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    Osteichthyes or Bony Fish

     
         
     

    Bony fishes, or the class Osteichthyes, are the largest group of fishes taxonomically, with over 20, 000 species worldwide. Like the other classes of fish (jawless and cartilaginous), bony fish may be generally defined as vertebrates that balance and propel themselves with fins and gather oxygen with gills.  True bones are the one character that sets the Osteichthyes apart from the other classes. 

     

    About sixty different species can be found on Stellwagen Bank with some regularity and others may visit sporadically depending upon variations in water conditions. At the southern end of the Gulf of Maine, Stellwagen often acts as a catch basin for a wide range of species: northern or boreal species mix with the more southern, warm temperate species of the Mid-Atlantic. Generally speaking, many Stellwagen species are of the boreal type and, are resident throughout the year with minor movements based upon temperature, such as sand lance, yellowtail flounder and cod. During the bounty of summer and fall, species diversity on the Bank is increased by migrants such as tuna, bluefish and summer flounder. 

     

    The few species descriptions that follow demonstrate the taxonomic and ecological diversity of the bony fishes of Stellwagen. It is, by no means, exhaustive. Included are some of the bony fishes commonly seen by whale-watchers or, describe animals that are known to be of importance to other aspects of the ecology of the Bank. 

     

    Species descriptions

     

    Sand lance, Ammodytes americanus, may be one of the most important bait fishes on Stellwagen Bank. Rarely over 15 cm (six inches) long, these skinny little fish form schools of thousands of writhing bodies that may be seen from the surface as they feed. 

    sand lance, roughly life-size

     

    Their long, tapered bodies and pointed chins make them superb burrowers: they can dive into sand and fine gravel in an instant to avoid predators, burrowing inches below the surface. At the same time, undulations along the length of their bodies (hence the common name, “sand eels”) quickly carry them through the water column for feeding. 

     

    Their favored foods are copepods and other zooplankters that swarm throughout summer and early fall. Since zooplankton may migrate up and down the water column throughout the day, schools of sand lance need to be highly mobile as they snap up individual prey items with their sharp jaws and keen vision. At the same time, this puts the sand lance in a vulnerable position: small fish are the favored food of everything else. 

     

    a swarm of sand lance at the surface

     

    Very few other fish, birds or marine mammals will pass up an opportunity to feed on sand lance when they are at the surface. The very fact that sand lance form schools to decrease the chances that any one individual will be caught, makes them the perfect find: where there is one, there is likely to be thousands.   

     

    Most years, shallow, sandy Stellwagen Bank seems to be a perfect place for sand lance. During the yearly, winter spawn, females release over twenty thousand tiny eggs that settle and attach to the surrounding sand. As the water temperature falls to forty-eight degrees F (88.8 degrees C) the eggs hatch, releasing clouds of larval fish that will drift with the tides, alongside copepods, for about two months. Those that escape the filter feeders, gaining in size to fight the currents, may join the breeding population by their second year. Life expectancy is rarely over five years. 

     

    Sand eels are gathered locally for bait. Peaks and valleys of sand lance populations probably represent cycles in water temperature and circulation and on the populations of their predators. At the same time, fluctuations of sand lance reverberate throughout the Bank (see Schiling, 1992). How over-fishing of their predators (cod and mackerel, for example) may affect the long term health of sand lance populations is still unknown. 

     

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    sleek shape of an alewife

    Alewife, Pomolobus pseudoharengus, are one of a number of closely related species in the herring tribe, including, in our area, herring, blueback, shad and menhaden. Most of these species are relatively small, rarely over 30 cm (12 inches) long, deep bodied and flattened vertically. Large eyes and mouths make them efficient hunters of some phytoplankton, zooplankton and smaller fish.   

     

    Moving around in massive, silvery schools throughout the year, bands of similar aged fish search for patches of productive water for feeding along the coast and shallow banks. Toward April and into June, mature adults gather outside the mouths of small streams and rivers before heading upstream. Almost any fresh water source may serve as a spawning site where massive amounts of milt and eggs mix creating bands of tiny pink eggs that attach to surrounding substrate. While spawning, adults fast, living off of the layers of fat laid down during winter feeding. After the spawn, spent fish head back out to sea to hunt voraciously. 

     

    Within a week, eggs hatch and clouds of tiny fry drift and feed toward the sea. By late summer and early fall schools of young alewife, 10 cm (4 inches) long become fully marine animals perhaps schooling with the same stream mates throughout their life. Eaten by just about anything larger than themselves, including other alewife, schools offer some protection against the onslaught of predators. The few that do survive to their third or fourth year may enter their natal stream to continue the anadromous cycle. Like the alewife, blueback and shad are anadromous while herring and menhaden spawn at sea. 

     

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    the distinctive shape of the cod: three dorsal fins, thread-like 
    ventral fins for sensing the bottom and a chin barbel for  
    sensing prey in dark waters

    Atlantic cod, Gadus callarias, stands out as one of the most famous fish species in the North Atlantic: numerous and good tasting, economies have been built upon their populations and regions have been named for them.   

     

    Cod belong to the Gadidae family, a diverse group of related species including pollock, haddock and hakes. Most are good sized predators with large eyes and mouths, three separate dorsal fins, a chin barbel, a broad, powerful tail for quick propulsion, and ventral fins modified into “feelers” for sensing the bottom. As a general rule, the Gadidae spend most of their life traveling close to the bottom over a variety of habitats and in large schools. Their body design gives them a certain amount of flexibility in hunting styles: they can sit and pounce on creatures passing by, root through sand or mud for invertebrates or actively hunt down schools of small fish. 

     

    The cod has an exceptionally varied diet but could be said to specialize to some degree on mollusks, especially large clams and snails swallowed whole. But just about anything can be taken: sea squirts, squid, comb jellies, shrimp, crabs, herring, sand lance and more. Some individuals tend to be rather sedentary, migrating to find fresh hunting grounds or spawning, while others actively school and carry out extensive movements throughout the year. Some individuals may carry out both lifestyles throughout their life. Their flexible diet and hunting styles has made them, pound for pound, one of the most important large predators in the Gulf of Maine (at least, historically). 

     

    During the heart of winter massive schools of cod congregate over definite spawning sites, one of which lies just to the west and parallel to Stellwagen. Teaming masses stimulate both sexes to spawn with large females releasing a few million eggs. These tiny, transparent eggs become part of the planktonic fauna and drift away on the great, counter clockwise currents of the Gulf of Maine. After about a month, larva hatch and continue their life as plankton for up to two months, growing on copepods and escaping planktivores like herring. Somewhere at about three months, young cod “seek bottom,” leaving the currents for a life among the protection of kelp beds and colonies of invertebrates

     

    Between three and eight years, depending upon their success at feeding, adults will find their way to the spawning grounds. Interestingly, cod spawned on the grounds near Stellwagen may produce adults swimming off of Canada and vice versa, reinforcing the idea that Stellwagen is nourished and nourishes a wider habitat. 

     

    Despite their adaptability, cod stocks have not been able to keep up with the demands we have placed upon them. A wide variety of fishing styles and technologies and sheer numbers of boats fishing have brought a number of cod stocks to “economic extinction.” Severe catch limits and all out moratoriums (in Canadian waters) have aimed at giving stocks a chance to rebuild. 

     

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    Bluefin tuna, Thunnus thynnus, are the largest of the Scombrids, a family of fishes that includes mackerel. Powerful and bullet-shaped, this family includes some of the swiftest predators in water.

     

    The bluefin is truly superlative: “giants” may push 640 kg (1400 pounds) and reach speeds topping 88 km (55 miles) per hour. The shape of a bluefin is very telling. Sickle-shaped dorsal and anal fins lend stability. A retractable first dorsal fin can be hidden to reduce drag and long, blue pectoral fins give maneuverability. Thick, bright red muscle powers the half moon caudal fin and large gills gather massive amounts of oxygen for activity. The body design describes a highly mobile predator built for the chase. 

     

    the unique shape of a bluefin tuna lends speed and efficiency

     

    What is not outwardly visible, is one of the most remarkable characteristics of bluefins: unlike most fish, they can maintain a relatively high body temperature. Thermoregulation allows these animals to forage further north, into more productive habitats and warm muscles increase potential speeds for hunting. Like whales, sheer mass and less surface area reduces the heat lost (heat generated by calories burned through swimming) to the surrounding water. This is referred to as “gigantothermy.” Additionally, a counter current blood system, where veins and arteries are enmeshed allows the cool blood of the veins, returning from the body, to be warmed by blood in the arteries heading out to the body. 

     

    Despite their abilities to maintain higher than water body temperatures, bluefin are sensitive to temperatures. Small tuna rarely venture into surface waters colder than 16 degrees C (60 degrees F) while giants may hunt in waters ten degrees colder. Aside from seeking out comfortable waters, tuna are constantly on the move for prey. Depending upon their size class and local environs, tuna chase down schools of sand lance, herring, mackerel, bluefish and just about anything else (one tuna caught off Provincetown contained, among other things, a young harbor seal). 

     

    Bluefins are a summer visitor on Stellwagen Bank, returning from their winter spawning grounds in the Gulf of Mexico where eggs are released as plankton. Tuna boats impatient for their northern run, watch the local water temperature rise in anticipation of the first schools. Perhaps by early June, bluefin crash onto the Bank to bulk up for the next winter season and the long migration. 

     

    On Stellwagen, the tuna boats of summer are a familiar sight: a tall tower for the captain and a long pulpit off the bow allow tuna to be hooked and harpooned efficiently. This style of fishing takes one high quality fish at a time as opposed to nets that gather whole schools at once. New restrictions on the overall catch are hoped to replenish dwindling stocks. 

     

     

    Ocean sunfish, Mola mola, may be the most commonly seen large fish from whale-watch boats. They also create the most confusion: they can be very large, their body design is unlike any other fish and their behavior tends to elicit sympathy. Whale-watchers find it difficult to tell which end is up when watching these fish. 

     

    the unmistakeable shape of a mola mola or ocean sunfish

    Like many marine fish, mola molas start out life drifting on the currents as plankton. Perhaps for protection or flotation, their tiny bodies are studded with large spines. As they grow, their caudal fin (tail) splits and begins to migrate: the top half heads toward the back, the bottom half towards the belly, leaving nothing but a ruffled lobe of skin where the tail once was. Massive animals, sunfish may grow up to 3 m (10 feet) and weigh just over 900 kg (over a ton). Looking somewhat like a giant, flat head with a large dorsal and anal fin, sunfish can move through the water with surprising speed and grace. 

     

    At the same time, speed is not a characteristic of their feeding style. The puckered mouth of the sunfish is designed for feeding on large plankters, especially jellyfish and combjellies. Cruising slowly through rich waters from the south during winter, sunfish are summer visitors to Stellwagen Bank. They are most frequently seen basking on their sides at the surface (hence 'sunfish'); warming up in the sun between bouts of feeding. This may leave them vulnerable to predators like large sharks. Despite their somewhat helpless appearance, sunfish do have defenses. Their skin is exceptionally tough and thick, up to 15 cm (6 inches) thick. The skin may be a light violet-gray or richly mottled brown and white.They can also swim quickly and have been known to breach. 

     

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