Fishery managers should consider compensatory processes: simulated responses to fishing

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Fishery managers should consider compensatory
processes simulated responses to fishing

• By
• Rob Day, David Bardos,
• Fabrice Vinatier and Julien Sagiotto
• Zoology Dept, School of Physics, University of
  Melbourne
• Agronomy, INA-PG, Paris, France

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Abalone are unusual fish

• Sedentary adults catch algae
• Short larval dispersal c 200m
• - Thus hundreds of stocks
• Juveniles cryptic under rocks

• Adults cannot be aged
• Aggregate to hotspots
• - Divers can target these
• - CPUE hyperstable

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Current Dynamic pool model

• Fitted to adult survey data 1992-present
• Nos, size distribution, catch
• Complete catch history known
• Stochastic growth model
• Average M, growth parameters used
• Fecundity based on size distribution
• Fits relation of Fecundity- Recruitment (50 mm)
• One year recruitment lag

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Experiments -with Industry help! on
compensatory mechanisms

• Settlement, Post-larval survival?
• Cryptic juvenile survival, growth?
• Size at maturity?
• Adult survival, growth?
• Adult Fecundity?

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What DD responses maintain stocks?
DD postlarval growth and survival DD cryptic
juvenile growth DD time to maturity DD growth of
smaller adults DD size-spcific fecundity in
larger adults
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A Simulation Approach

• Explore consequences of each DD mechanism
• compare responses to fishing
• NOT fitting a DD model to data
• Inspect the process like IBM approach
• Explore variations of the models
• to determine sensitivity to model structure

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Stage-Structured Matrix Models
xi,n is the population of size class i at time n
gij are growth transition probabilities from i
to j assuming survival fj is the number of
post-larvae produced / individual sj is the
survival probability over one time-step
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The growth transitions

• Fast abalone growth is modeled
• Best understood by a transition chart
• Some stages can grow 2-3 classes

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Each model has 1 DD mechanism

• Many possibilities!
• Specify which stage is affected by density
• Specify which stages affect them (biomass /
  numbers)
• Chose realistic options

e.g. DD fecundity fecundity of 3 adult sizes
affected by density of adults largest juveniles

• Used Beverton-Holt function
• Adj. ß to produce same initial equilibrium

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Effects of fishing, Part 1 Comparing DD models

• Start with pop. at 10, 000 adults (equilibrium)
• Start fishing at step 5
• 90 of largest size class fished
• 12 types of DD effect modeled
• Examine, explain compare responses
• Especially time to fished equilibrium

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DD Fecundity
After fishing Higher fecundity more
post-larvae per adult. Pop. stabilises in /- 16
yr
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DD Juvenile Mortality
Note changes in scales!
After fishing Juvenile No.s reduced, then
better survival stabilises population in lt10yr
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DD Juvenile Growth
(Depends on biomass density of population)
Prior to fishing large no. juveniles, as growth
is suppressed. After fishing Juvs begin to
grow faster. This increases, then stabilises
adult numbers Stability after /- 19 yr
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DD Growth - all stages
(Juvenile, small adult growth depends on pop.
biomass. Post-larvae growth depends on juvenile
biomass)
Prior to fishing Most adults small their
growth is suppressed, so fishing has less
effect After fishing increased growth of juvs
restores adults, but juv nos decline, so slow
decline of adults, over 30 years!
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Perturbation Analysis

• Perturbations of /- 10 to mortalities,
  fecundities and three growth parameters
• No changes substantially alter conclusions
• Altered fishing pressures
• At over 50 little change in dynamics or even
  equilibrium stocks under fishing

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Effects of fishing, Part 2 Variations of the
models

• Change growth reduction process
• 1st models set increasing to zero growth
• In type 2 models growth reduced by 1 step
• (some base growth transitions are 2 steps)
• From 1 to 2 adult size classes fished (90 pa)
• Fishing a set Quota
• Combine 2 DD responses (growth and mortality)

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Type 2 DD growth

• Type 2 compensation is weaker, as growth is not
  stopped more realistic
• But Nos. at equilibrium under fishing are similar
  for each model
• Times to equilibrium even longer!
• Because less change in recruitment to fished
  sizes as adults are fished.

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Changes to Fishing

• Few differences when 2 sizes fished
• but fewer adults remain under fishing, except DD
  adult growth models (few larger adults before
  fishing)
• Quota fishing is more realistic
• But real quotas are adjusted after the fish-down
  phase
• Time to mine down the stock below the quota was
  examined
• Thus sustainable quotas for each model found

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Quota Fishing Results

• Sudden transition sustainable to unsustainable
  quota
• DD growth based on biomass lowest quota
  transitions
• Type 2 DD models had even lower transitions
• (not shown)
• For each model, transitions reflect biomass under
  high fishing pressure i.e. recruitment
• With sustainable quotas, times decreases to
  equilibria under fishing similar to using F 0.9

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Two DD responses

• DD growth based on biomass DD mortality
• Realistic at high density growth reduces, then
  they die
• Effects appear additive, depend on ratio of ßs
• For growth ß 10 - 10000 x mortality ß
• slow approach to equilibrium under fishing
• Adult biomass remains at high level

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ß DD growth
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Simulation Summary

• DD mortality, fecundity rapid stabilisation
• Growth can lead to very slow stabilisation
• and complex responses
• speeds up the generation time
• This pattern holds for a range of models
• and for combined growth and mortality
• We know DD growth is strong in abalone
• DD growth perhaps best established
• Beverton Holt 1957
• most models assume simple, rapid DD recruitment

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The Assessment problem

• About 60 sites surveyed annually
• Adult Nos size distribution
• Cannot survey every reef
• Growth rates differ widely between reefs
• Size at maturity varies greatly
• 160 mm vs 70 mm
• Model cannot fit DD growth effect
• Model cannot be fitted at local scale

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Assessment at the local scale?

• Every reef requires different management
• Local scale management by industry?
• Now happening in Victoria
• Based on diver perceptions, guesses
• Catch history at local scale known
• But no time series except sample reefs
• Can experiments reveal enough about dynamics?
• Can simulations guide local management?