mistakes of that nature can have extreme consequences. (For more information on blue-water diving, see the article “Blue-Water Diving: The Closest Thing to Outer Space” in the May 1998 issue of Dive Training.)

Many blue-water divers opt for a tethering system, using a safety line that connects them to the boat. Tethering systems are used by some operators in Hawaii and Southern California. But not all operators use lines. For example, the Andros Buoy out of Nassau in the Bahamas is a popular open-ocean dive where free-swimming divers often see dorado, silky sharks and a variety of other open-sea creatures. The buoy is located in an area known as the Tongue of the Ocean, where the Atlantic meets the Caribbean.

Despite the demands and careful preparations required for blue-water diving, the experience can prove well worth the effort. Blue-water divers enjoy sights that few divers see firsthand. In many ways, that possibility is the essence of our sport.


Story and photos by Marty Snyderman