Living fishes  The living fishes (not a monophyletic group) include: the jawless fishes (e.g. lampeys), the jawless fishes (e.g. lampeys), cartilaginous fishes (e.g. sharks and rays), cartilaginous fishes (e.g. sharks and rays), bony, ray-finned fishes (most of the bony fishes such as trout, perch, pike, carp, etc) and bony, ray-finned fishes (most of the bony fishes such as trout, perch, pike, carp, etc) and the bony, lobe-finned fishes (e.g. lungfishes, coelacanth). the bony, lobe-finned fishes (e.g. lungfishes, coelacanth).

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16.2

Bony fishes: Osteichthyes  The term osteichthyes does not describe a monophyletic group, but is a term of convenience to describe the fishes whose skeletons are made of bone that replaces cartilage during embryonic development.  There are two classes the Actinopterygii (the ray-finned fishes) and the Sarcopterygii (the lobe-finned fishes)

General characteristics of bony fish  Skeleton made of bone of endochondral origin (derived from cartilage).  Paired and median fins supported by dermal rays.  Respiration mainly by gills. Gills covered with operculum.  Swim bladder often present.  Complex nervous, circulatory and excretory systems present

Class Actinopterygii (ray-finned fishes)  This is by far the larger of the two living classes of fishes with more than 27,000 species.  Includes probably every fish you can think of. E.g. salmon, cod, herring, tuna, marlin, pike, sardine, clownfish, goldfish.

Divisions of Actinopterygii  The Actinopterygii are divided into two groups Palaeonisciformes (formerly known as the “chondrosteans”), which includes the relic species just mentioned. Palaeonisciformes (formerly known as the “chondrosteans”), which includes the relic species just mentioned. Neopterygii, which includes the most derived and most recent group of ray-finned fishes the Teleostei and a number of primitive species including the gars and bowfins. These primitive Neopterygii were once grouped as the “holosteans.” Neopterygii, which includes the most derived and most recent group of ray-finned fishes the Teleostei and a number of primitive species including the gars and bowfins. These primitive Neopterygii were once grouped as the “holosteans.”

 Ancestral ray finned fishes in the Devonian were small and heavily armored with ganoid scales (thick, bony, non- overlapping, relatively inflexible scales) and heterocercal tails (shaped like that of modern sharks).

Palaeonisciformes “Chondrosteans”  A few relic species still possess such characteristics.  These include sturgeon and paddlefish which are included in the Acipenseriformes and the bichirs (Polypteriformes).

Palaeonisciformes  The relatively few surviving Palaeonisciformes are the remnant of what was once a much more diverse group.  Palaeonisciformes were the first bony fishes and were most diverse in the Carboniferous and Permian.

Palaeonisciformes  Extinct palaeoniscids were mostly small < 0.5m with a fusiform shape which suggests they were active foragers.  They were covered with small diamond- shaped scales.  The base of each scale was made of bone, the middle of dentin and the surface with an enamel-like substance called ganoine. Hence the name ganoid scales.

Sturgeons and Paddlefish  These lack ganoid scales except for in sturgeons the rows of enlarged scales that run along the sides of the body.  The skeleton is almost entirely cartilaginous, which has resulted from the loss of mineralization.

Paddlefish  The two species of Paddlefish are found in fresh water in North America and China. The Chinese species is nearly extinct.  About 2m long paddlefish possess an elongated flattened rostrum, which is believed to be used to detect tiny, electric fields.  The North American paddlefish is a planktivore.

Sturgeons  There are 24 species of sturgeons and all are large fish that reach up to 6m in length.  They have a protrusible jaw (evolved independently of the teleosts) which they use for suction feeding.  They are commercially important for their meat, but especially their eggs (caviar).

Bichirs  There are 11 species of bichir and they are considered the most primitive surviving group of the ray-finned fishes.  They are heavily armored with dermal bone and a thick layer of ganoid scales.  Occur in swamps and streams in Africa and have a swim bladder that acts like a paired ventral lung. They will drown if unable to gulp air at the surface.

Armored Bichir

Bichirs  Because bichirs have paired fleshy pectoral fins and lungs they were formerly classified with the lungfishes, but are now considered to have evolved these traits independently.

Neopterygii: “holosteans”  There are two genera of primitive Neopterygians that were previously grouped together as holosteans.  Both have more flexible jaws than Palaoniscids, but less flexible than those of more advanced Neopterygians.  These are the seven species of gars (Lepisosteiformes) and the single species of bowfin (Amiiformes).

Gars  Gars are medium to large (1-4m) predatory fish with a distinctive elongated body and long jaws.  They have hard, interlocking, multilayered ganoid scales which provide excellent protection and are similar to the scales of many extinct Paleozoic and Mesozic actinoptrygians.

Longnose gar Shared/StaticFiles/animals/images/primary/gar.jpg

Bowfin  There is only one species of bowfin.  Its scales are of a single layer of bone as in teleosts, but the caudal fin is asymmetric and similar to that of more primitive fishes.

Bowfin

Teleosts  The vast majority of modern fishes are “teleosts.”  They have replaced the heavy armored scales of their ancestors with much lighter more flexible scales that overlap each other and also have evolved homocercal symmetrical tails.

Teleost characters  Homocercal tail  Circular scales without ganoine  Ossified vertebrae  Swim bladder  Skull with complex jaw mobility

Teleost classification  How the Neopterygii should be subdivided differs greatly from authority to authority.  We will use the text’s division of the teleosts into three large groups: Teleostei Teleostei Euteleostei Euteleostei Acanthopterygii Acanthopterygii

Diversity of bony fishes: Teleostei  There are three major clades of the Teleostei  Osteoglossomorpha: [greek bony tongue]. About 220 species of tropical freshwater fish. Includes from the Amazon Osteoglossum or Arawana, and Arapaima the largest purely freshwater fish (regularly 3m long, but up to 4.5 m).  Also includes the African elephant nose fish, which are bottom feeders and that use weak electric signals to communicate with each other

Arawana

Arapaima

midorcas/animalphysiology/websites/2003/Wilson/cfunspics/ elephant_nose.jpg&imgrefurl= animalphysiology/websites/2003/Wilson/ GalONE.htm&usg=__yE31La06_D121J4Yga5NHWknr5Y=&h=467&w=1458&sz =57&hl=en&start=3&tbnid=xQ3Vx636CuW21M:&tbnh=48&tbnw=150&prev=/ images%3Fq%3Delephant%2Bfish%26gbv%3D2%26hl%3Den%26sa%3DG Elephant nose fish

Teleostei: Elopomorpha  Elopomorpha: includes tarpons, bonefishes, and eels.  Specialized leptocephalous [Greek small headed] larvae are a unique feature of the group. The larvae spend a long time adrift on the ocean being moved by ocean currents.

Bonefish Tarpon fishn/Tarpon-FISH-Justin- S-America-Venezuela-Los-Rogos.jpg

Eels  Most elopomorphs are eel-like and marine, but some tolerate freshwater.  The American eel has a very unusual life- cycle. The eels grow to sexual maturity in rivers and streams (taking 10 years or more) and then migrate downriver into the ocean to breed. (They are catadramous.)

Eels  They swim to the Sargasso Sea (an area of the North Atlantic between the Azores and West Indies) where they apparently spawn and die, presumably at depth.  Eggs and larvae float to the surface and drift on the currents until they reach the near the coast. Then they transform into miniature eels and travel up rivers to mature.

American Eel Anguilla-rostrata-2.jpg Eel larvae

Eels  European eels also spawn in the Sargasso Sea. Their larvae travel on clockwise currents mainly of the Gulf Stream and are distributed to North Africa, Northern Europe, the Mediterranean and as far as the Black Sea.  Because they drift in cooler waters European eels grow more slowly than American eels. Development is slowed less than growth however, and as a result European eels have more vertebrae than American eels.

Teleostei: Clupeomorpha  Are a commercially very important group of about 360 species of marine schooling, silvery fishes.  They include herring, shad, anchovies and sardines.  They feed on plankton which they gather using a specialized mouth and gill-straining apparatus.

Herring

Euteleostei  The next major division of the teleosts contains about 10,000 species.  There are four major groups of the Euteleostei Ostariophysi: carp, catfish, piranhas: about 7,900 species Ostariophysi: carp, catfish, piranhas: about 7,900 species Salmoniforms: trout, salmon and relatives: about 366 species Salmoniforms: trout, salmon and relatives: about 366 species Paracanthopterygii: cod and anglerfishes about 1,300 species. Paracanthopterygii: cod and anglerfishes about 1,300 species. “Stem Neoteleosts” not a monophyletic group, but includes just over 900 species of lanternfishes and relatives. “Stem Neoteleosts” not a monophyletic group, but includes just over 900 species of lanternfishes and relatives.

Euteleostei: Ostariophysi  Ostariophysi (from Greek for bone and bladder).  Represent about 30% of all living fishes, about 6500 species.  Display very diverse traits, but many have protrusible jaws and pharyngeal teeth act as second jaws.

Euteleostei: Ostariophysi  The group possesses two unique derived features: alarm substances in the skin and the Weberian apparatus.  When the skin is damaged, pheromones are released into the water and these stimulate a fright reaction in other members of the species and other ostariophysians. In response, they may quickly seek cover or school together.

Weberian Apparatus  Weberian apparatus: The name ostariophysian (Greek bone and bladder) refers to a series of small bones that connect the swim bladder with the inner ear.  The Weberian apparatus greatly enhances hearing in these fish and as a result they are more sensitive to sounds and can hear a wider range of sounds than other fishes.

Weberian apparatus  When sound waves strike the swimbladder it vibrates.  A bone (the tripus) in contact with the swim bladder then conducts this vibration via ligaments to two other bones, the second of which moves and compresses a section of the inner ear against a fourth bone.  This fourth bone (the claustrum) then stimulates the auditory region of the inner ear.

Weberian apparatus:

Euteleostei: Ostariophysi  The Euteleostei: Ostariophysi includes piranhas, tetras, carp and minnows, and catfishes.

Piranha piranha.jpg Carp others/carp jpg

Euteleostei: Salmoniforms  The group includes the esocid and salmonid fishes.  The salmonids include salmon and trout, which include many commercially important species.  Many species of salmon are anadromous and spend their adult lives at sea, but return to breed in freshwater.

Euteleostei: Salmoniforms  Trout are close relatives of salmon, but usually live their entire lives in freshwater.  Salmon and trout are important commercial and recreational species.

Coho Salmon Rainbow Trout: staticfiles/NGS/Shared/StaticFiles/ animals/images/primary/rainbow-trout.jpg

Euteleostei: Salmoniforms  The esocids are relatives of the salmonids and among the most primitive of euteleosteans.  They include pike, muskellunge, pickerels and relatives.  These fish (which superficially resemble gars) are voracious stealth-hunting predators and important freshwater game fish.

Northern Pike

Muskellunge Muskellunge.jpg

Euteleostei: Paracanthopterygii  Includes about 1,340 species of cod, toadfish and anglerfish.  Cod and their relatives (including pollock and haddock) are cold water marine fishes and the basis of some of the most historically important marine fisheries.

Atlantic Cod:

Anglerfishes  Anglerfishes are named for their method of foraging which involves using a lure to attract fish close to them.  The lure is a modified spine of the anterior dorsal fin and can be wiggled like a prey item.  In deep sea anglerfish the lure contains bioluminescent bacteria that help attract prey from a distance.  Some bottom-dwelling anglerfish depend on camouflage and these fish have arm-like pectoral fins that they use to move long the bottom.

Anglerfish Black devil Anglerfish deeplight/media/fig3b_600.jpg

Euteleostei: “Stem Neoteleosts”  About 916 species of lanternfishes and their relatives.

Lantern Fish photophores Barnard's lanternfish, Symbolophorus barnardi

Lanternfish  The lanternfish is a common resident of the upper portions of the deep-sea. It has a series of light-producing organs along its body, especially the belly.  The photophores can vary their intensity and the fish can tailor the illumination to break up its shadow and make it less visible to predators.  The photophores also appear to be used to attract mates.

Acanthopterygii  Includes two major groups:  Atherinomorpha: More than 1,600 species of silversides, killifishes, grunions, flying fish and relatives.  These are mostly small silvery fish that are surface feeders.  There are about 50 species of flying fish (mostly tropical) that are members of the Atherinomorpha and they use their enlarged pectoral fins to glide 50 to 400m (depending on updrafts from waves) to escape predators.

Silversides

Flying fish

Acanthopterygii  The second major group is the Perciformes: more than 13,000 species of perch and their relatives.  Range in size from 7mm to 5m long. A paraphyletic group there is no set of derived traits that groups them all together, but the they usually have dorsal and anal fins with anterior spiny portions, whereas the posterior spines are usually soft rayed. The two portions may be partially or completely separated.  Snook, sea bass, sunfish, perch, darter, snapper, cichlids, barracuda, tuna, most coral reef fish.

European Perch

Snook Black Seabass images/Black%20Sea%20Bass2.jpg

Barracuda images/cc_barracuda_national_park_service.jpg

SpeciaIizations of the teleosts  A major development in the teleosts is the conversion of jaws from simple devices for grasping to sophisticated suction devices.  An approaching fish can push prey away, but a rapid expansion of the orobranchial cavity creates a flow of water into the fishes mouth.

Protrusible jaws  Teleosts are characterized by having great mobility in the skeletal elements of the mouth. This allows the grasping portion of the jaws to be quickly extended forward.  Jaw protrusion is achieved by levering forward the premaxilla from behind.  The premaxilla is attached with ligaments that allow the bone to slide forward on top of the skull.

Protrusible Jaws  Based on anatomical comparisons of their structure in different groups it is clear that protrusible jaws have evolved independently multiple in different teleost clades.  Jaw protrusion is widespread among the perciform fishes, but also occurs in silversides, cods and anglerfishes, and in minnows.

Pharyngeal Jaws  Mobile and often powerful pharyngeal jaws have evolved several times in actinopterygians.  Ancestral ray finned fishes possessed many dermal tooth plates within the pharynx. Some toothplates over time became fused together and to parts of some gill arches.

Pharyngeal jaws  The earliest pharyngeal jaws were not very mobile, but could be used to hold prey before swallowing. Today a variety of pharyngeal jaws occur in different groups.  For example, in minnows the primary jaws lack teeth but the pharyngeal jaws are enlarged and close against a horny pad on the base of the skull. They are used to grind plant material.

Pharyngeal jaws  In many groups the upper and lower pharyngeal jaws can move independently of each other.  For example, in some moray eels the pharyngeal jaws can be extended from the throat into the oral cavity to grasp prey and pull it into the throat and esophagus.

These X-rays show the normal position of the pharyngeal jaws (upper), and how they can move forward into the mouth to seize food (lower). (Credit: Rita Mehta, Section of Evolution and Ecology and Candi Stafford, School of Veterinary Medicine, UC Davis.) Legend pasted from eleases/2007/09/ htm

Moray Eel Pharyngeal jaws

Pharyngeal jaws  The cichlids of Lake Victoria have diversified enormously into about 500 species in a period of only about 14,000 years.  The possession of pharyngeal jaws which can process food has allowed the outer jaws to be greatly modified to consume a wide variety of prey. Foods consumed include, other fish, plankton, algae, fish scales, bivalves, and diatoms.

Lobe-finned fishes: Class Sarcoptrygii  Primitive Sarcoptrygians were abundant in the Devonian, but have since declined to a handful of species.  Unlike in the actinopterygians (where the rays fan out from the base of the fin) the rays of the paired fins in Sarcopterygians extend from a central shaft of bones to support the fin web.

Fin structure

Lobe-finned fishes: Class Sarcoptrygii  Primitive Sarcopterygians were cm long and cylindrical.  They possessed two dorsal fins, paired pelvic and pectoral fins that were fleshy, scaled and possessed a bony central axis. The heterocercal caudal fin had a epichordal lobe.

Fossil Sarcopterygian

StaticFiles/animals/images/1024/coelacanth-swimming.jpg

Lobe-finned fishes: Class Sarcoptrygii  Sarcopterygian fishes also had massive jaw muscles in comparison to those of actinopterygians.  In addition, early sarcopterygians were covered with a dentine-like material called cosmine.

Lobe-finned fishes: Class Sarcoptrygii  Today the sarcopterygians are a very small group that includes only six species of lungfishes (Dipnoi) and two species of coelacanths (Actinistia).  However, all of the tetrapods (four-legged vertebrates) are descended from an extinct group of sarcopterygian fishes known as the rhipidistians.

Lungfishes  There are six species of lungfishes: one South American, one Australian and four African species.  As their name suggests, these fish, as all sarcopterygians do, possess alveolar lungs and can breathe air.

Lungfishes  Extant Dipnoi have lost the articulating toothed premaxillary and maxillary bones of the other Osteichthyes.  They have crushing dental plates with fan- shaped ridges and teeth scattered over the palate. In addition, strong muscles attach the lower jaw to the chondrocranium. Lungfishes are thus specialized to feed on hard foods such as crustaceans and molluscs.

Lungfishes  The dorsal, caudal and anal fins have fused into a single continuous fin that extends around the entire rear third of the body.  The change in body form of the lungfishes may be an example of paedomorphosis.  They were initially considered to be salamanders when first described.

Lungfishes  The Australian lungfish can gulp air and survive being in oxygen poor water, but cannot live out of water.  In contrast, the South American and African species can survive out of water for long periods of time.  The African species live in seasonal steams and ponds that dry out, but the lungfish survives by burrowing into the mud and forming a cocoon in which it survives until the water returns.

African lungfish

South American Lungfish

Australian Lungfish Australian%20lungfish%20copyright%20Jean%20Joss-370_12548_1.jpg

The discovery of living coelacanths  Coelacanths were believed to have been extinct for perhaps 50 million years (there are fossils identical in appearance that are 70 million years old) when one was caught by a South African fishing boat in  The curator of a small museum, M. Courtney- Latimer, recognized the fish was unusual and she brought it to the attention of the icthyologist J.L.B. Smith who after some delay in arriving identified the fish.

The discovery of living coelacanths  Unfortunately, the delay in arriving meant the fish had badly decomposed and many important structures had been lost.  Smith named the fish (Latimeria) in honor of Courtney- Latimer and then embarked on a 14-year quest to find another coelacanth.  But it wasn’t until 1952 that a second was caught off the Comoro Islands, north of Madagascar, which is where the fish occur naturally (the 1938 fish apparently had drifted far from its normal range). The story is told in Smith’s book “The search beneath the sea.”

Images from the rediscovery of the Coelacanth off the Comoros 1952.

Coelacanths  In 1998 another population of Latimeria [but a different species] was discovered off Indonesia (10,000km east of the Comoros).  Coelacanths are large fish up to about 5 feet long, blue-grey in color with white spots.  They live in deep (70-400m) cold water and are predators feeding mainly on lanternfish.

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Coelacanths  Coelacanths are readily identified from their fins. The caudal fin has a small median lobe. The caudal fin has a small median lobe. Each of the paired fins is very mobile and has a long fleshy basal lobe. Each of the paired fins is very mobile and has a long fleshy basal lobe. The anterior dorsal fin’s fleshy lobe is reduced and it possesses long protective hollow spines (coelacanth means “hollow spine”). The anterior dorsal fin’s fleshy lobe is reduced and it possesses long protective hollow spines (coelacanth means “hollow spine”). When they swim coelacanths move their pelvic and pectoral fins in the same pattern that tetrapods walk. When they swim coelacanths move their pelvic and pectoral fins in the same pattern that tetrapods walk.

Coelacanths  Because coelacanths possess an unusual suite of characters including fat-filled lungs, a high level of urea in the blood, a liquid filled notochord, lobed fins, ventral kidneys and a reduced brain there has been debate about their phylogenetic affinities.  The consensus today is that coelacanths are a sister group to the Rhipidistia which gave rise to the lungfish and tetrapods.  We will discuss the origins of the tetrapods shortly.

Global fisheries and conservation  Before moving on from fishes to the tetrapods, I want to devote some time to fishing.  It is a sad fact that global fish stocks have been enormously depleted and in most places fish populations are a pale shadow of their former abundance.

Global fisheries and conservation  For example: Captain John Smith describing tributaries of the Chesapeake in 1608 “… in diverse places that abundance of fish lying so thicke with their heads above the water, as for want of nets we attempted to catch them with a frying pan, but we found it a bad instrument to catch fish with. Neither better fish more plenty or variety had any of us ever seene, in any place swimming in the water than in the Bay of the Chesapeack, but there not to be caught with frying pans.”

Global fisheries and conservation  Captain John Smith again: Having grounded on an oyster bed in the Potomac as the tide was going out “…we spied many fishes lurking amongst the weeds on the sands, our captaine sporting himself to catch them by nailing them to the ground with his sword, set us all a fishing in that manner, by this devise, we tooke more in an houre than we all could eat.”

Global fisheries and conservation  Clearly a different level of fish abundance than we encounter today.  Similarly abundant numbers of fish were described in the waters off New England and eastern Canada.

The Grand Banks fishery  John Cabot voyaged to Newfoundland in The Milanese ambassador to London reported what he had heard from Cabot about the fishing there: “they assert that the sea there is swarming with fish, which can be taken not only with the net, but in baskets let down with a stone, so that it sinks in the water. I have heard this Messer Cabot state so much.”

The Grand Banks fishery  Two centuries later Pierre de Charlevoix in 1719 described the Grand Banks of Newfoundland “What is called the great bank of Newfoundland … you find on it a prodigious quantity of shell- fish, with several other sorts of fishes of all sizes, most part of which serve for the common nourishment of the cod, the number of which seems to equal that of the grains of sand which cover this bank. For more than two centuries since, there have been loaded with them two to three hundred ships annually, notwithstanding the diminution is not perceivable.”

The Grand Banks fishery  The rich fishing grounds off the northeastern U.S. and eastern Canada result from a combination of factors.  The various banks (the Grand Banks, George’s Bank, Brown’s Bank and others) are deposits of moraine deposited by glaciers.

The Grand Banks fishery  The water above them is relatively shallow ( feet in most places) and they occur at the confluence of the cold nutrient rich northern Labrador current and the warm southern Gulf Stream.  The mixing of these currents combines warmth and nutrients to produce massive blooms of plankton that supported huge schools of mackerel and herring that in turn supported cod and other predators.

The Grand Banks Grand_Banks_Map.jpg

George’s Bank and Brown’s Bank

The Grand Banks fishery  In 1992 the Canadian Government placed a two year moratorium on cod fishing, which was extended indefinitely and remains in place today. In 2003 the two main populations of Atlantic cod were added to Canada’s endangered species list.  In U.S. waters cod populations have similarly plummeted.  What happened? Industrial fishing happened.

The Grand Banks fishery  Up until the early 20 th century, cod-fishing had been almost exclusively by schooners using hand lines but then steam trawlers were introduced to North America. With their greater fishing power the steam trawlers soon replaced the schooners and had become common by the 1920’s.  Around the same time fast-freezing technology was developed and the frozen fillet entered the marketplace.

Fishing schooner Olympic about 1911.

Norwegian Cod schooner EBD7-4EDF-8067-DA989C5D9ECD%7D

Steam Trawler Bellerophon bellerophon_large.jpg

The Grand Banks fishery  One of the first fish to be targeted by ships using the new technology was haddock.  Haddock freezes well (but salts poorly and previously had been thrown away by fishermen).  Huge spawning aggregations were discovered on the Georges Bank and heavily fished for.  Catches soared through the 1920’s peaking at 120,000 tonnes in 1929.

The Grand Banks fishery  In 1930 an estimated 37 million haddock were landed in Boston. However, even more were discarded because small mesh nets caught fish indiscriminately and more than two juvenile haddock were discarded for each adult landed.  Not surprisingly haddock numbers crashed falling to 28,000 tonnes by Landings of about 50,000 tonnes per year were sustained into the 1960’s but only because the fishermen began fishing in new waters.

The Grand Banks fishery  In the 1960’s fishing pressure increased immensely as distant-water fishing fleets from Europe moved in to fish (national fishing limits were only 3 miles).  Fleets from Britain, Spain, Portugal, Romania, France, West Germany, Poland, East Germany and Russia crowded into the fishing grounds.

The Grand Banks fishery  The European fishing fleets consisted of groups of factory trawlers supplying mother ships that processed the catch and these had immense fishing and processing capacity (thousands of tons a day) much greater than local fleets.  In an hour a single factory trawler could catch 200 tons of fish, twice as much as a 16th century ship could have caught in an whole season’s fishing.

The Grand Banks fishery  Onboard the mother ships, fish was machine- filleted and frozen or turned into fishmeal. These ships could fish in any kind of weather and stay at sea for months on end.  In 1965 the Soviet Union had 106 factory trawlers and 425 smaller trawlers supplying 30 mother ships and together these took 872,000 tonnes of fish.

Russian factory trawler photosvideos/photos/russian-factory-trawler-fishin.jpg

The Grand Banks fishery  Fishing fleets were able to work cooperatively to exhaust aggregations of fish. When a concentration of fish was found (using the most sophisticated available search equipment) the trawlers would aggregate to fish it into oblivion before dispersing again to seek new schools.

The Grand Banks fishery  By 1974 more than 1,000 European vessels were fishing the banks. Their catch was more than 2 million tonnes, which was 3x the Canadian catch and 10x the New England catch.  Everything was taken juvenile or adult, spawning or not regardless of the future impact on stocks.

The Grand Banks fishery  Two Canadian fisheries scientists, Jeffrey Hutchings and Ransom Myers, have estimated that about eight million tons of northern cod were caught between Cabot's arrival in 1497 and 1750, over the course of 25 to 40 cod generations.  Factory trawlers took the same amount in only 15 years, a period less than the lifetime of a single cod.

The Grand Banks fishery  Catches of fish far exceeded sustainable yields and fisheries began to collapse. The haddock fishery in the Gulf of Maine collapsed in the 1970’s.  In 1977 following Iceland’s lead the U.S. and Canada declared a 200-mile limit and excluded the foreign fishing boats.

The Grand Banks fishery  Instead of attempting to hold down fishing efforts both countries expanded their fleets.  Between 1977 and 1982 the number of New England trawlers increased from 825 to more than 1,400 boats. Domestic overfishing replaced foreign overfishing.

The Grand Banks fishery  By the early 1980’s fishing catches had risen to twice the level that was sustainable, but by investing in more sophisticated equipment fishermen could still make a living.  However, at this point fishermen were killing 60-80% of all the cod, haddock and flounder in the Gulf of Maine every year.

The Grand Banks fishery  In the mid-1980’s U.S. fisheries scientists saw the collapse coming and pushed for major cuts in fish landings, but the fishing industry resisted cuts and it wasn’t until the mid 1990’s that reductions were imposed.

The Grand Banks fishery  A similar process played out in Canadian waters. Canadian fisheries scientists overestimated sustainable yields of cod based on a series of bad assumptions. In the 1980’s 5x times as many cod were being taken as should have been removed.  Calls to cut back the fishery were ignored and by 1992 the fishery was finished.

The Grand Banks fishery  Estimates of the size of the original population suggest that there were about 7 million tonnes of cod off the Atlantic coast of Canada in  By 1992 the estimate was 22,000 tonnes (<1/3 of 1% of the original population.).

The Grand Banks fishery  Cod have not made a comeback and there is some debate about why.  However, habitat transformation almost certainly has played a major role.  Before trawling, the sea bottom on the banks was not a layer of mud. Rocks outcrops, boulders and stones provided structure, places for young fish to hide and rich communities of sponges, crabs, mussels, anemones, tube worms and other invertebrates flourished.

The Grand Banks fishery  A bottom trawler’s net is held open by large metal doors weighing thousands of pounds and the bottom of the bag is kept on the seabed by a weighted metal cable. Each pass of a net drags boulders and rocks, buries and crushes invertebrates and leaves behind a virtual moonscape.  Bottom trawling is the ecological equivalent of clear-cutting, but carried out on a much more massive scale and out of view.

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Sea bottom habitat in Canada (left) and Australia (right) before trawling (above) and after (below).

The Grand Banks fishery  After years of bottom trawling the sea bed has been converted from a rich diverse ecosystem to a sterile one.  Unfortunately, the tragedy of the cod fishery is just one example of failed fisheries and the pattern has been repeated worldwide.