Fish Density Estimates Research Articles

Effects of episodic stream dewatering on brook trout spatial population structure

Abstract Stream dewatering is expected to become more prevalent due to climate change, and we explored the potential consequences for brook trout (Salvelinus fontinalis) within a temperate forest ecosystem in eastern North America. We estimated fish density within stream pools (n = 386) from electrofishing surveys over 10 years (2012–2021) to compare a stream that exhibits episodic dewatering (Paine Run) against a stream of similar size that remains flow‐connected (Staunton River) within Shenandoah National Park, Virginia (U.S.A.). Annual surveys encompassed fluvial distances ranging from 2.6 to 4.4 km in each stream. Mean annual fish density (fish/pool m2) was not different between streams for juvenile or adult age classes, but spatial variation in density was greater in Paine Run for both age classes of fish. Paine Run also included a greater proportion of unoccupied pools than Staunton River and exhibited stronger spatial autocorrelation in fish density among nearby pools, suggesting dispersal limitation due to surface flow fragmentation. Fish density in pools increased during years with low summer precipitation, and this effect was observed in both streams but was stronger in Paine Run than Staunton River, further indicating the importance of fish movement into pools in response to low‐flow thresholds. Our results indicate the importance of pools as ecological refuges during low‐flow conditions and that episodic dewatering may affect extirpation risks for brook trout by sequestering more fish into fewer areas. Our findings also highlight the importance of hydrological variation within stream networks because downstream river gages could not predict the observed spatial heterogeneity in fish density or pool occupancy.

Geospatial analysis for prediction of potential fishing zones in the Banggai Waters using hydroacoustic data and satellite remote sensing

Banggai waters are a strategic area rich in fish resources and a source of livelihood for most of the local population. Mapping fish stocks is important to ensure the sustainability of fishing in the area. One of them uses hydroacoustic data and satellite imagery to provide more comprehensive results on fish stocks and more accurate predictions of potential fishing zones. This study will spatially estimate fish density in Banggai waters using hydroacoustic data and investigate the distribution of chlorophyll-a and sea surface temperature using Himawari-8 imagery. Hydroacoustic data processing will produce target strength (TS) and volume backscatter strength (Sv) values to determine the value and distribution of fish density. Meanwhile, ZPPI results are obtained from the overlay of chlorophyll-a and sea surface temperature. From the results of mapping the potential fishing zone, the high category has the highest percentage of 43%, low 28% and medium 29%. The ZPPI point distribution map then shows the concentration of ZPPI points in the southern Banggai waters. The fish density distribution map shows that the spatial distribution tends to be around the coast of Banggai Peninsula, dominated by small and very small fish categories. Correlation of fish density point distribution with ZPPI results and oceanographic parameters has a positive influence on fish density from GAM modelling.

Combining hydro-acoustics and hydraulic modeling for evaluating fish entrainment risk

Individual fish may be involuntarily removed from hydropower reservoir populations due to dam operations, a phenomenon known as fish entrainment. Entrainment for two life stages (age-0 and age-1) of Kokanee salmon (Oncorhynchus nerka) was evaluated within the forebay of the Revelstoke Dam in the upper Columbia River, British Columbia, Canada. Intensive fish surveys using hydro-acoustics were conducted to estimate fish density in the forebay at two different times (day & night) and across three seasons (spring, summer and winter). The number of fish entrained into intakes was monitored using hydro-acoustics continuously over one year. Surveyed fish densities and entrainment numbers were combined with Computational Fluid Dynamic modeling of the velocity field around the penstock intakes to estimate the threshold velocity for entrainment where 50% of the fish within this volume of water would be entrained. Fish density decreased with increasing distance from the dam face and it also decreased with increasing water depths. Entrainment of age-0 kokanee was consistently higher compared to age-1 in all times and seasons evaluated. The threshold velocities resulting in entrainment of age-0 kokanee were consistently lower (range 0.24 to 0.59 m/s) compared to those estimated for age-1 kokanee (range 0.30 to 1.08 m/s). Additional modeled scenarios showed that fish entrainment risk increased both with increasing intake discharge and the number of intakes in operation. Results demonstrate combing computational fluid dynamics models with ecological information collected using hydro-acoustics allows age-specific entrainment risk to be evaluated for vulnerable species and warrants application to other species and hydropower facilities.

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.

MPA-FishMApp – A Citizen Science App That Simplifies Monitoring of Coral Reef Fish Density and Biomass in Marine Protected Areas

Monitoring changes in fish density and biomass inside marine protected areas (MPAs) through fish visual census (FVC) can determine if MPAs are achieving their goal of promoting fish population recovery. Simplified FVC methods have been developed for citizen scientists to enable them to monitor fish populations in MPAs. However, MPA monitoring programs led by local stakeholders remain rare and difficult to sustain due to technical barriers related to FVC data management. Here, we describe and evaluate a novel online app called MPA-FishMApp, which we developed to help stakeholders of MPAs that protect coral reefs in the Philippines efficiently store, analyze, and visualize FVC data. MPA-FishMApp is coupled to a simplified FVC method wherein the observer records only 21 reef fish species groups during surveys. The app provides a simple data entry interface, cloud storage, and algorithms to estimate fish density and biomass. Spatial and temporal trends in fish density and biomass can be instantaneously visualized in the app based on relative importance to fisheries. Field testing suggested that the MPA-FishMApp methodology (simplified FVC and app) is sensitive enough to detect qualitative patterns showing differences in density and biomass that may develop between MPAs and fished sites, especially in fishes that are highly important to fisheries. However, users must have sufficient training and experience in simplified FVC to produce reliable data. MPA-FishMApp may help reverse the lack of monitoring in MPAs across the Philippines and offers an accessible, transparent, and auditable venue for collaboration between citizen scientists and professional scientists.

Relationship of Acoustic Volume Backscattering of Fish Aggregations and Density of Pelagic Fish in The Waters of Pulau Baai Bengkulu City

The waters of Pulau Baai are unique waters that are closed or protected which have small pelagic fish resources, used as pelagic fishing areas for local fishermen so that they can help the community's economy. One method that can be used to describe the distribution of pelagic fish density is the use of hydroacoustic technology. This study aims to analyze the value of acoustic backscattering (volume backscattering strength, SV) of pelagic fish and associated with the estimation of the density of pelagic fish in the waters of Pulau Baai, Bengkulu City, through in-situ acoustic measurements. The results showed that in the waters of Pulau Baai the highest average SV value was -50.47 dB, which was thought to have a fish density of 2.57 ind/m3 while for a low average SV with a value of -57.48 dB, it was assumed to have a density of 0.04 ind/m3. The density of pelagic fish is acoustically estimated at 26.47 ind/m3 in the water column of Pulau Baai. The SV value affects the estimated fish density by 69% and is categorized as having a high correlation and a positive correlation direction.

A method for estimating fish density through the catches of gillnets

AbstractWe propose a new method for estimating fish density using catches obtained by gillnets of given mesh size. This method builds on work for estimating the number of fish approaching a gillnet by modelling the fish movement that leads to a gillnet encounter. The theoretical framework is developed, and the method is tested on catch data for cod Gadus morhua and Dolly Varden Salvelinus malma to demonstrate how the method can be applied. The method relies on knowledge of geometric properties of the net, and morphological and behavioural properties of the fish, so we explore the sensitivity of the estimates to some of these parameters. Estimates are most sensitive to the morphological parameter of the angle the mouth is open while swimming and largely invariant to estimates of uncertain parameters like maximum number of fish that can be caught by the particular net.

Comparison of Two Types of Survey Designs for Acoustic Estimates of Fish Resources in the Three Gorges Reservoir, China

Pre-planning for a rational survey design is essential to improve the reliability and efficiency of the acoustic assessment of fishery resources in large water bodies. In this study, we compared the differences in acoustic estimates of fish resources between triangular and parallel transect designs in the Three Gorges Reservoir (TGR, China), aiming to provide a reference for fishery acoustic survey planning in such canyon-shaped reservoirs. We conducted hydroacoustic surveys using an echosounder (SIMRAD EY60, 120 kHz) in the Yangtze mainstream near the dam and an adjacent tributary with triangular and parallel designs. The independent samples t-test showed that the acoustic estimates of fish density were not significantly different between these two types of survey designs for both mainstream and tributary. The Fisher’s exact test indicated that there was no significant difference in fish size distributions, as measured by target strength (TS), between survey designs either. In view of reducing time spent in nearshore areas to improve efficiency and ensure safety, we recommend that the triangular design with reliable coverage be given priority for hydroacoustic sampling in the TGR and similar canyon-shaped reservoirs, especially when sampling at night.

Using the hydroacoustic and mini trawl data for estimating fish density in the eastern part of Banyuasin coastal waters, South Sumatra of Indonesia

The hydroacoustic and mini trawl sampling was conducted to estimate a distribution pattern of fish density in the eastern part of Banyuasin coastal waters, South Sumatra, Indonesia. These surveys were carried out in September 2017 using a SIMRAD EK15 single‐beam echo sounder with a 200 kHz operating frequency. Catch data obtained from the mini bottom trawl were used to examine a dominant species for refining the TS value concerning the fish density estimation along the hydroacoustic track. The ponyfishes (Eubleekeria jonesi) from the Leiognathidae family were found at each station with the highest relative abundance (50.98%). Hence this species was used to refine target strength values for estimating the acoustic volume density. These volume densities of 206 ESDU ranged from 0 to 9048 fish 1000m–3, with an average of 930 fish 1000m–3. The results also described a distribution pattern of fish densities hence this information could be valuable to the fishery manager for improving sustainable management approaches.

Combined video–hydroacoustic survey of nearshore semi-pelagic rockfish in untrawlable habitats

Abstract New survey technologies are needed to survey untrawlable habitats in a cost-effective and nonlethal manner with minimal impacts on habitat and nontarget species. Here, we test the efficacy of integrating data from a suspended underwater camera with acoustic data to generate population estimates for nearshore Black (Sebastes melanops), Blue (Sebastes mystinus), and Deacon Rockfish (Sebastes diaconus). We surveyed Seal Rock Reef near Newport, Oregon, and compared our results to population estimates derived from a mark–recapture study conducted at the same reef. We compared fish density estimates from video deployments to those calculated from applying published target strength to length regression models to our acoustics data. Densities derived from the acoustics, using a generalized physoclist target strength to length model, were significantly different from densities derived from video; conversely, a rockfish-specific target strength to length model generated densities that were not statistically different from video densities. To assess whether, and how, fish behaviour was influenced by the presence of an underwater camera, we deployed our camera under the acoustic transducer. No statistical difference was observed in the acoustic density of fish before, during, or after camera deployment. Our work suggests that combining acoustic and stereo video data provided a similar population estimate to historic survey results, but an accurate acoustic density estimate was dependent on using the proper acoustic target–strength model. We contend that combining camera data with hydroacoustic data is effective for surveying rockfish in untrawlable habitats.

Spatial and vertical bias in down-looking ship-based acoustic estimates of fish density in Lake Superior: Lessons learned from multi-directional acoustics

Hydroacoustic surveys using hull-mounted down-looking transducers are useful for estimating pelagic fish densities; however, this method may miss shallow fish owing to the acoustic surface dead zone and vessel avoidance. Our objective was to compare pelagic fish density estimates acquired by a traditional down-looking acoustic survey to estimates obtained by a new multi-directional-towed sled capable of sampling the entire water column using upward-, sideways-, and downward-aimed transducers simultaneously. We deployed both systems concurrently in the western arm of Lake Superior during a period of stable stratification. We found the two survey approaches provided significantly different estimates of fish density in the upper water column layer (~4–9 m below the lake surface) with the sled up-looking transducer providing 56 times higher densities compared to the traditional ship down-looking method. Densities also varied significantly in the 9–14 m layer where densities were 6.2 times higher in the sled survey. Midwater trawl sampling indicated that cisco (Coregonus artedi) and rainbow smelt (Osmerus mordax) were the predominant species occupying the uppermost 14 m of the water column. The two acoustic approaches provided similar results at water column depths >14 m where rainbow smelt and kiyi (Coregonus kiyi) were predominant. Overall, the sled-based method estimates were, on average, 2.5 times higher for the whole water column. Our findings show that the new sled can reduce bias by better sampling the surface dead zone leading to more accurate estimation of pelagic fish densities for both management and research.

Towards more robust hydroacoustic estimates of fish abundance in the presence of pelagic macroinvertebrates

The inclusion of unwanted targets in hydroacoustic surveys biases estimates of fish abundance. Thus, knowledge of frequency-dependent responses of unwanted targets (e.g., pelagic macroinvertebrates) can help ensure that transducer frequencies are used that minimize this bias. We determined how fish density estimates varied across multiple frequencies when the larval stage of a midge, Chaoborus, was present in the water column. We hypothesized that fish density estimates would increase with increasing transducer frequency, owing to greater backscattering by Chaoborus at higher frequencies than lower ones, which allows it to be included with the backscattering caused by fish. We found that fish density estimates were always greater at higher frequencies (e.g., 120 and 200 kHz) compared to a lower one (70 kHz) in several productive north-temperate reservoirs. Furthermore, pairwise comparisons of total (i.e., fish plus Chaoborus) backscattering showed that significantly more backscattering occurred at higher rather than lower frequencies. We also found that fish density estimates varied between spring and summer, partially owing to inter-seasonal size variation in Chaoborus that influenced its backscattering. Beyond demonstrating why the presence of pelagic macroinvertebrates needs to be considered when estimating fish abundance with hydroacoustics, we provide methods to identify and reduce this bias.

Roving Divers Surveying Fish in Fixed Areas Capture Similar Patterns in Biogeography but Different Estimates of Density When Compared With Belt Transects

Fish abundance and diversity are core measurements taken by many nearshore marine monitoring projects. The most common approaches for counting fish include belt transects and timed counts by roving divers, each with its own limitations. Here we evaluate a fish counting method developed by the Channel Islands National Park’s Kelp Forest Monitoring Program (KFMP), in which roving observers make fish counts that are standardized both by the time taken (30 min) and the area sampled (2000 m2). This method is potentially very useful because it combines an advantage of simple timed counts – the ability to rapidly sample a large area – with the potential to calculate area-specific density of fish, not just their relative abundance. However, the method has not been comprehensively evaluated and it is uncertain whether fish can effectively be counted in such a large target area within the allotted time. Fortunately, many sites surveyed with this method are also sampled with a more standard fish counting approach of belt transects, both by the KFMP and by the Partnership for Interdisciplinary Studies of Coastal Oceans (PISCO). Here we compare estimates of fish density obtained through the area-standardized roving diver method and belt transect methods. In paired samples we find substantial and species-specific differences in densities estimated by each method. Considering all fish taxa together we find that roving divers are likely under-sampling the target area. Despite considerable species-level variation, the different methods produce similar estimates of average diversity and find similar regional and temporal patterns in fish abundance, demonstrating that they can successfully be used in parallel even if the datasets cannot be easily combined. These analyses can guide the interpretation of roving diver data for basic research and management decisions.

Fish density estimation using unbaited cameras: Accounting for environmental-dependent detectability

The fast development of camera technologies opens a breakthrough opportunity for animal ecology, particularly at the marine realm where observing wildlife is challenging. These outstanding technological advances are meeting with the impressive capabilities of artificial intelligence for enabling automatic extraction of relevant information from videos and images. Altogether, this may be a unique opportunity for a qualitative jump in marine wildlife assessment but substantial strengthening of the links between theorists, empiricists and engineers is still required. Specifically, a recent theory proposes that animal density can be estimated from (1) the counted animals per frame, (2) the area surveyed by the camera and (3) the probability of detecting an animal that is actually within the area surveyed by the camera. However, a potential drawback for applying this theory to the real world is that environmental dependencies of camera's detection probability may lead to biased estimates of animal density. Therefore, here we propose a sampling protocol and a statistical model of general application for estimating (and accounting for) the environmental factors affecting fish detectability when estimating fish density with cameras. The method implies one calibration sampling with cameras and with the preferred reference method at the same time and place. The relevance of this method is that, once calibrated, it can be used to obtain unbiased estimates of fish density at new sites and moments using only cameras. Thus, fish density could be estimated at the temporal and the spatial scale needed, but with substantially less cost-effort than any other reference methods (e.g., underwater visual censuses). As a proof of concept, we evaluated the dependence of camera's detection probability on habitat complexity (e.g., cavities, rocks, seagrass, etc.) as a proxy for the hiding capability of a small serranid. In that specific case, probability of detection seems to be independent of habitat complexity. However, the sampling protocol and the statistical model provided here open the opportunity to estimate fish density using underwater cameras at wider temporal and/or spatial scales, which will help to better understanding the ultimate drivers of marine fish population dynamics and further development of science-based management.

Tracking activity patterns of a multispecies community of gymnotiform weakly electric fish in their neotropical habitat without tagging.

Field studies on freely behaving animals commonly require tagging and often are focused on single species. Weakly electric fish generate a species- and individual-specific electric organ discharge (EOD) and therefore provide a unique opportunity for individual tracking without tagging. Here, we present and test tracking algorithms based on recordings with submerged electrode arrays. Harmonic structures extracted from power spectra provide fish identity. Localization of fish based on weighted averages of their EOD amplitudes is found to be more robust than fitting a dipole model. We apply these techniques to monitor a community of three species, Apteronotus rostratus, Eigenmannia humboldtii and Sternopygus dariensis, in their natural habitat in Darién, Panama. We found consistent upstream movements after sunset followed by downstream movements in the second half of the night. Extrapolations of these movements and estimates of fish density obtained from additional transect data suggest that some fish cover at least several hundreds of meters of the stream per night. Most fish, including E. humboldtii, were traversing the electrode array solitarily. From in situ measurements of the decay of the EOD amplitude with distance of individual animals, we estimated that fish can detect conspecifics at distances of up to 2 m. Our recordings also emphasize the complexity of natural electrosensory scenes resulting from the interactions of the EODs of different species. Electrode arrays thus provide an unprecedented window into the so-far hidden nocturnal activities of multispecies communities of weakly electric fish at an unmatched level of detail.

Gillnet catchability of Walleye (Sander vitreus): comparison of North American and provincial standards

Multi-mesh index gillnet surveys are widely employed for the assessment of fish populations, including Walleye (Sander vitreus), an important commercial and recreational species in North America. In this paper, we estimate gillnet catchability of Walleye for two standard sampling protocols: the North American (NA) and the Fall Walleye Index Netting (FWIN), allowing the conversion of Catch Per Unit Effort (CPUE) into absolute estimates of fish density or abundance. We combined mark-recapture studies from 14 lakes in Ontario and Quebec, 11 of which were concurrently sampled using NA and FWIN methods, totalling 53 recapture surveys from 1996 to 2012. To account for lake-specific influences on catchability, we analyzed the data with Generalized Linear Mixed Models (GLMMs) using lake as a random factor and a model-averaging approach to calculate mean estimates and variances. Catchability for Walleye ≥ 350mm total length was 1.04 ha/gang for the FWIN, meaning each fish caught in a FWIN net gang (1.8 x 61 m) represents roughly one fish per hectare. Catchability for one gang of NA net (1.8 x 24.8 m) was 0.52 ha/gang, which is 20% higher than FWIN catchability after accounting for net area. Two important sources of variation in catchability were: (i) lake-specific differences and (ii) seasonal changes within the NA protocol. The mechanisms underlying such variation are still not clear and will require additional studies covering a broader geographic range to reveal potential environment-catchability relationships. We provide recommendations for estimating Walleye density and confidence intervals, and for how to combine our catchability estimates with size-selectivity curves to predict density at any specific size interval.

Estimates of density of mesopelagic fish in the Southern Ocean derived from bulk acoustic data collected by ships of opportunity

The demand for information on mid-trophic level (MTL) organisms in open-ocean marine ecosystems has led to initiatives to collect acoustic data opportunistically in different regions around the world. Although, bulk acoustic data can provide information on the distribution patterns and dynamics of MTL organisms, it is necessary to convert acoustic-derived indices into biologically relevant quantities for parameterising and validating ecosystem and trophic models. A 7-year time series of acoustic data collected by ships of opportunity (SOOP) in the New Zealand sector of the Southern Ocean, information on species’ distribution derived from trawl samples collected in research voyages, and target strength (TS) estimates obtained using a resonance-scattering model and literature TS–length relationships, were used to obtain the first estimates of density of mesopelagic fish in this region. Estimates of mesopelagic fish density decreased from north to south reflecting changes in species composition and scattering properties across three latitudinal regions (Northern, Central and Southern). Density estimates ranged from 16.4–40.1 in the north to 4.4–13.4 g m−2 in the south. Catches revealed that the Northern region was dominated by Lampanyctodes hectoris and Protomyctophum sp. (Myctophidae) and Maurolicus australis (Sternoptychidae); the Central and Southern regions were dominated by the myctophids Electrona antarctica and Protomyctophum sp. and the Antarctic krill, Euphausia superba. Information on species composition was the main source of uncertainty in the density estimates, highlighting the need for more biological sampling. This study demonstrates that it is possible to integrate acoustic data collected opportunistically with auxiliary information from research voyages and literature to provide estimates of mesopelagic fish biomass in remote areas.

Effects of vessel size and trawling on estimates of pelagic fish backscatter in Lake Huron

Integrated acoustic and netting surveys are conducted in marine and freshwater environments to assess the status of fisheries resources and study fish population dynamics. Vessel avoidance by fish, due to noise generated by the survey vessel or from simultaneous trawl operations, can bias acoustic estimates of fish density. Unlike in marine systems, few studies have assessed vessel avoidance in freshwaters, including the Laurentian Great Lakes. Annual acoustic and netting surveys have been conducted in Parry Sound, a coastal embayment of Georgian Bay, Lake Huron (Ontario, Canada). Parry Sound supports one of the lake’s few rehabilitated native lake trout Salvelinus namaycush populations, as well as abundant pelagic prey fish. In 2014–2017, night-time fisheries acoustic surveys were conducted with a 120 kHz transducer while intermittently deploying a pelagic trawl from an 18.9 m long vessel, a typical size of vessel used for acoustic surveying on the Laurentian Great Lakes. In 2017, a simultaneous acoustic survey was conducted from a 7 m long vessel with a 120 kHz transducer. To assess potential bias associated with trawling, fish backscatter was compared before, during, and after trawl operation. Furthermore, to assess potential bias associated with vessel size, fish backscatter was compared between the large and small vessel surveys. Fish backscatter was significantly lower and deeper during and after trawl deployment compared to immediately before, suggesting fish avoided the trawling vessel. In contrast, there was no significant difference in fish backscatter between the large and small vessel, although mean backscatter depth was slightly deeper from the large vessel. These results show bias from avoidance is possible in the fish density estimates obtained from hydroacoustic and trawling surveys in large freshwater ecosystems.

Estimating habitat-specific abundance and behavior of several groundfishes using stationary stereo still cameras in the southern California Bight

The increasing use of underwater cameras to estimate fish abundance often does not account for the behavior of target species. These behaviors can affect detectability of fish and bias density estimates. This study estimated abundance and behavior of several rockfishes (Sebastes spp.) and lingcod (Ophiodon elongatus) at Footprint Bank, a small offshore bank using images from randomly deployed stationary cameras. Deployments collected images at 30-second intervals over ∼24 h periods to examine the behaviors of rockfish that might impact abundance estimates. The results showed that time of day and tidal change had a significant effect on the probability of presence, estimated abundance and species composition of fish, with densities highest for most species during daylight hours. The time elapsed since camera deployment did not have a significant effect on fish density. Fish density was significantly affected by habitat composition, an effect primarily driven by speckled rockfish (Sebastes ovalis) which exhibited a 5-fold increase in abundance in bedrock habitats. Speckled rockfish were the most abundant rockfish at depths less than 150 m, with an estimated abundance of 12,994 fish (SE = 6,722) on Footprint Bank. The abundance estimates and coefficients of variation were comparable to surveys conducted in 2011 and 2012 using remotely operated vehicles (ROVs) and manned submersibles. The implications of this study are that habitat and behavior as well as timing of the survey (day/night) are important considerations determining the perceived density of fishes from underwater image surveys.

Release of critically endangered crocodiles: Development and application of a food web approach to determine suitability of release habitat

Abstract The Siamese Crocodile (Crocodylus siamensis) is a critically endangered medium‐size crocodilian endemic to Southeast Asia. Extirpated from much of its natural range, conservation efforts in the Cardamom Mountains of Cambodia include the release of captive‐reared juveniles and sub‐adults into river reaches known to support adult C. siamensis populations. Despite conservation concerns, the biology of wild C. siamensis is not well studied and the ecology of ecosystems at release locations is poorly understood. Fish are thought to comprise a major component of the diet of C. siamensis. Here, the aim was to characterize fish communities within three potential C. siamensis release locations, focusing on community composition, density, size class structure and food web dynamics. The survey sites varied in both C. siamensis density and human fishing pressure, and the results are interpreted in light of these drivers. Genomic interrogation of fishes of the Cardamom Mountains distinguished 13 distinct fish species, contributing to genetic databases and adding to the documented taxon list for the region. The presence of two previously unconfirmed fish genera in the region is confirmed. The first estimates of fish density, biomass and size class distribution for three rivers in the Cardamom Mountains are provided. The three potential C. siamensis release reaches that were sampled showed clear differences in fish community composition, structural and trophic dynamics. Fish density and biomass were highest in the high‐density C. siamensis survey reach and lowest in the high human fishing pressure reach. Survey reaches with food webs that were more reliant on autochonously driven food webs supported higher densities and biomass of fish. These results have important implications for future C. siamensis conservation efforts in Cambodia and contribute valuable ecological information on a relatively unexplored region.