Fishery-independent Surveys Research Articles

Optimized stratified random surveys best estimate multispecies abundance in a rapidly changing ecosystem

Abstract Data from fishery-independent surveys are critical inputs to stock assessments, ecosystem-based fishery management, and applied ecological research. However, environmental change may affect species distributions and their availability to surveys, with consequences for the consistency and precision of abundance estimates over time. We investigated whether defining survey stratum boundaries by environmental conditions improves the precision and accuracy of abundance estimates in a multispecies survey. We fitted univariate spatiotemporal species distribution models to 16 stocks (14 species) using historical observations of fishery-independent bottom trawl survey catch-per-unit-effort and sea bottom temperature in the eastern and northern Bering Sea from 1982 to 2022. These spatiotemporal models were used to simulate species distributions and survey observations under a variety of environmental conditions and survey designs. The predicted density of each species at each location and time was passed to a multivariate optimization routine to determine whether this could increase the accuracy of estimates of abundance per unit of survey effort across species relative to traditional survey designs. Historical and projected future abundances for 10 of the 16 stocks were estimated more precisely under optimized designs–up to 4× as precise as the existing design. The accuracy of the estimate of abundance precision was always lowest for systematic sample allocation and highest for random or balanced random sampling within strata, suggesting that designs optimized with historical biological and environmental data lead to a better ability to quantify survey precision. The approach developed here can be applied in other ecosystems experiencing change to support the design of flexible survey designs that could increase the efficiency of sampling marine resources under current and future climates.

Climate Change Affects Multiple Coral Reef Fisheries Ecosystem Services

ABSTRACTCoral reef fisheries support livelihoods and provide an affordable source of essential dietary nutrients to coastal people. However, climate‐driven coral bleaching is dramatically altering reef habitats and changing reef fish composition, diversity, and productivity. We used data from fisheries independent surveys and artisanal trap landings to explore how reefs with different responses to coral bleaching differed in habitat configurations, fisheries catch rates, economic returns, and nutritional content. Coral‐associated fishes supported the highest catch rates, greatest species diversity, highest rates of economic returns for fishers, and a wide price range for consumers. Macroalgae‐associated fishes supported the greatest overall fisheries yields, which indicated higher fishery dependence on these habitats. Nutrient content was high in catches across all habitats, but concentrations varied by nutrient, which suggested that fishing across a range of reef habitats should provide the greatest variety of nutrients to coastal communities.

The spatial and seasonal patterns and stability of the Lake Whitefish fishery in Michigan waters of southern Lake Huron

AbstractObjectiveGiven that the harvested fish from five separate fishing grounds belong to one large mixed population, our objective is to develop an overall abundance index for Lake Whitefish Coregonus clupeaformis in Michigan waters of southern Lake Huron.MethodsWe analyzed catch‐and‐effort data for individual fishing trips to five separate fishing grounds and used Bayesian information criterion to select the best model struture.ResultThe best model included the year × area × month interaction as a random effect, representing the random component in fishery dynamics that were influenced by both fishing practices and fish abundance. The best model also included the fixed effects of area × month interaction, a result consistent with our hypothesis that the seasonal spatial migrations of Lake Whitefish provide the links among the fisheries in separate fishing grounds, and the fishery yields from these separate areas were all dependent on the production in southern Lake Huron as a whole. After adjusting for the random effect of year × area × month interaction and the fixed effects of area × month interaction, the estimated patterns for the fixed year effect indicated that the Lake Whitefish abundance in Michigan waters of southern Lake Huron declined during the late 2000s but has been relatively constant after 2012.ConclusionThe declines in annual fishery yield were due to and could be explained more by the decreases in fish abundance through 2012 but thereafter primarily reflected the declines in annual fishing effort as the fisheries continued to adjust to the changed fishing conditions. We also provided general lessons for studying fisheries without fishery independent surveys but with detailed seasonal and spatial catch‐and‐effort data.

Disentangling bottom-up and top-down controls on fish consumption of key prey in the Northeast US Shelf ecosystem

Abstract Exploited forage fishes serve a dual role in marine ecosystems by supporting directed fisheries and predator productivity, and thus both harvest and predatory removals should be accounted for when developing stock assessments and evaluating management trade-offs. Predator catch and stomach content data collected on the Northeast US Shelf from 1978 to 2019 by two fisheries-independent surveys were combined within multivariate spatiotemporal models to estimate time-series of consumptive removals during spring and fall for four commercially exploited prey; Atlantic herring (Clupea harengus), silver hake (Merluccius bilinearis), butterfish (Peprilus triacanthus), and longfin squid (Doryteuthis pealeii). Seasonal consumption trends were mostly synchronous for Atlantic herring and silver hake, asynchronous for butterfish and longfin squid, and predatory removals were generally greater during fall. Consumption has increased since the 1990s for all prey except Atlantic herring and butterfish during fall, which coincides with the widespread implementation of harvest constraints meant to rebuild predator and prey populations. These time-series were linked to hypothesized drivers using state-space regression models; prey availability (bottom-up; positive relationships) and commercial catch (top-down; primarily negative relationships) were the strongest predictors of consumption. Although the mechanisms underlying these relationships remain unresolved, these linkages highlight connections among the systemic drivers of productivity on the Northeast Shelf.

Accounting for spatio-temporal distribution changes in size-structured abundance estimates for a data-limited stock of Raja clavata

Abstract Vulnerability of elasmobranchs to fishing and declines in populations over the last decades have prompted calls for improved fisheries management and conservation efforts. The Raja clavata (Thornback ray) population in the Greater North Sea ecoregion is a population that has historically shown marked declines with increasing industrialized fishing, while a lack of robust catch data of commercial fisheries hampers assessment of population abundance. Using fisheries-independent survey catch data haul-by-haul surface area estimates, we employ integrated-nested Laplace approximation to estimate total and size-class abundances of R. clavata. By accounting for spatio-temporal changes in the population, size selectivity between survey gears, and minimizing bias from partially overlapping survey areas, we demonstrate major changes in the abundance and distribution over the past three decades. Notably, increases of abundance in the Eastern English Channel and south-eastern North Sea result in an overall increase in the abundance and biomass of the population. Our findings expand understanding of the spatio-temporal dynamics and exploitation of this data-limited stock, emphasizing the potential for improved population abundance estimates to inform future stock assessments.

How the collection of fish length data using electronic monitoring video review measures up to current methods used on a fishery-independent survey

How the collection of fish length data using electronic monitoring video review measures up to current methods used on a fishery-independent survey

Long-term effects of climate change on juvenile bull shark migratory patterns.

Seasonal variability in environmental conditions is a strong determinant of animal migrations, but warming temperatures associated with climate change are anticipated to alter this phenomenon with unknown consequences. We used a 40-year fishery-independent survey to assess how a changing climate has altered the migration timing, duration and first-year survival of juvenile bull sharks (Carcharhinus leucas). From 1982 to 2021, estuaries in the western Gulf of Mexico (Texas) experienced a mean increase of 1.55°C in autumn water temperatures, and delays in autumn cold fronts by ca. 0.5 days per year. Bull shark migrations in more northern estuaries concomitantly changed, with departures 25-36 days later in 2021 than in 1982. Later, migrations resulted in reduced overwintering durations by up to 81 days, and the relative abundance of post-overwintering age 0-1 sharks increased by >50% during the 40-year study period. Yet, reductions in prey availability were the most influential factor delaying migrations. Juvenile sharks remained in natal estuaries longer when prey were less abundant. Long-term declines in prey reportedly occurred due to reduced spawning success associated with climate change based on published reports. Consequently, warming waters likely enabled and indirectly caused the observed changes in shark migratory behaviour. As water temperatures continue to rise, bull sharks in the north-western Gulf of Mexico could forgo their winter migrations in the next 50-100 years based on current trends and physiological limits, thereby altering their ecological roles in estuarine ecosystems and recruitment into the adult population. It is unclear if estuarine food webs will be able to support changing residency patterns as climate change affects the spawning success of forage species. We expect these trends are not unique to the western Gulf of Mexico or bull sharks, and migratory patterns of predators in subtropical latitudes are similarly changing at a global scale.

Spatiotemporal dynamics of Atlantic reef fishes off the southeastern U.S. coast

AbstractUnderstanding the spatiotemporal dynamics of fish species is a central concern in fish ecology and crucial for guiding management and conservation efforts. We constructed a joint species distribution model (JSDM) to simultaneously estimate the spatiotemporal distributions and densities for 21 reef fish species in the southeastern United States (SEUS). The model separately estimates encounter probability and positive density, and accounts for unobserved spatial and spatiotemporal variation using latent factors, where the correlations among species are induced. We applied the model to video data collected from a large‐scale, fishery independent survey. A clustering method was applied to the results of the JSDM to group species based on spatial and spatiotemporal synchrony in encounter probability and positive density. We found strong spatial associations among most of the reef fish species. However, species did exhibit differences in occupied habitat that varied with latitude and/or depth. Within their area of occupied habitat, almost all the species share similar spatial pattern of average density. However, for some species, annual distributions were less correlated with their expected average distributions perhaps due to differing responses to underlying spatiotemporal drivers. Some species show significant declines in abundance, for example, black sea bass, red porgy, and blueline tilefish, while a small number of species showed evidence of shifts in distribution, for example, black sea bass. The findings suggest that spatiotemporal management strategies may be of limited utility for reducing bycatch in these highly mixed reef fisheries due to high spatial correlations in occupied habitat and spatial patterns in density. Species‐specific responses to environmental change may also influence the spatiotemporal structure of reef assemblages. This work suggests management attention is needed for some of the lesser known species as they are showing declining trends in abundance.

Fisheries independent surveys in a new era of offshore wind energy development

Abstract Fisheries independent surveys require rethinking because of increasing spatial restrictions and interactions with offshore wind energy development (OWD). Fisheries, protected species, and environmental data collections have been conducted by scientific institutions to meet societal demands for food security, conservation, and other marine uses. These data collections provide information on key resource measures, essential for fisheries, protected species, and ecosystem management. With the increase in pace and magnitude of OWD's industrialization of marine waters, disruptions in these long-term time series can be expected. These disruptions will impact the ability to support current and future management goals and objectives. This paper presents an expert survey on the perceptions of OWD interactions with common survey designs and survey methodologies in Europe and the U.S., along with a selected sample of 75 fisheries independent surveys in the U.S. and Europe providing an initial assessment and description of potential impacts from OWD. About 72% of the surveys sampled record interactions with operational, planned or future OWD. Four case studies demonstrate efforts to address these interactions within European regions that have operational OWD and the U.S. where development has just begun. Finally, we make recommendations for future research important to continue meaningful scientific-based management advice.

First record of the bluntnose sixgill shark Hexanchus griseus (Bonnaterre, 1788) in the Guatemalan Caribbean Sea

The Guatemalan Caribbean has a deepwater fishing area close to the shore around the Cayman Trench. This study reports the first record of the bluntnose sixgill shark (Hexanchus griseus) from this fishing area. Fishery-independent surveys using longlines at ~430-465 m depth, ~11 km northeast of El Quetzalito fishing village, were conducted in 2022 and 2023. Two bluntnose sixgill sharks were captured during these surveys. The sharks were females with total lengths of 300 and 310 cm, with morphological characteristics consistent with this species. These are the first confirmed records of bluntnose sixgill sharks in the western Caribbean Sea. Expanding coastal fisheries to deeper waters presents an emerging threat to deep-sea chondrichthyans in the region. Therefore, periodic fisheries monitoring is needed to estimate their vulnerability to fishing pressure.

Lessons from long-term monitoring of tropical rock lobsters to support fisheries management

Fishery-independent scientific surveys are highly valuable for monitoring lobsters and other fisheries. Yet, there are relatively few long-term monitoring surveys to support the provision of management advice. A number of challenges can prevent the continuity of long-term monitoring series. This paper uses the 35-year continuous annual dive surveys for Australia’s Torres Strait tropical rock lobster (TRL) to overview strengths and limitations of a long-term scientific survey, as well as lessons learnt from having overcome a range of challenges. Fishery-independent surveys are more reliable than fishery-dependent data and are more robust to external shocks such as market drivers and fuel prices. The primary purpose of the surveys is to provide a reliable index of stock abundance together with data to inform on size and age structure, plus model parameters and reference points. Here, we also highlight additional survey benefits including: yielding positive gains relative to cost, facilitating an adaptive management approach that serves as an early warning system, reducing and resolving inter-sector conflicts, reduced frequency of stock assessments, providing data from associated habitat monitoring, improved stakeholder buy-in, utility for development of Operating Models for Management Strategy Evaluation, contribution to achieving certification, contributing to biodiversity agreements and consistent cross-jurisdictional monitoring. We analyse lessons learnt from challenges such as changes in resource management, budget cuts, increased health and safety requirements, maintenance of capability and staff capacity, stakeholder buy-in, need to validate survey results, a pandemic and changing climate. Our experiences highlight why long-term scientific surveys are considered the gold standard for fisheries monitoring and we provide insights for building and maintaining key marine monitoring series.

Expansion of an established fishery-independent survey into the US Virgin Islands' upper mesophotic zone: feasibility and management implications

A three-year pilot study, the Deep Coral Reef Monitoring Program (DCRMP), expanded the National Coral Reef Monitoring Program's (NCRMP) established fishery-independent, diver-based reef fish visual survey to upper mesophotic reefs (>30 to 50 m) in the United States Caribbean for the first time. The new DCRMP sample domain (>30 to 50 m) encompassed 2.4 times more survey area than NCRMP (0 to ≤30 m) and collected high quality data (CV <20%) on coral reef fishes [three survey years, 29 (5) species; mean (standard deviation)]. For the four representative, fishery-targeted, analysis species selected (i. e., grouper, snapper, triggerfish, and parrotfish), domain-wide density and length comparisons between surveys showed similar or statistically higher abundances and larger lengths for fishes at deeper depths (>30 to 50 m). These results highlight the importance of surveying the entire insular shelf in St. Thomas and St. John, US Virgin Islands for fisheries management applications. Furthermore, the DCRMP survey leveraged NCRMP's methods and resources resulting in a seamless extension to deeper waters. However, if these programs were fully integrated and optimized within a single survey design, approximately half the sites would be needed to achieve the same level of precision, offering substantial time and cost savings. The principles of probabilistic sampling successfully used in the present fishery-independent survey design (0 to 50 m) can be applied more broadly to develop an "ideal" large-scale, multi-gear survey from 0 to about 500 m to encompass the entire depth ranges of managed species in the US Caribbean.

Integrating the US Caribbean Reef Fish Visual Census into fishery stock assessments

The United States Caribbean Reef Fish Visual Census (RVC) is a fishery-independent survey of the coral reef fish community, comprised of hundreds of species, designed to provide quantitative information for data-poor fisheries to support regional stock assessments. The Caribbean RVC, implemented in 2014 as part of the National Coral Reef Monitoring Program (NCRMP), uses a probabilistic stratified random sampling design capitalizing on the strong mean-variance relationship of population abundance dependent on hardbottom habitats and depths. Here we highlight RVC data for queen triggerfish in Puerto Rico, where survey precision improved from CVs of 20.6% in 2014 to 12.6% in 2021, which produced accurate and cost-effective estimates of key assessment metrics like size- structured relative population abundance and biomass. We estimated a relatively stable exploited-phase average population abundance for 2014–2021 at 2.3 million queen triggerfish with a biomass of 1246 metric tons. To support stock assessments using "statistical catch at age" models, we generated a 2001–2021 time series of several population metrics by a statistical conversion of spatially-restricted, pre- NCRMP data to the RVC frame. To cross-check assessment modeling, we combined 2014–2021 RVC results with life history demographics in an alternative length-based risk analysis (LBRA) model to evaluate stock sustainability consequences using two lifetime growth scenarios. LBRA models indicated low overfishing risks. We concluded that only a few quality fishery-independent surveys are needed to obtain understanding of stock sustainability status, and that RVC not only provides reliable data for single-species stock assessments, but also for design evaluation of marine protected areas and ecosystem-based fishery management.

Quantifying hurricane impacts on United States Virgin Islands reef fishes using a catchability invariant approach to compare uncalibrated survey indices

The United States Virgin Island's (USVI) coral reefs support many economically and ecologically important fish species. Located in the Caribbean Sea, they are subject to frequent severe weather disturbances, including two category 5 hurricanes, Irma and Maria, in 2017. The overarching goal of this study was to identify reef fish community impacts following these extreme events using discontinuous survey indices. Long-term (2001–2021), in situ, fishery-independent survey data using two methods were standardized to the extent possible in two regions of the USVI, St. Thomas/St. John (STT/STJ) and St. Croix (STX). Comparable data were assessed, 73 species collected on hard- bottom habitat, to identify the number of these species with significant changes in density and/or mean length between 2-yr survey intervals over the historic baseline (2001–2015), disturbance (2017–2019), and postdisturbance (2019–2021) periods. The results varied by region: STT/STJ had no disturbance impact and STX had a significant disturbance impact. In STX, 20 species had significant changes in density in the disturbance period compared to an average of 9.7 (SD 3.8) species for the baseline period. The proportion of species with significant density increases and decreases were similar suggesting that overall disturbance impacts are nuanced. Mean length observations were less informative, likely due to survey method and sample size changes. However, in combination with density they provided useful insights into the possible causes of population change. The successful use of discontinuous survey indices to obtain meaningful biological insights has broader applications to ecosystem and fishery datasets with similar limitations.

Recent Declining Trends in Pelagic Fish Catches in the Indian Ocean off Sri Lanka: Is Gill Oxygen Limitation Theory (GOLT) a Possible Explanation?

Recent trends in the pelagic fish landings of multi-day fishing fleets operated from Sri Lanka indicated significant declines in many fish species. Therefore, the present preliminary analysis investigates the perceptions of fishers on recent declining trends of pelagic fish landings from offshore areas of the Indian Ocean and further investigates whether the most common pelagic species landed in Sri Lanka conform to the gill oxygen limitation theory (GOLT) and to speculate GOLT as a possible explanation to such trends. According to the perceptions of 457 skippers of fishing vessels interviewed, such declines were possibly attributed to shifting of the areas of occurrence of pelagic fish species, making them less vulnerable to multi-day fishing vessels. As climate change and deoxygenation are major stressors affecting fish stocks, there is a challenging need for disentangling the impacts of these stressors from the effects of overfishing. The 18 most common pelagic fish species harvested from the Indian Ocean confirmed to the predictions from the GOLT, suggesting that shifting of these stocks could be due to deoxygenation which may have been triggered by increased sea surface and sub-surface temperatures. Therefore, fishery-independent surveys are needed to investigate the shifting of areas of occurrence of pelagic fishes in the Indian Ocean to understand their areas of occurrence the further investigate the relevance of GOLT for defining regional fisheries management plans.

Warming waters lead to increased habitat suitability for juvenile bull sharks (Carcharhinus leucas)

Coastal ecosystems are highly vulnerable to the impacts of climate change and other stressors, including urbanization and overfishing. Consequently, distributions of coastal fish have begun to change, particularly in response to increasing temperatures linked to climate change. However, few studies have evaluated how natural and anthropogenic disturbances can alter species distributions in conjunction with geophysical habitat alterations, such as changes to land use and land cover (LU/LC). Here, we examine the spatiotemporal changes in the distribution of juvenile bull sharks (Carcharhinus leucas) using a multi-decadal fishery-independent survey of coastal Alabama. Using a boosted regression tree (BRT) modeling framework, we assess the covariance of environmental conditions (sea surface temperature, depth, salinity, dissolved oxygen, riverine discharge, Chl-a) as well as historic changes to LU/LC to the distribution of bull sharks. Species distribution models resultant from BRTs for early (2003–2005) and recent (2018–2020) monitoring periods indicated a mean increase in habitat suitability (i.e., probability of capture) for juvenile bull sharks from 0.028 to 0.082, concomitant with substantial increases in mean annual temperature (0.058°C/yr), Chl-a (2.32 mg/m3), and urbanization (increased LU/LC) since 2000. These results align with observed five-fold increases in the relative abundance of juvenile bull sharks across the study period and demonstrate the impacts of changing environmental conditions on their distribution and relative abundance. As climate change persists, coastal communities will continue to change, altering the structure of ecological communities and the success of nearshore fisheries.

Network of nearshore protected areas provides important benefits to lake whitefish in the Apostle Islands region of Lake Superior

Aquatic protected areas are commonly used for fish conservation, restoration, and management and a spectrum of protection levels exist in the Great Lakes. Using fishery-independent survey data over 43 years, we evaluated whether lake whitefish (Coregonus clupeaformis) population dynamics differed among nearshore partially protected areas (PPAs; commercial fishing prohibited or severely limited), offshore no-take refuges, and unprotected areas in the Apostle Islands region of Lake Superior. Lake whitefish biomass and recruitment to the spawning stock increased at faster rates in nearshore PPAs and offshore refuges than unprotected areas during the initial phase after protected areas were established (1980–2000). Recruitment and biomass stabilized in all management areas in the post-rebuild phase (2002–2022), and mortality was lower in the nearshore PPAs. Mean size of adults decreased within protected areas as abundance increased but not in unprotected areas, suggesting a density-dependent growth response and spillover to the fishing grounds, which was reflected in commercial catch rates. However, nearshore PPAs still harbored larger, faster-growing, and earlier-maturing lake whitefish, likely due to underlying habitat differences. Tag recaptures indicated greater minimum distances traveled near an offshore refuge compared to a nearshore PPA, suggesting habitat gradients may influence boundary porosity. PPAs provided protection and benefits for lake whitefish in areas of higher vulnerability where ceasing all fishing was not reasonable. Great Lakes managers should consider implementing networks of protected areas across multiple habitats as tools for conserving spawning biomass, maintaining diverse population demographics, and preserving portfolio effects to enhance recruitment and population stability.

Development and application of a bioenergetics growth model for multiple early life stages of an ecologically important marine fish

Spatial and temporal variability in temperature and food availability are key drivers of growth of marine fishes. Growth during the early life stages (ELS's) is tightly coupled to survival, and in turn, can set year-class strength (i.e. annual recruitment) and overall stock productivity of populations and fished stocks. Ontogenetic changes in physiology, dietary preferences, and growth across ELS's can be accounted for within bioenergetics models, but existing models lack resolution within larval and early juvenile stages. We leveraged daily output from a coupled physical-biogeochemical model to force a highly resolved ontogenetic bioenergetics model parametrized for an ecologically important rockfish in the California Current System. Size-at-age predictions closely track empirical growth trajectories of the ELS's. Scenario testing revealed that growth performance is disproportionately driven by changes in temperature compared to food availability. We then expanded the model to incorporate spatial climatological differences in temperature and prey concentration and found that preflexion growth potential is maximized in areas of historical spawning, suggesting the timing and location of reproduction is an adaptive strategy that places larvae in habitat favorable for survival. Growth potential for late-stage larvae (postflexion) is greatest over a broad areal extent, implying that if a particle tracking algorithm was coupled to the bioenergetics model, a wide range of larval dispersal pathways would place postflexion larvae in habitat suitable for rapid growth. Finally, growth potential of pelagic juveniles is maximized over the continental shelf and shelf-break, aligning with high juvenile catch rates from a fisheries-independent survey. In summary, this study (i) serves as a proof of concept that a bioenergetics model with high ontogenetic resolution can reproduce life stage-specific growth trajectories even though the underlying physiology data for model parameterization is imperfect and (ii) can aid future studies aimed at understanding how ecosystem processes interact with ontogenetic growth and changes in year class strength of early life stages of marine fishes.

Performance evaluation of spatially balanced sampling designs in fishery-independent surveys

Cost-effective fishery-independent surveys are critical for obtaining high-quality data to support stock assessment and fisheries management. Classical probability-based sampling designs may exhibit limitations in spatial coverage and flexibility. Spatially balanced sampling (SBS) designs offer the ability to efficiently distribute sampling stations across a survey area. This study evaluates the performance of SBS designs in multispecies fishery-independent surveys through simulation studies, involving two widely used classical sampling designs (Simple Random Sampling and Stratified Random Sampling) and two SBS designs (Generalized Random Tessellation Stratified Model and Balanced Acceptance Sampling). The aim of the simulation study was to assess the performance of different sampling designs in estimating the abundance index for four target fish species across different seasons. The results showed that SBS designs had relatively high accuracy and precision in terms of relative estimation error, relative bias, and coefficient of variation. Among the four sampling designs, the Generalized Random Tessellation Stratified Model performed optimally at the same sample size. In conclusion, this study suggests that SBS designs can enhance the precision of abundance index estimation for different fish species. Given its ability to effectively distribute sampling stations and its flexibility, a better understanding and application of SBS designs in fishery-independent surveys are warranted.

Exploring the limits of spatiotemporal and design-based index standardization under reduced survey coverage

Abstract Indices of abundance derived from fisheries-independent surveys play a crucial role in sustainable fisheries management. While design-based methods provide unbiased indices in theory, logistical constraints may introduce biases in practice. Spatiotemporal models offer potential for mitigating such bias, but their limitations remain poorly understood. In this study, we compare the performance of model-based and design-based indices using 200 simulated populations representing two virtual species. We simulate stratified-random surveys under various scenarios, including constant effort and coverage, reduced set density, and reduced spatial coverage (e.g. closed areas). We consider three closed-area scenarios: no population recovery, population recovery, and population recovery with spillover. With consistent survey coverage, correctly specified spatiotemporal models demonstrated comparable bias, accuracy, and confidence interval coverage to design-based methods. Spatiotemporal models incorporating appropriate covariates and observation families could mitigate the impact of reduced spatial coverage. However, poorly specified models were sometimes outperformed by design-based methods. Our results, therefore, highlight the potential for spatiotemporal models to mitigate the effects of survey effort reduction on population assessment and the provision of scientific advice. However, they also present a cautionary tale about the critical importance of model evaluation and comparison.