Understanding Fish Stock Assessment. Gary Shepherd Northeast Fisheries Science Center Woods Hole, MA

Transcription

1 Understanding Fish Stock Assessment Gary Shepherd Northeast Fisheries Science Center Woods Hole, MA 1

2 What is Fisheries Stock Assessment? Ø Stock assessment is the process of assembling information to estimate fish population abundance using statistical inference. 2

3 Stock Assessment CATCH (landings and discards) RELATIVE ABUNDANCE BIOLOGY POPULATION MODEL ADVANCED MODELS HABITAT CLIMATE ECOSYSTEM MANMADE STRESS SOCIOECONOMICS STATUS FORECAST 3

4 In an ideal world we could determine population abundance by simply counting all the fish one at a time. 4

5 Unrealistic in a large marine system so we resort to using mathematical models, such as the Baronov catch equation: C(t) = F*(1- exp(-f-m)*n(t))/(f+m) Says catch (C) is related to abundance (N) and processes of fishing mortality (F) and natural mortality (M) 5

6 Ø Solving the equation can be quite complicated because we only see the fish after they are removed from the population. 6

7 Stock Assessment CATCH (landings and discards) COMMERCIAL, RECREATIONAL BIOLOGY AGE, GROWTH, MATURITY 7

8 Stock Assessment CATCH (landings and discards) COMMERCIAL, RECREATIONAL RELATIVE ABUNDANCE NOAA SURVEYS, FISHERY CATCH RATE POPULATION MODEL Calculates time series of Fish Abundance and Mortality ADVANCED MODELS HABITAT CLIMATE ECOSYSTEM MANMADE STRESS SOCIOECONOMICS FORECAST Annual Catch Limit 8

9 NEFSC Bottom Trawl Survey Sample hundreds of locations several times per year. Similar to taking a sample of voters to predict who will win an election 9

10 Biomass indices from NEFSC research vessel surveys 10

11 Stock Assessment BIOLOGY AGE, GROWTH, MATURITY The ages of the fish can be determined like trees. STATUS Overfishing? Overfished? POPULATION MODEL Calculates time series of Most in our region live years Redfish live years Sand eels live 3-5 years Fish Abundance and Mortality Squids live about 18 months Ocean quahogs estimated at 200+! FORECAST Annual Catch Limit ADVANCED MODELS HABITAT CLIMATE ECOSYSTEM MANMADE STRESS SOCIOECONOMICS 11

12 CATCH (landings and discards) COMMERCIAL, RECREATIONAL Stock Assessment RELATIVE ABUNDANCE NOAA SURVEYS, FISHERY CATCH RATE BIOLOGY AGE, GROWTH, MATURITY POPULATION MODEL Calculates time series of Fish Abundance and Mortality ADVANCED MODELS HABITAT CLIMATE ECOSYSTEM MANMADE STRESS SOCIOECONOMICS STATUS Overfishing? Overfished? FORECAST Annual Catch Limit 12

13 Estimating stock abundance Stock assessment models look for a solution that satisfies both: Ø the observed trend in relative abundance over time. Ø The observed absolute amount of catch that was removed. MT Stratified mean wt (kg) per tow Spring_kg_tow Fall_kg_tow year Commercial Recreational Year 13

14 Stock abundance Spawning Stock Biomass (mt) Recruits (age 0, millions) Year The model works better if we can divide the information into age categories

15 CATCH (landings and discards) COMMERCIAL, RECREATIONAL Stock Assessment ABUNDANCE NOAA SURVEYS, FISHERY CATCH RATE BIOLOGY AGE, GROWTH, MATURITY POPULATION MODEL Calculates time series of Fish Abundance and Mortality ADVANCED MODELS HABITAT CLIMATE ECOSYSTEM MANMADE STRESS SOCIOECONOMICS STATUS Overfishing? Overfished? FORECAST Annual Catch Limit 15

16 Stock Status theory CATCH MSY Catch < Production Allows Biomass Growth BIOMASS Catch > Production Decreases Biomass K Ø Maximum Population is carrying capacity (K). Productivity low. Ø Production is greatest if stock size at half its carrying capacity. Ø Associated catch is maximum sustainable yield (MSY). Ø If removals can be replaced by production each year, on average, the fishery is sustainable at that level. 16

17 Forestry example of sustainable yield Clear cut forest High yield, low biomass 17

18 Managed woodlot Optimal yield, medium biomass 18

19 Old growth forest No yield, high biomass 19

20 Stock Status Ø MSY is the amount of catch for optimal population production Ø SSB MSY is the biomass of mature spawning fish at the optimal level Ø F MSY is the removal rate to attain MSY Ø Overfishing is the rate of removal greater than optimal Ø Overfished is the condition of the stock below ½ of the optimal level 20

21 Stock status example Fishing Mortality/FMSY Overfished Overfishing F MSY Overfished 1/2 SSB MSY Overfishing SSB MSY High biomass, low mortality Spawning Stock Biomass/SSBMSY More fish 21

22 Model - Reality CATCH Nothing in life is certain except death and taxes Benjamin Franklin, 1817 Managers try to account for the uncertainty in the assessment and reduce the risk of overfishing or becoming overfished STOCK BIOMASS K

23 The end 23