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 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

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

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

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

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

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

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

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. β to produce same initial equilibrium

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

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

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

DD Juvenile Growth (Depends on biomass density of population) 20 40 60 80 100 120 1 4 7 10 13 16 19 22 25 28 31 34 TIME % INITIAL ADULTS Adults Juvs/100 Postlarvae/100 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

DD Growth - all stages (Juvenile, small adult growth depends on pop. biomass. Post-larvae growth depends on juvenile biomass) 20 40 60 80 100 120 1 4 7 10 13 16 19 22 25 28 31 34 TIME % INITAIAL ADULTS Adults Juvs/10 Postlarvae/100 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!

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

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)

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.

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

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

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

β DD growth

Simulation Summary Beverton & Holt 1957 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

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

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?