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.
