By Gerard M. Capriulo
Sharks in our waters...and elephant seals, sea otters & more
When I came to California in 1997 to join the faculty of Saint Mary’s College, I was amazed at the accessibility to — and preservation of — the wild, biologically rich and beautiful Pacific Coast. It was in marked contrast to my East Coast experiences of very limited access to mostly privately owned shoreline. Public access to pristine shores is one of California’s great hallmarks and blessings, made possible to a large extent by the tireless work of the California Coastal Commission.
The cornucopia of life forms and biological production in California’s coastal ocean waters fuel a robust marine food web leading up to the marine mammals and predator sharks that claim our waters as their home. But just how and why does this cornucopia exist; why is there so much biological production?
It starts just south of Monterey, surrounds, embraces and spills north past San Francisco and the Golden Gate Bridge, the Farallon Islands, Stinson Beach, Point Reyes and Bodega Head, stretching along the coast for some 120 miles. I am referring to the waters of the Red Triangle, one of three areas on earth sporting the highest-known concentrations of great white sharks, and where 45 percent of all recorded worldwide attacks by great white sharks occur. The other two areas include the waters around South Africa and those around part of the Australian coast.
At least since a girl’s final blood-curdling cry “Shark!!” in the opening great white shark attack scene from Peter Benchley’s Jaws, much of the world has been fascinated with and deathly afraid of sharks. For the last 30 years, largely in response to Jaws, sharks in all of the world’s oceans have been ruthlessly hunted and killed in the name of making us safer. Today, many of the key shark species are threatened with near extinction. Several factors play into this threat.
Most sharks are stingy reproducers and produce only a small number of young per year. Hunters target sharks for sport or safety concerns, or for their fins (to supply the ravenous and lucrative Asian shark-fin soup business). Pollution and ocean warming also harm sharks.
You might wonder why we should care about preserving sharks in our waters, and if indeed we would be better off without them. The necessarily short answer to this question is that top predators keep our ecological systems in needed balance. Top predators enhance biological diversity and keep habitats healthy and working. When we disturb the natural balance, we degrade the system.
No sharks in our ocean waters means disruption and negative alteration of our coastal and oceanic marine food webs, which negatively influences our climate. In short, we need the sharks for our own survival.
Little to Fear
The saddest part of all of this is that we really need not fear sharks any more than we fear dogs. We are not sharks’ natural prey. Most shark attacks are a case of mistaken identity (e.g., surfers on a board, from below, resemble seals and sea lions, favored prey of great whites). Attacks often occur in murky water where visibility is low, and are further stimulated by the presence of fish guts (e.g., from spear fishing) or other blood products often introduced due to human carelessness or design.
A Case of Mistaken Identity
Of the 200-plus species of sharks that exist on earth, less than 10 are considered man-eaters. Most attacks on humans are mistakes, and are rarely fatal. While 45 percent of the world’s great white attacks occur in the Red Triangle, California averages only one attack per year and one fatality per decade, hardly a statistic pointing to an indiscriminate people-killer. Two sharks to fear much more than the great white are bull sharks, which actually move into freshwater systems at will and pose a much greater danger to the unsuspecting, and tiger sharks.
When a great white attacks a person, seal, sea lion or whale, it generally ambushes from below and behind, bites and then backs away. This “cowardly” move helps protect the shark from harm that might be inflicted by its prey. After the attack, it backs off to await the prey’s bleeding to death and then finishes the deed. This tactic likely is the reason why many attack victims survive the encounter. Of course, given the size and strength of the fish, the first bite often causes much harm, even if the shark realizes that a human is not its usual prey and abandons the hunt.
There are many factors that contribute to the support of a large population of great white sharks in our Red Triangle coastal waters. The most significant is the extremely high number of marine mammals, including seals, sea lions, whales, dolphins and sea
otters in our waters.
When Sea Otters Are Healthy
Sea otters historically ranged from Alaska to Baja, Mexico, but have been more common on the central to northern parts of California’s coast. Sea otters have little blubber and are protected from the cold by their natural oils and thick fur, the thickest of any marine mammal. They eat more than 50 types of invertebrates but each otter has its favorites, usually including sea urchins and abalone but also lobster, crabs, snails, squid, octopus, scallops and mussels. They eat from 15 to 35 percent of their body weight per day. Their natural predators include sharks and humans. In the 1700s, their population was very high, but hunting over a 170-year period killed more than a million otters and brought them to near-extinction in the 20th century.
Since then, their numbers have recovered to about 2,500 in California waters. Yet, due to the low numbers left after their near-demise, their gene pool has been severely compromised, leaving them much more vulnerable to disease and population crashes. When sea otters are prevalent, they keep the sea urchin population in check.
Unchecked sea urchin populations decimate kelp beds and forests. When otters were nearly extinct, California’s kelp system largely disappeared with devastating ecosystem-wide effects on nearly all other marine life and coastal environments.
An Elephant Seal Comeback
Migratory Northern elephant seals seasonally populate California coastal waters and rocky shores. These mammals are known to dive to depths of nearly one mile and stay submerged for up to an hour. They are a preferred prey of the great white shark and are most vulnerable at high tide, when hauling out space is at a minimum. They often deep-dive to the bottom to get out of danger or move about at night when fewer attacks are initiated. Male bulls fight for dominance.
Northern elephant seals, once thought to be extinct, were very abundant along the California coast in the past, but their population declined to fewer than 100 individuals that survived on remote islands off of the Mexican coast. As numbers increased following the passage of the U.S. Marine Mammals Protection Act of 1972, these seals established a breeding colony on the Channel Islands. As their numbers increased further some males migrated north, followed by some females. This resulted in the establishment of a breeding colony at Año Nuevo followed by one at the Farallon Islands. Success there has now spawned a new colony at Point Reyes.
Rich Nutrients Aid Populations
Otters, elephant seals, sea lions and whales are here because of the large and healthy fish and invertebrate populations. Those populations result from healthy seaweed (macro-algae), phytoplankton (micro algae plant-like cells) and zooplankton (small micro-animal) populations. The seaweeds and phytoplankton result from the rich nutrients in our waters, which are present because of the lifting of nutrient-rich deeper waters to the surface by our winds and ocean circulation (see below). Rich phytoplanktonic life produces rich zooplanktonic life. Together, these factors create one of the richest habitats for marine life on earth — a cornucopia of life.
The process starts when deeper nutrient-rich waters are brought to the surface in a process known as upwelling. As surface water is pushed offshore by wind, colder, deeper water rises to replace the pushed-away water. This deeper, colder water brings with it rich nutrients.
Global ocean waters move for a few fundamental reasons. Most water on earth moves because of its density relative to other water, which results from differential heating of the waters by the sun. Colder and/or saltier, heavier water sinks and pushes warmer and/or less salty lighter water out of its way, in an interconnected way, moving water up, down and sideways. This moves the majority of the earth’s ocean waters to its deepest depths and sets up major deep ocean currents.
Tidal motions caused by the gravitational action of the moon and sun on the earth also move ocean waters, as do submarine earthquakes which cause tsunamis, and wind blowing over the ocean’s surface. Wind-driven water movement is responsible for much of the ocean’s surface water currents that sailors have relied on throughout history.
As with ocean currents, atmospheric wind also results from the differential heating of the earth’s surface by the sun. The sun is physically closer to the equator than the poles, so more heat enters the tropical latitudes than the polar ones. Hot air rises in the tropical latitudes and cold air sinks in polar and subpolar regions, setting up areas of high and low pressures. These pressure zones cause air masses to move relative to each other, producing weather. As air masses flow over water, they move it via friction.
During much of the year, winds blow from north to south off of California’s coast. Due to the earth’s spin, this prevailing wind direction forces water offshore. As the water is forced offshore, it is replaced by deeper upwelled waters that are rich in nutrients, including nitrogen, phosphorus and iron. These and other nutrients, along with sunlight, are needed by the microscopic algae and seaweeds for photosynthesis and their growth. The infusion of the nutrients fuels massive algal production, which supports equally impressive production of small planktonic animals. They in turn fuel massive production of fish and invertebrates, which support high numbers of marine mammals and their predators. In the end, all of this results in the development and maintenance of one of the most productive marine habitats on earth.
At the other end of California’s coastal ecosystems are the freshwater inputs and their substance-altering journeys that also shape the coast’s ecology in many ways.
More than 97 percent of the water on earth is seawater and 2 percent is freshwater ice, mostly locked up as glaciers. The remaining 1 percent is found primarily as lake water, groundwater and river water. Water’s journey begins as water vapor in the upper atmosphere that comes mostly from evaporative loss, primarily from oceans, as moved by atmospheric circulation. When the vapor stored as clouds reaches saturation point (dew point), it falls to earth, due to gravity’s pull, as precipitation. Water chemically interacts with the gases and particles of the atmosphere so that by the time it reaches the earth’s surface it already has gone through many chemical and physical alterations. Acid rain is one good example of such change.
Precipitation falls in patterns controlled by gravity, climate and weather, and continually runs down from mountains and hills, as snow or ice melt or rainfall. Some of it hits impervious surfaces, such as rock, concrete and asphalt, while some lands on soil, sand, oceans and other water bodies. Always it seeks the low spots and always, as it proceeds in its journeys over surfaces or through sediment, it continues to chemically and physically interact with its surroundings, forever being fundamentally altered, while slowly eroding the surfaces it interacts with.
As it flows downhill, it carves out small channels which grow larger over time. Soon the channels combine again to form small, then larger streams, and ultimately rivers, that run through and over everything from pristine forests to cities and rural communities, in a dance to the sea, picking up many substances in the journey. Groundwater continues to sink into pervious sediment until it reaches the barrier of solid rock. From there it forms aquifers which also move downhill toward the sea as underground flows.
All the sediment and rock, and any other substances encountered along the journey, continue to alter the chemistry of the water, as they themselves are altered by it. The changes can be good or bad, depending on the interactions. Ideally, water is purified in the journey. Alternatively, it is polluted with toxins. Additionally, the water picks up minerals from the rocks it flows over and delivers them to the ocean and its basins, ultimately blending with the previously existing, similarly formed, saltwater of our coastal ocean. This river-delivered water is thus the source of the ocean’s salt.
In the never-ending circle of life, the saltwater establishes the marine ecosystem, which supports the small life forms, which support the marine mammals, which ultimately feed the great white sharks.
Gerard M. Capriulo, Ph.D., is the Fletcher Jones Professor of Marine Biology and chair of the Biology Department at Saint Mary’s.