Effects of an erroneous natural mortality rate on a simple age-structured stock assessment

1999 ◽  
Vol 56 (10) ◽  
pp. 1721-1731 ◽  
Author(s):  
William G Clark
Keyword(s):  
Mortality Rate ◽  
Stock Assessment ◽  
Natural Mortality ◽  
Structured Model ◽  
Harvest Rate ◽  
Specific Selectivity ◽  
Age Structured ◽  
Age Structured Model ◽  
Size Estimates

The abundance of many stocks is estimated by fitting an age-structured model to catch-at-age and relative abundance data from the commercial fishery and scientific surveys. The natural mortality rate used in the model is usually estimated externally and its value is uncertain. An erroneous natural mortality rate will bias the stock size estimates obtained by fitting the model and will also bias the yield calculations that are done to choose a harvest rate and recommend quotas. This paper describes the general features of both effects by analyzing a simple age-structured model fitted to artificial data. It is shown that an erroneous natural mortality rate mainly affects the estimates of fishing mortality and hence abundance but not the estimates of age-specific selectivity. Errors in estimated abundance and target harvest rate are always in the same direction, with the result that, in the short term, extremely high exploitation rates can be recommended (unintentionally) in cases where the natural mortality rate is overestimated and historical exploitation rates in the catch-at-age data are low. A conservative (low) estimate of natural mortality can avoid that danger. Long-term yield under either an FMSY or F35% strategy is not very sensitive to error in natural mortality rate unless it is grossly underestimated.

An age-structured model with leading management parameters, incorporating age-specific selectivity and maturity

2008 ◽  
Vol 65 (2) ◽  
pp. 286-296 ◽  
Author(s):  
Robyn E Forrest ◽  
Steven J.D. Martell ◽  
Michael C Melnychuk ◽  
Carl J Walters
Keyword(s):  
Life History Traits ◽  
Stock Assessment ◽  
Maximum Sustainable Yield ◽  
Reference Points ◽  
Fish Stock ◽  
Structured Model ◽  
Specific Selectivity ◽  
Stock Recruitment ◽  
Age Structured ◽  
Age Structured Model

Previous authors have shown analytically that the optimal equilibrium harvest rate (UMSY) for an iteroparous fish stock is a function of the slope of the stock-recruitment curve at low stock size (α) and that UMSY can therefore be considered a direct measure of stock productivity. As such, it can be used as a leading parameter in stock assessment models and directly estimated using Bayesian or similar techniques. Here we present an alternative method for deriving α from UMSY that incorporates age-specific selectivity and fecundity, avoiding assumptions of knife-edged recruitment and maturity. We present an age-structured model with two fisheries reference points (UMSY and maximum sustainable yield, MSY) as its leading parameters. We show equilibrium properties of the model, chiefly in terms of its ability to show relationships between life history traits, density dependence, and UMSY. We also demonstrate a simple Bayesian estimation routine to illustrate estimation of UMSY and MSY directly from data. We compare our results to those from a structurally identical model with leading biological parameters. Using models with leading management parameters can improve communicability of results to managers.

An Analysis of the Hecate Strait Pacific Cod Fishery Using an Age-structured Model Incorporating Density-dependent Effects

1983 ◽  
Vol 40 (8) ◽  
pp. 1233-1243 ◽  
Author(s):  
D. A. Fournier
Keyword(s):  
Fishing Effort ◽  
Natural Mortality ◽  
Pacific Cod ◽  
Structured Model ◽  
Density Dependent ◽  
Gadus Macrocephalus ◽  
Stock Recruitment ◽  
Age Structured ◽  
Age Structured Model ◽  
Relationship Of

An age-structured model was used to analyze the Hecate Strait Pacific cod (Gadus macrocephalus) fishery for the years 1960–80. The data consisted of estimates of fishing effort, together with estimates of numbers of fish at age caught in each year. The latter estimates were derived from length–frequency analysis. A stock–recruitment relationship of the Ricker type with an additional environmental factor was estimated. The data is also analyzed for evidence of the existence of age-dependent trends in natural mortality, density-dependent trends in natural mortality and catchability, and for time-dependent trends in catchability. Evidence of a density-dependent trend in natural mortality was discovered. The average level of natural mortality was also estimated to be 0.65.

scholarly journals MULTIFAN-CL: a length-based, age-structured model for fisheries stock assessment, with application to South Pacific albacore, Thunnus alalunga

1998 ◽  
Vol 55 (9) ◽  
pp. 2105-2116 ◽  
Author(s):  
Daid A Fournier ◽  
John Hampton ◽  
John R Sibert
Keyword(s):  
Time Series ◽  
Growth Parameters ◽  
Stock Assessment ◽  
South Pacific ◽  
Environmental Data ◽  
Model Parameters ◽  
Structured Model ◽  
Thunnus Alalunga ◽  
Age Structured ◽  
Age Structured Model

We introduce a length-based, age-structured model, MULTIFAN-CL, that provides an integrated method of estimating catch age composition, growth parameters, mortality rates, recruitment, and other parameters from time series of fishery catch, effort, and length frequency data. The method incorporates Bayesian parameter estimation, estimation of confidence intervals for model parameters, and procedures for hypothesis testing to assist model development. We apply the method to South Pacific albacore, Thunnus alalunga, fishery data and demonstrate the incorporation of model structure such as spatial heterogeneity, age-dependent natural mortality and movement rates, time series trends and seasonal variation in catchability, and density-dependent growth. Consistency of the results of the albacore analysis with various exogenous sets of biological and environmental data gives credence to the model results.

An Age-Structured Model of Fish Population Enhancement

2002 ◽  
pp. 376-394
Author(s):  
Richard Langton ◽  
James Lindholm ◽  
James Wilson ◽  
Sally Sherman
Keyword(s):  
Fish Population ◽  
Structured Model ◽  
Age Structured ◽  
Age Structured Model

scholarly journals Null Controllability of a Nonlinear Age Structured Model for a Two-Sex Population

2021 ◽  
Vol 2021 ◽  
pp. 1-20
Author(s):  
Amidou Traoré ◽  
Okana S. Sougué ◽  
Yacouba Simporé ◽  
Oumar Traoré
Keyword(s):  
Null Controllability ◽  
Auxiliary System ◽  
Time Interval ◽  
Observability Inequality ◽  
Controllability Problem ◽  
Structured Model ◽  
Male And Female ◽  
System A ◽  
Age Structured ◽  
Age Structured Model

This paper is devoted to study the null controllability properties of a nonlinear age and two-sex population dynamics structured model without spatial structure. Here, the nonlinearity and the couplage are at the birth level. In this work, we consider two cases of null controllability problem. The first problem is related to the extinction of male and female subpopulation density. The second case concerns the null controllability of male or female subpopulation individuals. In both cases, if A is the maximal age, a time interval of duration A after the extinction of males or females, one must get the total extinction of the population. Our method uses first an observability inequality related to the adjoint of an auxiliary system, a null controllability of the linear auxiliary system, and after Kakutani’s fixed-point theorem.

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