How many conditional age-at-length data are needed to estimate growth in stock assessment models?

Xi He, John C. Field, Donald E. Pearson, and Lyndsey Lefebvre

Fisheries Ecology Division, SWFSC

Santa Cruz, CA

CAPAM Growth Workshop

Lo Jolla, CA

November, 2014

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Introduction

• Conditional age-at-length (CAAL) data are often used in stock assessment models to internally estimate growth and natural mortality

• Recent studies (Wetzel and Punt 2011, Hulson et al. draft, One et al. 2014) showed effects of quality and quantity of composition data on assessment outputs.

• Bocaccio is an important commercial species, especially in California. It has been assessed 12 times since 1986, and has been under rebuilding plan since the early 2000s. But no age data have been used in these assessments since it is extremely difficult to age

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When lengths are better than ages: The complex case of Bocaccio (Ralston and Ianelli 1998)

Ralston and Ianelli 1998 study: Difficulty in Bocaccio ageingUnable to detect recruitments due to ageing bias and imprecision

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Introduction – cont.

• Ageing criteria for Bocaccio are recently developed, and over 6000 thousands of fish have been aged – time consuming

• Ageing Bocaccio otoliths requires specific skills and is time consuming, as compared to ageing other rockfish

• Questions still remain: How many fish do we need to age and how do we chose which otolith samples to age?

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Study objectives

• Focused on numbers of fish that need to be aged and which annual age samples have high priority

• Examine effects of numbers of fish aged and numbers of yearly samples on stock assessment outputs

• To provide general guidelines for ongoing effort in Bocaccio ageing

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Methods

• Used three recent West Coast assessment models: 2011 Blackgill rockfish, 2013 Bocaccio, and 2013 Pacific sanddab

• Numbers of fish aged: randomly select 75%, 50%, 25% and 10% of age data used in the original assessment models. Each model with reduced data were run 100 times

• Numbers of annual age data: intermittently remove data from assessment models (i.e. removing data every other year, etc.)

• These real stock assessment models represent different levels of data availability and model complexity, and different life histories

• Results from model runs with reduced data were compared to the models with all available data, including estimated key parameters and stock status

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Blackgill rockfish

Max age: 60 (64 – 90?)

CAAL data: 1982-2009 (15 years)

Last assessed: 2011

M and steepness fixed

Bocaccio

Max age: 34 (45 – 57?)

CAAL data: 1985-2012 (20 years)

Last assessed: 2013 (update)

M fixed at 0.15 (too high?)

Pacific Sanddab

Max age: 11

CAAL data: 2003-2012 (9 years)

Last assessed: 2013 (first time)

M fixed at base-model values

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Key assumptions and limitations

• No time-varying growth• Natural mortality not internally estimated

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Comparisons: Use only portion of CAAL data

Spawning biomass Depletion

Blackgill: Spawning biomass and stock depletion

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Comparisons: Use only portion of CAAL data

Blackgill: Female growth

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Comparisons: Use only portion of CAAL data

Spawning biomass Depletion

Bocaccio: Spawning biomass and stock depletion

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Comparisons: Use only portion of CAAL data

Bocaccio: Female growth

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Comparisons: Use only portion of CAAL data

Spawning biomass Depletion

Sanddab: Spawning biomass and stock depletion

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Comparisons: Use only portion of CAAL data

Sanddab: Female growth

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Comparisons: Use only portion of CAAL data

Key assessment outputs

* Not estimatedNote: For reduced data set, values are medians of 100 runs

Species & Model lnR0 Steepness DepletionBlackgill all data (N=6258) 7.730 0.760* 0.314Blackgill 75% 7.750 0.760* 0.330Blackgill 50% 7.758 0.760* 0.330Blackgill 25% 7.741 0.760* 0.357Blackgill 10% 7.596 0.760* 0.213       Bocaccio all data (N=6073) 8.472 0.733 0.317Bocaccio 75% 8.481 0.728 0.318Bocaccio 50% 8.501 0.712 0.318Bocaccio 25% 8.519 0.688 0.316Bocaccio 10% 8.535 0.659 0.310       Sanddab all data (N=7970) 12.357 0.768 0.955Sanddab 75% 12.381 0.770 0.950Sanddab 50% 12.422 0.775 0.957Sanddab 25% 12.580 0.788 1.049Sanddab 10% 12.749 0.795 1.124

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Comparisons: Removal of annual CAAL data

Key assessment outputs

* Not estimated

Species & ModelNo. year with

CAAL dataNo. fish aged

for CAAL lnR0 Steepness DepletionBlackgill all data 15 6258 7.730 0.760* 0.314Blackgill 1 out 2 7 4356 7.746 0.760* 0.332Blackgill 1 out 3 5 1845 7.874 0.760* 0.384Blackgill 1 out 4 4 2505 7.706 0.760* 0.376Blackgill 1 out 5 3 481 7.884 0.760* 0.423 Bocaccio all data 19 6073 8.472 0.733 0.317Bocaccio 1 out 2 10 2977 8.495 0.706 0.322Bocaccio 1 out 3 8 2679 8.461 0.723 0.320Bocaccio 1 out 4 6 2229 8.511 0.714 0.320Bocaccio 1 out 5 4 634 8.528 0.659 0.316 Sanddab all data 11 7970 12.357 0.768 0.955Sanddab 1 out 2 5 3238 12.412 0.782 0.971Sanddab 1 out 3 3 2250 12.587 0.793 1.089Sanddab 1 out 4 2 1547 12.628 0.795 1.103

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Summary

• Important to obtain age data from early years• In case for Bocaccio, need to age otoliths from late 1970’s

and early 1980’s• More data for long lived species• May want to consider to estimate growth externally for long-

lived species when age data are limited

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