Allee Effects in Fisheries

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Allee Effects in Fisheries
Jo Gascoigne j.gascoigne_at_bangor.ac.uk Office 033
Westbury Mount, SOS
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Part I of lecture

• Density dependence what is density dependence?
  Competition
• Logistic population dynamics based on
  assumption of competition at high density
• What about low density? Not always good for
  organisms either
• If organisms suffer ( lower fitness) at low
  density get another kind of density dependence
  an ALLEE EFFECT

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Density dependence

• Population parameters depend on population
  density
• What are population parameters?
• GROWTH RATE (Von Bertalanffys k)
• TIME TO MATURITY
• REPRODUCTIVE OUTPUT
• MORTALITY RATE
• ? Depend on individual fitness

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Sidebar pop. size vs. density

• Density number per unit area (per divide by)
• Population size usually measured in a given area
  anyway
• i.e. as long as the area doesnt change in the
  middle of your calculations, population size and
  population density are EQUIVALENT
• Density a more meaningful measure hence DENSITY
  DEPENDENCE

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Population parameters
Bad conditions
Good conditions

• Growth rates
• Reproductive output
• Time to maturity
• Mortality rates
• Fitness

Population growth
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Competition main form of density dependence

• Low density plenty of food, space good
  conditions high fitness population gets
  bigger
• High density not much food, space etc to spare
  poor conditions low fitness population
  stays the same (at population carrying capacity)

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Traditional population dynamics logistic equation

• Logistic equation (if you only know one equation
  in ecology, this is the one to know)
• Verhulst (famous Belgian!)

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Assumption of logistic dynamics

• Logistic dynamics assumes that fitness declines
  as population density increases
• Assumes that fitness is maximum when population
  density is (close to) zero

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Logistic population dynamics
Per capita population growth rate (individual
contribution to population growth)
MEAN FITNESS
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Population size / density
Stable equilibrium (K)
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Population dynamics and fisheries management

• Fisheries management assumes that when you
  fish a population down, it can recover
• Fitness increases when population size or
  density decreases
• Sustainable fisheries management assumes
  logistic population dynamics

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Logistic population dynamics
Nearly all commercially fished stocks down to
10-20 of historic size (carrying capacity) 16
NE Atlantic stocks within safe biological limits
? now interested in low density end of graph
Fitness
0
Population size / density
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Fitness might be reduced at low density

• Broadcast spawning

Archaeogastropods abalone Haliotis spp.
Bivalves cockles Cerastoderma edule
Corals (and sponges) Dendrogyra sp.
Echinoderms Diadema antillarum
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Very strong relationship between fertilisation
success and density in broadcast spawners
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Fisheries for broadcast spawners

• Sponges (remember if youre sessile you dont
  have a choice!)
• Sea urchins (big fishery e.g. west coast of North
  America)
• Abalone (white abalone Haliotis sorenseni first
  marine invertebrate to be documented extinct?)
• All bivalves broadcast spawners - cockles,
  mussels, oysters
• Fish spawning aggregations e.g. for many
  grouper species (Epinephalus spp. and related
  genera) easy target for fishers
• Also non-fisheries conservation issues corals,
  Diadema

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Fitness might be reduced at low density

• Gregariousness mortality is lower where density
  is higher

Caribbean spiny lobster Panulirus argus
Queen conch Strombus gigas
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Fitness might be reduced at low density

• Sperm limitation when few males available

Blue crab
New Zealand rock lobster
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Fitness might be reduced at low density

• Protection from predators Cultivation effect

Cod Gadus morhua
Red sea urchin Strongylocentrotus franciscanus
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Reduction in fitness (or per capita population
growth rate) at low density ALLEE EFFECT
(depensation)
Prof. Warder Clyde Allee, University of Chicago,
1930s and 40s
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Caveat

• Need to consider whole life history

Caribbean spiny lobster Gregarious - but very
long larval stage high dispersal recruitment
decoupled from local density
Blue crab Sperm limitation but high levels of
cannibalism at high density
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Allee population dynamics
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Allee population dynamics
Allee effect
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Unstable equilibria
Any perturbation away from unstable equilibrium
one of two alternative stable states (stable
equilibria)
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Alternative stable state 2 extinction
Alternative stable state 1 carrying capacity
dN/Ndt
Density
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Take home messages from here

• Traditional fisheries models assumes that fitness
  maximum at low population density ? Allee effects
  say this might not be true
• Allee effects cause discontinuities (alternative
  stable states) in equilibrium population size ?
  critical density below which population will
  crash without warning

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Dont get the wrong idea

• Logistic equation is NOT completely wrong
  (heresy!)
• But need to be careful when dealing with
  populations which are very much reduced below
  carrying capacity
• Verhulst probably never had to deal with them but
  now we do

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Part II of lecture

• Allee effects and mortality Allee effects
  interact with fisheries (not in a good way)
• Evidence for Allee effects in practice? Some case
  studies
• Abalone
• Cod and other long lived fish species
• How to manage species with Allee effects

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Allee effects and mortality
Total mortality Z Natural mortality M Fishing
mortality F
Z 0.4
Per capita pop. growth rate (fitness)
Z 0.7
Density
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(No Transcript)
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Evidence..?

• Lots of mechanisms as discussed above (but
  mechanism Allee effect lobster and crabs)
• Hard evidence from population dynamics difficult
  to come by - mainly because population data for
  marine species is hard to get
• But there is some evidence case studies

/
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Abalone Haliotis spp.
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NB are landings really a good estimate of
population size?
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Grand Banks Cod (Gadus morhua)
Rose 2004 Canadian J. Fish. Aquat. Sci.
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Grand Banks Cod (Gadus morhua)
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Recent (sorry) history of Grand Banks cod fishery

• 1968 northern cod catch 810 000 tonnes
• 1975 catch 287 000 tonnes (down 65)
• 1989 scientists say TAC should be 125 000 but
  set at 266 000
• 1992 Jan TAC set at 120 000 July 2 year
  moratorium on northern cod
• 1993 indefinite moratorium

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Recent (sorry) history of Grand Banks cod fishery

• 1994 recreational fishery closed
• 1996 - Newfoundland (human) population down 3
  since 1991
• 2001 3 year TAC for northern cod set at 5 600
  tonnes. Newfoundland population down a further 7
• 2003 After 11 years of moratorium or very low
  TAC, no evidence that cod stocks are recovering

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Mechanisms for Allee effects

• Cultivation effect (low survival of juveniles in
  absence of adults)
• Reproduction at low density cod may not meet
  suitable mates any more
• NB recent paper suggests that climate change
  also a factor (Rose 2004 CJFAS)

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Not only cod
line of no recovery
Hutchings 2000 Nature
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Management of populations with Allee effects
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Management of populations with Allee effects
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Management of populations with Allee effects

• Fishermen tend to target high density areas (they
  are not stupid)
• Management needs to take into account patchiness
  of distribution
• Traditional management (quotas, closed seasons,
  size limits) doesnt do that
• Protected areas

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Marine protected areas

• Protect area at high density key if species has
  Allee effects
• Protect whole ecosystem from fishing damage
  (think about bycatch species, damage from fishing
  gear)
• BUT need to protect a big enough area (what is
  a big enough area???) and the right area (what is
  the right area???)
• Socially difficult

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Finding the right place is key
GOOD IDEA
BAD IDEA
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What if protecting gt1 species?
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Take home messages

• Allee effects low fitness at low density or
  small population size
• Allee effects can cause abrupt population crash
• Lots of possible mechanisms
• Cod and other fish spp., abalone likely
  candidates
• Allee effects interact badly with increased
  mortality (fishing)
• Management needs to consider Allee effects and
  density