MarineBio Newsletter 11
Featured Species: Fishes!
Issues in Marine Conservation: Cetaceans in Hot Water
Current Research: Overfishing Large Sharks Impacts Entire Marine Ecosystem
MarineBio is pleased to bring you the 11th edition of our newsletter! We welcome your feedback would love to hear what you'd like to see in future newsletters! Send your comments to: Joni@marinebio.org.
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Featured Species: Fishes!
We haven’t gone into too much depth about fish on MarineBio in favor of other, usually larger, species that seem to be more popular with our readers and better known to science. In this issue, however, the editor is using her prerogative to describe the wonderful world of fishes because they are some of the most fascinating and fun critters in the sea. It’s so entertaining to torment a damselfish, simply by swimming into her territory, who despite her diminutive size, defends her territory on the reef with the ferocity of a lion defending her cubs. Or the lovable little Nemo the clownfish who will bite any creature that threatens the space surrounding his anemone — no matter that the invading creature is at least 100 times his size and is an alien in his environment. And what about the all the tropical teleosts? The colorful reef fish that change sex from male to female, or female to male, or both?
The term "fish" is most precisely used to describe an animal with a backbone, that has gills present for the duration of its life, and if it has limbs, they are in the shape of fins. Fish, unlike taxonomic groups such as birds or mammals, are a paraphyletic (includes some, but not all, of the descendants of a single common ancestor) collection of taxa, that includes, sharks and rays, ray-finned fishes, coelacanths, hagfishes, and lampreys.
Most, but not all, fish are cold-blooded and have a body that is streamlined for rapid swimming. They extract oxygen from the water using gills, they generally have two sets of paired fins: 1-2 dorsal fins, an anal fin, and a tail fin, they have jaws of course, and the skin is often covered with scales. Most species lay eggs that are fertilized externally.
The various fish classes account for more than half of the known vertebrates. There are at least 30,000 known species of fish still in existence, of which about 27,000 are bony fish. The remainder consists of about 970 sharks, rays, and chimeras and about 108 hagfishes and lampreys. And there are thousands of species that remain to be identified. It is estimated that the total number of extant (still existing; not destroyed, lost, or extinct) fish species exceeds 32,500. That's a lot more fish identification that needs to be completed before they become endangered or lost entirely due to changes taking place in the marine environment.
Anatomy and Adaptations
Fish come in huge variety of shapes and sizes and colors. Some are very fish-like and some are stranger than fiction and highly adept at camouflage. The leafy sea dragon, for example, closely resembles floating seaweed with leaf-like appendages that enable them to blend in with the plants of their environment.
Some of the larger species, such as tuna and swordfish and some species of sharks have adaptations that make them able to warm their body and blood temperature significantly above that of the ambient water. The body shape and fin location is highly variable among fish species as is their size. Fish range in size from the largest fish in the sea, the whale shark, which reaches lengths up to 20 m to the stout infantfish, Schindleria brevipinguis, which measures 7-8 mm.
There are some fish that bear no resemblance to what most of us picture when we hear the word fish, such as seahorses, eels, and rays. And there are some species whose common names include the word fish, but they really aren’t fish at all: cuttlefish (cephalopods), jellyfish (cnidarians), and starfish (echinoderms). Marine invertebrates that are consumed such as shrimp, oysters, crabs, and lobsters are commonly called shellfish.
As mentioned above, fishes have a variety of body plans. In many species, the body is often fusiform, a streamlined body plan that tapers near the head, is wide in the middle, and tapers near the tail. This body shape is often found in fast-moving fish. Fish are also often either laterally compressed or vertically depressed (flat like flounders).
The head includes the eyes, the snout, which extends from the eyes to the forward most point of the upper jaw, the lower jaw, the operculum or gill cover, and the cheek. In most jawed fish there are three configurations of the mouth. It can be terminal, or found on the forward end of the head, it may be superior or upturned, or it can be subterminal or inferior, which means turned downwards or on the bottom of the head. The mouth locations is often what make some fish comical in appearance. Some like the balloonfish look like they're smiling, others, like groupers, look rather grumpy. Some fishes have modified sucker mouths adapted for clinging onto objects in fast-moving water.
The lateral line is a sense organ fish use to detect movement and vibration in the water and usually consists of receptors that run laterally along their sides. The caudal peduncle is the narrow part of the fish's body near the tail to which the caudal or tail fin is attached.
In bony fish, most fins may have spines or rays. A fin can contain only spiny rays, only soft rays, or a combination — if they sport a combination of fin types, the spiny rays are positioned anteriorly (in front). Spines are generally stiff and sharp. Rays are generally soft, flexible, segmented, and may be branched. This segmentation of rays is the main difference that separates them from spines; spines may be flexible in certain species, but they are not segmented.
Spines have a variety of uses. In catfish, they are used as a form of defense; many catfish have the ability to lock their spines outwards. Triggerfish also use spines to lock themselves in crevices to prevent being pulled out.
The placement of fins also varies dramatically among species. Fish can have up to 3 dorsal fins, which protect fish from rolling in the water and assist with sudden turns and stops. The caudal fin or tail fin is located at the end of the body behind the caudal peduncle. This fin is used to stabilize the fish while swimming. The paired pectoral fins are located on each side of the body and are used for direction. The paired pelvic or ventral fins are located ventrally below the pectoral fins. Some fast-swimming fish also have a horizontal caudal keel just in front of the tail fin. This is a lateral ridge on the caudal peduncle that provides stability and support to the caudal fin.
Many fish, often deep-sea fish, also have photophores like cephalopods — light-emitting organs that appear as luminous or lighted spots. The light can be produced from compounds produced during digestion, from specialized mitochondrial cells called photocytes, or they can be a result of symbiotic bacteria. Photophores are generally used to attract prey or to confuse predators.
Most fish also have a swim bladder. This internal organ helps control buoyancy and allows fishes to stay at desired water depths, or to ascend and descend without wasting energy swimming. Some fish, such as sharks, tuna, mackerel and many benthic species (bottom dwellers) do not have a swim bladder.
Fish gills serve as the respiratory organs used to extract oxygen from water and to excrete carbon dioxide. Most fish exchange gases by using gills which are made up of threadlike structures called filaments that are located on either side of the pharynx. The opening where the gills are found is usually hidden beneath a protective bony cover called an operculum. Each filament in the gills contains a network of capillaries providing a large surface area for the exchange of oxygen and carbon dioxide. Fish pull in oxygen-rich water through their mouths and pump it over the gills where the blood in the capillaries flows in the opposite direction to the water causing counter current exchange. They then push the oxygen-depleted water out through openings in the sides of the pharynx.
Because they have jaws, and often teeth, fish eat a wide variety of food, including plants and animals such as plankton, smaller fishes, and invertebrates. Some fish have gill rakers, which are bony, finger-like projections which serve the fish by helping them use their gills while filter-feeding. Food is ingested through the mouth, broken down in the esophagus, and then further broken down in the stomach. The liver and pancreas add enzymes and digestive chemicals as the food moves through the digestive tract, where the process of digestion and nutrient absorption is completed.
The vast majority of fish are oviparous meaning the eggs are fertilized outside of the female's body. The newly-hatched young of oviparous fish are called larvae. They are nourished by a relatively large yolk sac that they carry. They are usually very different in appearance to juvenile and adult specimens of their species, however the larval period usually lasts just a few weeks during which time the fish larvae rapidly change in size, appearance, and structure through a process called metamorphosis. During their metamorphosis, the yolk sac is depleted and the larvae begin to feed on zooplankton.
Some fish species are ovoviviparous where the females keep the eggs inside of their body after internal fertilization and each embryo develops in its own egg. Others, like some shark species, are viviparous where the embryos stay inside the mother's body, but in this case they receive nutrition directly from the mother's body rather than from an egg. The young of both ovoviviparous and viviparous species are born similar to mammal births.
As of 2006, the IUCN Red List describes 1,173 species of fish as being threatened with extinction. Included on this list are Atlantic cod, coelacanths, 20 species of groupers, 20% of all sharks and ray species, including great white sharks, blue fin tuna, black sea bass, giant sea bass, and dozens of other species threatened due to overfishing, habitat destruction, and bycatch. Because of their underwater habitat, fish populations are much more difficult to assess than terrestrial animals.
Popular commercial fishes such as cod and tuna are seriously threatened due to overfishing, which will eventually cause the collapse of fish populations when these species are unable to breed fast enough to replenish. There is great tension between fisheries science and the fishing industry because of the need to balance conservation with the livelihoods of fishermen. The fishing industry provides jobs for thousands world wide and seafood is an important source of protein, so sustainable solutions are needed that will protect the interest of the ocean and the viability of the fishing industry. Studies such as the recent Dalhousie study that predicts the collapse of fish stocks by 2048, the need for solutions is greater than ever. More than 70% of the world's fisheries are overexploited, which threatens the health, economy, and livelihoods of human communities all over the world as well as various marine ecosystems. The global fishing fleet is estimated to be 250% larger than needed to catch what the ocean can sustainably produce.
Habitat destruction is also a great stressor on marine ecosystems. This includes destructive fishing practices, development, and water pollution.
We’ll close this article with some numbers to serve as some fishy food for thought:
2048 = the estimated date at which stocks of commercial fishing species will collapse due to overfishing.
29% = the percentage of seafood species that have collapsed (their catch has declined by 90 percent or more) in the last 50 years.
13% = the percentage decline in global fishing yields since 1994.
146 pounds = the annual per capita consumption of fish and shellfish in Japan, the world’s biggest fish-eating nation.
17 pounds = the average per capita consumption of fish and shellfish in the U.S.
44,320,395 tons = the weight of fish caught each year by Chinese fishermen.
4.2 pounds = the average per capita consumption (in lbs) of shrimp in the U.S.
48 = the number of protected marine areas worldwide protected from over-fishing. Areas that have, according to the Dalhousie report, seen “dramatic” improvement in biodiversity and regeneration of fish stocks.
23% = the increase in biodiversity witnessed in the protected areas above.
1% = the area of the world’s oceans that is currently protected against over-fishing.
$80 billion = the annual worth of the worldwide fishing industry.
200 million = the number of people worldwide who depend on fishing for their livelihood.
1 billion = the number of people worldwide for whom fish is the main source of protein.
64 = the number of Large Marine Ecosystems (LME) in the world. LMEs are regions of ocean around the coastlines of the world’s continents. They account for over 90 percent of the world’s fish catch.
1,496 pounds = the weight of the largest tuna ever caught (a southern bluefin tuna, now a critically endangered species)
Issues in Marine Conservation
Cetaceans in Hot Water
The WWF Global Species Programme and the Whale and Dolphin Conservation Society recently published a report: The Impact of a Changing Climate on Whales, Dolphins, and Porpoises: A Call for Action, which highlights the increasing impact of climate change on cetaceans. Rising sea temperatures and the freshening of seawater due to ice melt and increased rainfall will lead to sea level rises, loss of pack ice habitat, and the decline of krill populations, the main source of food for many whale species.
Although cetaceans have some capacity to adapt to a changing environment, the rapidly changing climate doesn’t allow enough time for adaptations to occur and it is not known to what extent whales and dolphins will be able to adjust. Because climate change impacts the entire marine ecosystem, there is likely to be a devastating domino effect.
Currently, the impact of climate change is most visible in the Arctic and the Antarctic. Cetaceans such as belugas, narwhals, and bowhead whales that rely on polar waters for their habitat and food sources may be severely impacted by climate change and the consequent reduction of sea ice cover. In addition, as sea ice cover decreases, human activity will increase bringing more commercial shipping, oil, gas and mining exploration, development, and military activities to the previously unreachable areas of the Arctic. According to the report’s lead author, Wendy Elliott of the WWF’s Global Species Program, “This will result in much greater risks from oil and chemical spills, worse acoustic disturbance and more collisions between whales and ships.”
Another projected impact of climate change described in the report includes the acidification of the ocean as increasing CO2 in the atmosphere is absorbed. This may increase cetaceans' susceptibility to disease, reduced reproduction, and damages to the body condition and survival rates.
Though the IUCN lists many cetacean species as data deficient, there is evidence that climate change could lead to the decline of the last ~300 endangered North Atlantic right whales; their survival depends on the abundance of prey, which has been shown to vary due to climate change.
The WWF and WCDS are urging governments to cut CO2 global emissions by at least 50% by 2050. The latest report of the Intergovernmental Panel on Climate Change showed it was possible to stop global warming if the world’s emissions begin to decline before 2015.
In addition, the WWF and WCDS are calling on the International Whaling Commission to facilitate research on the impact of climate change on cetaceans as well as to increase efforts and resources to fight all the other threats to cetaceans. For the full report (2.46 MB): The Impact of a Changing Climate on Whales, Dolphins, and Porpoises: A Call for Action
Overfishing Large Sharks Impacts Entire Marine Ecosystem
The Pew Global Shark Assessment (PGSA) is an ongoing project in partnership with scientists at Dalhousie University to: 1) estimate the pre-exploitation population sizes of shark species, 2) estimate current population parameters, 3) predict the outcome of current management practices, 4) effectively communicate the results, and 5) make recommendations to ensure shark survival.
A study published by PGSA researchers in Science Magazine in March 2007 (Myers RA, Baum J, Shepherd TD, “Cascading Effects of the Loss of Apex Predatory Sharks from a Coastal Ocean,” Science Magazine, 30:1846-1850, March 30, 2007) revealed that though the impacts of chronic overfishing are evident in population depletions worldwide, the indirect ecosystem effects induced by predator removal from oceanic food webs remain unpredictable. The population sizes of 11 shark species that consume other Elasmobranchs (rays, skates, and small sharks) have fallen over the past 35 years resulting in an increase in the population sizes of their prey. The former prey species proliferate as their predators disappear and ecosystems become unbalanced. The study blames the collapse of the scallop fishery in the northwest Atlantic on the proliferation of cow nose rays and the resulting increased predation on bay scallops. Similar reductions in apex predators may result in similar consequences.
Put simply, according to a Pew Institute for Ocean Science press release “Fewer big sharks in the oceans mean that bay scallops and other shellfish may be harder to find at the market.”
The team of Canadian and American ecologists was led by the late world-renowned fisheries biologist Ransom Myers of Dalhousie University. They found that overfishing the largest predatory sharks, including bull sharks, great whites, duskys, and hammerhead sharks along the Atlantic Coast of the United States has led to an explosion of their prey species, which include rays, skates, and other small prey. “With fewer sharks around, the species they prey upon – like cow nose rays – have increased in numbers, and in turn, hordes of cow nose rays dining on bay scallops, have wiped the scallops out,” says co-author Julia Baum of Dalhousie. This research builds upon an earlier study by Myers and Baum published in 2003 that showed rapid declines in the great sharks of the northwest Atlantic since the mid-1980s. Now, an analysis of research surveys conducted between 1970-2005 along the eastern U.S. coast has shown that the original study underestimated the extent of the declines. Scalloped hammerhead and tiger sharks may have declined by more than 97%, bull, dusky, and smooth hammerhead sharks by more than 99%.
According to Baum, “Large sharks have been functionally eliminated from the east coast of the U.S., meaning that they can no longer perform their ecosystem role as top predators.” Baum blames the dramatic decline on the overexploitation of sharks in recent decades for their fins and meat.
Not only are sharks targeted by commercial fisheries, they also are caught as bycatch by fisheries targeting tunas and swordfish. As many as 73 million sharks are killed annually worldwide for the shark fin trade, largely based in China.
The effect of overfishing top predators has been difficult to study; however, ecologists have long predicted that overfishing top predators could lead to ecosystem imbalances and destructive consequences. This research is long overdue. According to Ellen Pikitch, executive director of the Pew Institute for Ocean Science, “This is the first published field experiment to demonstrate that the loss of sharks is cascading through ocean ecosystems and inflicting collateral damage on food fisheries such as scallops. These unforeseen and devastating impacts underscore the need to take a more holistic ecosystem-based approach to fisheries management.”
The east coast cow nose ray population is increasing by 8% per year and is estimated at a population size of 40 million. Rays eat large quantities of bivalves, including bay scallops, oysters, soft-shell and hard clams, which will lead to significant declines in these species.
Baum stated “Our study provides evidence that the loss of great sharks triggers changes that cascade throughout coastal food webs. Solutions include enhancing protection of great sharks by substantially reducing fishing pressure on all of these species and enforcing bans on shark finning both in national waters and on the high seas.”
Sharks must be protected not only to maintain their long term sustainability, but also to ensure healthy marine ecosystems. These studies dispel the myth of a vast and invulnerable ocean. They show an ocean where all marine life is interconnected, which means the depletion of apex predators must stop.
Deep Sea Imax: Dive in!
A sea full of wonders awaits. Famed oceanic filmmaker Howard Hall (Into the Deep) guides this immersive adventure that lets you swim alongside some of the most exotic creatures of the planet. Johnny Depp and Kate Winslet provide the narration. And an unusual array of finned and scaled stars is ready to steal every scene. Among them: green sea turtles who gather off Kona so that surgeonfish can strip harmful algae from their shells... an ominous, predatory Humboldt squid that changes color four times per second like a flashing strobe light... an underdog mantis shrimp, whose claws have the speed of a 22-caliber bullet, in battle against a hungry octopus (the shrimp wins!). So many creatures. So many amazing stories. Sea them all.
earthdive: This is a very cool idea that provides divers with the opportunity to contribute to marine conservation simply by sharing their diving experiences. earthdive is a revolutionary new concept in ‘citizen science’ and a global research project for millions of recreational scuba divers who can help preserve the healthand diversity of our oceans. At the heart of this unique research project is the earthdive Global Dive Log (GDL), which has been developed in partnership with the United Nations Environment Programme - World Conservation Monitoring Centre (UNEP-WCMC), Coral Cay Conservation (CCC), and marine biologists from all over the world. The GDL is a unique database into which divers (and snorkelers) log sightings of key indicator species and human induced pressures.
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