Transcript Slide 1

Review and Conclusions
D. Roberts
Queen’s University, Belfast
Since the appearance of humans Strangford Lough has
been exploited by them – first, in prehistoric periods, by
hunter-gatherers then by farmers
Because population density was low, early exploitation
was probably sustainable although the landscape started
to change as a result of agriculture
In Medieval times exploitation increased and this
continued into post-medieval and recent times with
increasing exploitation for food, minerals, recreation and
waste disposal and agricultural intensification
A number of changes have been documented over
the last 40-50 years and their causes and
consequences suggested
The most recent changes i.e. changes in benthic and
intertidal communities, as we have heard, led to the
present study which was charged with trying to identify
probable causes
The presentation is in two parts:
review longer historical perspective
review of present study [SLECI]
Historical Perspective (McErlean et al., 2002)
PERIOD
Prehistoric
10,000-1,600
BP
Mediaeval
1,600-400 BP
Molluscs
oysters,
limpets,
cockles,
winkles,
whelks
Middens
ND
CRUSTACEA Finfish
crabs, lobster, Herring,
prawns
salmon,
whitefish
ND
ND
ND
Fish
traps
Seaweed
Dulse,
Laminaria,
fucoids
ND
ND
Habitat/
species
changes
minimal
Fish
traps
Post- medieval
17th C
18th C
19th C
ND = no data
oyster
fishery
oyster
fishery
oysters
decline
ND
ND
ND
herring
ND
herring
decline
ND
seaweed
industry
seaweed
collapse
ND
seaweed
grids
ND
Historical Perspective
PERIOD
Molluscs
CRUSTACEA
Finfish
Seaweed
Habitat/
species
changes
20th C
1900-1920s
Collapse
of oysters
1930-1960s
ND
1970s-1980s
1990s-2000
ND = no data
Oyster
aquaculture;
scallop
fishing
Oyster
restoration
aquaculture:
Pecten;
Mytilus
ND
ND
Potting:
Cancer,
Homarus,
Nephrops
Potting:
portunids
Herring
decline
Herring
collapse
ND
ND
ND
ND
ND
ND
Spartina
introduced
Modiolus
damage
(Brown
’89); Skate
absent
experimental Modiolus
damage
seaweed
aquaculture confirmed;
Sargassum
Habitat/species changes confirmed/recorded
by SLECI
Subtidally the major documented changes between the
earlier studies and the present study have been in
biogenic reefs of Modiolus:
Large-scale reductions in the extent, reported by
recent surveys were confirmed by both direct
observation and broadscale mapping
Densites have declined and community structure
has changed
Habitat/species changes confirmed/recorded
by SLECI
However, population structure at the remaining
intact sites studied is comparable to earlier studies
and there is evidence of recruitment
Other subtidal communities were largely unchanged in
both species composition and abundance when
compared with earlier studies
Indeed all major habitat types/ biotope complexes
reported by Ulster Museum sub-littoral surveys in the
1970s and 1980s are still present
Habitat/species changes confirmed/recorded
by SLECI
Intertidally:
Zostera and Ostrea have declined and increased within
the last 100 years, Ascophyllum has declined over
recent years and the local distributions of Gibbula
umbilicalis and Osilinus lineatus have changed
Other Changes
We have witnessed a number of introductions of nonnative and non-local species, both
deliberate: Spartina, Crassostrea gigas, Ostrea edulis
and Pecten maximus
and
Accidental: Sargassum
There has been:

a general decline in commercial fishing

a steady but regulated expansion in aquaculture

no major changes in pollutant loadings apart from
tin and chromium levels which have both declined

blooms of some algal species
In the 1970s & 80s Modiolus beds were known to be
extensive in Strangford Lough (Seed & Brown, 1977;
Erwin, 1978, Erwin et al, 1990).
Trawling and dredging are known to damage biogenic
reefs and seabed communities (Watling & Norse, 1998;
Cranfield et al. 2003).
Brown (1989) published report suggesting trawling
has damaged M modiolus communities in Strangford
Lough
In the early 1990s certain areas, particularly those
occupied by the M. modiolus/ Chlamys varia community,
were impacted by mobile fishing gear (Service &
Magorrian, 1997).
As a result of recommendations by DARD (formerly
DANI a number of legislative measures were introduced in
1993 to manage fishing activity.
Judging by fishing returns for species such as scallops
(Pecten & Aequipecten), fishing using mobile bottom gear
peaked between the late ‘80s and early ’90s
In their 1997 paper Service and Magorrian suggested
that reduction in fishing activity would afford the
opportunity to follow recovery
SLECI found no evidence of recovery of Modiolus reefs
in impacted areas
In impacted areas we have heard that K-selected
(long-lived) species are generally decreasing and rselected (short-lived) species are increasing in both
Modiolus and infaunal communities
Communities damaged by bottom gear and discards
attract predators and may be subject to heavy predation
Evidence for this is provided by increasing starfish
abundance in visual fast counts of benthic video
surveys between 1993 and 2002 and the observation
of large starfish in the SLECI diving survey
Damage to the reef structure and its epifauna,
especially sponges, increases its vulnerability to
predation (see Bloom, 1975, Pitcher & Bingham, 2002)
Experience Elsewhere
Studies by Cranfield at al. -1999 & 2003 in the Foveaux
Strait, New Zealand, where oyster dredging has occurred
over 130 years, contribute to our understanding of
interactions between fishing and biogenic reefs :
Before dredging, bryozoan reefs and associated
oysters (Ostrea [Tisostrea] chilensis) were tidally
oriented in areas of strong current
As well as removing the reefs and oysters, dredging
resulted in the release of large volumes of biogenic
sediments which were deposited elsewhere and the
seafloor reverted to underlying pebble gravel so that
the substrates underlying biogenic reefs were exposed
The Foveaux Strait experiences strong tidal
currents which accelerate this process
Surveys in 1998 revealed that some sites which had
been unfished for 49 years showed no regeneration of
biogenic reefs
Other sites showed the development of biogenic
reefs of Modiolus areolatus which were also tidally
oriented
They concluded that attempts to rebuild a fishery may
be more successful if combined with habitat restoration
Relevance to the present study
Although the Foveaux Strait is in a near open ocean
situation and Strangford Lough is enclosed, there are
parallels in that both experience strong tidal flows
In Strangford Lough, strong tidal flows surge through
the Narrows and through the central channels
becoming dissipated in the upper reaches of the Lough
It is possible that any impact on Modiolus reefs will be
accelerated by tidal currents and the release of sediment
INTERTIDAL CHANGES
Zostera densities declined as a result of disease and
may be expanding now in response to improved water
clarity
Recent increases in Ostrea are probably a result
of restoration and over-summering commercial
stocks but this may have occurred fortuitously at the
time when Modiolus densities were declining so that
oysters can capitalise on primary production no
longer being used by Modiolus
Ascophyllum may have declined as a result of a
combination of increased grazing by limpets and changes
in wind patterns
SUMMARY and CONCLUSIONS
Strangford Lough is a complex system
It has been used by humans since prehistoric times
with changes in resource exploitation in response to
changes in demands and different levels of impact
from relatively benign to localised extinction of species
The most recent impact has been the severe decline in
Modiolus biogenic reefs and changes in their associated
communities
Other recent changes include increases in Zostera
and Ostrea ; decreases in Ascophyllum and
distributional changes in Gibbula umbilicalis and
Osilinus lineatus
Although apparently well studied our understanding
of the Lough is incomplete
Long-term data are fragmentary and monitoring has
been inadequately coordinated - i.e. fisheries and
environmental data, environmental/habitat monitoring
Acknowledgements
Principal Investigators
Bernard Picton, UM; Matthew Service, DARD
Investigators
Carys Ann Davies; Annika Mitchell; James
Strong; Heather Moore; Samantha Vize;Alex
Portig; Jane Preston; David Smyth