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     DEPARTMENT OF MARINE RESOURCES

 
A HERRINGS TALE
by Stanley Chenoweth

Maine Department of Marine Resources
Bureau of Marine Sciences
West Boothbay Harbor, ME 04575

1989

DMR Research Reference Document 89/12

The Atlantic herring has been an important part of Maine's coastal fisheries for a long time. Many of the herring that we harvest originate and spend most of their lives in our shallow, coastal waters. They spawn on level gravelly beds in the nearshore waters of eastern Maine and their young are dispersed westward along the coast where they spend their first two years of life. Finally, four years later, they join the mature, spawning population on the spawning grounds in eastern Maine.

The herring that spawn along our coast are of particular interest to biologists at Maine's Department of Marine Resources because the important phases of their life history are spent within the State's waters; we know this spawning unit contributes directly to our fisheries and its welfare can depend on our actions. In recent years the Department has spent considerable effort investigating the life cycle of this particular group of herring - their spawning, the dispersal of their larvae, the life of the early pre-commercial juveniles, and the movements of commercial size fish - in order to determine how this spawning unit replenishes itself.

Lets look at the life of a typical herring spawned in eastern Maine as we can best piece it together from research data and what we know of the fishery.

SPAWNING

It is late summer, 1988 and the events affecting the life of our herring (we will call her Sally) begin before she is conceived. During July and August, schools of mature herring begin to form in eastern Maine, around Grand Manan Island and in the channel between Grand Manan and the mainland (see Figure 1). By mid August the herrings' gonads are full of eggs or sperm as they move into the relatively shallow waters along the eastern Maine coast. Their spawning grounds are small, isolated areas of gravelly bottom between Roque Island and Cutler Harbor. The large tides in this region provide a steady flow of water over the egg beds, which is essential for good survival. Also, as we shall soon see, the area is strategically situated for the maximum dispersal of the young.

On an evening in late August, Sally's mother moves with the other herring onto a piece of bottom just off Cutler Harbor to lay her eggs. She deposits about 100,000 eggs, so that they and those from thousands of other female herring form a mat several inches thick which extends over a square kilometer of bottom. The males quickly spread their milt over the egg bed and Sally's life begins.

The first ten days of life go well for sally. She has the luck to avoid being eaten by the hungry codfish that continually nibble at the egg bed and her position near the surface of the bed allows access to oxygen so that she does not smother, as many of her not-so-fortunate cohorts do deeper within the egg mass. About the tenth day Sally is ready to hatch. She is a little over 1/4 inch long, worm-like and almost transparent. She has a sac under her head that contains yolk which should give her a weeks supply of food while she learns to catch her own food. Sally hatches and life begins in earnest.

THE LARVAL STAGE

When Sally hatches she has two concerns - to eat and not to be eaten. She is at a distinct disadvantage because she is a weak swimmer and has to learn to feed in a very hostile environment. During the first week or so she hovers over the bottom near the egg bed making stabs at passing organisms by coiling her body and striking in a manner similar to the striking motion of a snake. Her prey are the tiny larval stages of small crustaceans called copepods; these copepods when they are juveniles and adults will be her principal prey throughout life. As the days go by she becomes more successful at catching her food but her reserve of yolk is rapidly being exhausted. Survival is a race to reach the point where she can support herself by capturing prey before she becomes too weak from lack of nourishment to hunt. Whether or not she survives depends a lot on chance, such as the concentration of prey within her reach. Many of her cohorts are unable to make this important transition, but Sally does and as she grows stronger she moves up in the water column and away from the egg bed.

Sally has hatched out in a unique area as far as water movements are concerned. If you were a spawning herring and wanted to spread your offspring as far as possible so at least some would find an area suitable for survival, then the spawning ground of eastern Maine would be the place. The main current that comes out of the western side of the Bay of Fundy passes to the east of Grand Manan Island and then sweeps in and down the coast towards Penobscot Bay (Figure 1). The water near Grand Manan Channel, the area where the herring spawn, is in somewhat of an eddy, at least during August and early September. Later there seems to be a more definite movement out of the channel and on down the coast. If Sally had hatched a bit earlier she might have remained in the waters that eddy within the Channel and as she grew and became more mobile she most likely would have moved into the nearshore waters of eastern Maine or found her way around to Passamaquoddy Bay. But, by the time Sally moved up into the water column she and her cohorts were part of a water mass moving south towards the main coastal current. By mid-September, when she is almost an inch long, she is swept up in a "fast express west" water mass and is rapidly transported down the coast. By the first of October, Sally's water mass approaches the islands and shoals that stretch out from Penobscot Bay, which include Matinicus, Wooden Ball and Seal Islands. The speed of the water mass is slowed and it begins to break up.

By this time Sally is well over an inch long and is able to swim some distance through the water. Although she cannot yet swim against the currents, she does begin to move vertically and as she enters deeper waters she is caught up in the bottom water that moves into the western passage of Penobscot Bay to replace the surface water that flows out. Sally is joined by a cohort of smaller larvae that have just hatched from eggs laid by herring that spawned around Sea Island in late September. At this point we lose track of Sally. Presumably she remains within the coastal area of Penobscot Bay during the winter, perhaps moving offshore into a little deeper water during the coldest months. We suspect that she may pass through another critical period during the winter when her diet changes from larval copepods to small adult copepods. By spring she has grown to 1 1/2 inches and appears with the remaining members of her cohort as they begin to concentrate in loose schools.

THE EARLY JUVENILE STAGE

In May and June Sally begins metamorphosis, a major transformation in her life when her body changes from a thin, transparent larva to a small but recognizable herring. Within a couple of months her body will deepen, she will begin to grow rapidly, scales will form along her body and the typical blue-green and silvery colors will appear. During this period her schooling instinct will also develop and she will join other members of her cohort and move with them in dense shoals near the surface of the water.

It is June, 1989 and Sally is in Castine Harbor well inside Penobscot Bay. She is about 2 inches long now and is swimming in a small school close to shore with other members of her cohort. This is another critical period in her life when the "eat but not be eaten" problem is most acute. She is still a weak swimmer and will be for another month. She is also larger and has lost her transparent invisibility so that her list of predators has grown. Her refuge is to stay near the surface and hug the shoreline. Her recent growth and activity has also increased her body's demand for food, so she must increase her food consumption. Fortunately this is the time her copepod prey multiply in response to the seasonal increase in their phytoplankton food supply. For Sally everything depends on timing. If she is in phase she will have adequate food, if she is out of phase, or if the numbers of herring in her cohort are too many and consume the food that she requires, then she will starve. Again Sally is lucky and survives, although almost 99% of the herring that hatched with her from her original egg bed have died.

As July approaches there is a dramatic increase in Sally's swimming ability. Along with the other herring in her school she moves off into deeper water where she will remain for the rest of the summer. She will stay with herring of her own age, remaining separate from the juveniles a year or two older than she is, and her group will stay within the Penobscot Bay area.

As fall approaches we again lose track of Sally. By now she is about 4 inches long and approaching the size that is suitable for commercial harvest, but cold weather arrives and she moves off into slightly deeper water to over-winter before she becomes vulnerable to the fishery.

THE COMMERCIAL JUVENILE STAGE

The spring of 1990 finds Sally a robust 5 inches and part of a large school of juveniles hanging out just outside of Matinicus Island off Penobscot Bay. She is about to add another major predator to her list - man. Along the Maine coast herring fishermen have been preparing their twine for the upcoming fishing season. They are looking for 6-7 inch herring, the most desirable size for sardine canning. In eastern Maine the weir fishermen are setting their nets on the weir fences in hopes that this year the herring will move into their areas. Likewise the stop seiners in western Maine are loading their twine into dories, ready to shut off their coves if the herring should come. The purse seiners too have their boats ready to move in on the schools should they move into areas where the stop seines and weirs cannot get them. All of these harvesters depend on the fish moving into depths that can be reached with their gear, but whether they do depends on two circumstances completely beyond the control of the fishermen. The first is how many of Sally's cohort have survived and how many juveniles that were hatched from more distant spawning grounds move into the Maine coast, in other words how many juvenile herring there are to catch. The second is what combination of driving forces are present this year that will cause the herring to be available to the fishermen in shallow, coastal waters.

As it turns out the past year has not been a good one for Sally's year class. A relatively small number have survived and these survivors are beginning to move in schools towards the coast searching for dense patches of copepods that are just beginning to develop in the shallow waters. At the same time a particular set of circumstances develops that does not bode well for the fishermen this year. Through a complex set of events in the coastal water column, the copepod populations are distributed farther offshore than usual. Also the usual groundfish predators: cod, haddock and whiting, are in short supply along the coast. The finback whales which feed on juvenile herring are also absent and to top it off, the coves and bays by mid summer are full of pogies and bluefish.

What this means is that Sally is perfectly content to remain offshore in deeper water all summer. If her cohort were larger or there were more predators around she might have been forced inshore and into some fisherman's net, but as it turns out Sally avoids the sardine can and as fall approaches she again moves into deeper water for the winter.

In the spring of 1991 Sally begins her third year of life and again she avoids being caught. The next year class to enter the fishery was spawned in 1989. It is a big one and there are plenty of fish to go around. Sally is about 8 inches long and a bit larger than the preferred size. She now begins a seasonal pattern of movement that will become stronger as she grows older. During the previous winter she moved a little farther to the west and over- wintered in deep water off Cape Elizabeth. This spring as the water temperatures warm she moves eastward past Penobscot Bay and spends the summer along the eastern Maine coast. Late summer finds her near the spawning grounds. Although her gonads have not yet fully matured (that will come next year), she is beginning to display adult behavior patterns, moving to the spawning grounds in the late summer and fall and then southwest to the wintering grounds.

THE ADULT STAGE

In the spring of 1992 we find Sally in Massachusetts Bay where she has been over-wintering in warmer waters. She is now a sleek, mature sea herring and measures about 10 inches long. During the winter she has managed to evade the Maine purse seine fleet that followed her south to her wintering grounds. A Russian factory ship has laid in the vicinity all winter to buy herring from the purse seiners and the toll on Sally's cohort has been heavy. But now the vernal warming begins in the southwest Gulf of Maine, starting with an intense bloom of plant life followed by a rapid multiplication of zooplankton. The herring begin to move north in search of food and the fishery ends.

As Sally moves north she is searching for dense swarms of a large copepod called Calanus finmarchicus. This copepod is the most abundant animal in the Gulf of Maine and will be her main food supply for the rest of her life. During the spring and summer she slowly makes her way along the coast but remains mostly in deeper water where Calanus is usually found. She gorges herself on the rich food supply, for during the summer her ovaries begin to develop so she has to eat enough to sustain her basic body needs and also her developing eggs. Sally's predator list has dwindled somewhat. She is still vulnerable to the larger fishes and mammals, including man, but her size has eliminated many of her previous smaller predators and even the commercial fishery does not threaten her as long as she stays offshore.

As late summer approaches Sally's ovaries begin to ripen rapidly. As a four-year-old herring she will be capable of laying only about 40 thousand eggs, but as she grows older her fecundity will increase, so by the time she is a ripe old age of 10 years she will be able to produce about 200 thousand eggs. The older a spawning herring is, the more valuable it becomes to the population.

By mid August Sally has joined a large school of pre-spawning herring near the entrance to Grand Manan Channel and is about to complete the cycle that her mother started four years before. In late August she moves on to the spawning grounds and lays her eggs on a level, gravelly bottom just outside Cutler Harbor. By some unknown means the location of her origin has been imprinted on her and she has been guided back to the eastern Maine spawning ground to reproduce.

Spawning is another critical period for Sally, but it is probably even more critical for the survival of the spawning group. At this time the herring are very near to shore, are tightly schooled and vulnerable. The continued replenishment of the spawning group depends on spawning success; wiping them out would mean the reproductive cycle is broken and the herring in this group would disappear. Fortunately, the eastern Maine coast is closed to fishing for herring during the spawning period, so that Sally and her cohorts are able to reproduce undisturbed.

Now Sally has finished spawning. She is in poor physical shape because most of her food during the past months has gone into producing eggs. Her body is soft and flabby, she needs rest and food. She moves off the spawning grounds and begins to swim slowly westward along the coast towards the wintering grounds in Massachusetts Bay. Late one afternoon she and a few of her fellow post-spawners are passing by Petit Manan Island when disaster strikes. A harbor seal swims out past the island, near the bottom, and sees the silhouettes of the herring from below. The seal swiftly swims to the surface, catching Sally off guard. A quick pass, jaws open, a final lunge and Sally is gone!

Friends, lets not dwell on Sally's fate. She has not died, she has merely been recycled.

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