Mean Target Strength Research Articles

Sustainable utilization of mixed healthcare waste as aggregate in lightweight concrete mixes: mechanical behavior, water absorption and leaching studies

Wastes generated from healthcare practices are a serious problem for humans as well as for the environment. In this study, an attempt was made to use mixed solid medical waste (WPN) which consisted of shredded waste plastic and chopped waste needles for partial replacement of natural fine aggregate (sand) in concrete mixes at 0%, 2%, 4%, 6%, 8% and 10% by weight. To examine the validity of the suggested approach, the fundamental concrete properties including workability, fresh and hard densities, water absorption, compressive strength, and flexural strength as well as a leaching test were performed. The curing ages for the concrete mixes were considered to be 7, 14, and 28 days. Fifty-four cubes were molded for compressive strength test, and 72 prisms were cast for flexural strength, whereby 162 cubes were prepared for density tests as well as water absorption test. The results revealed that WPN-concrete mixes exhibited improvement in the workability and steadily decreased both the compressive and flexural strength values of the WPN-concrete mixes which were higher than the mean target strength of lightweight concrete grade M17. Leaching test results indicated the absence of the target components in the leachant. The outcomes of this experimental study demonstrated that reusing WPN as a sand-substitution aggregate in concrete is a good approach to reducing the adverse impact resulting from the improper management of healthcare and medical solid waste.

Effects of season and diel cycle on hydroacoustic estimates of density, Target Strength, and vertical distribution of fish in Yudong Reservoir, a plateau deep water reservoir in southwest China in a plateau deep

Hydroacoustics is a non-invasive fish stock assessment sampling technique that plays an important role in fishery science and management. However, non-standard hydroacoustic surveys could lead to biased results, and the factor of the sampling period (e.g., season and diel cycle) is extremely critical as it can greatly affect hydroacoustic results. Efforts to improve the accuracy and credibility of the hydroacoustic survey results are getting more and more attention. Thus, we conducted two diel hydroacoustic surveys in situ in summer and winter to detect whether there were diel and seasonal differences in density, Target Strength (TS) and vertical distribution of fish. The results indicated that nighttime had significantly higher fish mean density than daytime in summer and winter. No significant difference between summer and winter daytime, however, significant difference between summer and winter nighttime, but this bias could be accepted from the fisheries management perspective; The mean TS of the summer daytime was significantly higher than that of summer nighttime, winter daytime and winter nighttime, but there were no significant differences among summer nighttime, winter daytime, and winter nighttime, mainly due to mean TS may be overestimated from fish schooling behavior during summer daytime; The fish vertical distribution had significant seasonal correlations and was more dispersed in different water layers during the nighttime, proving that the assessment was better at nighttime than during the daytime. Consequently, the hydroacoustic surveys in Yudong Reservoir and other similar plateau deep water reservoirs should be performed at nighttime, which will yield relatively accurate density and TS, and dispersed vertical distribution of fish.

The time-area fishing closure impacts on fish stock; Qiantang River before and after a four-month fishing closure

Fishing closures, commonly used to manage fisheries’ catch, involve temporarily closing a body of water to particular fishing gears to control fishing effort and protect feeding and spawning areas. In recent years in Qiantang River of China, with the socio-economic development, protection of fish stock has become increasingly urgent. The year 2019 was the first year that Qiantang River was included in the unified fishing ban system for the south of Yangtze River basin. Here, fish captures and hydroacoustic surveys were carried out in the research area of Qiantang River in order to present comparative descriptions of the dominant fish species, the temporal changes of fish size, density, biomass, and distribution affected by the four-month fishing closure in 2019. The results showed that Pseudobrama simoni (Bleeker, 1864) was the most dominant species both before and after the closure by using the traditional capture method. The mean target strength (TS) of overall fish after closure was –50.28 ± 0.19 dB, which was lower than that before, resulting in a significantly shorter derived mean length (13.42 ± 0.74 cm). The mean fish density and calculated biomass after closure were both significantly higher than that before it. More than 50% of fish species were distributed in the water of 5–20 m depth after the closure, which likely occurred in water deeper than 20 m before. Meanwhile, fewer outliers were found in different depth categories after closure. It is concluded that the four-month closure in 2019 had a positive effect on fish size, density, and biomass, leading to protection of pelagic fishes and a more even distribution of fish.

In situ broadband acoustic measurements of age-0 walleye pollock and pointhead flounder in Funka Bay, Hokkaido, Japan

Measurements of the broadband acoustic backscattering from fish should improve acoustic discrimination between species. The pulse compression processing of broadband systems can be used to measure acoustic backscattering with high range resolution and improve signal-to-noise ratio. This may increase opportunities for in situ target strength (TS) measurements, the preferred method of collecting TS data. To evaluate the availability of TS spectra for acoustic discrimination, three Simrad EK80 wideband transceivers and split-beam transducers of 70, 120, and 200 kHz were used to collect in situ frequency responses of TS from age-0 juvenile walleye pollock and pointhead flounder, a swimbladderless flat-fish, distributed in and around Funka Bay, Hokkaido, Japan. The single echoes were extracted from backscattering data, and the TS spectra of the two species were obtained. However, processing of the broadband acoustic data is under discussion. To ensure the reliability of our data, the TS spectra of standard targets were also calculated. The measured TS spectra of the standard targets were close to the theoretical TS spectra. The individual TS spectra of two species included complex frequency response; however, the mean TS spectrum was relatively stable. The different characteristics of the frequency responses observed for the two species were in good agreement with reported observations. The results available for the acoustic discrimination were similar to the multi-frequency method, which uses two or more single frequencies. The raw TS spectra should aid in not only discriminating between species but also Q3 estimating the size of the fish.

Analysis of Concrete Block by Partial Replacement of Cement with Fly Ash

The objective of this research is to enhance the properties of concrete by using Fly Ash as a partial substitution of concrete. Tests carried out on cement are Initial and Final setting time and on aggregates, specific gravity and sieve analysis. The mean target strength with given compressive strength at 28 day and quality control level is calculated. Concrete blend proportions for the first tryout mix is calculated and casting of cube of 15mm size with curing for 28 days, and then test for strength using UTM/CTM machines. C-S-H gel as well as Calcium hydroxide as bi product when reacts with water. C-S-H gel has an ability to keep the ingredients together by making a proper bond whereas lime which is freely available can react with atmospheric moisture and cause efflorescence. In such cases if we use fly ash, it reacts with free lime and produces C-SH gel again and water as bi product. We have used Fly ash as a partial replacement of cement with variation of fly ash in percentage. In this study fly ash add in increment of 5%, 10%, 15%, 20%, 25%, 30% and 35% replacement of cement which has shown satisfactory results in the strength of the concrete. Fly Ash improved the workability of the concrete, decreases the bleeding, surface finish and increases the cohesiveness. Compressive strength is comparatively increased. The initial strength of concrete with fly ash has lower strength but acquires higher strength after 56 days; which shows that more the nos. of curing days more will be the strength. Thus, it can be used in areas of construction such as dams, pavements etc.

Analysis of Concrete Block by Partial Replacement of Cement with Fly Ash

The objective of this research is to enhance the properties of concrete by using Fly Ash as a partial substitution of concrete. Tests carried out on cement are Initial and Final setting time and on aggregates, specific gravity and sieve analysis. The mean target strength with given compressive strength at 28 day and quality control level is calculated. Concrete blend proportions for the first tryout mix is calculated and casting of cube of 15mm size with curing for 28 days, and then test for strength using UTM/CTM machines. C-S-H gel as well as Calcium hydroxide as bi product when reacts with water. C-S-H gel has an ability to keep the ingredients together by making a proper bond whereas lime which is freely available can react with atmospheric moisture and cause efflorescence. In such cases if we use fly ash, it reacts with free lime and produces C-SH gel again and water as bi product. We have used Fly ash as a partial replacement of cement with variation of fly ash in percentage. In this study fly ash add in increment of 5%, 10%, 15%, 20%, 25%, 30% and 35% replacement of cement which has shown satisfactory results in the strength of the concrete. Fly Ash improved the workability of the concrete, decreases the bleeding, surface finish and increases the cohesiveness. Compressive strength is comparatively increased. The initial strength of concrete with fly ash has lower strength but acquires higher strength after 56 days; which shows that more the nos. of curing days more will be the strength. Thus, it can be used in areas of construction such as dams, pavements etc.

First target strength measurement of Trachurus mediterraneus and Scomber colias in the Mediterranean Sea

Knowing the species-specific target strength (TS) allows converting volume backscattering strength to numerical abundance. Since the acoustic surveys conducted for biomass assessment in the Mediterranean Sea currently focus on the echoes of two or three target pelagic species, the TS of non-target species has seldom been investigated in this basin. This is the first study of the TS of two pelagic species – Mediterranean horse mackerel (Trachurus mediterraneus) and Atlantic chub mackerel (Scomber colias) – in the Mediterranean Sea. A pilot approach using tethered live fish but not involving hooks and anesthetic was tested in experiments using a split-beam scientific echosounder operating at 38, 120, and 200 kHz. The mean TS was estimated for 29 live fish. The relationship between TS and fish length was determined with a standard linear regression model; the b20 conversion parameter was obtained with the slope forced to 20. b20 was computed at all frequencies for both species. The key values at 38 kHz were −71.4 dB for T. mediterraneus and −71.6 dB for S. colias. Although these results differ from those obtained with in situ and ex situ experiments using Pacific chub mackerel and other species of the genus Trachurus, they have the potential to provide new reference values for T. mediterraneus and S. colias biomass assessment in the Mediterranean Sea. The proposed method removes some potential biases due to the unnatural behavior of anesthetized fish. Moreover, it provides an alternative to hooks, although the use of a piece of rope instead of the hook seems to increase the acoustic reflectivity of the tethering apparatus.

Sustainable concrete: Potency of sugarcane bagasse ash as a cementitious material in the construction industry

The aim of this research is to determine the mechanical and durability properties of sugarcane bagasse ash (SCBA) as a partial replacement to Ordinary Portland Cement (OPC) in concrete. The SCBA was partially replaced at the percentage of 5 %, 10 %, 15 % and 20 % by weight of cement for a mean target strength of 27 MPa. A slump range of 130–150 mm was maintained constant throughout the experiment procedures. A total of 120 cubes and 30 rectangular beams were tested and the results were compared with control concrete. To evaluate the behaviour of SCBA on concrete, different tests were conducted on the concrete specimens namely, compressive strength, flexural strength, water absorption, water penetration, carbonation and ultrasonic pulse velocity. The results demonstrated that at 120 days of curing, compressive strength increased by 2.6 % and 1.7 % for 5 % and 10 % replacement level. The water absorption increased by 255 %, 390 %, 438 % and 488 % for 5 %, 10 %, 15 % and 20 % replacement level. The other tests showed decreased flexural strength and ultrasonic pulse velocity and increased water penetration and carbonation depth. The study inferred that 10 % replacement of OPC by SCBA exhibited positive performances and can be considered a suitable cementitious material in the construction industry.

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.

Estimating individual fish school biomass using digital omnidirectional sonars, applied to mackerel and herring

Abstract Economic profitability and responsible fisheries are objectives of fishermen and fisheries managers. In purse seine fisheries, an accurate biomass estimate of the targeted school is crucial to accomplish this. For this study, omnidirectional fisheries sonar was used to estimate individual school biomass of Norwegian spring spawning herring (Clupea harengus) and Atlantic mackerel (Scomber scombrus). A sonar sampling design based on professional skipper’s experience provided detailed information on school dimensions and acoustic backscattering. Using calibrated digital sonar data, school volume and fish densities were obtained, and school biomass computed. A positive linear relation was found between the estimated sonar school biomass and purse seine catches for both species (r2 = 0.92; residual standard error, RSE = 4.7 t). Large variability in volume backscattering coefficient and uncertainty in side-aspect target strength (TS) are the main sources of discrepancy between the estimates and the catch. Using a 4 dB (39%) weaker mean TS for mean side-aspect TS than the normal mean dorsal aspect TS was needed for optimizing the 1:1 relationship between sonar biomass estimate and catch. Accurate estimation of single school biomass can reduce the catch of unexpectedly large schools, leading to improvements in economic efficiency and reduced release of dead or dying fish.

Corrigendum to: In situ target strength measurement of the black triggerfish Melichthys niger and the ocean triggerfish Canthidermis sufflamen

Triggerfish are widely distributed in tropical waters where they play an important ecological role. The black triggerfish Melichthys niger may be the dominant species around oceanic tropical islands, whereas pelagic triggerfish, such as the ocean triggerfish Canthidermis sufflamen, can assemble around fish aggregating devices (FADs) where they are a common bycatch of tuna fisheries. In this study we combined acoustic and optical recordings to provide the first in situ target strength (TS) measurement of black and ocean triggerfish. Data were collected in the Archipelago of Fernando de Noronha off north-east Brazil. The mean TS of a 27.8-cm-long black triggerfish at 70 and 200kHz was –39.3dB re 1m2 (CV=14.0%) and –38.9dB re 1m2 (CV=14.4%) respectively. The mean TS values of ocean triggerfish (with a size range of 39–44cm) at 70 and 200kHz were –36.0dB re 1m2 (CV=15.7%) and –33.3dB re 1m2 (CV=14.0%) respectively. This work opens up the field for acoustic biomass estimates. In addition, we have shown that TS values for ocean triggerfish are within the same range as those of small tunas. Therefore, acoustic data transmitted from FADs equipped with echosounders can introduce a bias in tuna acoustic biomass estimation and lead to increased rates of bycatch.

Исследование акустической силы цели японской скумбрии в северо- западной части Тихого океана in situ

Significant concentrations of Chub mackerel (Scomber japonicas) were found in 2014 in the area of the Southern Kuril Islands (Northwestern Pacific Ocean (NWPO)). Commercial fishing for Chub mackerel was resumed in the NWPO in 2016. In this connection, exploratory work and acoustic-trawl surveys are carried out to identify distribution and abundance of Chub mackerel in the NWPO. Acoustic abundance estimates of the stock size of Chub mackerel is important for fishing. However, the reflectivity of Chub mackerel is not well understood. Collections of in situ target strength (TS) signals and corresponding fish size measurements for Chub mackerel were obtained as a result of research in the NWPO (2015–2017). The calibrated scientific echo-sounder “EK‑60 Simrad” with two split-beam echo sounders of respectively 38 and 120 kHz was used to collect sonar data. Measurement of the TS of Chub mackerel was conducted at the trawl layer, which corresponds to the trawl opening per the trawling area. The echogram selection process and the filtering process of single targets of Chub mackerel are realize in the “SALTSE” software package (“TINRO”). The dependence of mean target strength (TS, dB) on mean fish length (L, cm) for was estimated as 20 log (L) — 72,8 dB at 38 kHz and as 20 log (L) — 73,6 dB at 120 kHz. The obtained values backscattering strength (Sv) of Chub mackerel in the NWPO are compared with the values Sv of other studies of the reflective properties of Scombridae.

Spatiotemporal Distribution and Species Composition of Fish Assemblages in the Transitional Zone of the Three Gorges Reservoir, China

The river–reservoir transitional zones play an important role in maintaining riverine fish biodiversity and abundance in highly regulated rivers. The Upper Yangtze River, which contains 286 fish species, is experiencing intense hydropower development. However, few studies have been conducted on the spatial distribution of fish in the river–reservoir transitional zones in the Upper Yangtze River, and it is therefore difficult to predict how this development will impact the region. In this study, a 200-kHz split-beam acoustic system in combination with biological samplings was used to investigate the composition and spatiotemporal distribution of the fish in the transition zone of the Three Gorges Reservoir (TGR) in 2017. The biological samplings comprised 7434 individuals from 61 species, 38 genera, 15 families, and 5 orders. The dominant fish species were Rhinogobio cylindricus, Coreius guichenoti, Hemiculter bleekeri, and Pelteobagrus vachelli. Acoustic detection identified the mean target strength (TS) to be −55.6 in May, −57.8 in August, and −55.8 dB in December. Accordingly, the average fish density was 4.96, 6.96, and 4.42 ind/1000 m3. Significant differences were found in the TS and fish density between flood season (August) and non-flood season (May and December). Spatially, fish preferred the meandering sections at 10–20 m deep. A size-dependent vertical distribution pattern was observed, with larger fish in the deeper layers and smaller fish in shallower waters. Lastly, our results suggest that meandering deep channels should be assigned high priority with respect to fish conservation in the Upper Yangtze River.

Influence of fish backbone model geometrical features on the numerical target strength of swimbladdered fish

Abstract The method of fundamental solutions has been applied to evaluate the influence of fish models geometrical features on the target strength (TS) directivity and TS frequency response of swimbladdered fish. Simplified models were considered for two fish species: gilt-head sea bream (Sparus aurata, Linnaeus 1758) and Atlantic salmon (Salmo salar, Linnaeus 1758), and different geometrical details of their morphology were studied, such as backbone presence, and its curvature or the inclusion of vertebrae modulation. Swimbladder shape and tilt, together with the inclusion of backbone (and its realistic curvature) for dorsal measurements were the most important features for proper estimation of mean TS. The estimation of mean TS is considered including the effect of fish tilt, the echosounder frequency, and the fish-to-transducer distance.

Verification of the Backscattering Strength Based on the Swimming Behavior of Antarctic Krill

This study provides an estimate of the SDWBA-modelled mean target strength for given sets of krill length and swimming orientation. The range of the dB identification window for krill length was between 28 mm and 67 mm and the distribution of swimming orientation N[11.0°, 4.0°] was estimated to be between 0.06–10.97 dB, while the range estimated using N[53.1°, 21.4°], i.e., the distribution of orientation obtained under the brightest illuminance level, was 5.21–9.82 dB. The range of the dB window calculated using N[45.2°, 23.0°], obtained under the second brightest illuminance level, was 3.83–11.76 dB, and the range calculated with N[48.1°, 23.0°], obtained under the third illuminance level, was 3.83–11.76 dB. The range of the dB window calculated by N[45.9°, 23.0°] was 3.41–11.75 dB, and the range of the dB window for krill length ranging from 28 mm to 67 mm was 4.05–11.34 dB. In all the swimming orientations, the smaller the krill size was the lower the averaged target strength value was; likewise, the larger the krill size was the higher the averaged target strength value was. The result indicated that swimming orientation impacted greatly on the range of frequency differences of Antarctic krill.

Distribution of Fish Target Strength in Malang Rapat Seawater of Bintan Island, Kepulauan Riau Province

Malang Rapat is an area located in east Bintan Island. As a part of coastal communities, fisheries were the primary sector for public revenue in Malang Rapat. A qualified method is needed to determine the abundance and distribution of fish were required. Hydroacoustic technology is one method to solve this problem. This research aimed was to determine the value of fish target strength and to determine the pattern of fish distribution behavior in Malang Rapat, Kepulauan Riau Province, on September 23 and 24, 2016, using scientific echosounder Simrad EK15. The result indicated that fish target strength in Malang Rapat was -60 dB to -42 dB. This value was useful to estimate the length of fish ranged from 3 cm to 31 cm. The highest mean target strength based on depth was -48 dB at 10 m in the daytime and -52 dB at 3 m in the nighttime. The abundance of fish was found at night, precisely 3 meters from the surface of the water. The highest frequency appearance target strength range from -60 dB to -58 dB with 3.94 to 4.95 cm estimated fish length.

In situ target strength measurement of the black triggerfish Melichthys niger and the ocean triggerfish Canthidermis sufflamen

Triggerfish are widely distributed in tropical waters where they play an important ecological role. The black triggerfish Melichthys niger may be the dominant species around oceanic tropical islands, whereas pelagic triggerfish, such as the ocean triggerfish Canthidermis sufflamen, can assemble around fish aggregating devices (FADs) where they are a common bycatch of tuna fisheries. In this study we combined acoustic and optical recordings to provide the first in situ target strength (TS) measurement of black and ocean triggerfish. Data were collected in the Archipelago of Fernando de Noronha off north-east Brazil. The mean TS of a 27.8-cm-long black triggerfish at 70 and 200kHz was –39.3dB re 1m2 (CV=14.0%) and –38.9dB re 1m2 (CV=14.4%) respectively. The mean TS values of ocean triggerfish (with a size range of 39–44cm) at 70 and 200kHz were –36.0dB re 1m2 (CV=15.7%) and –33.3dB re 1m2 (CV=14.0%) respectively. This work opens up the field for acoustic biomass estimates. In addition, we have shown that TS values for ocean triggerfish are within the same range as those of small tunas. Therefore, acoustic data transmitted from FADs equipped with echosounders can introduce a bias in tuna acoustic biomass estimation and lead to increased rates of bycatch.

Acoustic reflectivity of a harbor porpoise (Phocoena phocoena).

Acoustic backscatter measurements were conducted on a stationary harbor porpoise (Phocoena phocoena) under controlled conditions. The measurements were made with the porpoise in the broadside aspect using three different types of signals: (1) a 475 μs linear frequency-modulated (FM) pulse with a frequency range from 23 to 160 kHz; (2) a simulated bottlenose dolphin (Tursiops "truncates") click with a peak frequency of 120 kHz; and (3) a simulated killer whale (Orcinus orca) click with a peak frequency of 60 kHz. The measurement with the FM pulse indicated that the mean target strength at the broadside aspect decreased from -26 to -50 dB as the frequency increased from 23 to 120 kHz in a nearly linear fashion (on a logarithm plot). Target strength variation with frequency was similar to a previous backscatter measurement on a bottlenose dolphin over a comparable frequency range (23-80 kHz). The porpoise seems to be a stealth body with low backscatter properties. The target strength of the porpoise was also about 15-16 dB lower than that of the bottlenose dolphin. The difference in lung volume of the two species when expressed in dB was also approximately 15 dB.

Target strength of a common kelp species, Saccharina japonica, measured using a quantitative echosounder in an indoor seawater tank

Knowledge about kelp forests spatial distribution and biomass is necessary for ecosystem conservation and sustainable use of fishery resources in coastal waters. The acoustic method has been suggested useful for estimating kelp forest thickness and spatial distribution in a large region; and is expected to be useful for biomass estimation; however, echo intensity per unit of kelp [i.e., target strength (TS)], which is a necessary parameter for estimating biomass, has not been revealed. The objective of this study was to obtain TS of the most common and commercially important kelp species, Saccharina japonica. We sampled 12 specimens of various weights in the coastal waters off Hakodate, Hokkaido, Japan, and measured the length, wet and dry weights, target strength of the blade body individually after they were folded and fixed to the same shape (0.3 × 0.3 m) using a quantitative echosounder at 120 kHz frequency in an indoor seawater tank. The TS of each specimen varied when they swayed slightly owing to the derived water currents, and a positive correlation was confirmed between the TS and length, wet weight (p < 0.05). Mean TS (TSmean) of samples were from –58.23 to –41.6 dB re 1 m2. Moreover, a positive linear correlation was found between TSmean and the logarithm weight (TSWmean ＝ 15.3 logWkg − 43.4, r2 = 0.79), which indicates the possibility for estimating biomass of kelp forests using TSmean by acoustic method, and the confirmation and accuracy increasing is expected.

Bioacoustic application on fisheries management in an artificial reefs’ ecological reserve of Bohai Gulf China

Various techniques have been used in marine fishery resources assessment. Among those, acoustic techniques have high efficiency and wide adaptability, and are environment friendly. Therefore, acoustic techniques played a key/indispensable role in the field of ecosystem-based fishery management. Acoustic surveys were conducted during the wet (May) and dry (November) seasons in 2016 in the artificial reefs ecological reserve of Bohai Gulf and a nearby control region to understand the current situation of fishery resources and explore the effectiveness of underwater artificial constructions by a dual-frequency identification sonar (DIDSON) and a scientific split-beam echosounder (120 KHz, Simrad EY60). Fish densities in various seasons and regions showed significant differences and interaction, with a maximum value of 358020.34 ind/n.mile2 in the artificial reefs’ region and a minimum value of 71898.35 ind/n.mile2 in the control region during the wet season. Fish densities in defined parallel transects revealed a roughly southward migration trend of small-size individuals from wet season to dry season. Single echo detections in the artificial reefs region demonstrated a wider target strength (TS) range (− 60 ~ − 30 dB vs − 60 ~ − 51 dB) and higher mean TS values (− 55.87 dB vs − 58.31 dB) than the control region both in the wet and dry seasons, which indicated a more complicated and stable fish community structure in the artificial reefs area. The vertical distribution of single echo detections tracked by the echoview post-processing system showed an apparent downward gathering effect. A positive correlation between TS and water depth was discovered from Pearson correlation analysis in the artificial reefs region during the dry season.