Fish [Pices]

Marine vertebrates include fish, as well as certain groups of mammals, birds, and reptiles. Of these, only fish have gills for breathing in the water. Gills are filaments of finely divided blood vessels. The blood circulating through the vessels has hemoglobin to absorb oxygen molecules from the water.

The two general divisions of this phylum are the jawless fish (containing only two groups), and the jawed fish. Great Bay Estuary has one jawless species, the sea lamprey, which is seen only at the end of its life as it migrates upstream in spring to spawn and die. Its long, tubular body has a round sucking mouth lined with teeth and a rasping tongue, a cartilaginous skeleton, no scales, seven round gill openings behind the eyes, and no paired fins (just two dorsals and a small tail).

Of the jawed fish, the two main classes are the cartilaginous fish (sharks, skates, rays) and the bony fish. Two skates come to Great Bay Estuary: the little skate Raja erinacea, and the winter skate Raja ocellata. They are bottom-dwelling fish with broad flat bodies, wing-like fins, and thick tails with small dorsal fins on top. Water enters their gills through slits just behind their eyes, and exits through five pairs of slits on their underside.

About 90 percent of all fish have bony skeletons with some cartilaginous pieces. Their skeletons provide points of attachment for muscles. Muscles play a big part in their locomotion, since fins are moved by their contractions. A fish is propelled forward by contractions on alternating sides of its body, which undulate the tail (caudal) fin. It brakes and steers in any direction with paired pectoral fins and pelvic fins on its sides. It is prevented from rolling to either side by the dorsal fin down the back and the anal fin behind the belly. These fins consist of membranes extending from the skin, supported by spiny or soft rods, called rays.

Unique to bony fish is an organ called the air bladder or swim bladder, which keeps the fish from sinking. It sits just above the intestines, and is filled with gas produced by a gland. By inflating or deflating its sack, a fish is able maintain buoyancy relative to the pressure of surrounding water. To deflate the sack, it contracts muscle surrounding the bladder. To inflate the sack it relaxes the muscles.

A covering of protective scales grows from pockets in the skin. Additional protection is provided by mucus secreted from cells in the skin.

The colors of fish of all groups (excepting a few individual species) is reflected in two ways. The various hues (color names like brown, red, yellow) are created by chemicals called pigments. These are in the form of tiny granules in cells called chromatophores, or pigment cells. Light is also reflected and refracted by other kinds of chemicals in reflective cells, called leucophores and iridophores. Pigments are mainly in cells of the epidermis, a thick lower layer of skin.

The silvery white sheen, say on the bellies of herring, is created by reflective cells alone. The stripes on the striped bass are created by rows of black pigment cells. Black pigments also play a part in the shading that creates iridescent greens and blues.

A flounder camouflages itself by changing color to blend with the sediments it is on. Color changes like this are possible because the pigment granules can either spread out or aggregate within these cells. Expanding intensifies a color, aggregating to a central dot fades a color.

Fish can also sense vibrations in the water from things like currents, sounds and the movements of nearby fish, with a series of cells down its side called the lateral line. These cells contain minute hairs called cilia, which pick up these vibrations and trigger electrical impulses that travel up the spinal cord to the brain.

Biologists have identified 52 species of fish in Great Bay Estuary. Here is a look at some of them.

Sea Lamprey
Petromyzon marinus
Lampreys are one of two groups referred to as the primitive fish, because they have no jaws. Instead, their mouths are circular, suction disks with rows of hooked teeth in a concentric pattern.

As adults out in the ocean, the sea lamprey uses its oral disk to latch onto other fish. Penetrating the victim’s skin with its toothed tongue, the lamprey feeds on its blood and body fluids. The lamprey maintains this flow by secreting an anti-coagulant from its mouth, preventing the victim’s blood from clotting. The oral disk shown at right is seen through a piece of Plexiglas.

Sea lampreys have long, tubular bodies like eels, but with cartilaginous skeletons. They have two elongated dorsal fins, but no paired fins on their sides. Their skin is mottled brown, has no scales and is covered by mucus to protect them from infection. They breathe through two sets of gill openings, in the form of seven portals in a line just behind each eye.

Each spring sexually mature lampreys return to the estuary and head upriver to spawn. They are found primarily in the Exeter and Cocheco rivers, as they bypass the dams by swimming up the fish ladders operated by the New Hampshire Fish & Game Department. By the time they start their spawning run, lampreys have stopped feeding. At this point their digestive tract and teeth are breaking down, and they may latch onto hard surfaces, but only for rest or security. When they finally reach clear, shallow waters with gravelly bottoms, the females set about arranging rocks into arc-shaped nests. There, protected from the current, she lays and the male fertilizes, up to 400,000 eggs. After spawning, they die.

About a week later, larvae emerge from the eggs. After about a month in the nest, they drift downstream where they burrow into silty sediments. Poking their heads out, they filter algae and detritus from the water for about the next four years. At about six inches, with their oral disks developed, they begin their journey to the ocean. There they take up their carnivorous, parasitic lifestyle. By the time they are sexually mature, they are up to three feet long.

American Eel
Anguilla rostrata
American eels are easily recognizable with their tubular bodies, very thick greenish-brown skin with embedded scales, and continuous anal and dorsal fins. The females grow longer than the males, to about three feet.

They are nocturnal bottom dwellers in estuaries and rivers. They feed on a wide variety of fauna, including insects, frogs, other fish, crustaceans, mollusks and worms.

In the late summer and fall, mature American eels from all along the Atlantic coast head out to sea. They assemble for spawning from February to April in the Sargasso Sea, off the northeast coast of South America. Though the adults are presumed to die after spawning, their larvae are carried back up the Atlantic coast by the Gulf Stream. These remain off the coast for nearly a year until they grow eel-shaped bodies, though without pigment. At this stage they are called “glass eels.” Most reach the New England coast by spring. They typically they don’t gain their pigment until they move into estuaries and migrate up rivers. At this stage they are then called “elvers.”

Once they have grown seven to ten inches long, they become sexually differentiated and are called “yellow eels.” They remain in estuaries and rivers until they mature for spawning. This takes at least three or four years, commonly ten to twelve years. Males usually prefer to remain in the estuaries. Before they head back out to sea, they again metamorphose into silver eels. The changes that occur in the last stage of their life, including thicker skin, a metallic, bronze-black color, and larger eyes with altered pigments, are thought to be adaptations for swimming at oceanic depths.

It is common for eels to leave the water to slither through wet vegetation, over slimy concrete walls, or across other surfaces. Though not as efficient as breathing with gills in water, eels can also absorb oxygen through their moist skin.

Bald eagles, gulls and other fish-eating birds prey on American eels. We also harvest them for food and as bait fish.

American Shad
Alosa sapidissima
At two and a half feet long when full grown, this is the largest of the herring family in this region. Shad are a bluish-green color on top, fading to silvery white on their sides and belly. Like other herring they have a forked tail. One way to distinguish shad from local river herring is the profile of their lower jaw. The local river herring jaw is deeply concave with a sharp angle. The shad’s is slightly concave with no sharp angle.

American shad from all along the Atlantic coast spend the summer and fall in the Gulf of Maine. When winter comes, they disperse. By early spring they are returning to the rivers where they originated to spawn. Spawning has been found to take place during the latter part of the day, when the water temperature is around 45° to 50° F. After this they immediately swim back to sea. In northern rivers, many shad survive to spawn again. But from North Carolina on south, they die after spawning.

The shad eggs hatch and their planktonic larvae mature as they flow downstream. By the fourth week they are juveniles, feeding on tiny crustaceans and aquatic insects. They stay in the river until fall, when they head out to sea to mature. There they feed mainly on zooplankton, small fish, fish eggs and amphipods. Three to five years later the males are old enough to spawn. Four to six years later, the females are. They typically live to be five to seven years old.

After pollution, dams and over fishing contributed to a decline in the shad population, the New Hampshire Fish and Game Deapartment began in 1980 to stock spawning adults in the Lamprey, Cocheco and Exeter rivers. The stocking has since stopped and adults have been returning the rivers, though not in the numbers that had been stocked. Commercial shad fishers harvest shad eggs. They may be catching some of those fish as they return, but before they have a chance to spawn. Seals also prey on shad.

Atlantic Herring
Clupea harengus
Though not as common in the estuary as other fish species, at least some juvenile Atlantic Herring have been caught at Fox Point. They can generally be found in tide pools and salt marshes. These marine fish migrate in from the ocean, where they are most abundant north of Cape Cod. They have iridescent bluish bodies with silvery sides and bellies.

They have been found to spawn in large numbers in late summer and fall in the Gulf of Maine at Jeffreys Ledge, a 33-mile long shallow stretch off the coast, which reaches from northern Massachusetts to southern Maine. Currents carry the larvae into estuaries or along the coast. In April they begin to metamorphose into juveniles, and start swimming in schools toward the shore. There they feed on zooplankton. After spawning, the adults head to warmer water (some south of Cape Cod) for winter. They return to the Gulf of Maine in spring to feed on particular euphausiid and copepod species of zooplankton. Their schools have been estimated to contain hundreds of thousands of similar-sized fish.

The main commercial harvest of adult Atlantic Herring is located at Jeffreys Ledge. Juveniles, or “sardines,” are caught in coastal waters from Maine to New Brunswick. These herring are also food for Atlantic cod, haddock, swordfish and bluefin tuna.

Atlantic Menhaden
Brevoortia tyrannus
Atlantic Menhaden swim in and out of Great Bay Estuary at different times of the year. These marine fish migrate up and down the Atlantic coast, using estuaries along the way as nursery areas.

From November to March each year, Atlantic Menhaden of all ages meet up off the coast of North Carolina, and many of them spawn there. The adults then move inshore and migrate northward with some spawning along the coast and in embayments. The oldest and largest of these go the greatest distance, as far as the Gulf of Maine.

Larvae in the North Atlantic migrate into estuaries from May to October. As they grow into juveniles they are feeding on zooplankton and swimming in the freshwater shallows of estuaries, especially among marsh grasses. In their first year these fish do not go far. When they do, usually the next year, schools of juveniles emigrate from the estuaries between August and November. Their adulthood is reached when they transition to filter feeding. They loose their teeth and gain complex gill rakers for straining zooplankton, phytoplankton and diatoms. Most Atlantic Menhaden hauled in commercial catches are about four to six years old, though they can live from eight to ten years old.

This fish is an example of a link between estuarine productivity and the offshore food web. Atlantic Menhaden consume large amounts of estuarine plants and animals, and transfers a portion of this productivity to the shelf water predators like bluefish, striped bass, bluefin tuna and sharks.

Atlantic Silverside
Menidia menidia
The most common resident finfish of Great Bay Estuary, the Atlantic silverside, tolerates salinities ranging from freshwater to 37.8‰ (ocean water is 35‰). Groups of these fish move in schools along sand and gravel areas, tidal creeks and flooded marsh vegetation. As omnivores, they consume a range of biota including algae, amphipods, detritus, diatoms, fish eggs, insects, mysids and worms.

Silversides spawn every couple of weeks at high tide, starting during a full moon in early spring. Schools move inshore at peak high tides and release their eggs and milt onto substrates like cordgrass, eelgrass or filamentous algae. Depending on water temperature it can take anywhere from three to 27 days for the eggs to incubate. The yolk-sac larvae stage follows, lasting for up to five days until the yolk is consumed. The juvenile stage begins once their anus migrates from their front end to their back end. They remain as juveniles until they stop growing in late fall. Most silversides live only one more year.

About six inches long, silversides are important forage fish for striped bass, Atlantic mackerel and bluefish. Schools of them are easily spotted in late summer from the dock at Hilton Park in Dover.

Atlantic Tomcod
Microgadus tomcod
With its chin barbell and body shape, the Atlantic tomcod could be mistaken for the Atlantic cod, but for its smaller size (about 12 inches), narrower ventral fins, and rounder caudal fins. This common estuarine fish spawns from November to February in fresh water at the heads of estuaries from the Hudson River north. After hatching in summer, the larvae remain in these low-salinity waters as they grow to be juveniles. Depending on the estuary they are in, these juveniles consume some combination of copepods, amphipods, mysids and crustaceans. Adults eat small crustaceans, worms, mollusks and fish. They use a variety of habitats, swimming about a mile offshore into coastal waters, through estuarine eelgrass beds, tidal marshes and mudflats, and up rivers beyond the tides. They range to Virginia. Not much is known of tomcod predators, though striped bass have been found to prey on juvenile tomcods. There is a commercial tomcod fishery in the Great Bay and Hampton/Seabrook estuaries.

Bluefish
Pomatomus saltatrix
Notable features of this popular sport fish include a bluish-green color that fades to silvery white on its belly, a forked caudal fin, a black spot at the base of its pectoral fin, and a length of up to three feet.

Bluefish prefer water temperatures around 50°F. In the winter they range south of Florida to the south Atlantic. They return north along the outer continental shelf (30 to 60 miles offshore) in two spawning migrations. Some schools spawn from April to May along the shelf from Florida to North Carolina. The others spawn from June to August farther up the shelf from North Carolina to Long Island. Afterward many adults move into coastal areas up to Long Island Sound. Bluefish venturing into estuaries north of Cape Cod are two years or older. Spring-spawned juveniles are carried by the Gulf Stream until warm waters allow them to swim toward mid-Atlantic estuaries. Most summer-spawned juveniles remain offshore until the fall migration south.

When young-of-the-year bluefish make their initial migration to estuaries the following spring or summer, they feed on shrimp, killifish and silversides. Adult bluefish are voracious predators, feeding during daylight on other fish including herring, menhaden, mackerel, and even smaller bluefish. The bluefish is another example of a species that transports estuarine productivity offshore. Sharks, tuna and swordfish are possible predators.

Flounders
Smooth Flounder (Pleuronectes putnami)
& Winter Flounder (Pleuronectes americanus)
These are types of flat fish, characterized by compressed bodies. At a certain point in the larval stage of flat fish, one eye migrates around to a permanent position right next to the other eye. At this point the fish swims on its side across substrates, with its eyes on top. (Some flounder are “lefteye” species; their eyes are on the left side of their bodies when viewed from above. Smooth and winter flounder “righteye” species.)

The two resemble each other. But smooth flounder (pictured) are usually dark brown, with dorsal and anal fins angled to a central peak, and scaleless skin between their eyes. Winter flounder color varies, and they have scales between their eyes. They are commonly found on soft sediments and among eelgrass beds from the intertidal zone to channel bottoms.

A flounder feeds only during the day when there is enough light to see prey. Lying partially buried in sediment, it props its head up with its dorsal fin to watch quietly for small invertebrates like crustaceans, cnidarians (e.g. hydroids and jellyfish), worms and mollusks. When they are within range, the flounder lunges at them. Winter flounder also swim onto sand and mud flats as the tide comes in, nipping at the clam siphons extending out of the sediments.

Winter flounder spawn from January to May on the sandy bottoms of inshore and coastal waters with salinities of 31‰ to 33‰. Juveniles are most abundant in the estuary between late spring and early fall. North of Cape Cod, winter flounder have been found to stay inshore where water salinities are between 5‰ and 35‰. The larger adults migrate out of Great Bay Estuary each fall and return in the spring. Bluefish, striped bass and other large fish prey on them. Recreational and commercial catches of winter flounder have declined significantly due to heavy commercial fishing of this species in the Gulf of Maine.

Mummichog
Fundulus heteroclitus
From spring through fall you can find plenty of mummichogs close to the shore all over the estuary. They are a relatively stress-resistant fish, tolerating salinities ranging from seawater to freshwater, and rapid temperatures swings. This helps them survive in marsh pannes which, given their small size and occasional isolation from regular tides, have these harsh physical extremes.

Mummichogs have been found in various studies to have a wide range of food items in their stomachs, including tiny shrimp, copepods, amphipods, worms, snails, small fish, fish eggs, bivalves, diatoms, algae and eelgrass. Though they also consume lots of detritus, they do not get significant energy from it.

They spawn during spring tides at least eight times per season, casting eggs on a variety of substrates, including cordgrass stems, ribbed mussel shells, algal mats and shallow pits covered by the female. The eggs incubate in the open until the next spring tide, which triggers hatching of developed eggs. The young fish stay in the intertidal zone until they grow to a little over a half inch long. Then they venture into the marsh with adults. These fish stick close to the marsh creek or other water body that is their home range. In winter, some stay in pools where they burrow six to eight inches down in the mud.

Though they can live four years and grow five inches long, fewer than eight percent of them finish their second growing season and grow beyond two or three inches.

Eels, white perch and summer flounder are some of the fish that prey on mummichogs.

Rainbow Smelt
Osmerus mordax
Smelt spend the winter in the estuary waiting for water temperatures to rise so they can swim up rivers to freshwater spawning areas. They spawn at night in high-velocity water flows, and then swim back downstream to the estuary or coastal waters. Later, river currents carry the larvae down to the estuary. Schools of juvenile smelt have been found to swim into higher salinity waters as they grow, spending their days in deeper channel areas and eelgrass beds. They feed on small crustaceans, worms and fish. Later when adults return from the coast, juveniles join them in the upper estuary. Adults feed on small fish and crustaceans.

When you see the little bob houses sitting on the ice covering the estuaries rivers and Great Bay, you know that it is smelt fishing season. Smelt fishers drill holes in the ice and fish through them with small rods or lines hanging from wood beams across the holes. Live smelt have transparent olive green tops and silvery bellies, and are said to have a vague cucumber smell when they are caught.

Seals and large fish like striped bass and bluefish prey on rainbow smelt.

River Herring
Alewife (Alosa pseudoharengus) & Blueback (Alosa aestivalis)
You can get a look at these fish in spring as they head up rivers to spawn in fresh water. They can be seen swimming and jumping as they wait to move up the ladders that bypass damns in the rivers. The fish usually return from mid-April to mid-May to spawn in the estuary's tributaries. The alewives run three to four weeks ahead of the bluebacks, and swim only during daylight hours (mostly between 3 p.m. and 5 p.m.). They travel further upstream than the bluebacks, and spawn in the quiet waters of river bends, lakes and ponds. The bluebacks prefer to spawn in fast currents.

A school of Alewives waiting to exit the Lamprey fish ladder.

As females release their eggs onto the substrate, the males release their milt to fertilize the eggs. Within five days of spawning, both the males and the females migrate back to the sea, leaving the eggs to incubate. These hatch larvae equipped with a yolk sac for feeding, which transform to free swimming larvae, and then to juveniles nearly three quarters of an inch long. After swimming upriver for a time, juveniles of both species then migrate into the estuary and out to the coast. Things like heavy rains and dropping water temperature prompt large schools of juveniles to migrate out of the rivers between June and November.

Both species feed mostly on zooplankton. The larger adults also feed on small fish, insects and crustaceans. In the food web, herring link zooplankton to gulls and terns, as well as bluefish, cod and striped bass. They also have commercial value: as lobster bait; as oil, protein and fish meal (especially for animal foods); and as salted, smoked or fresh people food.

Sticklebacks
Pungitius pungitius
These small fish, usually about three inches long, have no scales. Their most prominent features are three or more spines on their backs, as well as a spine on their bellies. The colors can vary, even within species, depending on the season and habitat they are in.

They can be found in fresh, brackish and saltwater on both sides of the northern Atlantic, as well as the shores of the northeastern Pacific. Though they can sometimes be found offshore in floating vegetation, most sticklebacks swim along the vegetated areas of the shore, such as eelgrass beds and marsh creeks. They feed on small invertebrates, insect larvae, fish eggs and fry fish.

The male of most species builds a barrel-shaped nest of vegetation in shallow areas. It collects the material with its mouth and cements it together with mucous threads. Females deposit eggs in the inch-wide opening. After fertilizing the eggs the male guards them, using its spines as weapons. When the eggs are ready to hatch, he’ll take the nest apart and guard the fry. Many males die after spawning.

The nine-spine stickleback (Pungitius pungitius) has a long, thin body colored olive with bars of slightly darker color. The spines can number from seven to ten, though most do have nine. It is mainly an estuarine species.

The three-spine stickleback (Gasterosteus aculeatus) has a stouter body and with two spines midway down its back, and a third smaller one right by its dorsal fin. The color of its back varies, but it has a silvery belly. It also has a series of bony, vertical plates on its sides. When breeding from May to June, the male has red a belly and blue eyes. He zigzags in front of his nest to attract multiple female mates.

The four-spine stickleback (Apeltes quadracus) has a stout body, which tapers at both ends, and is olive or brown with darker splotches. It has four to five spines (the last being attached to the dorsal fin). When it spawns from May to June, its nest is less elaborate than the three-spine stickleback.

Striped Bass
Morone saxatilis
Striped bass are seasonal migrants in Great Bay Estuary, from mid to late May until October. Though some have been caught here in winter and early spring, they are generally thought to migrate in the fall to their main spawning grounds in the mid-Atlantic region, particularly tributaries of the Chesapeake Bay.

Mid-Atlantic striped bass spawning takes place from April to June, depending on the river or estuary. The resulting juveniles remain for about two years in the same waters, swimming in schools ranging from a few individuals to thousands. When they reach two or three years old, most will head out to the coast for migrations that move north in summer and south in fall and winter. Some remain close to where they originated. Though some striped bass have been found to live into their 20's, most live to about 12 years old.

Juveniles feed on a diet that can include amphipods, decapods, mysids, insects and bay anchovies. Adults consume these items, as well as lobsters, assorted crabs and bivalves, and other fish species, including herring, mummichog, rainbow smelt, white perch and American eel. Rather than feeding continually, schools of stripped bass feed intermittently, focusing on the most abundant prey.

Striped bass larger than 32 inches are a popular commercial and sport fish catch.

White Perch
Morone Americana
White perch is a relative of the striped bass (Morone saxatilis). Some of its distinguishing features include no stripes, a deeper, flatter body, and dorsal fins that are joined at their base. It can handle fresh or salt water habitats.

As temperatures rise in spring, marine and estuarine white perch typically spawn in the freshwater rivers, marshes and subtidal areas of estuaries with very low salinity levels. It can take nearly five days for eggs to hatch. Then the larvae drift along in the flow until they develop into juveniles.

The estuary may serve as a nursery for up to a year, supplying them with zooplankton to eat. In some studies, juveniles swim estuary creeks and shallow zones where mud and plants cover the bottom. In other studies, schools of white perch were found to sometimes move offshore in daylight and return to protected water at night. As they grow older, they consume amphipods, shrimp, crabs and fish eggs. Juveniles spend the winter in the deep parts of rivers, tidal creeks and bays. By the time they are nine inches long they eat only other fish. Larger fish, in turn, prey on them.

Copyright © 2006 Barbara Driscoll.