Implementing operational assessment models that account for spatial structure and movement dynamics is challenging, especially with limited tagging data. Random effects on numbers-at-age (NAA) transitions in state–space models offer a potential solution to circumvent direct movement estimation by attributing movement variation to NAA random effects. However, whether this approach reliably achieves desirable management outcomes remains unclear. In this study, we conducted a management strategy evaluation that emulated a generic medium-lived fish that exhibit natal homing dynamics, using assessment models with varying levels of spatial complexity. We compared the performance of each spatial implementation with and without NAA random effects to evaluate their effectiveness in achieving management outcomes. Our results showed that models with NAA random effects consistently outperformed those without, although the benefits of NAA random effects degraded at high rates of movement. Therefore, NAA random effects could serve as a practical intermediate solution when explicit movement modeling is not feasible due to insufficient movement information. Our findings suggest that incorporating NAA random effects should be a default starting point in state–space stock assessments.

Management strategy evaluations (MSEs) are helpful simulation tools for exploring the expected benefits and trade-offs among management objectives produced by various management procedures. Few spatially explicit MSE tools exist, however, to facilitate evaluation of spatial complexity in stock assessment and management. We describe a generalized MSE R package tool (Spatial Processes and Stock Assessment Methods MSE, SPASAM-MSE) that integrates spatial population dynamic options in both the operating and assessment models to aid implementation of spatial MSE applications. The tool facilitates exploration of assessment and management performance across conditions that create spatial structure (e.g., biocomplexity, connectivity, demographics, fisheries, and management) while utilizing contemporary statistical methodologies (e.g., random effects and state-space modeling features). An example application is provided to demonstrate the utility of understanding trade-offs in spatial management decisions. SPASAM-MSE provides a straightforward interface to consider spatial complexity in biological processes, assessment configurations, and management actions. We envision that the SPASAM-MSE tool will help facilitate increased operational implementation of robust spatial management procedures and aid management decision-making.

The Gulf Menhaden (Brevoortia patronus) stock assessment uses an age-structured statistical catch-at-age model that includes natural mortality rates estimated from a tagging study. These estimates are based on work by Ahrenholz et al. in the 1970s. In this work, we develop contemporary analytical methods to analyze data from a 19-year tagging study conducted from 1970 to 1988 in which adult and juvenile Gulf Menhaden were tagged, released, and recovered in fish processing plants. We constructed a Bayesian mark-recovery model using the negative binomial distribution to estimate natural mortality, catchability, and an overdispersion factor. We then explored sensitivity analyses to evaluate the precision of parameter estimates. The base model estimated a constant instantaneous natural mortality for Gulf Menhaden of 1.082 y(-1) (95% CI: 1.03 y(-1) to 1.13 y(-1)), which is comparable to the estimate by Ahrenholz (1981) of 1.0935y(-1). The sensitivity analyses indicated that the parameter estimates were robust to assumptions. The inclusion of updated natural mortality rates based on a longer time series and newer tag-recoveries models will directly improve the assessment model for the Gulf Menhaden stock and enhance management reference points and advice.

Small Pelagic Fish (SPF) exhibit large fluctuations in abundance and distribution in response to environmental variability. To maintain the resilience of fishing communities and develop effective and equitable climate adaptation strategies, improved understanding of how the fishing industry responds to spatio-temporal shifts within and across SPF populations is of critical importance. In this paper, we examine the responses of the fishing industry and resource managers to shifts in SPF availability worldwide and identify the resulting socioeconomic impacts. Leveraging SPF case studies from around the globe, we synthesize and compare the social-ecological linkages and feedbacks mediating how SPF fisheries respond to changes in marine ecosystem structure and function associated with (1) spatial shifts in species distribution and habitat availability, (2) ‘boom and bust’ population dynamics, or (3) changes in fish size and quality. Our case studies illustrate multiple paths towards the resilience of small pelagic fisheries and the fishing industry dependent upon them while emphasizing the need for increased coordination and cooperation across sectors and scales as climate change progresses. Drawing from the lessons offered by historical responses, as environmental variability increases, efforts to increase the flexibility and dynamism of SPF harvest portfolios and management strategies, licensing regimes, and international catch and allocation agreements may be required to ensure resource sustainability and human well-being.

Simulation studies are useful for determining the implications of available data and mis-specified model structure on the accuracy of model estimates of abundance and fishing mortality. Spatially-explicit multi-stock, age-structured stock assessment models have not been evaluated to understand how the accuracy of model estimates is affected by different assumptions of stock structure, age composition, and movement, compared to current methods used to inform management decisions. Our objective was to evaluate the accuracy of spatially-explicit multi-stock and spatially-implicit single stock assessment models for striped bass under alternative scenarios of data availability and quality. We conducted a simulation study to evaluate the accuracy of spatial assessment models under alternative scenarios of data quality and appropriateness of assumptions including investigating the effect of ageing error on model accuracy. A range of estimation models with alternative assumptions about spatial dynamics, stock composition, and ageing error were fitted to the datasets. Spatially-explicit estimates were approximately unbiased in estimating abundance and fishing mortality when they closely matched the assumptions of the data generating model and accounted for ageing error. Data to inform stock composition or informative priors on occupancy probabilities were necessary for the spatially-explicit models, but estimates were sensitive to correct specification of the occupancy probability priors. Models that ignored potential ageing error in datasets resulted in biased and inaccurate estimates of abundance and fishing mortality. All models provided inaccurate estimates of reference points, although estimates from spatially-explicit models were the least biased. Spatially-explicit stock assessments can potentially improve accuracy of estimates when they match spatial dynamics of fish populations and when ageing error was corrected.

Objective Atlantic Striped Bass Morone saxatilis are one of the most economically important fish species on the U.S. East Coast; however, trends in growth have not been recently evaluated despite concerns about the health and status of the stock. Our objective was to use otolith-aged fish from a combination of fishery-independent and fishery-dependent sources to evaluate growth of Striped Bass from 1998 to 2019 in the mid-Atlantic region of the U.S. East Coast.Methods We used general linear models to describe temporal changes in weight at age and length at age for Striped Bass by sex and a linear mixed effects model to characterize temporal changes in Striped Bass weight at length.Results Striped Bass length and weight at age increased primarily for ages 10-14 between 1998 and 2019 for both sexes. Increases in length at age during 1998-2019 for male Striped Bass ages 10-14 averaged 8.9 cm (+/- 5 cm; range 0.9-14.9 cm), and female Striped Bass ages 10-15 averaged 7.8 cm (+/- 0.5 cm; range 7.0-8.4 cm). For male and female Striped Bass ages 10-14, there was an almost 3 kg (+/- 0.94 kg) increase in mean weight across the time series, and the percentage of change in the mean weight of males was nearly two times as great for females. Additionally, we found that female Striped Bass continued to increase in both length and weight up to ages 22 or older. The relationship between length and weight changed during 1998-2019 such that Striped Bass of a given length weighed more later in the time series. Density-dependent effects on length and weight at age were identified.Conclusions Striped Bass of intermediate ages increased in size at age from 1998 to 2019, and female Striped Bass weight continued to accumulate through approximately age 22. The method used to age Striped Bass influenced the characterization of growth such that otolith-based weight at age is generally lower than scale-based or a mixture of scale- and otolith-based weight at age. Striped Bass size at age for intermediate ages and both sexes increased during 1998-2019. Otolith-based size at age was generally lower than scale-based. Given bias and imprecision of scale ages, we recommend that Striped Bass age determination transition from scales to otoliths coastwide.

Ecosystem approaches to fisheries management are being explored worldwide, but few evaluations of multispecies harvest control rules (HCRs) exist. Our goal was to perform a simulation test of a suite of HCRs using an age-structured predator–prey model to represent the dynamics of a small pelagic fish, Atlantic menhaden ( Brevoortia tyrannus), and its predator, striped bass ( Morone saxatilis). We evaluated a suite of static and dynamic single and multispecies HCRs to estimate effects on stock performance metrics. No single HCR achieved ecosystem management objectives for both stocks given their current reference points, but HCRs that involved the “40–10 rule” for striped bass performed well across all predator performance metrics. The most influential factor determining performance of striped bass HCRs was striped bass fishing mortality, and relatively few HCRs achieved target SSB for Atlantic menhaden. Our study indicated that some HCRs recommended for forage fish management may not be effective in systems with generalist predators, and that ecosystem management objectives might be achievable by simultaneously adopting HCRs for both predator and prey stocks that complement one another.

Gulf menhaden ( Brevoortia patronus ) support the largest fishery by yield in the Gulf of Mexico (GoM) and are a key forage species for many marine predators. While menhaden stock assessments indicated that overfishing was not likely to have occurred in the past, concerns have been raised regarding the possible effects of menhaden fishing on their predators. In this study, we used a US Gulfwide Ecopath with Ecosim (EwE) model to explore the predicted effects of increased menhaden harvest on the GoM ecosystem and focused our analyses on Gulf menhaden predators. Key menhaden predators identified included king mackerel ( Scomberomorus cavalla ), Spanish mackerel ( Scomberomorus maculatus ), sea trout ( Cynoscion spp.), red drum ( Sciaenops ocellatus ), and pelagic coastal piscivores [e.g., bluefish ( Pomatomus saltatrix )]. As expected, these predators exhibited reduced biomass in response to increased Gulf menhaden harvest, with a predicted 11% decrease in predator biomass at simulated fishing levels near historical highs. Our results indicate strong relationships between the effects of menhaden fishing and the predator fishing mortality for king mackerel and intermediate relationships for Spanish mackerel, blacktip shark ( Carcharhinus limbatus ), red drum, large coastal sharks, and pelagic coastal piscivores. Biomass of predator groups such as demersal coastal invertebrate feeders [e.g., drums and croakers (Sciaenidae)] are more affected by menhaden harvest (through trophodynamics interactions and bycatch removal) compared to the isolated effect of their fishing mortality. For almost all the groups examined in the trade-off analysis, with the exception of sea trout, current biomass (2016) was higher than their target biomass representing 75% of their biomass at maximum sustainable yield. In comparison to the time series of fishing mortality rates estimated by the most recent Gulf menhaden stock assessment, the mean ecological reference point (ERP) of 0.862 was exceeded in all but 1 year from 1977 to 2007; however, neither the target nor threshold upper ERP value has been exceeded since 2008. The observed Gulf menhaden landings from 2003 to the present were generally within the range of the projected equilibrium landings (i.e., within confidence intervals) at both the ERP target and threshold values except for three recent years.

Multispecies models have existed in a fisheries context since at least the 1970s, but despite much exploration, advancement, and consideration of multispecies models, there remain limited examples of their operational use in fishery management. Given that species and fleet interactions are inherently multispecies problems and the push towards ecosystem-based fisheries management, the lack of more regular operational use is both surprising and compelling. We identify impediments hampering the regular operational use of multispecies models and provide recommendations to address those impediments. These recommendations are: (1) engage stakeholders and managers early and often; (2) improve messaging and communication about the various uses of multispecies models; (3) move forward with multispecies management under current authorities while exploring more inclusive governance structures and flexible decision-making frameworks for handling tradeoffs; (4) evaluate when a multispecies modelling approach may be more appropriate; (5) tailor the multispecies model to a clearly defined purpose; (6) develop interdisciplinary solutions to promoting multispecies model applications; (7) make guidelines available for multispecies model review and application; and (8) ensure code and models are well documented and reproducible. These recommendations draw from a global assemblage of subject matter experts who participated in a workshop entitled "Multispecies Modeling Applications in Fisheries Management".

Objective: Atlantic Menhaden Brevoortia tyrannus support fisheries that yield the largest landings by volume on the U.S. East Coast and fulfill a critical ecological role as a forage species. The spawning reference point of the stock assessment model that is routinely applied to this species requires information on total annual fecundity. The goal of this study was to generate a contemporary, histology-based evaluation of the reproductive biology and fecundity of female Atlantic Menhaden.Methods: Female Atlantic Menhaden (n = 559) were collected between Cape Cod, Massachusetts, and Cape Hatteras, North Carolina, from 2013 to 2019. Ovarian tissues were prepared using standard histological techniques which, when coupled with oocyte size-frequency and count data, were used to classify reproductive mode and estimate batch fecundity, spawning frequency, and maturity. Monthly gonosomatic indices were combined with published female reproductive information and spatiotemporal patterns in larval and juvenile abundance to designate spawning seasonality.Result: Histological preparations and oocyte size-frequency patterns of female Atlantic Menhaden ovaries were consistent with indeterminate batch spawning. Batch fecundity increased with fork length, while spawning seasonality extended from September 15 to April 15 with a spawn every 7.5 +/- 2.3 days. Female maturity probabilities transitioned from 0.2 to 0.8 over fork lengths of 214.8-226.5 mm (approximately ages 2.0-2.3 years). Estimated mean per capita female annual fecundity varied from 465,757 to 3,250,135 oocytes for fish ranging from 215.8 to 284.8 mm fork length (ages 2-6 years), which represented a 614-2267% (mean = 1656%) increase in annual female reproductive output relative to previous estimates.Conclusion: Female Atlantic Menhaden exhibit indeterminant batch spawning while spawning seasonality and mean per capita female annual fecundity were appreciably greater than previously reported. This new reproductive information illuminates the robust reproductive productivity of this species and will aid routinely conducted stock assessments.

Spatially stratified integrated population models (IPMs) can account for fine-scale demographic processes and support spatial management for complex, heterogeneous populations. Although spatial IPMs may provide a more realistic representation of true population dynamics, few studies have evaluated the consequences associated with incorrect assumptions regarding population structure and connectivity. We utilized a simulation-estimation framework to explore how mismatches between the true population structure (i.e. uniform, single population with spatial heterogeneity or metapopulation) and various parametrizations of an IPM (i.e. panmictic, fleets-as-areas or a spatially explicit, tag-integrated model) impacted resultant fish population estimates. When population structure was incorrectly specified in the IPM, parameter estimates were generally unbiased at the system level, but were often biased for sub-areas. Correctly specifying population structure in spatial IPMs led to strong performance, whereas incorrectly specified spatial IPMs performed adequately (and better than spatially aggregated counterparts). Allowing for flexible parametrization of movement rates (e.g. estimating age-varying values) was more important than correctly identifying the population structure, and incorporation of tag-recapture data helped movement estimation. Our results elucidate how incorrect population structure assumptions can influence the estimation of key parameters of spatial IPMs, while indicating that, even if incorrectly specified, spatial IPMs can adequately support spatial management decisions.

Diagnosis of ageing error is critical to the proper interpretation of age data used in fisheries science and management. However, the influence of sample size and number of age classes on the characterization of ageing error has not been thoroughly evaluated. We conducted a simulation study of ageing error diagnostics for paired-age comparisons across 648 scenarios differing in 1) number of age classes, 2) total number of samples aged, 3) trend in sample size by age, and 4) magnitude and type of imprecision and bias. Imprecision was identified by comparing average coefficient of variation (ACV) with two common thresholds. Bias was evaluated using maximally (McNemar's), diagonally (Evans & Hoenig), and unpooled tests of symmetry (Bowker's). Imprecision was identified less frequently at low to moderate (x¯=6% of runs) levels of random vs high (x¯=55% of runs) error, and ACV was artificially inflated in the presence of bias. McNemar's and Evans & Hoenig bias tests outperformed Bowker's (x¯=4% vs 29% false positives), particularly at large sample sizes, and its use is strongly discouraged. This study can help guide the interpretation of ageing error studies and their products (e.g., ageing error matrices) used to inform stock assessment and management.

Understanding spatial population structure and biocomplexity is critical for determining a species' resilience to environmental and anthropogenic perturbations. However, integrated population models (IPMs) used to develop management advice for harvested populations have been slow to incorporate spatial dynamics. Therefore, limited research has been devoted to understanding the reliability of movement parameter estimation in spatial population models, especially for spatially dynamic marine fish populations. We implemented a spatial simulation-estimation framework that emulated a generic marine fish metapopulation to explore the impact of ontogenetic movement and climate-induced distributional shifts between two populations. The robustness of spatially stratified IPMs was explored across a range of movement parametrizations, including ignoring connectivity or estimating movement with various levels of complexity. Ignoring connectivity was detrimental to accurate estimation of population-specific biomass, while implementing spatial IPMs with intermediate levels of complexity (e.g. estimating movement in two-year and two-age blocks) performed best when no a priori information about underlying movement was available. One-way distributional shifts mimicking climate-induced poleward migrations presented the greatest estimation difficulties, but the incorporation of auxiliary information on connectivity (e.g. tag-recapture data) reduced bias. The continued development of spatially stratified modelling approaches should allow harvested resources to be better utilized without increased risk. Additionally, expanded collection and incorporation of unique spatially explicit data will enhance the robustness of IPMs in the future.

Atlantic menhaden ( Brevoortia tyrannus ) support the largest fishery by volume on the United States East Coast, while also playing an important role as a forage species. Managers’ and stakeholders’ increasing concerns about the impact of Atlantic menhaden harvest on ecosystem processes led to an evolution in the assessment and management of this species from a purely single-species approach to an ecosystem approach. The first coastwide stock assessment of Atlantic menhaden for management used a single-species virtual population analysis (VPA). Subsequent assessments used a forward projecting statistical catch-at-age framework that incorporated estimates of predation mortality from a multispecies VPA while analytical efforts continued toward the development of ecosystem models and explicit ecological reference points (ERPs) for Atlantic menhaden. As an interim step while ecosystem models were being developed, a series of ad hoc measures to preserve Atlantic menhaden biomass for predators were used by managers. In August 2020, the Atlantic States Marine Fisheries Commission formally adopted an ecological modeling framework as a tool to set reference points and harvest limits for the Atlantic menhaden that considers their role as a forage fish. This is the first example of a quantitative ecosystem approach to setting reference points on the United States Atlantic Coast and it represents a significant advance for forage fish management. This case study reviews the history of Atlantic menhaden stock assessments and management, outlines the progress on the current implementation of ERPs for this species, and highlights future research and management needs to improve and expand ecosystem-based fisheries management.

Age determination is important for estimating productivity and status in fisheries stock assessments. Aging methods must balance advances in technology with continuity of data for long-term sampling programs. The long-term sampling program for Atlantic menhaden (Brevoortia tyrannus) and Gulf menhaden (B. patronus) has resulted in a database of ages that were determined by using an Eberbach projector, a now outdated technology. The objective of this study was to compare ages determined with an Eberbach projector to those determined with a more recent technology, the stereo microscope. Scales from Atlantic (total number of fish [N]=1317) and Gulf menhaden (N=1569) were aged by using both an Eberbach projector and a stereo microscope, and results were compared by using percent agreement, average percent error, Chang's average coefficient of variation, bias tests, and simultaneous multinomial confidence intervals. The results from these measures for comparing age estimates were generally within standard, expected levels. Some bias was detected between estimates from the use of the 2 devices but was likely due to the use of poorly preserved scales or images of low quality on the Eberbach projector. Our results indicate that the use of a microscope will help maintain continuity in age estimates over time for long-term monitoring and for stock assessments.

Although many countries have formally committed to Ecosystem-Based Fisheries Management (EBFM), actual progress toward these goals has been slow. This paper presents two independent case studies that have combined strategic advice from ecosystem modeling with the tactical advice of single-species assessment models to provide practical ecosystem-based management advice. With this approach, stock status, reference points, and initial target F are computed from a single-species model, then an ecosystem model rescales the target F according to ecosystem indicators without crossing pre-calculated single-species precautionary limits. Finally, the single-species model computes the quota advice from the rescaled target F, termed here F-eco. Such a methodology incorporates both the detailed population reconstructions of the single-species model and the broader ecosystem perspective from ecosystem-based modeling, and fits into existing management schemes. The advocated method has arisen from independent work on EBFM in two international fisheries management systems: (1) Atlantic menhaden in the United States and (2) the multi species fisheries of the Irish Sea, in the Celtic Seas ecoregion. In the Atlantic menhaden example, the objective was to develop ecological reference points (ERPs) that account for the effect of menhaden harvest on predator populations and the tradeoffs associated with forage fish management. In the Irish Sea, the objective was to account for ecosystem variability when setting quotas for the individual target species. These two exercises were aimed at different management needs, but both arrived at a process of adjusting the target F used within the current single-species management. Although the approach has limitations, it represents a practical step toward EBFM, which can be adapted to a range of ecosystem objectives and applied within current management systems.

Fisheries policy inherently relies on an explicit definition of management boundaries that delineate the spatial extent over which stocks are assessed and regulations are implemented. However, management boundaries tend to be static and determined by politically negotiated or historically identified population (or multi-species) units, which create a potential disconnect with underlying, dynamic population structure. The consequences of incoherent management and population or stock boundaries were explored through the application of a two-area spatial simulation–estimation framework. Results highlight the importance of aligning management assessment areas with underlying population structure and processes, especially when fishing mortality is disproportionate to vulnerable biomass among management areas, demographic parameters (growth and maturity) are not homogenous within management areas, and connectivity (via recruitment or movement) unknowingly exists among management areas. Bias and risk were greater for assessments that incorrectly span multiple population segments (PSs) compared to assessments that cover a subset of a PS, and these results were exacerbated when there was connectivity between PSs. Directed studies and due consideration of critical PSs, spatially explicit models, and dynamic management options that help align management and population boundaries would likely reduce estimation biases and management risk, as would closely coordinated management that functions across population boundaries.

Atlantic menhaden is an important forage fish and the target of the largest fishery along the US East Coast by volume. Since 1999, managers at the Atlantic States Marine Fisheries Commission, stakeholders, and scientists have been interested in developing ecological reference points (ERPs) that account for menhaden’s role as a forage species. To accomplish this, we developed a suite of modeling approaches that incorporated predation on menhaden and changes in productivity over time and allowed for evaluation of trade-offs between menhaden harvest and ecosystem management objectives. These approaches ranged in complexity, from models with minimal data requirements and few assumptions to approaches with extensive data needs and detailed assumptions. This included a surplus production model with a time-varying intrinsic growth rate, a Steele-Henderson surplus production model, a multispecies statistical catch-at-age model, an Ecopath with Ecosim (EwE) model with a limited predator and prey field, and a full EwE model. We evaluated how each model could address managers’ objectives and compared outputs across the approaches, highlighting their strengths, weaknesses, and management utility. All models produced estimates of age-1 + biomass and exploitation rate that were similar in trend and magnitude to the single-species statistical catch-at-age model, especially in recent years. While the less complex models were relativity easy to implement and update, they lacked key elements needed to manage multiple species simultaneously. More complex models required a wider array of data and were more difficult to update within the current management time-frames, but produced a more useful framework for managers. Ultimately, an EwE model of intermediate complexity coupled with the existing single-species assessment model was recommended for use in management.

Atlantic menhaden (Brevoortia tyrannus) are an important forage fish for many predators, and they also support the largest commercial fishery by weight on the U.S. East Coast. Menhaden management has been working toward ecological reference points (ERPs) that account for menhaden’s role in the ecosystem. The goal of this work was to develop menhaden ERPs using ecosystem models. An existing Ecopath with Ecosim model of the Northwest Atlantic Continental Shelf (NWACS) was reduced in complexity from 61 to 17 species/functional groups. The new NWACS model of intermediate complexity for ecosystems (NWACS-MICE) serves to link the dynamics of menhaden with key managed predators. Striped bass (Morone saxatilis) were determined to be most sensitive to menhaden harvest and therefore served as an indicator of ecosystem impacts. ERPs were based on the tradeoff relationship between the equilibrium biomass of striped bass and menhaden fishing mortality (F). The ERPs were defined as the menhaden F rates that maintain striped bass at their biomass target and threshold when striped bass are fished at their Ftarget, and all other modeled species were fished at status quo levels. These correspond to an ERP Ftarget of 0.19 and an ERP Fthreshold of 0.57, which are lower than the single species reference points by 30–40%, but higher than current (2017) menhaden F. The ERPs were then fed back into the age-structured stock assessment model projections to provide information on total allowable catch. The ERPs developed in this study were adopted by the Atlantic menhaden Management Board, marking a shift toward ecosystem-based fishery management for this economically and ecologically important species.