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Dynamic food webs and climate are changing lobster ecology and management. American lobsters (Homarus americanus) evolved in the North Atlantic under conditions of intense predation from large finfish such as Atlantic cod (Gadus morhua). Lobster’s relatively extended brood period and large larval size result in high per capita pelagic phase survival, which, coupled with settlement habitat selection for predator refugia, contributes to the species’ high lifetime reproductive success. However, the western North Atlantic is an extremely low diversity ecosystem prone to booms and busts. Extirpation of coastal predators released past constraints on lobster population growth such that lobster landings increased three- to five-fold since 1980 in Canada and the US. Climate change may stress lobsters in some regions and enhance stocks elsewhere, but it also facilitates warm-water species distribution shift northward. As lobster population densities and water temperatures increase, so do risks and consequences of disease. In the future we must expect the unexpected. “Equilibrium” conditions on which traditional fisheries management depends simply do not exist. This creates new challenges for managing this species regionally and into the future.
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PERSPECTIVE
American lobster dynamics in a brave new ocean
1
Robert S. Steneck and Richard A. Wahle
Abstract: Dynamic food webs and climate are changing lobster ecology and management. American lobsters (Homarus americanus)
evolved in the North Atlantic under conditions of intense predation from large finfish such as Atlantic cod (Gadus morhua). Lobster’s
relatively extended brood period and large larval size result in high per capita pelagic phase survival, which, coupled with settlement
habitat selection for predator refugia, contributes to the species’ high lifetime reproductive success. However, the western North
Atlantic is an extremely low diversity ecosystem prone to booms and busts. Extirpation of coastal predators released past
constraints on lobster population growth such that lobster landings increased three- to five-fold since 1980 in Canada and the US.
Climate change may stress lobsters in some regions and enhance stocks elsewhere, but it also facilitates warm-water species
distribution shift northward. As lobster population densities and water temperatures increase, so do risks and consequences of
disease. In the future we must expect the unexpected. “Equilibrium” conditions on which traditional fisheries management
depends simply do not exist. This creates new challenges for managing this species regionally and into the future.
Résumé : Des réseaux trophiques dynamiques et le climat modifient l'écologie et la gestion du homard. Les homards américains
(Homarus americanus) ont évolué dans l'Atlantique Nord dans des conditions de prédation intense par des grands poissons a
`
nageoires comme la morue (Gadus morhua). La période d'incubation relativement longue des homards et la grande taille de leurs
larves se traduisent par un taux de survie relativement élevé au stade pélagique qui, jumelé a
`une sélection de l'habitat de
résidence axée sur les refuges contre les prédateurs, contribue a
`expliquer le succès de reproduction élevé de l'espèce a
`l'échelle
de la durée de vie des individus. L'Atlantique Nord occidental constitue toutefois un écosystème d'une diversité extrêmement
faible qui le rend susceptible a
`des expansions et des effondrements rapides. La disparition locale de prédateurs côtiers s'est
traduite par l'atténuation de facteurs qui, par le passé, contrôlaient la croissance des populations de homards, de sorte que les
débarquements de homards ont triplé, voire quintuplé, depuis 1980, au Canada et aux États-Unis. Les changements climatiques
peuvent exercer un stress sur le homard dans certaines régions et accroître les stocks ailleurs, mais ils favorisent également le
déplacement vers le nord des aires de répartition d'espèces d'eau chaude. L'augmentation de la densité de population des
homards et de la température de l'eau s'accompagne d'une augmentation des risques de maladie et de leurs conséquences. À
l'avenir, il faut s'attendre a
`l'inattendu. Les conditions « d'équilibre » dont dépend la gestion traditionnelle des pêches n'existent
tout simplement pas. Cela crée de nouveaux défis en ce qui concerne la gestion régionale de cette espèce. [Traduit par la
Rédaction]
Introduction
At the turn of the last century after Maine’s American lobster
(Homarus americanus) landings had declined from 10 000 to about
6000 metric tons (t), Francis Herrick worried that the American
lobster was on the verge of “commercial extinction.” He went
on to wonder, “What is the matter with the lobster?” (Herrick
1909). Nearly a century later with Maine’s landings approaching
20 000 t, with proportional increases in Canada, Canadian fisher-
ies scientist Robert Miller openly wondered, “Why are there so
many American lobsters?” (Miller 1994). In 2012, Maine landings
had more than doubled to nearly 56 000 t and in July of that year
The New York Times ran an article entitled “In Maine, More Lob-
sters Than They Know What to do With”.
Arguably, the American lobster fishery throughout the western
North Atlantic is the only fishery in the world targeted for
150 years but has higher landings today than ever before (Fig. 1). It
is the only fishery to have had a serious crisis entirely based on the
economics of fishing during a time when abundance, catch rates,
and spawning stock biomass have been steadily increasing for
over three decades to record high values (Steneck et al. 2011). The
recent lobster glut is far from the commercial extinction pre-
dicted by Herrick (1909), but Miller’s question of why there are so
many lobsters also remains unresolved.
The persistent increase in lobster landings over the past three
decades to record abundances recorded in the US are almost per-
fectly matched by landings in Canada (http://www.dfo-mpo.gc.ca/
fm-gp/, accessed 4 June 2013). Importantly, there had been no
major changes in how lobsters were managed in either country
preceding this increase; both countries had prohibited landing
egg-bearing lobsters for decades, and both maintained about the
same minimum harvestable size (e.g., Acheson and Steneck 1997).
Nevertheless, there were differences in how the two countries
managed their lobster stocks. Canada held fishing effort relatively
constant, and fishing seasons are open only a few months per year
in each of its over 40 lobster fishing areas. Maine (the largest
lobster-producing state in the US), in contrast, had no limits on
effort at the time landings began to increase, and fishing was and
is practiced year-round, but “v-notch” protection of egg-bearing
lobsters and protection of all lobsters above 127 mm carapace
length (CL) only existed in Maine. Thus, the synchrony and mag-
nitude of the increase in lobster abundance and landings (i.e., a
3.6- and 4.8-fold increase since 1975 for Canada and US, respec-
Received 15 February 2013. Accepted 27 August 2013.
Paper handled by Associate Editor B. Sainte-Marie.
R.S. Steneck* and R.A. Wahle. University of Maine, School of Marine Sciences, Darling Marine Center, Walpole, ME 04573, USA.
Corresponding author: R.S. Steneck (e-mail: steneck@maine.edu).
*Present address: Darling Marine Center, Walpole, ME 04353, USA.
1This article is one of a series of papers published in the special issue “The American Lobster in a Changing Ecosystem”.
1612
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tively) suggests factors other than specific management measures
are demographically important to this species.
Nevertheless, until recently, fisheries managers in the US con-
cluded that the American lobster was overfished and at risk of
collapse (ASMFC 2000;Steneck 2006a). Perhaps a more important
question is, why do lobsters continually surprise us? While there
is no simple or conclusive answer, there are several unique fea-
tures about this species and the ecosystem in which it lives that
may shed light on this enigma. Specifically, we consider how the
geological history of the North Atlantic Ocean, the American lob-
ster, and the species with which it evolved created unusual con-
ditions that drive lobster dynamics today. We take a long
temporal view of the ecosystem centered in the Gulf of Maine,
which today is close to the demographic center of this species’
distribution and is the region having the highest landings per
length of coastline (Harding et al. 1983). The Gulf of Maine is the
only lobster-producing region where standardized annual trawl
surveys have been conducted annually in both US and Canadian
waters. We review selected studies in this core of their range that
are relevant to the dynamics of this species, the ecosystems in
which it lives, and the implications of climate change as we look
towards the future. Most of our examples draw from our work in
Maine, but we think they should be applicable over most of the
range of this species.
We explicitly include humans as part of this ecosystem, since
they are demonstrably capable of affecting its structure and func-
tion. We will suggest that one of the unintended consequences of
unsustainable fishing on large coastal predatory finfish has been
the creation of a predator-free zone in which lobsters and other
former prey species have markedly increased. This has created a
lucrative lobster monoculture, but it also increases the social–
ecological risks throughout the region should lobster stocks sud-
denly decline (Steneck et al. 2011).
Setting a Spartan stage in the western North
Atlantic
The North Atlantic is one of the world’s youngest and least
species diverse oceans (Crame 2004;Estes et al. 2013). It began to
form during the Mesozoic Era at a time when most of the world
was tropical. However, after polar cooling during the mid-
Miocene around 14.8 and 14.1 million years ago (Ma), a distinct and
diverse cold-water biota evolved in the North Pacific (Raymo and
Ruddiman 1992). The endemic diversity in the North Atlantic was
much lower (Vermeij 1991). When the Bering Strait opened about
5 Ma, an asymmetrical species interchange occurred, with many
more North Pacific species entering the North Atlantic than the
reverse (Vermeij 1991). Many species characteristic of the North At-
lantic today, including groundfish (such as Atlantic cod (Gadus
morhua), pollock (Pollachius virens), and Greenland halibut (Reinhardtius
hippoglossoides)), northern shrimp (Pandalus borealis), Cancer crabs,
green sea urchins (Lytechinus variegatus), and phocid (earless) seals,
had their evolutionary roots in the North Pacific. Some potentially
important species arrived in the North Atlantic but failed to
persist. For example, a North Atlantic sea otter (Enhydra reevi), a
close relative of the iconic shellfish-eating North Pacific sea
otter (Enhydra lutris), lived on the coast of England 2.6–1.75 Ma,
but failed to persist in the Atlantic (Willemsen 1992).
Clawed lobsters are one of the relatively few higher taxa to be
endemic in the North Atlantic (Estes et al. 2013). There is no evi-
dence of lobster ancestors in the North Pacific Ocean. Homarus
likely evolved in the North Atlantic at least 130 Ma ago. The two
extant species, Homarus gammarus and H. americanus live in the
eastern and western North Atlantic, respectively. The two species
are closely related and have similar diets, but H. americanus main-
tains much higher population densities and is overall much more
abundant than its eastern North Atlantic counterpart (Butler et al.
2006;Wahle et al. 2012).
The incomplete inoculation of species from the North Pacific,
together with the much greater impacts of glaciation to coastal
zones of the western North Atlantic (Wares and Cunningham
2001;Adey and Steneck 2001), resulted in remarkably low species
diversity. In fact, the Gulf of Maine ranked last in a study compar-
ing species richness of rock-dwelling invertebrates in regions
around the world (Witman et al. 2004). In another study compar-
ing the species richness of marine fishes, the western North
Atlantic diversity was significantly lower than the eastern North
Fig. 1. Maine lobster landings, 1880 to 2011. Data from MDMR (2012).
0
10000
20000
30000
40000
50000
1880 1900 1920 1940 1960 1980 2000 2020
Maine Lobster Landings 1880-2011
Landings (t)
Year
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Atlantic, and both shores had declining species richness with in-
creasing latitude (Frank et al. 2007). In short, the western North
Atlantic is a species-depauperate shore within a species-depauperate
ocean (Crame 2004).
The nature and strength of interactions
The incomplete inoculation of species from the North Pacific to
the North Atlantic resulted in food webs that differ in structure
and function. Marine mammals dominate coastal zones of the
North Pacific coasts and are well represented in prehistoric Indian
middens there dating back 1000 to over 10 000 years ago. Specifi-
cally, bones of fish-eating seals, sea lions, and fur seals are com-
mon in middens of the Pacific Northwest (e.g., Rick et al. 2008). In
the Aleutian archipelago, sea otters were commonly found in
prehistoric middens (Simenstad et al. 1978). Fur seals and sea lions
are especially important because they are large coastal predators
capable of consuming large fish such as mature sockeye salmon
(Oncorhynchus nerka) and Pacific cod (Gadus macrocephalus)(Misarti
et al. 2009). This contrasts with the smaller phocid seals that
entered the North Atlantic. These seals usually consume fish by
swallowing them whole and thus feed on smaller species such
as Atlantic herring (Clupea harengus) and American sand lance
(Ammodytes americanus)(Payne and Selzer 1989). The diversity of
marine mammals including sea lions that can eat the largest fish in
coastal zones results in a mammal rather than fish dominated food
webs in the North Pacific (Estes and Steinberg 1988;Estes et al. 2013).
The first people colonizing coastal Maine more than 4000 years
ago ate large predatory fish such as Atlantic cod, swordfish
(Xiphias gladius), Atlantic sturgeon (Acipenser oxyrinchus), pollock,
and Atlantic wolffish (Anarhichas lupus), with seal bones being rel-
atively rare (Fig. 2;Bourque et al. 2008). Atlantic cod comprise the
most abundant bone fragments found in middens (Spiess and
Lewis 2001), and they may well have been the most important
predator in coastal zones of the North Atlantic. Atlantic cod are
the world’s largest gadoid, approaching 100 kg in maximum size.
They also have a relatively large mouth, remarkably broad trophic
latitude, and can eat virtually all mobile–benthic invertebrates
and most fish they encounter (Collette and Klein-MacPhee 2002).
Cod averaged nearly a metre in length for thousands of years
based on bones found in Indian middens in Maine (Fig. 3). As a
result, cod and other large predatory fish may have been superior
trophic competitors over seals in coastal zones. Large cod can
consume all prey required by seals, but unlike seals they are not
depth-limited air breathers. Thus, it is possible that large preda-
tory finfishes exerted the strongest ecological pressure or “inter-
action strength” in coastal zones of the region.
Interaction strength is measured as the per capita demographic
impact one species has on another species or community (Paine
1992). Large consumers often exert the greatest interaction strength
(Sala and Graham 2002), especially in ecosystems having naturally
low species diversity. For example, all known keystone species
(sensu Power et al. 1996) are found in ecosystems having relatively
low species diversity (i.e., none are found in the diverse tropics).
Therefore, the naturally low species diversity in coastal zones of
the western North Atlantic will likely have high interaction
strength from large predatory finfish such as Atlantic cod.
Ecological success in a sea of large predators
Ecological success relates to a species’ capacity to persist and
thrive under prevailing environmental conditions (Poulin et al.
2002). Obviously, the American lobster survived and likely evolved
under conditions of intense fish predation despite having an ex-
posed abdomen and being relatively slow in its capacity to escape
from harm via tail-flips (Butler et al. 2006). Their success relates to
Fig. 2. Percentage of bone fragments from all strata (dated from 4500 to 400 years before present (BP)) of the Turner Farm archaeological sites
data from Spiess and Lewis (2001).
Atlantic cod
Flounder spp.
Sea mink
Fish unidentified
Winter flounder
Softshell clams
Swordfish
Sculpin
Sturgeon
Yellowtail flounder
Tomcod
Harbor seal
Seal (unidentified)
Grey seal
American dab
Dogfish
Cunner
Herring
Blue mussel
Haddock
Halibut
Pollock
wolffish
Sand flounder
Salmonids
American Eel
Sea urchin
Harp Seal
Alewife
Waved whelk
Quahog
Great white shark
Harbor porpoise
Cusk
Right whale
Mackerel
Bluefish
010 20 30 40
Average percent fragments from all strata
1614 Can. J. Fish. Aquat. Sci. Vol. 70, 2013
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nocturnal foraging and diurnal shelter-dwelling behavior during
periods when visual predators are active. They also exhibit high
parental care by carrying their egg broods for 9 to 11 months. This
allows clutches of large eggs to produce relatively few but large
and relatively advanced larvae that quickly progress through
three larval stages before becoming strong swimmers in their
settling postlarval (PL) stage (Cobb et al. 1989). Swimming allows
settling PLs to actively select safe benthic nursery habitats where
they prey on small invertebrates and may suspension-feed for
several years (Lavalli and Barshaw 1989;Cobb and Wahle 1994;
Sainte-Marie and Chabot 2002). All of this results in high per cap-
ita rates of postsettlement survival. Lobsters can also have a very
long reproductive life (i.e., more than half a century). Taken to-
gether, the American lobster’s ecological success may result from
its predator avoidance behavior, high per capita recruitment suc-
cess, and a very long reproductive life, which allows it to persist
even under high rates of adult mortality.
Human domestication of the American lobster
ecosystem: changes in patterns and processes
“Humans … have domesticated landscapes and ecosystems
in ways that enhance our food supplies, reduce exposure to
predators and natural dangers, and promote commerce.”
Kareiva et al. 2007
While “ecosystem domestication” (sensu Kareiva et al. 2007) can
be created by design and in fact is a subtext of most management
plans of commercial fisheries, it can also be the unintended result
of human activities. The long history of fishing in the western
North Atlantic could have domesticated, and thus fundamentally
changed, the ecosystem in which the American lobster lives.
Humans began fishing almost as soon as they arrived on the
coast of Maine between about 5000 to 10 000 years ago (Bourque
et al. 2008). Carbon and nitrogen analysis of fish and human bones
show people in coastal Maine subsisted on fish living in nearshore
environments (Fig. 4). The same study suggests there could have
been a very nearshore depletion in cod prior to the arrival of the
first Europeans (Fig. 5;Bourque et al. 2008). Importantly, no crab
or lobster carapaces have been found in Indian middens in the
western North Atlantic (e.g., Fig. 2). Their absence is not likely due
to poor preservation because lobsters are found in middens else-
where (e.g., spiny lobster (Jasus lalandii) carapaces are abundant in
South African middens; Jerardino et al. 2008), and they readily
fossilize (Bishop 1985).
The first important export from the New World was Atlantic cod.
This is a species easy to catch and easy to preserve. The earliest
naturalist description of marine organisms described “great Cod and
Haddocke” being caught wherever they went along the coast of
Maine (Rosier 1605). Around the turn of the last century, Francis
Hobart Herrick (1909) asserted “Next to man with his traps, the cod-
fish is probably the most destructive enemy of the lobster…” By the
mid-1930s, mechanized trawling and refrigeration caused cod spawn-
ing aggregations to be targeted and extirpated (Ames 2004).
Centuries of unsustainable fishing serially extirpated large
predatory groundfish beginning in coastal zones (Fig. 5) and ex-
tending farther offshore over time (Rosenberg et al. 2005). In re-
cent decades, fish stocks such as cod have continued to decline
along Maine’s coastal zone (Fig. 6). Large predators such as Atlan-
tic cod and wolffish were not driven biologically extinct, but ar-
guably they became ecologically extinct (sensu Estes et al. 1989)in
that they lost their function as predators. Given how few predator
species naturally occur in the Gulf of Maine, the loss of a few of the
largest and strongest interacting species creates “trophic level
dysfunction” (sensu Steneck et al. 2004). This is the functional loss
of an entire trophic level and can weaken top-down demographic
control of lower trophic levels (Estes et al. 2011).
Trophic-level dysfunction in the Gulf of Maine may explain the
inverse correlation between lobster landings and the decline of
predatory finfish over the past four decades (Boudreau and Worm
2010). Although Atlantic cod had the most lobsters in their stom-
ach, they were but one of the 17 species found to have consumed
Fig. 3. Average size of coastal Atlantic cod reconstructed from vertebrae collected in middens from the Turner Farm 4500 to 500 years BP.
More recent data are from Bigelow and Schroeder (1953),Hacunda (1981), and Ojeda and Dearborn (1989) (modified from Jackson et al. 2001).
050010001500200025003000350040004500
0
20
40
60
80
100
120
5000
Atlantic cod body length (cm)
Years Before Present
Steneck and Wahle 1615
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lobsters in groundfish trawl surveys (Boudreau and Worm 2010).
Several studies in coastal zones experimentally tested for dysfunc-
tion among higher-order carnivores in the Gulf of Maine by teth-
ering lobsters and other common mobile benthic invertebrates to
featureless substrates as a measure of the ecosystem’s predation
potential. The tethered adult crabs, sea urchins, and lobsters were
all found to be largely free from attacks by finfishes in coastal
Maine; however, all were attacked at offshore sites that had large
predators (Witman and Sebens 1992;Vadas and Steneck 1995;
Steneck 1997, respectively). On an offshore ledge where large
predators were significantly more abundant, tethered lobsters of
all sizes were readily attacked, with largest lobsters suffering the
highest attack rate (Fig. 7a). This supports the idea that the ab-
sence of crabs and lobsters from Indian middens in Maine reflects
the predator-induced rarity of these decapods in the top-down-
structured ecosystem of the past.
Today the largest groundfish in the Gulf of Maine (i.e., halibut,
cod, wolffish, and haddock (Melanogrammus aeglefinus)) are ecolog-
ically extinct from coastal zones (Steneck 1997). Because of their
extirpation and truncation in body size (e.g., Fig. 3), the remaining
predatory fishes in shallow lobster nursery habitats are small and
largely commercially unimportant species. These small meso-
predators, including rock gunnels (Pholis gunnelus), radiated shan-
nies (Ulvaria subbifurcata), grubbies (Myoxocephalus aenaeus), and
juvenile cunner (Tautogolabrus adspersus), may well have been prey
for large groundfish when they were abundant (Steneck and
Carlton 2001;Wahle et al. 2013a,2013b). However, today in shal-
low cobblestone nursery habitats, attack rates on lobsters are
highest at or soon after PLs settle to the benthos (Fig. 7b;Wahle
and Steneck 1991,1992).
The loss of large predatory finfish (Fig. 6) signifies both a change in
ecosystem structure and how it functions. Importantly, when pred-
ators were abundant, the risk of predation extended well into lobster
adulthood (e.g., Fig. 7a). Whereas under the current predator envi-
ronment, attacks are confined to a relatively brief period at or soon
after lobster settlement (Fig. 7b;Wahle and Steneck 1992). By the
time they exit their predator-free cobblestone nursery habitat, the
small lobsters have outgrown their potential predators (Wahle 2003;
Wahle et al. 2012;Fig. 7b). The requirement for predator refugia pri-
marily at the time of settlement may create a demographic bottle-
neck for lobsters.
Where predatory fish remained diverse and relatively abundant,
such as in southern New England, rates of predation were relatively
high and juvenile lobsters were restricted to shelter (Wahle et al.
2013b). However, in much of the Gulf of Maine, juvenile lobsters and
even the lobster fishery is much less habitat-restricted. Under a re-
laxed demographic bottleneck, lobsters are less constrained by shel-
ter availability, and thus patterns in PL settlement will more likely
translate to populations of adolescent and adult lobsters in the fu-
ture. This “settlement driven demography” (Palma et al. 1999) repre-
sents a shift in the ecological processes that govern population
lobster dynamics. Relatively localized settlement “hotspots” thus
translate to regions of higher lobster population densities (Fig. 8;
Steneck and Wilson 2001;Wahle et al. 2013b).
The naturally low species diversity characteristic of western North
Atlantic marine ecosystems could have intensified interaction
strength among species by creating strong top-down control on
Fig. 4. The average stable isotope composition (±SD) of bone collagen
specimens dating between 4000 and 400 years BP. Collections were
made from the Turner Farm midden on North Haven Island, Maine.
The isotopic fields occupied by marine verses terrestrial organisms are
indicated (shaded regions). The carbon isotope values suggest
organisms were from shallow, nearshore environments dominated by
sea grass, kelp, or other seaweed primary producers. The human
isotope ratios (from a 3500 BP cemetery plot very close to the marine
field) suggest that humans ate coastal marine fish, contrasting
distinctly from deer and bear bone isotope ratios. Figure is modified
from Bourque et al. (2008).
MARINE
Cod
Human Sculpin
Flounder
18
16
14
12
10
8
6
4
2
-26 -24 -22 -20 -18 -16 -14 -12 -10 -8 -6
TERRESTRIAL
Deer
Bear
15N
13C
Fig. 5. The proportion of bone fragments in middens with median
dates ranging from 4350 to 400 BP separated by their fractional
trophic levels (i.e., TL, as listed in Steneck et al. 2004). (A) Top (i.e.,
“apex”) predators, TL ≥ 4.0. (B) Lower (i.e., “meso”) predators, TL > 3
and <4. Figure is modified from Bourque et al. 2008. Importantly,
this suggests early coastal populations ate seafood. Cod lived in
shallow coastal zones, but apex predators had declined prior to
European contact (Bourque et al. 2008).
010002000300040005000
20
40
60
80 Apex Predators (TL > 4.0)
0
Atlantic Cod
Dogfish
Sea Mink
Seals (3 spp)
Swordfish
0
10
20
30
40
50
60
70
80
90
010002000300040005000
Flounder
(4 spp)
Fish (UID)
Sculpin
Tomcod
Sturgeon
Groundfish (6 spp)
Pelagic fishes (5 spp)
Mesopredators (TL > 3.0, < 4.0)
Years Before Present
% Bone Fragments% Bone Fragments
(A)
(B)
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Fig. 6. Decadal trends in the distribution and abundance of Atlantic cod. Figure is modified from Sosebee and Cadrin (2006). Circles represent
the number of fish caught per standard tow in the US National Marine Fisheries Service autumn trawl surveys. Black points are empty tows.
Absolute abundances are less important than the decadal trend evident in this figure. Note that the contiguous band of cod in the 1970s was
reduced to three small clusters by the 2000s. Coastal Maine is virtually devoid of cod.
1970s
Maine
NS
1980s
Maine
NS
2000s
Maine NS
1990s
Maine
Catch mass (kg)
Fig. 7. Attack rates on tethered lobsters of varying body sizes tethered (A) offshore at Cashes Ledge, where large predatory fishes were present
(data from Steneck 1997), and (B) along coastal Maine, where large predatory fishes were absent (data from Wahle and Steneck 1992).
Tethering experiments were conducted 3 and 4 years in succession at offshore and inshore sites, respectively.
10060 8002040
Lobster Body Size (mm CL)
Attack Rate (no. eaten·day–1)
0
.4
.8
1.2
1.6
2.0
2.4
2.8
3.2
3.6
4.0
4.4
Offshore ledge
With large predatory fishes
A.
Attack rate (eaten·h–1)
0
10
20
30
40
50
60
70
020406080100
Lobster Body Size (mm CL)
Coastal Maine
Without large predatory fishes
B.
Steneck and Wahle 1617
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most mobile benthic invertebrates. If so, when predation was
relaxed by fishing, population increases were evident among
several important mobile invertebrate prey, including lobsters,
crabs, and sea urchins (Frank et al. 2005;Boudreau and Worm
2010;Steneck et al. 2013).
Lobster distribution and abundance today likely reflects cascad-
ing processes in which recruitment, predation, and shelter com-
petition all play a role. Logically, this cascade begins with elevated
recruitment success into a world relatively free of large predators.
Settling PL lobsters increase their per capita survival by actively
selecting predator-safe cobblestone nursery habitats (Wahle and
Steneck 1991,1992). There, young-of-year lobsters can live at den-
sities exceeding 5·m
−2
(Wahle et al. 2013b). Early benthic phase
lobsters (<35 mm CL) usually remain confined to shelters for the
first few years and possibly longer if they detect the chemical
presence of predatory fishes (Wahle 1992). Regional recruitment
success thus translates to extremely high population densities of
adolescent and adult lobsters (i.e., >2·m
−2
;Fig. 8).
As lobsters grow, they require larger and more limited shelter
space, which escalates intraspecific competition. (Steneck 2006b;
Fig. 9a). Shelter competition intensifies at high shelter densities,
resulting in an increase in vacant shelters (Fig. 9b). Since larger
lobsters can detect conspecifics at distances of at least1m(Atema
1986), under coastal Maine’s high lobster population densities
(Fig. 8), shelter competition is likely to be both intense and wide-
spread. The strong agonistic behavior of larger lobsters at higher
population densities (Fig. 9A) results in those lobsters vacating
regions of highest population densities (Steneck 2006b). This be-
havior of leaving regions of highest lobster population densities
to regions of lower densities is called “demographic diffusion”
(sensu Steneck 2006b). This may be a contributing factor to the
observed increase of larger reproductive phase lobsters in Maine’s
offshore waters (Fig. 10; i.e., >90 mm CL).
The domesticated ecosystem for lobsters may involve more
than just the reduction in predators (Fig. 6) because there is also a
significant trophic contribution to lobsters from trap bait
(Grabowski et al. 2010). This, along with optimal temperatures
(discussed below), may be creating a reinforcing feedback loop
that is escalating lobster population growth. Increasing coastal
recruitment and offshore expansion translates into increasing repro-
ductive capacity of lobsters (Fig. 10) that may result in greater larval
supply and possibly greater lobster recruitment (Wahle and Incze
1997;Incze et al. 1997;Fig. 8). To date, the lobster population increase
outpaces the fishing capacity as is evident in the steadily increasing
catch per unit effort (i.e., catch per trap haul) in recent decades
(Fig. 11). Thus, the American lobster with its high capacity to sustain
adult mortality when released from that predation can outpace the
rate at which fishermen catch them.
Fig. 8. Location of lobster settlement hotspots (gray shading in
Maine coastal figure) and the population density of all lobsters
quantified in boulder fields at 91 locations studied from 1989 to 1999
(n= 10 503 m
2
quadrats). Note the lobster abundances are plotted
relative to the longitude conforming to the above coastal zone. The
area of peak lobster settlement corresponds well with the area of
highest population densities of all lobsters (modified from Steneck
and Wilson 2001).
Fig. 9. Demographic responses of lobsters to variously spaced,
identically sized PVC shelters (modified from Steneck 2006b).
Abscissas for panels A–C are distance in metres between five
opposing and adjacent shelters. “2S” refers to the linear string of
shelters set 2 m apart without opposing shelters. Bonferroni
multiple comparison was used on square-root-transformed data to
determine significant differences (indicated by lowercase letter)
among shelter spacing treatments at P≤ 0.05 level (means of daily
observations with variance expressed as standard error).
(A) Population density. (B) Shelter occupancy. (C) Proportion of
lobsters >60 mm CL.
0.25 0.50 1.00 1.50 2.00 2 S
0.0
0.1
0.2
0.3
0.4
0.5
0.6
a
b
c
d
ab
c
c
Proportion Big Lobsters
> 60 mm CL
Shelter Spacing (m)
C
Proportion of lobsters No.m-2
0
1
2
3
b
c
c d dd
AaPopulation Density
0
10
20
30
40
50
60
70
80
90
a
b
ccc
b
Competitive
Exclusion
Shelter Occupancy
B
Percent Shelters Occupied
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Changing dynamics and risks of a lucrative
monoculture
The increase in landings and value of the American lobster fishery
is widely viewed as a rare fisheries success story (e.g., New York Times
article mentioned above). However, the economic diversity of Maine’s
fisheries hints at a larger looming problem. Economic diversity had
increased from 1960 until 1990 as a result of new global markets for
some species such as sea urchins (Steneck et al. 2011). Over the past
two decades, economic diversity from all other harvested species has
declined to a record low level (Fig. 12) as the American lobster became
Maine’s lucrative monoculture (Steneck et al. 2011).
The rise in lobster abundance and landings likely result from more
than simply predator declines. The development of lobster larvae is
temperature-sensitive (Harding et al. 1983;MacKenzie 1988), and the
sounding and settling behavior of PLs (stage IV) is much greater at
warmer temperatures above the summer thermocline (Boudreau
Fig. 10. The abundance of lobsters by body size (from National Marine Fisheries Service (NMFS) offshore trawl surveys) at the start of the
population increase (1982–1983; open circles) and more recently (2000–2002; solid squares). Population density of reproductive-phase lobsters
has been steadily rising since the early 1980s (see Steneck 2006a). This change does not correspond with any major change in lobster
management (i.e., minimum legal size was 81 mm carapace length (CL) from 1958 to 1979 and was increased 1.6 mm in 1989 to the current
82.6 mm CL). Figure is modified from Steneck (2006a).
16015014013012011010090807060504030
0.00
0.01
0.02
0.03
0.04
Reproductive phase
Body Size (mm CL)
Mean number of lobsters per tow
2000-2002
1982-1983
Fig. 11. CPUE between 1975 and 2004 suggesting that lobster populations are expanding despite increases in effort. Data from NMFS offshore
trawl surveys (Steneck 2006a).
Catch per unit effort
4 000
5 000
6 000
7 000
8 000
9 000
10 000
2005200019951990198519801975
(Landings in metric tons per million trap hauls)
Year
Steneck and Wahle 1619
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For personal use only.
Fig. 12. The economic diversity of Maine’s fisheries from 1950 to 2006 on the basis of Shannon diversity index (H=) and the percentage of economic
value of all landings (34 commercial species) attributable to American lobster (Homarus americanus). Figure is modified from Steneck et al. (2011).
1.0
1.5
2.0
2.5
3.0
3.5
0
10
20
30
40
50
60
70
80
90
2010200019901980197019601950
Year
Shannon Index of Diversity (H)
% Lobster
( )
Fisheries
( )
Fig. 13. Temperature effects on postlarval behavior. (A) The cumulative time (bars) postlarvae spent along a naturally occurring thermal
gradient in the water column from diver observations of postlarval swimming behavior. (Figure is modified from Annis (2005); copyright 2013
by the Association for the Sciences of Limnology and Oceanography, Inc.) The vertical dashed line reflects a possible thermal threshold below
which there is little or no postlarval settlement. The decline at warmer temperatures suggests an optimum settlement temperature around
16 °C. (B) Possible depth regulation of available nursery habitats by thermally mediated sounding behavior of competent postlarval lobsters
(after Annis 2005). Warmer-than-average years or regions could increase the settlement success at greater depths of American lobsters.
Temperature C)
10 12 14 16 18 20
Cumulative Time (min)
0
50
100
150
200
250 Thermal threshold
For settlement?
Too warm
Too cold
Subtidal
nursery
habitat
Cooler regions or year
Settlement area (> 12oC)
Thermal threshold
Too cold
Sea level
Subtidal
nursery
habitat
Warmer regions or year
Settlement area
(> 12oC)
Thermal
threshold
Too cold
Sea level
A
B
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et al. 1992;Wahle et al. 2013b). Settlement of lobster PLs is greatest
at or above 20 m along the midcoast of Maine (Wilson 1999;Wahle
et al. 2013b). The depth limit for PL sounding may result from
cessation of sounding in cold water (Annis 2005). Studying PL
behavior in the field, Annis (2005) found that PLs sought an opti-
mum temperature of about 16 °C and showed declines at both
cooler and warmer temperatures, reaching zero settlement at
about 12 and 20 °C, respectively (Fig. 13a). Since temperatures
warmer than 12 °C facilitate settlement in sounding PLs, then
warming temperatures may facilitate the expansion of nursery
habitat (Fig. 13b;Steneck 2006a).
Sea surface temperatures in the midcoast of Maine have been
steadily rising over the past century (Fig. 14). Sea temperature
in the western North Atlantic (including the Gulf of Maine) is
expected to rise at a rate greater than the global average
(Hoegh-Guldberg and Bruno 2010). Current average summer tem-
peratures along coastal Maine are very close to the optimum for
lobster larvae (Fig. 15), as determined by Annis (2005). As water
temperatures warm, the region cooler than the thermal threshold
will likely move northward. Consistent with this, over the past
two decades, lobster larval settlement and landings in Maine
show a clear northward trend (MDMR 2010).
However, the warming trend also has its limits. Sea tempera-
tures over 16 °C are less preferred by settling PLs (Fig. 13a) and may
be increasingly stressful for lobsters (Dove et al. 2005). Sea tem-
peratures approaching or exceeding 20 °C stresses lobsters and
likely contributes to the rise of lethal disease (Dove et al. 2005;
Castro et al. 2006;Glenn and Pugh 2006). Summer temperatures
in southern New England are exceeding 20 °C much more fre-
quently in recent decades. In the late 1990s, a shell disease that
had been present over much of the lobster’s range suddenly be-
came epizootic, causing inshore lobster populations in Rhode Is-
land to crash (Castro et al. 2006).
The disease-related decline of lobsters in southern New England
effectively clouded the predictive relationship between lobster
settlement and fishery recruitment described above, until the
shell disease prevalence was included as a proxy for the height-
ened levels of postsettlement mortality (Wahle et al. 2009). Warm
temperatures in southern New England were the prime reason
why some fisheries managers described the lobster fishery there
to be collapsed (Glenn and Pugh 2006).
Thermogeography (i.e., geographic patterns of ocean tempera-
tures) drives biogeography (e.g., Adey and Steneck 2001). In the
North Atlantic, the diversity of fish species declines northward,
largely as a result of fewer species being adapted to the cooler
temperatures found in the Arctic (Macpherson 2002;Frank et al.
2007). As global warming continues, anticipated changes in long-
time resident species other than lobsters will likely shift north-
ward. For example, the anticipated (conservative) expected
increase of 2–4 °C in the Gulf of Maine and Georges Bank will
likely make those large marine ecosystems stressful for Atlantic
cod (Drinkwater 2005). However, the diversity of predators inhab-
iting lobster nurseries to the south is much greater than in today’s
Gulf of Maine (Wahle et al. 2013b), and with the higher predator
diversity comes higher attack rates on lobsters (Fig. 16;Wahle
et al. 2013a,2013b). Already some species such as red hake (Urophycis
chuss) have increased in abundance in the Gulf of Maine (Nye et al.
2009). During the record warm summer of 2012, Maine lobster
fishers were reporting seeing mid-Atlantic species such as black
sea bass (Centropristis striata) and blue crabs (Callinectes sapidus)in
their traps (R.S. Steneck, personal communication).
Exactly how climate change (and ocean acidification) will affect
lobsters is unknown. Ocean warming is complex, and as coastal
zones continue to warm, we may continue to see increases in
lobster recruitment, predator diversity, physiological stress, and
disease … or not. This suggests that lobster management in the
“brave new ocean” may be less certain in the future, making it
more difficult to manage.
Equilibrium-based, single-species models may not serve us well
into the future. Traditional approaches to fisheries management
that calculate maximum sustainable yields assume an equilib-
rium or steady-state exists in which catch is equal to the surplus
production at a given level of fishing effort (Jennings et al. 2001).
Such equilibria require that none of the big demographic drivers
Fig. 14. Average sea surface temperature measured in Boothbay Harbor, Maine (1905–2012). Data from Maine’s Department of Marine
Resources. Line is a best-fit third-order polynomial (r
2
= 44).
6
7
8
9
10
11
12
13
1900 1920 1940 1960 1980 2000 2020
Boothbay Harbor Sea Surface Temperature
Year
(1905 - 2012)
Mean Annual Temp. (°C)
Steneck and Wahle 1621
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For personal use only.
such as recruitment or natural mortality (i.e., predation) change
(Hilborn and Walters 1992). However, that is clearly not the case
for lobsters as we have described above, and as a result, managers
and policy makers are continually surprised by this species. While
single-species predictive models may have unacceptably high un-
certainty, linking monitoring of early and adult life stages over
the geographic range of the species allows spatially explicit, short-
term lobster dynamics to be approximated. However, it is unlikely
that detailed analyses of the target species alone will improve our
ability to predict their population dynamics in the future. We
need to move towards multispecies, ecosystem-based manage-
ment at nested scales that accounts for the key physical and bio-
logical drivers of the food web (e.g., Steneck and Wilson 2010).
The American lobster was uniquely suited for a world of intense
predation and high rates of adult mortality. Unsustainable fish-
ing on groundfish predators from this already low diversity western
North Atlantic ecosystem triggered cascading processes and related
feedback mechanisms that fundamentally altered the ecosystem
Fig. 15. Mean summer sea surface temperatures (20032005) determined from satellite data (contours) and shallow subtidal temperatures
over the same period from thermistors (arrows). Isotherm contour map was generated by the University of Maine Satellite Oceanography Data
Lab. Average summer temperatures at specific sites correspond with offshore temperatures except well up Penobscot Bay (i.e., 17 °C). Note
regions listed as too cool and too warm are based on postlarval behavior (i.e., Fig. 13). Figure is modified from Stephenson et al. (2009).
15.95
(±0.05)
15.14
(±0.02
)
16.00
(±0.02)
14.11
(±0.03)
14.25
(±0.05)
12.42
(±0.02)
11.73
(±0.02)
11.63
(±0.07)
17.00
(±0.05)
3
2
)
1
1.
6
3
(
±0.07
)
0
.0
5
)
Maine Too cool
< 12
o
C
Too warm
> 18
o
C
Average Summer Temperatures
Fig. 16. Latitudinal gradients in lobster population densities (left histogram) and predator densities (right histogram) determined from visual
surveys (lobster) and stationary video surveillance (predatory fish). Figure is modified from Wahle et al. (2013a).
0
0.2
0.4
0.6
0.8
1.0
1.2
Lobster Density
(No. .m-2)
Maine
New
Brunswick
MA
NH
RI
Predator Density
(predators per frame)
0
500
1000
1500
2000
2500
3000
1622 Can. J. Fish. Aquat. Sci. Vol. 70, 2013
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structure and function in the coastal Gulf of Maine. This domestica-
tion of the Gulf of Maine has contributed to the rise of lobster stocks
and socio-economic reliance on this species as a lucrative monocul-
ture. However, at high population densities and with ocean warm-
ing, we may see a rise in physiological stress and disease in the
warmer zones of this species’s range. Further, with warming seas the
diversity of predatory fishes will likely increase, as southern species
move north faster than northern species retreat. All of these changes
to climate, ocean acidification, and food webs suggest that lobster
management will likely be more challenging in the future. As unwel-
come as this may sound, we must expect the unexpected. Fisheries
management will have to assess stocks at finer spatial scales to cap-
ture their dynamics and to be able to react to unexpected events,
such as climate-driven range shifts of numerous species and diseases
into and out of management areas. In short, we must find ways to be
more “agile” in dealing with these and other new challenges of our
brave new ocean.
Acknowledgements
We have had help and guidance from many people over the past
several decades. Funding was from the University of Maine Sea
Grant program, National Science Foundation, UpEast Foundation,
and the Island Institute, with considerable help from lobster fish-
ers along the coast of Maine. Myriad summer interns and numer-
ous graduate students helped us gather the data used in the
studies cited here. Throughout this time, the University of Maine’s
Darling Marine Center was and is the hub of lobster research on
the coast of Maine. Elizabeth Stephenson and two anonymous
reviewers made useful suggestions to improve this paper. We
received editorial assistance from Jenn McHenry, Alison Hamlin,
and Molly Wilson. To all, we are grateful.
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... American lobster landings in the Gulf of Maine area have been increasing since 1940 with no appearance of population overexploitation (Steneck and Wahle 2013;Fisheries and Oceans Canada 2018), most likely due to the high per-capita recruitment success, long reproductive life, and a relaxation of predation pressure due to overexploitation of groundfish species (e.g., Atlantic cod, Gadus morhua) (Boudreau and Worm 2010;Steneck and Wahle 2013). Lobster landings along the Scotian Shelf have also been increasing (Fisheries and Oceans Canada 2020), although the long-term sustainability of this fishery is uncertain due to the effects of climate change (Greenan et al. 2019). ...
... American lobster landings in the Gulf of Maine area have been increasing since 1940 with no appearance of population overexploitation (Steneck and Wahle 2013;Fisheries and Oceans Canada 2018), most likely due to the high per-capita recruitment success, long reproductive life, and a relaxation of predation pressure due to overexploitation of groundfish species (e.g., Atlantic cod, Gadus morhua) (Boudreau and Worm 2010;Steneck and Wahle 2013). Lobster landings along the Scotian Shelf have also been increasing (Fisheries and Oceans Canada 2020), although the long-term sustainability of this fishery is uncertain due to the effects of climate change (Greenan et al. 2019). ...
... Many small coastal communities in Nova Scotia rely heavily on the roughly $880 million derived from the American lobster fishery, which comprised 57.6% of the value of all seafisheries landings in the province in 2019 (Fisheries and Oceans Canada 2021b). The heavy socioeconomic reliance on a single species has inherent risks given the uncertainties of climate change, such as the northward expansion of warm-water predators, lobster redistribution away from warmer waters, physiological stress and disease due to warmer water, the increasing prevalence of disease, and ocean acidification (Pearce and Balcom 2005;Steneck and Wahle 2013). To offset the risk of heavy reliance on a single species fishery, Nova Scotia has promoted a diversified seafood sector, which includes aquaculture, to ensure greater resiliency in the face of climate change (Doelle and Lahey 2014). ...
Article
Full-text available
Salmonid aquaculture occurs in coastal Atlantic waters around the Canadian Maritimes and can overlap with the American lobster ( Homarus americanus) fishery, the most profitable fishery in the region. There has been debate around whether there is potential for salmonid aquaculture to negatively affect the fishery that has been heated in both the scientific community and public news media. This review and resultant commentary explore the developing research approaches used in the Canadian Maritimes to examine known and inferred interactions between these two important industries. We re-examine some inferences of previous research and identify low-oxygen environments and improper use of therapeutants as having the greatest potential to adversely affect lobsters, although there are knowledge gaps. We further discuss the implications of whether localized lobster displacement from a farm area would have any measurable impact on the lobster fishing industry as a whole, using examples from Nova Scotia, Canada. In most instances, existing regulatory compliance addresses the drivers that have the largest potential to adversely impact lobster.
... The American lobster, Homarus americanus Milne Edwards, is North America's most valuable single-species fishery (DFO, 2021;NMFS, 2021), over 80% of the US production comes from the GoM (NMFS, 2021), and the fate of lobster in the GoM has been fundamentally shaped by anthropogenic impacts to suitable habitats. Overfishing over the past century drove successive regime shifts and facilitated the emergence of benthic habitats ideal for lobster proliferation while ocean warming expanded the extent to which this proliferation could occur (Steneck et al., 2011;Steneck & Wahle, 2013). Temperature affects embryonic and larval development, spawn timing, patterns of connectivity, sounding behavior, habitat suitability, and fishery recruitment (Campbell, 1983;1986;Aiken & Waddy, 1986;MacKenzie, 1988;Annis, 2005;Steneck, 2006;Xue et al., 2008;Castro et al., 2012;Tanaka & Chen, 2016). ...
... At the core of projected landings declines are patterns of reduced post-larval recruitment and warming-induced reductions in recruitment potential Steneck & Wahle, 2013;Le Bris et al., 2018). The American Lobster Settlement Index (ALSI) has been indicating widespread declines in settlement in the GoM since the mid-2000s (Figure 1-2) despite increases in spawning stock biomass and early larval stage abundance (Carloni et al., 2018). ...
... However, one fishery remains an outlier, the American lobster. Lobster is one of the only fisheries in the world that has been harvested for centuries and currently has landings higher than ever before (Steneck & Wahle, 2013). The anthropogenic forcings that decimated other fisheries lead to a niche expansion for lobster through complex interactions between reduced demographic bottlenecks and top-down controls (Wahle & Steneck, 1991;Steneck & Wahle, 2013). ...
Article
Full-text available
The Gulf of Maine has been fundamentally altered by anthropogenic forcings for decades and offers an ideal study system to monitor response to change. Through complex interactions between ocean warming, altered demographic bottlenecks, and reduced top-down controls, the American lobster (Homarus americanus Milne Edwards) capitalized on favorable conditions and proliferated within the Gulf of Maine. These changes catalyzed the expansion of the lobster fishery, elevated its status as North America’s most valuable marine resource, and shifted coastal communities towards a virtual lobster monoculture. The same processes that facilitated lobster to capitalize on favorable conditions may come with unintended consequences and have implications for sustainability in a continually changing ocean environment. As such, evaluating the anthropogenic impacts by the American lobster fishery and to lobster demographic processes is critical for effective fisheries management. This dissertation research developed, and implemented, several modeling frameworks to assess how anthropogenic impacts have fundamentally altered the American lobster fishery, how ocean change affects the demographic processes of larval and postlarval lobster, and the implications of these relationships to the sustainability of this species under climate change.
... The American lobster, Homarus americanus Milne Edwards, is North America's most valuable single-species fishery (DFO, 2021;NMFS, 2021), over 80% of the US production comes from the GoM (NMFS, 2021), and the fate of lobster in the GoM has been fundamentally shaped by anthropogenic impacts to suitable habitats. Overfishing over the past century drove successive regime shifts and facilitated the emergence of benthic habitats ideal for lobster proliferation while ocean warming expanded the extent to which this proliferation could occur (Steneck et al., 2011;Steneck & Wahle, 2013). Temperature affects embryonic and larval development, spawn timing, patterns of connectivity, sounding behavior, habitat suitability, and fishery recruitment (Campbell, 1983;1986;Aiken & Waddy, 1986;MacKenzie, 1988;Annis, 2005;Steneck, 2006;Xue et al., 2008;Castro et al., 2012;Tanaka & Chen, 2016). ...
... At the core of projected landings declines are patterns of reduced post-larval recruitment and warming-induced reductions in recruitment potential Steneck & Wahle, 2013;Le Bris et al., 2018). The American Lobster Settlement Index (ALSI) has been indicating widespread declines in settlement in the GoM since the mid-2000s (Figure 1-2) despite increases in spawning stock biomass and early larval stage abundance (Carloni et al., 2018). ...
... However, one fishery remains an outlier, the American lobster. Lobster is one of the only fisheries in the world that has been harvested for centuries and currently has landings higher than ever before (Steneck & Wahle, 2013). The anthropogenic forcings that decimated other fisheries lead to a niche expansion for lobster through complex interactions between reduced demographic bottlenecks and top-down controls (Wahle & Steneck, 1991;Steneck & Wahle, 2013). ...
Thesis
Full-text available
The Gulf of Maine has been fundamentally altered by anthropogenic forcings for decades and offers an ideal study system to monitor response to change. Through complex interactions between ocean warming, altered demographic bottlenecks, and reduced top-down controls, the American lobster (Homarus americanus Milne Edwards) capitalized on favorable conditions and proliferated within the Gulf of Maine. These changes catalyzed the expansion of the lobster fishery, elevated its status as North America’s most valuable marine resource, and shifted coastal communities towards a virtual lobster monoculture. The same processes that facilitated lobster to capitalize on favorable conditions may come with unintended consequences and have implications for sustainability in a continually changing ocean environment. As such, evaluating the anthropogenic impacts by the American lobster fishery and to lobster demographic processes is critical for effective fisheries management. This dissertation research developed, and implemented, several modeling frameworks to assess how anthropogenic impacts have fundamentally altered the American lobster fishery, how ocean change affects the demographic processes of larval and postlarval lobster, and the implications of these relationships to the sustainability of this species under climate change.
... Climate change is already provoking changes in the spatial distribution of lobster species and therefore has the potential to alter territorial behaviour of fishermen and their landings as a consequence (Briones-Fourzań and Lozano-Álvarez, 2015). Such changes have already been reported in lobster populations worldwide, and are mainly related to sea warming (Cockcroft et al., 2008;Pecl et al., 2009;Caputi et al., 2010;Steneck and Wahle, 2013;Wahle et al., 2015;Rheuban et al., 2017;Le Bris et al., 2018). Boavida-Portugal et al. (2018) projected that clawed lobsters will contract their climatic envelope between 40 and 100% by the end of the century. ...
... In this study, a positive and strong correlation was observed between winter (JFM) and spring (AMJ) sea bottom (1958) detected a strong correlation between long-term catch rates of the American lobster and sea surface temperature (SST) at the largest spatial scales, with lags of 0-3 years. More recently, Zhao et al. (2019) also reported that a temperature rise in Gulf of Maine led to increased catchability of American lobster over many years, with an expanded juvenile habitat in the north (Steneck and Wahle, 2013;Tanaka and Chen, 2016). These environmental changes have also been accompanied by the decline of large predators (Le Bris et al., 2018). ...
Article
Full-text available
The study describes recent decadal changes (2008-2017) in the landing biomass, fishing effort and CPUE (kg/day) data of European lobster Homarus gammarus in the eastern Adriatic Sea region, and relates these changes to increases of sea bottom temperatures detected at long-term in situ stations and modelled by an ocean numerical model (ROMS, Regional Ocean Modelling System). Modelling results were further used to quantify spatial and temporal differences of bottom temperature changes over different fishing zones. Trends of sea bottom temperature were positive and statistically significant between stations. Temporal trends of landing, effort and CPUE were also positive and significant for the northern Adriatic. Correlation analysis was used to test the relationship between winter and spring sea bottom temperatures and CPUE data of H. gammarus, separately for the northern and central Adriatic Sea, resulting in statistically significant correlations for both areas. Whether the increased CPUE in the northern Adriatic is due to increased abundance or catchability is discussed. The observed temperature changes likely reflect climate system changes recognised at the regional level and as such, lobster management measures will need to be revised and updated in the future.
... Flexible management approaches should be adopted to try to maintain the system resilience high in order to prevent potentially unpleasant surprises for the coupled socio-ecological system (Ingeman et al., 2019). Indeed, over the last decades, marine ecosystems have drastically changed, and in some cases have reached new simplified structures, for example crustaceans dominated communities (Daskalov et al., 2017;Pauly et al., 1998;Pershing et al., 2015;Steneck et al., 2011;Steneck & Wahle, 2013). In many cases, major changes in the environment and climate will hinder the recovery of the ecosystems towards previous states, and thus new, possibly moving targets need to be defined and adaptively reached (Ingeman et al., 2019). ...
Article
Full-text available
1. Cumulative human pressures and climate change can induce nonlinear discontinuous dynamics in ecosystems, known as regime shifts. Regime shifts typically imply hysteresis, a lacking or delayed system response when pressures are reverted, which can frustrate restoration efforts. 2.Here, we investigate whether the northern Adriatic Sea fish and macroinvertebrate community, as depicted by commercial fishery landings, has undergone regime shifts over the last 40 years, and the reversibility of such changes. 3.We use a stochastic cusp model to show that, under the interactive effect of fishing pressure and water warming, the community reorganized through discontinuous changes. 4.We found that part of the community has now reached a new stable state, implying that a recovery towards previous baselines might be impossible. Interestingly, total landings remained constant across decades, masking the low resilience of the community. 5.Our study reveals the importance of carefully assessing regime shifts and resilience in marine ecosystems under cumulative pressures and advocates for their inclusion into management.
... The American lobster (Homarus americanus) fishery in the Gulf of Maine is a clear example of a fishery for which a range of locally and globally derived human and natural drivers are known to influence abundance and decline (Steneck and Wahle 2013). The lobster fishery is consistently among the most valuable in the United States (Le Bris et al. 2018), with increasing landings since the early 1990s (Acheson and Steneck 1997; Maine Department of Marine Resources 2021) and with strong stakeholder investment in its continued success (Steneck et al. 2011). ...
Article
Full-text available
The degree to which human actions affect marine fisheries has been a fundamental question shaping people’s relationship with the sea. Today, divergences in stakeholder views about the impacts of human activities such as fishing, climate change, pollution, and resource management can hinder effective co-management and adaptation. Here, we used surveys to construct mental models of the Maine lobster fishery, identifying divergent views held by two key stakeholder groups: lobster fishers and marine scientists. The two groups were differentiated by their perceptions of the relative impact of pollution, water temperature, and fishing. Notably, many fishers perceive the process of fishing to have a positive effect on fisheries through the input of bait. Scientists exhibited a statistically significantly stronger concern for climate change and identified CO 2 as one of the dominant pollutants in the Gulf of Maine. However, fishers and scientists agreed that management has a positive impact, which appeared to be a change over the past two decades, possibly due to increased collaboration between the two groups. This work contributes to the goal of decreasing the distance between stakeholder perspectives in the context of a co-managed fishery as well as understanding broader perceptions of impacts of human activities on marine ecosystems.
... At the southern extent of the NES, the onset of epizootic shell disease is more rapid and severe at higher temperatures (Castro et al., 2006;Steneck et al., 2011) contributing to recent reductions in reproductive potential and settlement (Wahle et al., 2015). In the Gulf of Maine, however, rapid expansion of suitable thermal recruitment habitats (Goode et al., 2019;Le Bris et al., 2018) attributable to a thermal regime shift starting in 2008 (Friedland et al., 2020) (Steneck & Wahle, 2013) and limited ability to recover due to ocean warming (Pershing et al., 2015), hence providing an additional feedback on MAB lobster populations. ...
Article
Full-text available
The Cold Pool feature of the Middle Atlantic Bight (MAB) is a body of cold bottom water that develops in the spring and persists through the summer‐autumn months. It is maintained by northerly currents and can be traced back to Arctic water masses. The Cold Pool provides habitat for many boreal species at latitudes far south of their normal range and plays an important role in the population dynamics of lower and upper trophic level organisms. Here, we describe changes in the extent and thermal properties of the Cold Pool using both observations and models. Two indices are developed based on a gridded, interpolated bottom temperature dataset; the first is a mean temperature indicator, and the second is a spatial extent indicator. The temperature indicator showed a significant increasing trend over the study period 1968–2019 and a single change point in 2008. Similarly, the area indicator declined significantly, also displaying a change point in 2008. Cold Pool maximum temperature and minimum size were observed in 2017, which is also known as a heatwave year in the MAB. The indices presented here support the view of a rapidly warming Cold Pool that is being limited in its spatial extent. Changes in Cold Pool hydrography will likely affect boreal species distributions and total ecosystem productivity.
... The spatial distribution of sublegal lobsters from June to August is influenced by many biotic and abiotic factors (Steneck and Wahle, 2013;Boudreau et al., 2015). A generalized additive model (GAM) with a Tweedie distribution has been recommended and widely used to quantify the relationships between lobsters and environmental variables in the inshore GOM (Chang et al., 2010;Tanaka et al., 2019). ...
Article
The stock assessment of American lobster (Homarus americanus) plays an important role in managing the fishery in the Gulf of Maine (GOM). Various fishery-dependent and fishery-independent data are required in the stock assessment to estimate key fisheries parameters that define the population dynamics of American lobster. In the 2015 benchmark stock assessment, ventless trap survey (VTS) data were included for the first time to provide information about the sublegal lobster (carapace length < 83 mm) dynamics. However, the effectiveness of VTS data in monitoring sublegal lobsters has not been evaluated and we have little information on whether the VTS sampling design can capture sublegal lobster dynamics. The primary goal of this thesis research was to evaluate and determine whether the data collected from the Maine VTS provide robust estimation of design-based sublegal lobsters abundance index in the inshore GOM. To achieve this goal, I (1) estimated and evaluated variations in catch rates derived, respectively, from the first, second, and third ventless trap per site; 2) predicted sublegal lobster population at a high spatial resolution using generalized additive models (GAMs); (3) sampled the simulated sublegal lobster population following the sampling protocol used in the VTS program to derive a simulated VTS abundance index; and 4) compared the simulated VTS abundance index with the predicted population abundance index in the simulated sublegal lobster population. The spatial scale of the study was defined by the National Marine Fisheries Service (NMFS) statistical areas in Maine, areas 511, 512, and 513. The lobster data used to develop the GAMs were from the Maine-New Hampshire Inshore Bottom Trawl Survey (BTS) from years 2006-2016. The VTS data from 2006-2016 were sourced as the observed VTS abundance index. VTS catch rate per trap was considered during the step of sampling the simulated sublegal lobster population using the VTS sampling protocol, and the predictive variables considered included depth and temperature. This study showed that there were no significant differences in abundance, sex ratio, and size composition of the juvenile lobsters caught by the three traps in a trawl used in a VTS and that the correlation between abundance indices from subsampling scenarios and corresponding observed abundance indices were all greater than 0.99. I conclude that the VTS provides a robust estimation of sublegal American lobster abundance index in the inshore GOM.
... Abrupt and unexpected transitions between alternative system states are often the consequence of climate change, overexploitation or a combination of both (Benson & Trites, 2002;Daskalov, 2002;Hare & Mantua, 2000). Commercially exploited fish species that experienced population collapse are prominent examples with important socio-economic ramifications (Beisner et al., 2003;Frank et al., 2011;Myers & Worm, 2003;Steneck & Wahle, 2013). Studying the population dynamic and the effect of the population structure on the population change of two co-occurring fish in the Norwegian sea-Barents Sea system ( Figure 1), Rouyer et al. (2011) concluded that the NSS herring and NEA cod stocks were responding to environmental forcing differently before compared to after the collapses. ...
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Both the Norwegian Spring Spawning herring (Clupea harengus) and the Northeast Arctic (NEA) cod (Gadus morhua) are examples of strong stock reduction and decline of the associated fisheries due to overfishing followed by a recovery. Cod and herring are both part of the Barents Sea ecosystem, which has experienced major warming events in the early (1920–1940) and late 20th century. While the collapse or near collapse of these stocks seems to be linked to an instability created by overfishing and climate, the difference of population dynamics before and after is not fully understood. In particular, it is unclear how the changes in population dynamics before and after the collapses are associated with biotic interactions. The combination of the availability of unique long-term time series for herring and cod makes it a well-suited study system to investigate the effects of collapse. We examine how species interactions may differently affect the herring and cod population dynamic before and after a collapse. Particularly we explore, using a GAM modeling approach, how herring could affect cod and vice versa. We found that the effect of cod biomass on herring that was generally positive (i.e., covariation) but the effect became negative after the collapse (i.e., predation or competition). Likewise a change occurred for the cod, the juvenile herring biomass that had no effect before the collapse had a negative effect after. Our results indicate that the population collapses may alter the inter-specific interactions and response to abiotic environmental changes. While the stocks are at similar abundance levels before and after the collapses, the system is potentially different in its functioning and may require different management action.
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Climate change is fueling unprecedented warming in marine environments. Crustaceans demonstrate strong physiological responses to rising temperatures including smaller molt increments, more frequent molting, and decreased size-at-maturity (SAM). Despite the potential for these changes to affect population and fisheries dynamics, there is limited research quantifying potential impacts. This is concerning, as crustacean fisheries are becoming increasingly important globally. Using an individual-based model for the valuable American lobster (Homarus americanus) fishery in the Gulf of Maine (GOM), we sought to address pressing questions about how climate-driven changes to growth and maturity could impact the population and fishery dynamics. To do so, we used counterfactual simulations informed by the literature and an expert survey to quantify the effects of smaller molt increments, increased molting, and decreased SAM on lobster spawning stock biomass (SSB) and landings. We found that all three changes to life history, combined, increased the SSB and landings relative to the base case. Notably, SSB in the terminal year of the time-series was 278.3%, 505.1%, and 748.5% greater than the base case under small, moderate, and extreme changes, respectively, to life history. However, when molt increment size and molting probability were changed but SAM was not, SSB and landings decreased relative to the base case, emphasizing the importance of SAM as a driver of productivity. Overall, these findings indicate that climate-driven changes to growth and maturity may favorably impact the population and fishery dynamics of lobster, contributing to sustained fishery yield in the GOM. This study emphasizes the utility of simulating the effects of climate-forced life history change for informing climate-ready fisheries management. Future work should explore the effects of other climate-driven life history changes alongside alternative management strategies for lobster and other crustaceans.
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The natural diet of 506 American lobsters (Homarus americanus) ranging from instar V (4 mm cephalothorax length, CL) to the adult stage (112 mm CL) was determined by stomach content analysis for a site in the Magdalen Islands, Gulf of St. Lawrence, eastern Canada. Cluster and factor analyses determined four size groupings of lobsters based on their diet: <7.5 mm, 7.5 to <22.5 mm, 22.5 to <62.5 mm, and ≥62.5 mm CL. The ontogenetic shift in diet with increasing size of lobsters was especially apparent for the three dominant food items: the contribution of bivalves and animal tissue (flesh) to volume of stomach contents decreased from the smallest lobsters (28% and 39%, respectively) to the largest lobsters (2% and 11%, respectively), whereas the reverse trend was seen for rock crab Cancer irroratus (7% in smallest lobsters to 53% in largest lobsters). Large lobsters also ate larger rock crabs than did small lobsters. This study is the first to examine the natural diet of shelter-restricted juveniles (SRJs, <14.5 mm CL), which were thought to be principally suspension feeders and to a lesser degree browsers or ambush predators in or near their shelter. However, at our study site no planktonic organisms were identified from the stomachs of SRJs, whereas formaniferans, crustacean meiofauna, and macroalgal debris that could be derived by browsing, together represented only 10-14% by volume of stomach contents. We infer that SRJs obtained bivalves by predation and flesh by exploiting larger lobsters' meal scraps or food reserves. Some implications of these findings for lobster artificial reef programs and for the conservation of lobster stocks are discussed.
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Food resource utilization was investigated among longhorn sculpin, Myoxocephalus octodecemspinosus; winter flounder, Pseudopleuronectes americanus; windowpane, Scophthalmus aquosus; yellowtail flounder, Limanda ferruginea; little skate, Raja erinacea; Atlantic cod, Gadus morhua; red hake, Urophycis chuss; and ocean pout, Macrozoarces americanus. Despite the dominance of polychaetes and mollusks in the benthos, crustaceans composed the major prey group in all predators. There was considerable trophic similarity among the fishes, and the amphipods Unciola sp. and Leptocheirus pinguis were the most important prey in 7 of the 8 predators. Resource partitioning by prey size is related to different mouth morphologies for closely related species (winter flounder, yellowtail flounder, windowpane), and unrelated species with similar mouth morphologies may overlap in prey size use (longhorn sculpin, Atlantic cod).-from Author
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When the Bering Strait between Alaska and Siberia opened about 3.5 Ma during the early Pliocene, cool-temperate and polar marine species were able to move between the North Pacific and Arctic-Atlantic basins. In order to investigate the extent, pattern, and dynamics of this trans-Arctic interchange, I reviewed the Recent and fossil distributions of post-Miocene shell-bearing Mollusca in each of five northern regions: (1) the northeastern Atlantic (Lofoten Islands to the eastern entrance of the English Channel and the northern entrance of the Irish Sea), (2) northwestern Atlantic (southern Labrador to Cape Cod), (3) northeastern Pacific (Bering Strait to Puget Sound), (4) northwestern Pacific (Bering Strait to Hokkaido and the northern Sea of Japan), and (5) Arctic (areas north of the Lofoten Islands, southern Labrador, and Bering Strait). I have identified 295 molluscan species that either took part in the interchange or are descended from taxa that did. Of these, 261 are of Pacific origin, whereas only 34 are of Arctic-Atlantic origin. Various analyses of the pattern of invasion confirm earlier work, indicating that there is a strong bias in favor of species with a Pacific origin. A geographical analysis of invaders implies that, although trans-Arctic interchange contributed to a homogenization of the biotas of the northern oceans, significant barriers to dispersal exist and have existed for trans-Arctic invaders within the Arctic-Atlantic basin. Nevertheless, trans-Arctic invaders in the Atlantic have significantly broader geographical ranges than do taxa with a pre-Pliocene history in the Atlantic. Among the possible explanations for the asymmetry of trans-Arctic invasion, two hypotheses were explicitly tested. The null hypothesis of diversity states that the number of invaders from a biota is proportional to the total number of species in that biota. Estimates of Recent molluscan diversity show that the North Pacific is 1.5 to 2.7 times richer than is the Arctic-Atlantic, depending on how faunistic comparisons are made. This difference in diversity is much smaller than is the asymmetry of trans-Arctic invasion in favor of Pacific species. Rough estimates of regional Pliocene diversity suggest that differences in diversity during the Pliocene were smaller than they are in the Recent fauna. The null hypothesis was therefore rejected. The hypothesis of ecological opportunity states that the number of invaders to a region is proportional to the number of species that became extinct there. The post-Early Pliocene magnitude of extinction was lowest in the North Pacific, intermediate in the northeastern Atlantic, and probably highest in the northwestern Atlantic. The absolute number and faunistic importance of post-Early Pliocene invaders (including trans-Arctic species, as well as taxa previously confined to warm-temperate waters and western Atlantic species that previously occurred only in the eastern Atlantic) was lowest in the North Pacific, intermediate in the northeastern Atlantic, and highest in the northwestern Atlantic. Further support for the hypothesis of ecological opportunity comes from the finding that hard-bottom communities, especially those in the northwestern Atlantic, show a higher representation of molluscan species of Pacific origin, and are likely to have been more affected by climatic events, than were communities on unconsolidated sandy and muddy bottoms. Support for the hypothesis does not rule out other explanations for the observed asymmetry of trans-Arctic invasion. A preliminary study of species-level evolution within lineages of trans-Arctic invaders indicates that anagenesis and cladogenesis have been more frequent among groups with Pacific origins than among those with Atlantic origins, and that the regions within the Arctic-Atlantic basin with the highest absolute number and faunistic representation of invaders (western Atlantic and Arctic) are the regions in which speciation has been least common among the invaders. The asymmetry of invasion is therefore distinct from the asymmetry of species-level evolution of invaders in the various northern marine regions.