Size Composition Data Approach in Stock Assessment
Explore a comprehensive approach to analyzing size composition data in stock assessment, including growth patterns, population distribution, expected catch calculation, sampling processes, and discard analysis.
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Presentation Transcript
Generalized Approach to Size Composition Data in SS
Possible Size Data Size by fine, even categories: mm Size by cruder categories, inches Mean body weight = bin weight / N fish Catch weight sold by processor category A model can calculate the expected value for any of these from an estimate of the size composition of the catch
Growth 250 Carapace Length (mm) 200 150 100 Female with +/- 2sd 50 Male 0 0 5 10 15 20 Age (months) Start point is the growth curve
Size Distribution at - Age POPULATION Step 2 is the population size-at- age matrix, ALK Step 3 uses population numbers-at-age to scale the ALK to get population numbers at age and length at time t 22 20 17 15 13 AGE 11 9 7 5 3 101 60 52 46 40 34 SIZE 28 22 16
Create the Expected Catch Step 5: apply selectivity at age and length and catchability to get the expected sample as a matrix of age x length This matrix has a particular length bin structure as defined in the input files and is the foundation on which transformations create expected values for the available data
Sampling & Observation Processes 1.0 0.8 SELECTIVITY 0.6 CATCH AT TRUE AGE 0.4 0.2 23 0.0 21 25 30 35 40 45 19 SIZE 17 15 13 AGE 11 9 7 5 3 101 60 52 46 40 34 SIZE 28 22 16
Discard and Retained Catch 0.09 Optionally, a retention function partitions the total catch into discarded and retained portions Each can then be compared to corresponding data 0.08 obs exp Retained 0.07 0.06 Est & Obs 0.05 0.04 0.03 0.30 0.02 obs exp 0.01 0.25 0.00 Discard 0.20 20 40 60 80 Est & Obs 0.15 0.10 0.05 0.00 20 40 60 80
Size Composition Options In earlier SS versions, the size composition data needed to represent proportions at length in these same length bins Now, the length bin structure can differ from the population length bin structure But it cannot be finer And, a generalized size structure approach adds ability to define multiple size structure methods
Generalized Size Composition For each size composition (here labeled WtFreq) method, enter the following: 2 25 4 2 1 3 2 0.00001 -1 40 5 # N WtFreq methods nbins per method units per each method (1=biomass; 2=numbers) scale per each method ((1=kg; 2=lbs; 3=cm; 4=inches) mincomp to add to each obs (entry for each method) N observations per wtFreq method
Example: Gulf of Mexico Pink Shrimp Catch weight categories for shrimp Early analyses used growth curve to slice these data into monthly age composition for use in a VPA With SS the process is reversed: estimated population age comp and selectivity produce estimate of catch size composition which is accumulated into the data categories
Shrimp Catch Size Categories Size Bin 1 2 3 4 5 6 7 8 Shrimp/lb Lower edge of bin in lbs. >67 1E-06 1E-06 51-67 1/67 0.015 41-50 1/50 0.020 31-40 1/40 0.025 26-30 1/30 0.033 21-25 1/25 0.040 15-20 1/20 0.050 <15 1/14 0.071 Bins defined in terms of heads-off shrimp count per pound Data are pounds landed in each category Nearly 500 months of data available since 1960
From Age to Weight Bins 0.18 25 Normal Dist. Shown for age 20 0.16 20 0.14 Mean Size- at-Age >67 shrimp tails per lb. 0.12 Body weight (tails - lbs) 51-67 15 Age at Size 41-50 0.10 31-40 26-30 0.08 21-25 10 Body Weight- at-Length 15-20 0.06 <15 Female age-at-size 0.04 5 Size at age 20 Apportion body weight at each 5 mm size bin into 1 of 8 weight categories 0.02 0.00 0 0 50 100 150 200 250 Carapace Length (mm)
Population N-at-Length and Selectivity 100 Population Numbers 1.0 Total Female Selectivity 80 0.8 Selectivity 60 0.6 40 0.4 20 0.2 0 0.0 110 125 140 155 170 185 200 215 5 20 35 50 65 80 95 Carapace Length (mm) Dimorphic growth Selection at age 3 months for females and 4 months for males, but all data are combined gender
Catch Numbers-at-Length 50 40 Catch Numbers 30 Total Female 20 10 0 110 125 140 155 170 185 200 215 5 20 35 50 65 80 95 Carapace Length (mm)
Catch Weight by Category 300 >67 shrimp tails per lb. 51-67 250 41-50 31-40 26-30 200 Catch weight (tails - lbs) 21-25 15-20 <15 150 100 50 0 5 15 25 35 45 55 65 75 85 95 105 115 125 135 145 155 165 175 185 195 205 215 Body Size (mm)
Compare to Observation 0.35 0.30 fraction of catch (by weight) 0.25 Obs Exp 0.20 0.15 0.10 0.05 0.00 >67 51-67 41-50 31-40 26-30 21-25 15-20 <15 Shrimp tails per pound
