Assessment Of Fish Populations Research Articles

A problem for the ages: Achieving reliable fish age information through quality management

Age information is central to assessment and management of fish populations. Age information must be reliable to have value, which depends on its quality. Quality assurance (QA) and quality control (QC) are processes often applied to some aspects of producing age information (e.g., annulus validation). However, we advocate for a more holistic approach which leverages QA/QC measures across all phases of the age information-generating process. Systematic implementation of QA/QC measures in a repetitive process is common in the manufacturing industry where it is known as a quality management system (QMS) but this framework is not well described in the fisheries literature. We designed and implemented a QMS that incorporates QA/QC measures across all phases of fish age information development: Collection, Interpretation, and Distribution. These measures are guided by six principles: Train, Simplify, Validate, Compare, Record, and Improve. In our QMS, the Train, Simplify, and Validate principles are largely guidance for QA measures, while Compare, Record, and Improve guide QC measures. We provide examples of common errors (or sources of error) in each phase, and how the guiding principles in our QMS address these errors. This is a QMS crafted as a holistic approach to managing the quality of fish age information; however, it has broad application as a conceptual framework for other repetitive processes in fisheries.

Six decades of ecohydrological research connecting landscapes and riverscapes in the Girnock Burn, Scotland: Atlantic salmon population and habitat dynamics in a changing world

AbstractLong‐term data are crucial for understanding ecological responses to climate and land use change; they are also vital evidence for informing management. As a migratory fish, Atlantic salmon are sentinels of both global and local environmental change. This paper reviews the main insights from six decades of research in an upland Scottish stream (Girnock Burn) inhabited by a spring Atlantic salmon population dominated by multi‐sea‐winter fish. Research began in the 1960s providing a census of returning adults, juvenile emigrants and in‐stream production of Atlantic salmon. Early research pioneered new monitoring techniques providing new insights into salmon ecology and population dynamics. These studies underlined the need for interdisciplinary approaches for understanding salmon interactions with physical, chemical and biological components of in‐stream habitats at different life‐stages. This highlighted variations in catchment‐scale hydroclimate, hydrology, geomorphology and hydrochemistry as essential to understanding freshwater habitats in the wider landscape context. Evolution of research has resulted in a remarkable catalogue of novel findings underlining the value of long‐term data that increases with time as modelling tools advance to leverage more insights from “big data”. Data are available on fish numbers, sizes and ages across multiple life stages, extending over many decades and covering a wide range of stock levels. Combined with an unusually detailed characterization of the environment, these data have enabled a unique process‐based understanding of the controls and bottlenecks on salmon population dynamics across the entire lifecycle and the consequences of declining marine survival and ova deposition. Such powerful datasets, methodological enhancements and the resulting process understanding have informed and supported the development of fish population assessment tools which have been applied to aid management of threatened salmon stocks at large‐catchment, regional and national scales. Many pioneering monitoring and modelling approaches developed have been applied internationally. This history shows the importance of integrating discovery science with monitoring for informing policy development and assessing efficacy of management options. It also demonstrates the need to continue to resource long‐term sites, which act as a focus for inter‐disciplinary research and innovation, and where the overall value of the research greatly exceeds the costs of individual component parts.

Good Practices for estimating and using length-at-age in integrated stock assessments

Estimating growth (increase in size with age) is an integral component of fish population assessment. The use of integrated assessment models combined with the influence of misfitting size composition data on results have led to renewed interest in how growth is modeled in the assessment process. The types of data available to describe the growth process control how the length-at-age relationship will be estimated. Many factors contribute to the complexity of estimating length-at-age, including multiple sources of biological variability and difficulties in getting representative samples. The growth process in the population dynamics model is linked to all other processes and data but most directly influences the assessment model through 1) converting numbers into weight and vice versa, 2) productivity, and 3) modifying fits of size composition data. In some cases, an assessment may be insensitive to moderate levels of misspecification of the growth process, and therefore, relatively simple treatments may be adequate. However, in many cases, especially those where the fit of size composition is influential in estimating scale, a more thorough treatment of the growth process is needed. A complete treatment of growth will estimate the most important forms of biological variability, including individual, sex-specific, temporal, and spatial variability. Several types of sampling bias, including selectivity, length-stratified sampling, and spatial and measurement error, will likely also need to be addressed. When sufficient data are available, assessment authors should consider estimating the growth process as part of the integrated assessment model or consider empirical approaches for situations with high biological variability and sampling bias.

An empirical comparison of sex ratio, condition factor and length–weight relationships in Mediterranean horse mackerel from the Sea of Marmara, the Middle and Eastern Black Sea

Fish length–weight relationships are essential for estimating biomass and assessing fish population health, typically showing low variation within and between neighbouring stocks. This study investigates the length–weight relationships of Mediterranean horse mackerel (Trachurus mediterraneus) collected from the Sea of Marmara, the Middle Black Sea, and the Eastern Black Sea in Türkiye during 2022. In addition, the study assessed the sex ratio and condition factor of Mediterranean horse mackerel at each station, alongside an examination of length-frequency samples, providing a comprehensive analysis. The one-way ANCOVA analysis showed no significant differences in length–weight regression between the sexes. However, it identified a considerable area effect, with samples from the Sea of Marmara significantly differing from those in the middle Black Sea and Eastern Black Sea. Fish samples from the latter areas displayed moderate length–weight variation, which was close to the significance threshold with a p-value of 0.069. The growth pattern of Mediterranean horse mackerel consistently exhibited negative allometry across all sampled areas for both males and females, except for female Mediterranean horse mackerel from the middle Black Sea, which tended to follow an isometric growth pattern. This study revealed consistent results within neighbouring areas (Middle Black Sea and Eastern Black Sea) and significant variations when comparing the Black Sea and Sea of Marmara Mediterranean horse mackerel, emphasising notable large-scale spatial trends.

Evaluating effects of changing sampling protocol for a long-term ichthyoplankton monitoring program

Abundance indices play a crucial role in monitoring and assessing fish population dynamics. Fishery-independent surveys are commonly favored for deriving abundance indices because they follow standardized or randomized designs, ensuring spatiotemporal consistency in representative and unbiased sampling. However, modifications to the survey protocol may be necessary to accommodate changes in survey goals and logistic difficulty. When the survey undergoes changes, calibration is often needed to remove variability that is unrelated to changes in abundance. We evaluated a long-term monitoring program, the Long River Survey (LRS) in the Hudson River Estuary (HRE), to illustrate the process of calibrating survey data to account for the effects of changing sampling protocol. The LRS provided valuable ichthyoplankton data from 1974 to 2017, but inconsistencies in sampling timing, location, and gears resulted in challenges in interpreting and comparing the fish abundance data in the HRE. Generalized Additive Models were developed for five species at various life stages, aiming to mitigate the impact of sampling protocol changes. Model validation results suggest the consistent performance of the developed models with varying lengths of time series. This study indicates that changes in the sampling protocol can introduce biases in the estimates of abundance indices and that the model-based estimates can improve the reliability and accuracy of the survey abundance indices. The model-estimated sampling effects for each species and life stage provide critical information and valuable insights for designing future sampling protocols.

Calibration of fish biomass estimates from gillnets: Step towards broader application of gillnet data

Fish are an important component of aquatic ecosystems, thus representative and reliable assessments of their population variables are essential for a variety of ecological applications, management and conservation. Determining Fish Density per actual Spatial Unit (volume or area, FDSU) as a measure of absolute fish quantity is of particular interest. Gillnets are undoubtedly one of the most common and important methods for assessing fish populations in large lentic waters. However, direct calculating of FDSU from gillnet catches is impossible because of the passive nature of this method, and to date there is no reliable model for calculating FDSU from gillnet catches. This weakness largely limits the use of gillnet data for applications requiring FDSU estimates.The aim of this study was to calibrate gillnet catches using FDSU obtained by active methods (beach seine nets and hydroacoustics) to develop a tool for assessing FDSU from gillnet catches. To achieve this goal, we compared gillnet biomass to fish biomass estimated from the active methods, both of which cover similar spatiotemporal niches. This comparison was performed using a statistical approach based on the recognition of non-negligible random measurement error in both the explanatory (active methods) and response (gillnets) variables.We found a strong positive linear relationship between fish biomasses sampled with gillnets and with active methods. The slope of the fitted linear model was similar when comparing gillnets with the two active methods. The statistical method used allowed for the inclusion of error in the biomass estimates with gillnets and active methods, refining the credible intervals of the estimated relationship. The effect of gillnet effort on model accuracy was simulated to show how increased effort narrows the credible interval. Finally, comparison with previously published relationships revealed a large but explainable discrepancy between our model and previous models.Our study showed that conversion of gillnet biomass to biomass per actual spatial unit is possible. The effective sampling area of one square meter of gillnet was determined to be 8 m2 of waterbody surface area when European standard 12 mesh-sizes gillnets are used, and 5 m2 when four larger meshes are added to the European standard gillnets. Our model further stressed the impact of increased sampling effort on reducing estimation variability and shows that the model may be dependent on the fish community.

Optimizing data quantity and quality for side-looking echosounder surveys in large rivers

The increased use of side-looking echosounder surveys in large rivers necessitates the creation and evaluation of standardized sampling protocols to effectively sample these habitats. Using data from five Ohio River pools, we evaluated the effects of habitat and transect direction on the quality and quantity of mobile, side-looking, split-beam echosounder data to determine how these factors may affect survey design in large rivers. Split beam echosounder data were collected using two multiplexing 200 kHz transducers offset in angle to maximize water column coverage. Using mixed-effects models, we evaluated the effects of habitat (main channel vs. tributary) and transect direction (upstream vs. downstream movement of the vessel) on fish density and the number of targets per fish track (series of acoustic targets from an individual fish). We also used a skewness test in conjunction with a Kolmogorov-Smirnov test to determine the distribution of fish orientations relative to the transducer and examine differences in the distribution of fish orientations among habitats, transect directions, and years. Mean fish densities were greater in tributary habitat relative to main channel habitat, whereas main channel habitat had a higher mean number of targets per fish track when compared to tributaries. Upstream-facing transects had greater mean fish density estimates and mean number of targets per fish track than downstream-facing transects. Although most fish were oriented perpendicular to the transducer, we identified habitat, transect direction, and annual differences in the distribution of fish orientations. These results suggest that researchers and managers that plan to use side-looking echosounders to assess fish populations in large rivers should ensure that standardized protocols include main channel and tributaries, and are expanded to encompass other relevant habitats (e.g., backwaters and side channels). Further, managers should use fish orientation data to select or develop TS-L equations that best matches the fish orientation distribution of each survey to ensure accurate density and size estimates.

Do angler catch and harvest rates differ between open water and ice anglers in Wisconsin?

Understanding seasonal differences in species-specific vulnerabilities to recreational angling can be important for informing sustainable fisheries management practices, like fishing seasons or season-specific regulations. However, comparisons of angler catch and harvest rates among seasons with disparate modes of fishing, like open water and ice angling in north-temperate lakes, are lacking. We used all available Wisconsin Department of Natural Resources point-intercept creel survey data during 1990–2020 to test for seasonal differences in the fisheries for five species targeted by open water and ice anglers in Wisconsin, USA, lakes (black crappie Pomoxis nigromaculatus, bluegill Lepomis macrochirus, northern pike Esox lucius, walleye Sander vitreus, yellow perch Perca flavescens). Specifically, we tested for: 1) species-specific differences in mean open water versus ice angling catch and harvest rates; 2) trends in species-specific mean open water versus ice angling catch and harvest rates during 1990–2020; and 3) monthly patterns in mean species-specific angler effort and catch rates. Mean angler catch rates of the five study species were significantly higher during the open water season and mostly temporally stable – only black crappie and bluegill open water catch rates and black crappie ice season catch rates significantly increased during 1990–2020. Mean angler harvest rates were significantly higher in the open water season for black crappie, bluegill, and walleye. Harvest rates were significantly higher during the ice season for northern pike and did not differ between seasons for yellow perch. Harvest rates were mostly temporally stable across species, although harvest rates declined for bluegill ice fishing and yellow perch in both seasons. Species-specific angler effort and catch rates mostly corresponded with seasonal (spring/early summer) vulnerabilities of our study species, although higher winter effort and catch rate patterns for northern pike and yellow perch may suggest unique harvest preferences for those species. Our results suggest that ice angling should not be assumed to have a negligible influence on fish populations in north-temperate lakes, fisheries-dependent and -independent monitoring data should be jointly considered when assessing fish population status, research on the human dimensions of ice fisheries is critically needed, and angler knowledge of seasonal fish vulnerabilities are generally coupled with higher fish catch rates.

Investigating the Potential Utility of Environmental DNA to Provide a Relative Abundance Index for the Depleted Teleost, Mulloway, Argyrosomus japonicus

Non-invasive, low-cost methods for censusing depleted fish populations are being prioritised among many jurisdictions worldwide. Collecting environmental DNA (eDNA) could offer one such option for augmenting fish population assessments. However, candidate species need to be carefully selected because species-specific DNA shedding and decay rates are affected by many biotic and abiotic factors that may influence relative abundance estimates. In this study, we sought to ascertain if the eDNA of a depleted Australian teleost, mulloway, Argyrosomus japonicus, reflects its weight under controlled aquaria conditions. With four experiments, we investigated the relationships between mulloway eDNA concentrations and their weight tank−1 as a function of: (1) time post-tank establishment; (2) water temperatures (within the species’ tolerance range); (3) stocking densities; and (4) among individual, similar-sized fish. The concentrations of eDNA in tanks stabilised after six days, and a positive relationship was found between fish weight and eDNA concentration, despite some variability in shedding rates by similar-sized fish. There was also a positive effect of water temperature on eDNA concentrations, which reinforces the need to control for such abiotic factors. We conclude that there is strong utility in applying eDNA concentrations as an index of relative abundance for mulloway under controlled conditions, which justifies future field-based investigations.

Trends in abundance indices derived from commercial fisheries for priority marine stocks in the Azores.

Large-scale assessments of fish and shellfish populations may be valuable for identifying the impacts of fishing pressure on stocks, but they are rarely possible due to a lack of long-term datasets. In this study, a two-part mixed-effect model for lognormal data was used to obtain historical abundance indices for 16 priority fish and shellfish stocks in the Azores region derived from catch per unit effort (kg per day at sea per vessel) and landing per unit effort (kg per landing per vessel). This data was obtained over the past 30 years under the EC Data Collection Framework and, in addition to information on effort and catch, included details about the fishing operation such as quarter, vessel length and fishing gear. This information was analysed to investigate how abundance indices changed over the years, verify if trends from different data sources were different and, if possible, relate these results to the population and fishery dynamics. The abundance indices derived from both datasets were generally in agreement with each other. Declining trends have been observed for some commercially important species, such as forkbeard Phycis phycis, European conger Conger conger, parrotfish Sparisoma cretense, red scorpionfish Scorpaena scrofa, offshore rockfish Pontinus kuhlii, common spiny lobster Palinurus elephas, splendid alfonsino Beryx splendens and alfonsino B. decadactylus, which is concerning because they are particularly slow-growing, long-lived and have low natural mortality, making them susceptible to overfishing. The results of this study are very important and should help future stock assessment and management initiatives.

Relationship between fish length and otolith dimensions of Yellow striped goatfish, Upeneus vittatus (Forsskal, 1775) along the Indian coast

Otoliths are widely used to estimate the age and growth of fishes and to identify fish species. This information plays a vital role in fisheries management and population dynamics studies. The information about fish length and otolith dimensions relationship of Upeneus vittatus is lacking from the Indian water. In this study, a total of 237 numbers of individuals were collected from October 2018 to March 2019 and left otolith was studied since there were no significant differences between right and left otoliths. The relationships between total length and otolith dimensions were estimated by linear regression and found a strong positive correlation. The maximum correlation was found between fish length and otolith length for Mumbai populations (R² = 0.9342). In contrast, Kakinada populations had the maximum correlation between fish length and otolith width (R² = 0.8957), and Puri populations had the maximum correlation between fish length and otolith weight (R² = 0.9101). Present study found a strong positive correlation between fish length and otolith dimensions. This finding provides new insights into the field and can be used for the growth study, fish population assessment, and fisheries management.

Proposed Standard Weight Equation and Standard Length Categories for Lake Chubsucker

AbstractStandard weight equations have been proposed for a variety of fishes, including many nongame species. Here, we developed standard weight (Ws) equations for Lake Chubsucker Erimyzon succetta using the traditional regression‐line‐percentile technique and the newer empirical‐percentile method. Length and weight data for 19,727 Lake Chubsuckers from 8 to 42 cm TL and 129 populations were used in the final analysis. These fish were collected from 1977 to 2020 by various agency and university personnel from most known populations across the species’ range in North America. This species is considered vulnerable, imperiled, or extirpated in several Midwestern and northern states, but it appears to be secure in the Southeast. The resulting Ws equations were log10W = −5.0962 + 3.1125 × log10L and log10W = –5.9735 + 3.8828 × log10L −0.1701 × (log10L)2 for the RLP and quadratic empirical‐percentile method techniques, respectively, where W is weight (g) and L is total length (TL, mm). The regression‐line‐percentile equation␣exhibited a greater length‐related bias especially at the ends of the length spectrum, so we recommend the quadratic empirical‐percentile equation␣for this species. We also developed standard length categories for assessing fish population size structure. We propose minimum lengths for five standard length categories of 11, 18, 24, 28, and 35 cm TL for stock, quality, preferred, memorable, and trophy sizes, respectively. Development of a Ws equation␣and standard length categories will aid biologists in assessing condition and size structure of wild Lake Chubsucker populations and those being propagated for supplemental stocking into trophy bass ponds for native forage or in areas with depressed native populations. Additionally, by evaluating this nongame species that occupies the same environments as higher profile game fish species, a more holistic approach of evaluating the overall health of systems can be examined.

Does Type, Quantity, and Location of Habitat Matter for Fish Diversity in a Great Plains Riverscape?

Fisheries professionals frequently measure habitat type and amount, but less often measure the importance of where those habitats are located and in what combinations. We address this challenge by testing whether the individual and combined type, quantity, and location of habitat affects fish diversity in the upper Neosho River basin, Kansas, as a different approach to measuring habitat heterogeneity. Habitat type mattered in that species richness increased in areas of higher riffle density. Furthermore, variation within habitat type also influenced fish diversity; specifically, slower, shallower riffles had more species of fish. The spatial arrangement (i.e., impact of neighbor habitats) influenced fish diversity patterns in that riffle–run and riffle–glide pairings altered riffle habitat characteristics. The study illustrates a useful approach by measuring the type, amount, and arrangement of habitats to assess fish populations and could be adapted to other stream ecosystems.

A comparison of passive and active gear in fish community assessments in summer versus winter

Fish populations and communities are monitored using a variety of sampling gears, each with their own inherent biases. Gear biases can arise from a number of factors, such as fish species characteristics (e.g., body shape/size, physiology, and behaviour), species habitat requirements, as well as the abiotic characteristics of sites sampled. Such factors and their effects on gear selectivity are also heavily influenced by seasonality. Consequently, understanding the effects of seasonal changes on gear selectivity is of vital importance, especially during the winter—a season seldom studied in freshwater systems. Here, we compared the selectivity, efficiency, and degree of biodiversity in fish communities sampled using three gear types: minnow traps, Windermere traps, and electrofishing during summer and winter in Hamilton Harbour, ON, Canada. Catch per unit effort was similar among gear types in the summer, whereas in the winter, minnow traps captured the most fish. Electrofishing samples were the most species rich and species diverse, but only during the summer. Additionally, sampling efficiency and the number of different species encountered was highest when all gear types were used in combination, followed by electrofishing alone, Windermere traps alone, and minnow traps alone in both seasons. Each gear type differed in its selectivity for certain species, which was further influenced by seasonality. This resulted in the fish communities caught within each gear type being dissimilar from one another. Our study highlights the importance of understanding gear type selectivity, particularly under different climatic conditions, and outlines the importance of incorporating multiple gear types in ecological assessments of fish populations and communities.

Fishing and habitat condition differentially affect size spectra slopes of coral reef fishes.

Marine food webs are structured through a combination of top-down and bottom-up processes. In coral reef ecosystems, fish size is related to life-history characteristics and size-based indicators can represent the distribution and flow of energy through the food web. Thus, size spectra can be a useful tool for investigating the impacts of both fishing and habitat condition on the health and productivity of coral reef fisheries. In addition, coral reef fisheries are often data-limited and size spectra analysis can be a relatively cost-effective and simple method for assessing fish populations. Abundance size spectra are widely used and quantify the relationship between organism size and relative abundance. Previous studies that have investigated the impacts of fishing and habitat condition together on the size distribution of coral reef fishes, however, have aggregated all fishes regardless of taxonomic identity. This leads to a poor understanding of how fishes with different feeding strategies, body size-abundance relationships, or catchability might be influenced by top-down and bottom-up drivers. To address this gap, we quantified size spectra slopes of carnivorous and herbivorous coral reef fishes across three regions of Indonesia representing a gradient in fishing pressure and habitat conditions. We show that fishing pressure was the dominant driver of size spectra slopes such that they became steeper as fishing pressure increased, which was due to the removal of large-bodied fishes. When considering fish functional groups separately, however, carnivore size spectra slopes were more heavily impacted by fishing than herbivores. Also, structural complexity, which can mediate predator-prey interactions and provisioning of resources, was a relatively important driver of herbivore size spectra slopes such that slopes were shallower in more complex habitats. Our results show that size spectra slopes can be used as indicators of fishing pressure on coral reef fishes, but aggregating fish regardless of trophic identity or functional role overlooks differential impacts of fishing pressure and habitat condition on carnivore and herbivore size distributions.

Zero, one or more broods: reproductive plasticity in response to temperature, food, and body size in the live-bearing rosy rockfish Sebastes rosaceus

Patterns of reproduction, such as size-fecundity relationships used in models to assess fish populations, are generally treated as static through time and invariant to environmental change. However, growing evidence suggests that changes in ocean conditions, such as warming water temperatures and reduced primary productivity, affect life history traits, including reproduction. Under controlled experimental conditions, we documented reproductive plasticity in the live-bearing rosy rockfish Sebastes rosaceus in response to different temperature and feeding regimes, with maternal size as a covariate. Rosy rockfish occur throughout the California Current, a highly dynamic ecosystem for which increased environmental variability is predicted with climate change. Females produced 0-5 larval broods annually. Larger females had disproportionately higher fecundity than smaller females by producing larger-sized broods and a greater number of annual broods. Warmer water temperature decreased the time interval between brood releases, likely reflecting faster egg and larval development. However, warmer temperature did not increase the total number of broods, potentially reflecting a tradeoff with increased metabolic demand. Well-fed females had better body condition and higher annual fecundity compared to poorly fed females, primarily due to a greater number of broods. Conversely, females with poor body condition at the start of the reproductive season did not reproduce, providing possible evidence of delayed maturation at smaller sizes or skipped spawning at larger sizes. Reproductive plasticity (in terms of whether and how many broods are produced per year) in response to the environment likely contributes to high inter-annual variation in population larval production. Understanding the causes and consequences of reproductive plasticity is critical to developing sustainable management strategies and predicting population response to changing climate conditions.

Hydroacoustic survey standardization: Inter-vessel differences in fish densities and potential effects of vessel avoidance

Hydroacoustics is used broadly to assess fish populations in marine and freshwater systems. Large-scale surveys often employ multiple vessels to complete a survey. Vessels can be a source of variation in multi-vessel surveys, and accounting for this variation is critical to precise and accurate assessments, whether as indices or measures of absolute abundance. We examined areal and volumetric density estimates, area backscattering coefficient (ABC), and mean target strength (TS) produced by three vessels in Lake Erie along three cross-lake transects in July 2014. Our study showed positive correlation in areal ABC, mean TS, and density among vessels; however, there was an almost 30 % higher density obtained with the largest vessel, on average. As a result, vessel choice could contribute to variation and/or bias in forage fish surveys deploying multi-vessels conducted on Lake Erie. Additionally, in some situations, differential depth patterns of fish contributed to vessel related differences in survey results suggesting a difference in vessel avoidance behaviors among targeted forage species, primarily emerald shiner Notropis atherinoides and rainbow smelt Osmerus mordax. This work demonstrates the potential magnitude of differences among vessels and highlights the need to identify and account for or otherwise control for these effects when monitoring or managing fisheries through standardized hydroacoustic assessments surveys.

Environmental DNA (eDNA) as a tool for assessing fish biomass: A review of approaches and future considerations for resource surveys

AbstractEnvironmental DNA (eDNA) has revolutionized our ability to identify the presence and distributions of terrestrial and aquatic organisms. Recent evidence suggests the concentration of eDNA could also provide a rapid, cost‐effective indicator of abundance and/or biomass for fisheries stock assessments. Globally, fisheries resources are under immense pressure, and their sustainable harvest requires accurate information on the sizes of fished stocks. However, in many cases the required information remains elusive because of a reliance on imprecise or costly fishery‐dependent and independent data. Here, we review the literature describing relationships between eDNA concentrations and fish abundance and/or biomass, as well as key influencing factors, as a precursor to determining the broader utility of eDNA for monitoring fish populations. We reviewed 63 studies published between 2012 and 2020 and found 90% identified positive relationships between eDNA concentrations and the abundance and/or biomass of focal species. Key influencing biotic factors included the taxon examined as well as their body size, distribution, reproduction, and migration. Key abiotic factors mostly comprised hydrological processes affecting the dispersal and persistence of eDNA, especially water flow and temperature, although eDNA collection methods were also influential. The cumulative influence of these different factors likely explains the substantial variability observed in eDNA concentrations, both within and among studies. Nevertheless, there is considerable evidence to support using eDNA as an ancillary tool for assessing fish population abundance and/or biomass across discrete spatio‐temporal scales, following preliminary investigations to determine species‐ and context‐specific factors influencing the eDNA abundance/biomass relationship. Advantages of eDNA monitoring relative to other approaches include reduced costs, increased efficiencies, and nonlethal sampling.

High-Resolution Acoustic Cameras Provide Direct and Efficient Assessments of Large Demersal Fish Populations in Extremely Turbid Waters

Monitoring fish species populations in very turbid environments is challenging. Acoustic cameras allow work in very poor visibility but are often deployed as a fixed observation point, limiting the scope of the survey. A BlueView P900-130 acoustic camera was deployed in rocky marine habitats off the coast of French Guiana in order to assess the total abundance, size structure and spatial distribution of a demersal fish population. The relevancy of using an acoustic camera to achieve these three objectives was evaluated by comparing acoustic data to those obtained from fishing surveys. The detection and identification of large demersal fish species were possible with the shape and size of the acoustic signal and acoustic shadow silhouette as well as swimming behavior. Mobile surveys combined with stationary surveys increased the probability of distinguishing individuals from inanimate objects. Estimated total length based on the acoustic signal underestimated the actual length of fish measured on deck, but the data showed the same trends in spatial and temporal variation. Acoustic cameras overcame the extreme lack of visibility by increasing knowledge of fish use of habitat, therefore providing much more efficiency in the effort, more accurate data on the abundance, size structure and spatial distribution than the fishing method. Thus, despite few limitations, acoustic camera surveys are far superior to fishing surveys in evaluating large demersal fish stock status.

Detection of protection benefits for predatory fishes depends on census methodology

Abstract Marine protected areas (MPAs) are used as fisheries management and conservation tools. Well‐enforced no‐take zones allow the rebuilding of natural populations of exploited species; however, there is still controversy on the role of buffer zones. The effectiveness of MPAs could be underestimated, as fish population assessments depend largely on traditional methodologies that have difficulties in detecting predatory fish because of their low abundances, their patchy distribution, and their reaction to the presence of divers. The performance of different census methods was compared in assessing the protection benefits for large predatory fishes under different protection levels (i.e. no‐take and buffer zones) in five Mediterranean MPAs. Specifically, conventional strip transects (CSTs, 50 × 5 m2) and tracked roaming transects combined with distance sampling (TRT + DS, variable lengths) were compared, including a series of TRT‐derived estimators with variable transect lengths and fixed widths of 20, 10, and 6 m (TRT20, TRT10, and TRT6, respectively). Additionally, the effectiveness of the MPAs studied and protection levels for conserving large predatory species was evaluated. Transects covering larger areas (i.e. TRT + DS and TRT20) allowed the detection of a greater number of species and yielded more accurate estimates of density and biomass than transects of narrower fixed widths, particularly the CSTs, which were associated with the lowest richness detection capability, accuracy, and precision. On average, both no‐take zones and buffer zones appeared effective for the conservation of predatory fishes, indicating that multiple protection areas were ecologically effective. Differences between MPAs were also observed, however, probably arising from both local environmental and management factors. We suggest the implementation of methodologies with larger transects for the study of large predatory fish, combined with CSTs for the rest of the fish community, in order to avoid biases in predatory population assessments, which are key indicators of MPA effectiveness.