Fish Life In The Sea Essay Competition

The air we breathe, life-saving medicines and a critical supply of food for our population all come from marine life. The ocean takes care of us and now needs our help.

Did you know?

  • Tiny phytoplankton in the ocean produce more oxygen than all the forests and plants on Earth.
  • Plant-like animals, called Bugula neritina, are the source of a family of chemical compounds currently being studied to treat a variety of cancers.
  • An estimated 1 billion people worldwide depend upon fish and shellfish as their main source of protein.

Take Action Against Marine Life Decline

Marine life is being threatened by a variety of human activities that result in habitat loss, unsustainable fishing and water pollution. The good news is that there are steps we can take every day to help marine life recover and thrive.

  • Make smart seafood choices. Buy seafood that you know is being harvested sustainably and doesn’t contain heavy metals, such as mercury, that pose a risk to human health. Consult the Monterey Bay Aquarium’s seafood guide that identifies the best choices to make to help preserve these fish stocks for future generations. Apps are also available from MBAqua regarding sustainable seafood.
  • Take shorter showers and install low-flow shower fittings like EPA’s Water Sense products.
  • Don’t purchase items that exploit marine resources unnecessarily such as coral jewelry and supplements such as coral calcium and shark cartilage. The nutrients these supplements allegedly provide are easily obtained from other food sources such as green leafy vegetables.
  • Join a marine mammal rescue center and volunteer your time.
  • Take the pledge. Return the favor by taking our pledge to protect the ocean.
  • Click here for more ways you can take action against marine life decline.

Causes and Implications of Marine Life Decline


Scientists report that 90% of large fish, such as tuna and swordfish, have been removed from the ocean through fishing . Experts estimate that 25 to 30% of the world’s major fish stocks are overexploited . Despite the U.S. having strong fishery laws, of the 267 major fish stocks, roughly 17% are not being fished sustainably . Inadequate information for another 30% of the major fish stocks and virtually all of the minor fish stocks makes it difficult to manage these fisheries sustainably.

Worldwide, for every four pounds of fish caught, more than a pound of other animals are also caught unintentionally. Many animals such as sea turtles and sharks are inadvertently caught in fishing gear. Often the unintended catch, or “by catch”, is killed in the nets or simply thrown overboard to die. For some types of gear, like shrimp trawls, the ratio is even worse: For every pound of shrimp, four or more pounds of unwanted animals are caught and discarded.

You can help protect fish populations and marine habitat by choosing only sustainably caught seafood. The Monterey Bay Aquarium has developed a guide to help consumers choose sustainable seafood alternatives. Consult their Web site for an interactive guide to choosing sustainable seafood or download their wallet-size guide. In 2008, the aquarium also published a guide to choosing sustainable sushi.

Another resource to help you choose sustainable seafood is the National Marine Fisheries Service’s Fish Watch website. For detailed information on the state of the world’s fisheries, check out the United Nations Food and Agriculture Organization’s biennial State of the World Fisheries and Aquaculture report located at the bottom of its webpage:

Coastal Development Threats to Salmon and Steelhead in California

Human development has a number of different effects on the marine environment. Population pressures can negatively affect marine life by destroying habitats, increasing pollution, and contributing to global climate change. Salmon and steelhead trout are anadromous fish, meaning that they live in fresh and ocean water at various stages of life. Salmon and steelhead hatch in small freshwater streams and then migrate downstream to estuaries to mature. They then spend two to six years in the open ocean where they mature, eventually returning to the same streams where they were hatched to spawn and ultimately die.

California Department of Fish and Wildlife release a chinook salmon into Butte Creek.
Photo courtesy

Twenty-eight species of salmonids and steelhead trout have been listed as either threatened or endangered on the West Coast. Dams, water diversions, habitat loss, water pollution, and changing ocean conditions all contribute to the declining of California salmonid species. Salmon populations may improve if more water is available in our streams and rivers. Do your part by reducing the amount of water that you use.


Diseases interact with other pressures on marine species and contribute to marine life decline. The southern sea otter (Enhydra lutris nereis) was decimated in the 1700’s and 1800’s by hunters seeking their pelts. The California population has grown from a group of about 50 survivors off Big Sur in 1938 to just over 2,700 today. Sea otters still face serious risks, often from disease. Recent studies show Toxoplasma gondii, a water- borne pathogen that is hosted by cats and spread through their feces, is a major cause of sea otter mortality in California by causing brain damage and seizures. You can help by properly disposing of kitty litter in trash receptacles instead of flushing it down the toilet. Microcystin, a product of blue-green algae, is another fatal toxin that can be a byproduct of algae blooms. Nutrient runoff and other land-based pollution can increase the number of harmful algae blooms and toxic events.

Sea otters. Photo: Madralynn Haye

Invasive Species

Invasive species are organisms that have been brought by humans (either intentionally or not) to areas where they do not historically occur, and that cause harm to the environment, create economic costs, or create risks to public health. Some invasive species cause harm by competing with, feeding on, or parasitizing native species. As of 2005, of the species listed as threatened or endangered all over the world, 42% were listed because of negative interactions with invasive species (like competition). Once invasive species arrive in a new location, they may be difficult or impossible to eradicate.

In California, the invasive overbite clam was introduced from Asia and has been linked to the decline of endangered delta smelt. The delta smelt is recognized as an important indicator of ecosystem health, and is also threatened by water pollution. Researchers believe that the overbite clam removes key microorganisms from the water, which form the basis of marine food chains. Without these microorganisms, smelt and other fish species starve.

Another invasive species that has caused marine life decline is the North American comb jelly, introduced to the Black Sea from North American waters. This invasive species has contributed to declines in important commercial fish stocks in the Black Sea by competing with native fish for food and consuming fish eggs.

Marine Protected Areas

As of December 2012, California has established a network of 124 marine protected areas (MPAs) that cover approximately 16% of state waters. This historic achievement used strong science and input from stakeholders from across the state to protect our ocean for future generations. MPAs address some reasons for the decline in marine life populations and the health of ocean ecosystems by designating where some human activities are prohibited, such as oil drilling or fishing, in order to protect the ecosystems and marine life around that area. Marine reserves, sometimes known as “no-take” areas, are one type of MPA in which the extraction of marine resources is prohibited and comprise 9.4% of California’s state waters.

You can find information about the different types of MPAs, their location, and the history of how they were developed at the California Department of Fish and Wildlife’s website: Learn about your local MPAs and go for a visit!

Interpretive panel located at Ano Nuevo State Marine Conservation Area and Greyhound Rock State Marine Conservation Area along the Central Coast of California (

California is also home to five National Marine Sanctuaries off the West Coast, which boast 36 species of marine mammals, including whales, dolphins, seals, sea lions and sea otters. Some of these species of marine mammals and seabirds are listed as endangered under the federal Endangered Species Act (ESA), including the Short-tailed Albatross, Marbled Murrelet, California Clapper Rail, southern sea otter, Steller sea lion and several species of whale.

Photo: Humpback whales. Photo: Silke Stuckenbrock

For more information on the critters that live off California’s coast, please visit the West Coast Field Guide of the National Marine Sanctuaries.

For more information regarding MPAs, please visit our new MPA web page dedicated to this issue. MPAs will not protect against all types of human impacts affecting the ocean. They are a tool that should be used to complement other marine management policies, such as fishing limits, gear restrictions and regulations on pollutant discharges into the ocean.

Many resources are available on the web for information on marine invasive species. Here is a good place to start:

Laws and Government Actions that Combat Marine Life Decline

Clean Water Act – Administered by the EPA, the Clean Water Act is federal government’s primary tool to governing pollution. It can be found here: http://

Marine Protected Areas (MPAs) – MPAs are areas where some causes of marine life decline are limited or banned. Information on MPAs can be found on our MAP page: /threats/marine-life-decline/marine-protected-areas/

Marine Mammal Protection Act – The Marine Mammal Protection Act bans any take of marine mammals in US waters and also bars US citizens in international waters. More information can be found here:

Aldo Palmisano is a research chemist at the Western Fisheries Research Center of the U.S. Geological Survey Biological Resources Division and is affiliated with the University of Washington in Seattle. Here is his answer.

The reason some fish normally live in freshwater and others live in seawater is that one or the other environment provides them with opportunities that have traditionally contributed to their survival. An obvious difference between the two habitats is salt concentration. Freshwater fish maintain the physiological mechanisms that permit them to concentrate salts within their bodies in a salt-deficient environment; marine fish, on the other hand, excrete excess salts in a hypertonic environment. Fish that live in both environments retain both mechanisms.

SALMON and other so-called anadromous fish species spend portions of their lives in both fresh and saltwater.

Life began evolving several billion years ago in the oceans and since that time, living things have maintained an internal environment closely resembling the ionic composition of those primeval seas. Presumably, the ionic conditions in which life began are uniquely appropriate to its continuation. Laboratory studies support the view that the various chemical phenomena on which life depends--including the interactions of nucleic acids with each other and with proteins, the folding and performance of proteins such as enzymes, the functioning of intracellular machines such as ribosomes, and the maintenance of cellular compartments--are critically dependent on the ionic milieu in which the reactions take place.

Given time, ocean-dwelling creatures took advantage of untapped resources, such as relatively safe spawning habitats or new food sources, that were available to them only by colonizing other environments, like freshwater and land. Colonization was facilitated, if not necessitated, by geological events, such as the movements and collisions of land masses (plate tectonics) and volcanic activity, which served to isolate portions of very similar populations of a single species from one another. Such geological change forced some populations to either adapt or face extinction. Time and natural selection due to physical and environmental variation worked in concert with isolation to foster adaptations. In some cases, these adaptations became permanent and led to species differentiation.

One important aspect of environmental variation is the ionic composition of bodies of water utilized as habitat. Chloride cells in the gills of marine fish produce an enzyme, called gill Na+/K+ ATPase, that enables them to rid their plasma of excess salt, which builds up when they drink seawater. They use the enzyme to pump sodium out of their gills at the cost of energy. Additionally, their kidneys selectively filter out divalent ions, which they then excrete. An alternative set of physiological mechanisms allows freshwater fish to concentrate salts to compensate for their low salinity environment. They produce very dilute, copious urine (up to a third of their body weight a day) to rid themselves of excess water, while conducting active uptake of ions at the gill.

Certainly, other adaptations contributed to the capability of isolated populations to adapt more fully to their circumstances. With different sets of predator and prey organisms present in the differing habitats, and different physical ranges available to them, behavioral changes would be required; perhaps a smaller or larger body size or body part would be favored. The accumulation of these kinds of physiological, behavioral and physical changes ultimately led to new species. Isolation may have forced them to conserve their newly developed adaptations among their own descendants, rather than distribute them more broadly. For some, the rift eventually became complete and there could no longer be any cross-breeding between populations that once interbred.

Not unreasonably, there were multiple instances of colonization of the freshwater environment by seawater species of fish; some were more or less complete. The ability to escape an environment may have been seasonal, or periodic in some other way, or intermittent, and the ability to osmoregulate in freshwater need not have excluded the capacity to revert to a seawater mode of osmoregulation, as long as the capacity could be utilized by a substantial portion of the population, and selected for, rather than simply lost.

Salmon spend a relatively short time in freshwater before developing the capacity to osmoregulate in seawater, where they live for the majority of their lives. Some species of salmon, like pink salmon, migrate to sea as soon as they emerge from the gravel as free-swimming juveniles. Others, such as sockeye and coho and some chinook salmon, remain in freshwater for one or two years or more before the urge to migrate downstream overcomes them, in a sequence of physiological and physical events that coincides with the development of their capacity to osmoregulate in seawater. So the different species of salmon exploit different aspects of the freshwater environment, but evidently they all enjoy better life prospects if they are spawned in a freshwater habitat and spend their adult lives in seawater.

Other related species, like trout, are physiologically less tolerant of salty water. Most have permanently adapted to life in freshwater. They have probably also lost characteristics (e.g., mating behaviors) that might enable them to lead a successful life in the marine environment. For reasons that may relate to their geographic distribution, the characteristics that once made life in seawater natural to them eventually became excess baggage and fell into disuse and disrepair.

William A. Wurts is an aquaculture specialist in Kentucky State University's cooperative extension program. He provides additional insight on fish evolution and physiology.

The various species of fish found in oceans, lakes, rivers and streams have evolved over millions of years and have adapted to their preferred environments over long periods of time. Fish are categorized according to their salinity tolerance. Fish that can tolerate only very narrow ranges of salinity (such freshwater fish as goldfish and such sea water fish as tuna) are known as stenohaline species. These fish die in waters having a salinity that differs from that in their natural environments.

Fish that can tolerate a wide range of salinity at some phase in their life-cycle are called euryhaline species. These fish, which include salmon, eels, red drum, striped bass and flounder, can live or survive in wide ranges of salinity, varying from fresh to brackish to marine waters. A period of gradual adjustment or acclimation, though, may be needed for euryhaline fish to tolerate large changes in salinity.

It is believed that when the newly formed planet Earth cooled sufficiently, rain began to fall continuously. This rainfall filled the first oceans with freshwater. It was the constant evaporation of water from the oceans that then condensed to cause rainfall on the land masses, which in turn, caused the oceans to become salty over several billion years. As rain water washed over and through the soil, it dissolved many minerals--sodium, potassium and calcium-- and carried them back to the oceans.

Vertebrate animals (fish, birds, mammals, amphibians and reptiles) have a unique and common characteristic. The salt content of their blood is virtually identical. Vertebrate blood has a salinity of approximately 9 grams per liter (a 0.9 percent salt solution). Almost 77 percent of the salts in blood are sodium and chloride. The remainder is made up primarily of bicarbonate, potassium and calcium. Sodium, potassium and calcium salts are critical for the normal function of heart, nerve and muscle tissue.

If the salinity of ocean water is diluted to approximately one quarter of its normal concentration, it has almost the same salinity as fish blood and contains similar proportions of sodium, potassium, calcium and chloride. The similarities between the salt content of vertebrate blood and dilute seawater suggest a strong evolutionary relationship among vertebrates and with the primordial oceans.

Indeed, it seems likely that vertebrate life evolved when the oceans were approximately one quarter as salty as they are today. As the oceans became saltier and vertebrates evolved further, several groups of vertebrates (birds, mammals, reptiles and amphibians) left the oceans to inhabit the land masses, carrying the seawater with them as their blood. They maintained their blood salt concentrations by drinking freshwater and absorbing salts from food.

But fish stayed in the aquatic environment. To adapt, they had to either remain in low salinity environments, such as bays and estuaries, or they had to evolve mechanisms to replace water lost through osmosis to the seawater and to remove salts absorbed from the increasingly saline oceans. To inhabit freshwater, fish had to replace salts lost through diffusion to the water and eliminate excess water absorbed from the environment. Kidney function had to be altered accordingly for fish to survive in these different habitats.

In seawater, fish must drink salt water to replace lost fluids and then eliminate the excess salts. Their kidneys produce small volumes of fluid containing high concentrations of salt. Freshwater fish produce large volumes of dilute urine, which is low in salt. Less demand is placed on the kidneys to maintain stable concentrations of blood salts in brackish or low salinity waters.

Ultimately, fish adapted to or inhabited marine, fresh or brackish water because each environment offered some competitive advantage to the different species. For instance, it has been suggested that euryhaline fish are able to eliminate external parasites by moving to and from fresh and saltwaters. Habitats of differing salinity offered new or more food, escape from predators and even thermal refuge (stable temperatures).

Steven K. Webster, marine science advisor to the Monterey Bay Aquarium in California adds some perspective on fish that move between salt and freshwater.

The approximately 22,000 species of fishes alive today live in virtually all sorts of marine and aquatic habitats that are not unduly toxic. Some, including salmon, lampreys, shad, sturgeon and striped bass, move between freshwater bodies and the ocean at least once in their lives to spawn. Many of these anadromous species do so annually, finding conditions needed for reproduction in one realm and those needed for feeding and growth in the other.

These fishes have to switch over their salt balance physiology when they move from fresh to saltwater and back again. They typically make these adjustments in a brackish estuarine environment--which lies on the way between salt water and freshwater habitats.

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