1、Are the apparent rapid declines in top pelagic predators real?,Mark Maunder, Shelton Harley, Mike Hinton, and othersIATTC,Outline,Definitions and terms Why the declines dont make sense Japanese longline catch and effort (EP0) Population dynamics Explanatory hypotheses Regime change Ecosystem Spatial
2、 distribution of effortHabitat and gear distribution Stupid fish hypothesis Summary,Definitions,CPUE = catch-per-unit-of-effort Nominal CPUE Fitting a model Carrying capacity Average abundance in the absence of fishing,Basic assumption,C = EqA C/E = qAWhere : C= catch, E = effort, and A = abundance,
3、Fit model to data,PellaTomlinson-model with Nmsy/N0 = 30% (based on numbers) Project population dynamics from an unexploited condition using observed catch Fit to CPUE data as a relative abundance index (assume CPUE proportional to abundance) Use only Japanese longline CPUE and Catch data,Remove fir
4、st few data points,Only fit to decline,Regime change hypothesis,Blue marlin production residuals,Change in productivity, yellowfin example,Maunder 2002. IATTC Stock Assessment Report 3,Change in productivity, bigeye example,Maunder and Harley 2002. IATTC Stock Assessment Report 3,Ecosystem model,Hig
5、h Adult Biomass,Low Adult Biomass,Ecosystem model,K = carrying capacity N = numbers f(N) is the single species production,Ecosystem results,Fits the data significantly better Nearly all improvement from fit to bluefin tuna data Also improves fit to blue marlin data,Bluefin tuna,Single Species,Ecosys
6、tem,Blue marlin,Single Species,Ecosystem,Spatial expansion of the longline fishery,Spatial hypotheses,Time,Highest CPUE,Highest Profit,Equal distribution,Simulation of effort expansion,Effort increases by 100 units every 5 years Movement of effort only occurs every five years No movement of fish amo
7、ng areas In highest profit hypothesis, new effort goes into new area and old effort stays in the same area In highest CPUE hypothesis, all effort goes into new area,CPUE trends,Expansion of the fishery,30% of striped marlin catch,Limited stock distribution: striped marlin,Habitat, gear, and fish beh
8、avior,Fish have habitats that they prefer Habitat changes with the environment Fishermens behavior determines where the gear fishes Gear, habitat, and fish behavior have to match for fishing to be successful Gear, habitat, and fish behavior have to be taken into consideration when interpreting CPUE,
9、Bigelow et al. 2000,Environment,Thermocline,Depth of gear,50-150,50-400,Current,Current,Increasing depth of longlines,EPO examples,Bigeye tuna,Yellowfin tuna,From Keith Bigelow,Habitat standardization,Used to remove the changes of gear depth and the environment from the relative index of abundance M
10、ethod developed by Hinton and Nakano 1996 Applied to bigeye and yellowfin tuna by Bigelow et al. 2002 Used in the assessments of yellowfin tuna, bigeye tuna, blue marlin, striped marlin, and swordfish in the EPO,The “stupid” fish hypothesis,The stupid fish hypothesis under historic effort,CPUE vs ab
11、undance,The big “stupid” fish hypothesis (size-specific vulnerability),Size-specific vulnerability,The size specific vulnerable hypothesis under historic effort,Blue marlin example,Historic yellowfin length frequency data,60 cm,150 cm,Purse seine,Longline,Suda and Schaefer 1965,Size at 50% maturity,
12、Other Hypotheses,Multiple stocks (e.g. northern and southern albacore) Fraction of stock (bluefin) Stock distribution limited (e.g. Striped Marlin, Swordfish, sailfish, shortbill spearfish) Gear saturation/interference Increase in fishing power Targeting (swordfish, bait, setting at night) Age speci
13、fic natural mortality Fishing regulations (e.g. EEZ),Myers Dalhousie Group,Soak time increases current CPUE Has slightly increased Increased soak time increases CPUE Shark damage increases current CPUE 25% in early data and about 4% in recent data Lower shark damage increases CPUE Hook saturation -
14、increases current CPUE Bait loss due to catching other species has decreased More bait available increases CPUE Depth of gear Ecosystem Also looking at non-pelagic species and dada from trawl World wide patterns similar,Summary,Regime change Same for all species? Implies that the stocks are depleted
15、 New management values for new regime Ecosystem Implies that the stocks are depleted Does not explain all increase in production Spatial distribution of effort Spatial expansion occurred when the rapid declines occurred CPUE declines faster than abundance Final depletion level is the same,Summary co
16、ntinued,Habitat and gear distribution Did not change during period of depletion Current abundance may be underestimated for most species explaining current catches Stupid fish hypothesis Both stupid fish and age-specific vulnerability could explain some of the decline Indicates stocks are less deple
17、ted Limited distribution of stock Probably explains increase in CPUE for striped marlin, swordfish, sailfish, and spearfish,Conclusions,Regime change, ecosystem, and spatial distribution result in high depletion levels Longline depth, stupid fish hypothesis, and age-specific vulnerability result in
18、lower depletion levelsEcosystem, spatial distribution, longline gear depth, and age-specific vulnerability most likely,Yellowfin and Bigeye selectivity,Bigeye,Yellowfin,Age in quarters,Age in quarters,60 cm,150 cm,Size at 50% maturity,Current yellowfin length frequency data,125 cm,Age-specific selectivity and recruitment residuals,
