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Fishery managers should consider compensatory processes: simulated responses to fishing

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. Abalone are unusual fish.

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Fishery managers should consider compensatory processes: simulated responses to fishing

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  1. 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

  2. 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

  3. 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

  4. 3 m 3 m 2 m Experiments -with Industry help! on compensatory mechanisms • Settlement, Post-larval survival? • Cryptic juvenile survival, growth? • Size at maturity? • Adult survival, growth? • Adult Fecundity?

  5. 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

  6. 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

  7. 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

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

  9. 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 Used Beverton-Holt function • Adj. βto produce same initial equilibrium e.g. DD fecundity: fecundity of 3 adult sizes affected by density of adults + largest juveniles

  10. Effects of fishing, Part 1Comparing 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

  11. 125 Adults Juvs Post-larvae 100 Adults Adults Juvs Juvs Postlarvae Postlarvae 75 % INITIAL ADULTS 50 25 0 1 4 7 10 13 16 19 22 25 28 31 34 TIME DD Fecundity After fishing: Higher fecundity: more post-larvae per adult. Pop. stabilises in +/- 16 yr

  12. 250 Postlarvae/100 Juvs/10 Adults 200 250 Adults Juvs/10 Postlarvae/100 200 150 150 % INITIAL ADULTS 100 % INITIAL ADULTS 100 50 0 50 1 4 7 10 13 16 19 22 25 28 31 34 TIME 0 1 4 7 10 13 16 19 22 25 28 31 34 TIME DD Juvenile Mortality Note changes in scales! After fishing: Juvenile No.s reduced, then better survival stabilises population in <10yr

  13. 120 Adults Juvs/100 Postlarvae/100 100 80 120 Adults Juvs/100 Postlarvae/100 100 60 80 60 % INITIAL ADULTS % INITIAL ADULTS 40 40 20 0 20 1 4 7 10 13 16 19 22 25 28 31 34 TIME 0 1 4 7 10 13 16 19 22 25 28 31 34 TIME 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

  14. 120 Adults Juvs/10 Postlarvae/100 100 80 60 % INITAIAL ADULTS 40 20 0 1 4 7 10 13 16 19 22 25 28 31 34 TIME 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!

  15. 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

  16. 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)

  17. 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.

  18. 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

  19. 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

  20. 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 βs • For growth β = 10 - 10000 x mortality β: • slow approach to equilibrium under fishing • Adult biomass remains at high level

  21. β DD growth

  22. 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

  23. 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

  24. 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?

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