A review of estimation methods for natural mortality and their performance in the context of fishery stock assessment

Maunder, Mark N.; Hamel, Owen S.; Lee, Hui-Hua; Piner, Kevin R.; Cope, Jason M.; Punt, André E.; Ianelli, James N.; Castillo-Jordán, Claudio; Kapur, Maia; Methot, Richard D.

doi:10.1016/j.fishres.2022.106489

Cited by 39 publications

(8 citation statements)

References 163 publications

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“…The total mortality is the sum of natural and fisheries mortality (Z=M+F). In data-poor stocks, however, natural mortality is an inherently difficult parameter to estimate (Maunder et al 2023) and in data-poor stocks it is thus seldom known. An approximate estimate can be obtained from life-history invariants or, if available, be derived from studies on related stocks/species (see papers by Pauly 1980;Chen & Watanabe 1989;Lorenzen 1996;Gislason et al 2010;Thorson et al 2017;Thorson 2020;Dureuil & Froese 2021;Hamel & Cope 2022;Lorenzen et al 2022).…”

Section: Mortality Related Indicatorsmentioning

confidence: 99%

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Developing management goals and associated assessment methods for Sweden’s nationally managed fish stocks : a project synthesis

Naddafi¹,

Sundblad²,

Sandström³

et al. 2023

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This report summarizes and synthesizes results from the Swedish Agency of Marine and Water Management (SwAM, or HaV) funded project “Förvaltningsmål för nationella arter (Management goals for nationally managed species)”. The objectives of the project have been to promote the development of management goals and associated status assessment methods and indicators, as well as reference points, for some nationally managed fish stocks both in coastal as well as freshwater areas. The report focusses largely on species and stocks that can be defined as data-poor. Such stocks are characterised by marked limitations in data availability and/or resources allocated to detailed analytical stock projections. Data-poor stocks also often lack carefully formulated management goals and associated methods and indicators for assessing stock status. In this report, we provide an overview of potential assessment methods and indicators and try to synthesise how they work and what the strengths and weaknesses are by applying them to selected data poor stocks such as pikeperch, pike, whitefish, and vendace. We also discuss how they relate to different potential management goals and provide recommendations for their application. We grouped the indicators and assessment methods by the three categories that are now used in the yearly status assessment framework provided by SLU Aqua (Resursöversikten/Fiskbarometern) – i) mortality, ii) abundance/biomass and iii) size/age structure. The results are also described for these three main categories of assessment indicators. Included is also a status report from a size- and age-based population dynamics model (Stock Synthesis 3) that is being developed for pikeperch in Lake Hjälmaren. An important experience from the project is that to improve the assessment methods for Swedish national fish stocks, it is important that managers develop both general as well as more detailed quantitative goals for the individual stocks. This should ideally be conducted in various forms of collaboration with the main stakeholders and scientists involved with assessment as participatory processes foster legitimacy. Carefully articulated management goals, which are possible to translate into quantitative targets, will facilitate the development of various approaches and methods to monitor stock statuses. Given the strong and complex interactions of fish and their environments it is also important to consider other pressures than fisheries when developing indicators and assessment methods. Our synthesis highlights a number of areas where the assessment of data-poor stocks can be improved: 1. Apply precautionary principles for data-limited stocks, particularly ones that are known to be vulnerable to exploitation. 2. Tailor approaches to how fisheries are managed in Sweden. Swedish nationally managed fish stocks are not managed by quotas (with one exception, vendace in the Bothnian Bay) and do not aim for maximum sustainable yield. Instead, the coastal and inland fisheries are managed by regulating the effort in the small-scale commercial fisheries (number of fishers/licenses and amount of gear). Regulation of recreational and subsistence fisheries effort, in terms of licenses or number of fishers) is not applied, nor possible since the fisheries is lacking obligatory notification and reporting systems. All national fisheries, however, are regulated by various technical measures (closed areas, size-limits, bag-limits, gear restrictions etc). Thus, goals and assessment methods that result in harvest limits or quota recommendations expressed in e.g. biomass/numbers are difficult to use as basis for management. Instead, there is a need for alternative management goals and associated assessment methods. 3. Use best practice methods and indicators and adapt as scientific knowledge is developed. Data-limited methods are developing rapidly, and new methods/approaches are proposed in the scientific literature every year. It is thus important to be updated on the most recent developments. 4. Clearly describe limitations/assumptions of methods used. It is important to be aware of and critically evaluate the assumptions underlying the analyses, and to carefully communicate uncertainty together with the stock status assessment. 5. Be particularly careful with low sample numbers. Many indicators and methods can be applied also on small sample sizes, however, the accuracy and precision of the estimates risk being low in such cases. 6. Accept that there is no "gold standard" for fisheries assessment. Each case study is unique and needs to be balanced against data availability, local needs and other important factors. This also means that analysts need to be careful when using generic reference levels or “borrowing” data from other stocks. 7. If possible, use several different methods/indicators. Although several indicators aim to measure similar aspects of the stock, small methodological differences can support the overall interpretation of individual indicator values. It is particularly important to incorporate many aspects and indicators (size/age/abundance/mortality) in order to produce a balanced assessment. 8. Develop means of communication. Indicators and goals should be easy to understand. However, interpretation of results from multi-indicator frameworks can be challenging. There is thus a need for finding ways of communication that can convey complicated results in a simple-to-understand manner. 9. For details on additional improvements, we refer the reader to the sub-header “recommendations for the future” found under each chapter. The implementation of Stock Synthesis for pikeperch in Lake Hjälmaren showed that it is possible to develop a more ambitious and detailed stock assessment model for a relatively data-poor stock. The model results partly support earlier interpretations of the development of the stock and the importance of the changes in regulations in 2001 (increased minimum size, increased mesh size and reduced mortality of undersized pikeperch). Before the model can be implemented and used for practical management, a number of actions for improvement are needed, which are highlighted in the relevant chapter. The most important next step is establishing management goals and reference levels for this stock. We recommend that such a dialogue is initiated by managers. The fisheries management goals should consider both biomass, fisheries mortality and size-based targets. To conclude, we stress the importance of improving all ongoing aspects related to the assessments of data-poor Swedish stocks. Strong local stocks and sustainable fisheries are vital for a variety of fisheries-related businesses and practices, particularly in rural areas, providing economical and societal value. Fishes also have important roles in aquatic food-webs and it is important that ecological values are managed wisely in order to reach targets for water quality, ecosystem structure and diversity. Given the strong and complex interactions of fish and their environments it is also important to consider other pressures than fisheries when developing indicators and assessment methods.

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Section: Mortality Related Indicatorsmentioning

confidence: 99%

“…A recent paper by Mainguy & Moral (2021) suggests another related approach to perform catch-curves. Examples of other methods are length/biomass cohort analysis (Jones 1990;Zhang & Megrey 2006), mark-recapture models (Miranda and Bettoli 2007) and analytical stock assessment models (Hamel et al 2023).…”

Section: How Do the Indicators Workmentioning

confidence: 99%

Developing management goals and associated assessment methods for Sweden’s nationally managed fish stocks : a project synthesis

Naddafi¹,

Sundblad²,

Sandström³

et al. 2023

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show abstract

“…Natural mortality and survival are notoriously challenging to estimate regardless of stage or age-based assumptions (e.g. Maunder et al, 2023;Punt et al, 2021). To capture the savings reflected in the time of harvest between CBA versus typical fishing we assumed a Type III survivorship relationship, the common association reported in the literature (Barnett et al, 2017;Jørgensen & Holt, 2013;Lorenzen, 1996Lorenzen, , 2000 (Figure 1), by increasing discrete natural mortality by 0.2 for earlier life stages.…”

Section: Capture-based Aquaculture Replacement Scenariomentioning

confidence: 99%

Biological life‐history and farming scenarios of marine aquaculture to help reduce wild marine fishing pressure

Froehlich,

Montgomery,

Williams

et al. 2023

Fish and Fisheries

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Aquaculture (freshwater and marine) has largely supplemented fisheries, but in theory could help reduce fishing pressure on wild stocks. Although not the sole factors, some potential benefits depend on aquaculture pressures on fished species, including collection of wild ‘seed’ material—earlier to later life stages—for rearing in captivity and the capacity of aquaculture to increase. Here we first classify 203 marine (saltwater and brackish) animal species as being produced by either open‐cycle capture‐based aquaculture (CBA) or closed‐cycle domesticated aquaculture (DA)—based on their likely reliance on wild seed—and assess the extent to which these forms of aquaculture could support seafood production and greater wild biomass. Using a data‐limited modelling approach, we find evidence that current aquaculture practices are not necessarily helping reduce fishing to sustainable levels for their wild counterparts—consistent with emerging scientific research. However, if some wild capture species (87 equivalent spp.) were instead produced through CBA, almost a million extra tonnes could theoretically be left in the wild, without reducing seafood production. Alternatively, if reliance on wild seed inputs is further reduced by shifting to DA production, then a little less than doubling of aquaculture of the overexploited species in our study could help fill the ‘production gap’ to support fishing at maximum sustainable levels. While other ecological (e.g. escapes), social and economic considerations (e.g. market substitution) are important, we focused on a critical biological linkage between wild fisheries and aquaculture that provides another aspect on how to improve management alignment of the sectors.

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“…Natural mortality is a major driver of fish population dynamics and is therefore an essential parameter used in stock assessments and underlying stock management strategies (Clark, 1999; Maunder et al, 2023). A key component of natural mortality is predation; however, it can be challenging to estimate the share of natural mortality attributable to predation because predation may be the proximate but not the ultimate cause of death.…”

Section: Introductionmentioning

confidence: 99%

Impacts of predation by Eurasian otters on Atlantic salmon in two Norwegian rivers

et al. 2023

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1. The return of the Eurasian otter (Lutra lutra) to western Norway has sparked human-predator conflicts because otters prey on vulnerable Atlantic salmon (Salmo salar) populations. Although predation may not be the ultimate cause of salmon population declines, otters that kill adult salmon in rivers before they spawn impact the salmon spawning stock, with potential consequences for stock recruitment and ecosystem services (especially fisheries).2. To gain insight into impacts of otter predation on salmon populations, we quantified the predation by otters on adult salmon in two rivers in western Norway using a combination of radiotelemetry and temperature loggers. We tagged 30 salmon in Aureelva and 30 salmon in Søre Vartdalselva, and tracked the salmon until they died or left the river.3. This method identified the fates of 95% of tagged salmon. Estimated predation rates on adult salmon were 32% in Aureelva and 95% in Søre Vartdalselva.The salmon stock in Søre Vartdalselva was well below the spawning target, partly attributable to putatively additive mortality from predation by otters.Notwithstanding, we found no evidence that otters selectively killed salmon based on sex, length, health status, or activity level. 4. Salmon in Søre Vartdalselva had greater predation risk compared to salmon in Aureelva, possibly due to differences in habitat types such as availability of holding pools. The presence of more holding area in Aureelva probably provided predation refuges for adult salmon that buffered the effectiveness of otter predation. 5. Our findings emphasise that management decisions should be guided by riverspecific evaluations of impacts of predation on salmon. Otters are a very visible predator operating at the final phase of the life cycle before spawning, so predation is liable to be controversial given that salmon are now Red Listed in Norway.

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A review of estimation methods for natural mortality and their performance in the context of fishery stock assessment

Cited by 39 publications

References 163 publications

Developing management goals and associated assessment methods for Sweden’s nationally managed fish stocks : a project synthesis

Developing management goals and associated assessment methods for Sweden’s nationally managed fish stocks : a project synthesis

Biological life‐history and farming scenarios of marine aquaculture to help reduce wild marine fishing pressure

Impacts of predation by Eurasian otters on Atlantic salmon in two Norwegian rivers

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