scholarly journals Age-specific natural mortality rates in stock assessments: size-based vs. density-dependent

2014 ◽  
Vol 71 (7) ◽  
pp. 1629-1637 ◽  
Author(s):  
Joseph E. Powers
Keyword(s):  
Single Species ◽  
Dynamic Parameter ◽  
Rate Function ◽  
Reference Points ◽  
Natural Mortality ◽  
Density Dependent ◽  
Ecological Variability ◽  
Stock Assessments ◽  
Dependent Model ◽  
Two Parameters

Abstract Natural mortality (M) rates are difficult to measure empirically and are often specified in stock assessments based on life history characteristics. More recently, these specifications have included M as a function of the size or age of a fish. However, natural mortality is a dynamic parameter that will change with the suite of predators and, thus, indirectly with cohort size and age. As an alternative, a density-dependent M rate function is derived and compared with the commonly used Lorenzen model, where M at age forms an allometric relationship with weight-at-age. The density-dependent model expresses M as a function of two parameters: one density dependent and one density independent. Properties of the two models (size-based vs. density-dependent) were explored to indicate conditions where the results are and are not similar. Associated catch equations, equilibrium analyses, and non-linear replacement lines in stock–recruitment theory are examined. Just as with density-independent values of M, most assessment data are not sufficient to provide precise estimates of density-dependent M parameters. However, the density-dependent model provides a basis for incorporating ecological variability into single-species assessments, noting the differing dynamics between short- and long-lived species. The incorporation of dynamic natural mortality has implications when estimating abundance trends and stock status, and ultimately setting management reference points.

Bayesian integrated survey-based assessments: an example applied to North Sea herring (Clupea harengus) survey data

2011 ◽  
Vol 68 (8) ◽  
pp. 1387-1407 ◽  
Author(s):  
Richard Hillary
Keyword(s):  
North Sea ◽  
Survival Probability ◽  
Assessment Method ◽  
Reference Points ◽  
Clupea Harengus ◽  
Natural Mortality ◽  
Mortality Trends ◽  
Management Procedure ◽  
Management Procedures ◽  
Stock Assessments

Stock assessments that use only fisheries independent data have been developed from a number of different standpoints over the last 15 years. While the ability of such stock assessments to avoid the use of potentially compromised or hard to interpret commercial data or making certain assumptions that are common to traditional assessments has been established, little has been made of their potential for detecting and estimating complex mortality trends over time or their potential utility in survey-based management procedures. Using North Sea herring ( Clupea harengus ) data as an example, a Bayesian survey-based assessment method that is able to estimate all the key population variables is detailed. However, survival probability, and not fishing mortality that is conditional on natural mortality, is the key parameter. Reversible jump Markov chain Monte Carlo routines were developed to explore the range of ages over which survival separates into year and age effects (a common assumption in many stock assessments). Post hoc estimates of natural mortality suggest that changes over years and ages may have occurred in relativity to historic levels. The derivation of reference points based on survival probability and surplus biomass production are detailed as proxies for more common F-based reference points. The potential role for the outputs of such assessments in a management procedure sense is discussed.

scholarly journals Effects of multispecies and density-dependent factors on MSY reference points: example of the Baltic Sea sprat

2017 ◽  
Vol 74 (6) ◽  
pp. 864-870 ◽  
Author(s):  
Jan Horbowy ◽  
Anna Luzeńczyk
Keyword(s):  
Baltic Sea ◽  
Gadus Morhua ◽  
Maximum Sustainable Yield ◽  
Reference Points ◽  
Stochastic Simulations ◽  
Natural Mortality ◽  
The Baltic Sea ◽  
Density Dependent ◽  
Dependent Growth ◽  
The Baltic

In this paper, maximum sustainable yield (MSY) parameters for the Baltic Sea sprat (Sprattus sprattus) are estimated in relation to pressure from cod (Gadus morhua) predation and the influence of density dependence on sprat growth. This study is based on long-term deterministic and stochastic simulations in which sprat density-dependent growth and predation mortality are considered. The resultant model is a relatively simple tool that allows for streamlined analyses of problems typically approached using complex multispecies models. The analysis indicates that estimates of the MSY parameters (i.e., MSY and FMSY) and equilibrium biomass differ significantly between approaches that hold growth and natural mortality constant and those that allow for density-dependent growth and natural mortality. Based on the cod biomass observed in the 1980s, the MSY parameters estimated by a model that accounts for density-dependent growth and by a model assuming constant growth may differ by a factor of 2. As such, the MSY parameters decline (approximately linearly) with the size of the cod stock.

Fishing catch shares in the face of global change: a framework for integrating cumulative impacts and single species management

2010 ◽  
Vol 67 (12) ◽  
pp. 1968-1982 ◽  
Author(s):  
Isaac C. Kaplan ◽  
Phillip S. Levin ◽  
Merrick Burden ◽  
Elizabeth A. Fulton
Keyword(s):  
Global Change ◽  
Ocean Acidification ◽  
Fishery Management ◽  
Single Species ◽  
Ecosystem Model ◽  
Careful Consideration ◽  
Reference Points ◽  
Cumulative Impacts ◽  
English Sole ◽  
Modeling Tools

Any fishery management scheme, such as individual fishing quotas (IFQs) or marine protected areas, should be designed to be robust to potential shifts in the biophysical system. Here we couple possible catch scenarios under an IFQ scheme with ocean acidification impacts on shelled benthos and plankton, using an Atlantis ecosystem model for the US West Coast. IFQ harvest scenarios alone, in most cases, did not have strong impacts on the food web, beyond the direct effects on harvested species. However, when we added the impacts of ocean acidification, the abundance of commercially important groundfish such as English sole ( Pleuronectes vetulus ), arrowtooth flounder ( Atheresthes stomias ), and yellowtail rockfish ( Sebastes flavidus ) declined up to 20%–80%, owing to the loss of shelled prey items from their diet. English sole exhibited a 10-fold decline in potential catch and economic yield when confronted with strong acidification impacts on shelled benthos. Therefore, it seems prudent to complement IFQs with careful consideration of potential global change effects such as acidification. Our analysis provides an example of how new ecosystem modeling tools that evaluate cumulative impacts can be integrated with established management reference points and decision mechanisms.

scholarly journals Risk assessment of cartilaginous fish populations

2014 ◽  
Vol 72 (3) ◽  
pp. 1057-1068 ◽  
Author(s):  
Enric Cortés ◽  
Elizabeth N. Brooks ◽  
Kyle W. Shertzer
Keyword(s):  
United States ◽  
Risk Assessment ◽  
The United States ◽  
Cartilaginous Fish ◽  
Assessment Methods ◽  
Reference Points ◽  
Assessment Process ◽  
Model Complexity ◽  
Stock Assessments ◽  
Demographic Methods

Abstract We review three broad categories of risk assessment methodology used for cartilaginous fish: productivity-susceptibility analysis (PSA), demographic methods, and quantitative stock assessments. PSA is generally a semi-quantitative approach useful as an exploratory or triage tool that can be used to prioritize research, group species with similar vulnerability or risk, and provide qualitative management advice. Demographic methods are typically used in the conservation arena and provide quantitative population metrics that are used to quantify extinction risk and identify vulnerable life stages. Stock assessments provide quantitative estimates of population status and the associated risk of exceeding biological reference points, such as maximum sustainable yield. We then describe six types of uncertainty (process, observation, model, estimation, implementation, and institutional) that affect the risk assessment process, identify which of the three risk assessment methods can accommodate each type of uncertainty, and provide examples mostly for sharks drawn from our experience in the United States. We also review the spectrum of stock assessment methods used mainly for sharks in the United States, and present a case study where multiple methods were applied to the same species (dusky shark, Carcharinus obscurus) to illustrate differing degrees of model complexity and type of uncertainty considered. Finally, we address the common and problematic case of data-poor bycatch species. Our main recommendation for future work is to use Management Strategy Evaluation or similar simulation approaches to explore the effect of different sources of uncertainty, identify the most critical data to satisfy predetermined management objectives, and develop harvest control rules for cartilaginous fish. We also propose to assess the performance of data-poor and -rich methods through stepwise model construction.

Marine ecosystem assessment in a fisheries management context

2002 ◽  
Vol 59 (9) ◽  
pp. 1429-1440 ◽  
Author(s):  
Jason S Link ◽  
Jon K.T Brodziak ◽  
Steve F Edwards ◽  
William J Overholtz ◽  
David Mountain ◽  
...  
Keyword(s):  
Fisheries Management ◽  
Marine Ecosystem ◽  
Single Species ◽  
Reference Points ◽  
System Level ◽  
Multivariate Statistical Methods ◽  
Multivariate Statistical ◽  
Management Context ◽  
Ecosystem Level ◽  
Ecosystem Based Fisheries Management

We examined a suite of abiotic, biotic, and human metrics for the northeast U.S. continental shelf ecosystem at the aggregate, community, and system level (>30 different metrics) over three decades. Our primary goals were to describe ecosystem status, to improve understanding of the relationships between key ecosystem processes, and to evaluate potential reference points for ecosystem-based fisheries management (EBFM). To this end, empirical indicators of ecosystem status were examined and standard multivariate statistical methods were applied to describe changes in the system. We found that (i) a suite of metrics is required to accurately characterize ecosystem status and, conversely, that focusing on a few metrics may be misleading; (ii) assessment of ecosystem status is feasible for marine ecosystems; (iii) multivariate points of reference can be determined for EBFM; and (iv) the concept of reference directions could provide an ecosystem level analog to single-species reference points.

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