Fish Abundance Research Articles

Assessing the combined effects of catchment land use and runoff on estuarine fish assemblages

Connectivity between land and sea through the movement of species, energy and nutrients means that land-based impacts can affect the structure and functioning of nearby coastal ecosystems. As the interface between land and sea, estuaries are often faced with increasing pressures from catchment modifications (e.g. the removal of terrestrial vegetation) which can alter the overall health of estuaries (e.g. the degradation of coastal water quality due to an increase in terrestrial runoff) and change the value of habitat for many marine species. These landscape-scale impacts can, however, be mediated through variations in the context and connectivity of seascapes (e.g. greater extent of vegetated ecosystems help to filter nutrients). We surveyed fish across five estuaries over four years using underwater videography and sought correlations between fish biodiversity and abundance with variables indexing catchment land use, seascape connectivity, and coastal water quality. Fish assemblages were more abundant and diverse at sites which had lower chlorophyll-a concentrations and were nearer to the estuary mouth, in conjunction with catchments containing higher percent of natural land and heterogenous seascapes. Sites with lower concentrations of chlorophyll-a supported indicator species from more diverse functional groups that had greater fisheries values, in comparison to indicator species from sites with high concentrations of chlorophyll-a. This research can help direct coastal management to better prioritise management in coastal ecosystems, thereby enhancing fish richness and abundance, functional group richness and fisheries value.

Study of Microplastic Abundance in Animal Fish (Stolephorus Sp) With Microplastic Content in Stunting Clown's Breast Milk

This study, conducted in Kragan District, investigated the prevalence of microplastics in Stolephorus sp. and their occurrence in the breast milk of stunted infants. Microplastics, emerging as a global oceanic pollutant, pose significant challenges due to their small size and potential impacts on marine and human health. The study focused on their possible transfer to infants via breast milk, raising concerns about associated health risks, including toxicity and infection. Stunting, a critical health issue affecting 14% of Rembang Regency's population, provided the backdrop for this research. The study included all Stolephorus Sp. Sold in Kragan Subdistrict and collected breast milk samples from 11 stunted infants. Employing a descriptive quantitative approach with a case study design, the researchers utilized total sampling based on predefined criteria. Analysis revealed microplastics in both Stolephorus Sp. and breast milk, predominantly as fibers and filaments in black and blue colors. Particle sizes varied widely, with Stolephorus sp. containing 1.00–2.13 particles per specimen and breast milk containing 0.20–3.00 per milliliter. The study identified a correlation between microplastic abundance in Stolephorus sp. and its presence in breast milk among stunted infants. However, no correlation was found between Stolephorus sp. Consumption frequency and microplastic levels in breast milk. These findings underscore the potential pathways of microplastic exposure to infants and highlight the need for further investigation into their health impacts, particularly among vulnerable populations. Future research should continue exploring these dynamics to understand better and mitigate the risks associated with microplastic contamination in food chains and human health.

Exploring potentials to enhance fish sauce industry in Sri Lanka: A review

Fish sauce, an amber-colored liquid derived from the fermentation of fish with sea salt, is a vital condiment in Southeast Asian and East Asian cuisines. Despite its culinary importance, fish sauce remains underutilized in Sri Lanka, where it is predominantly imported, resulting in high costs and substantial foreign exchange outflows. The technology for fish sauce production is relatively simple, presenting an opportunity for local production using abundant fish resources. Various types of fish, including marine, freshwater, and shellfish, are used globally in fish sauce production, and it could harness the nutritional benefits of fish, transforming underutilized species into a valuable condiment. The production process of fish sauce involves salting, fermenting, maturing, filtering, and blending, resulting in a product rich in hydrolyzed proteins and essential minerals. Quality standards are regulated to ensure safety and purity, with a focus on maintaining the distinctive fishy odor and taste characteristic of pure fish sauce. Establishing a local fish sauce industry in Sri Lanka promises economic growth and culinary enrichment. By leveraging local fish resources and employing cost-effective production methods, Sri Lanka can reduce imports of fish sauce, and can utilize fish in a sustainable manner.

Opportunity to Export Fish Directly From the Outermost Islands of Indonesia: Exploring Value Chain and Power Dynamics in Fisheries

The fishers in the outermost islands of Indonesia have an abundant fish catch but local markets have limited capacity to absorb large quantities of fish which limits the potential economic returns to the fishers. This research examines the current fishing value chain in the Sangihe and Talaud islands, which are considered the outermost locations in Indonesia. The study also investigates the roles and influence of each participant within the value chain and the potential for direct export of fishery goods from these islands, bypassing transit points. The contemporary fishing value chain, power dynamic, and fishery direct export issue were investigated through a series of interviews with relevant actors, field observations, and focus group discussions. The results show that the fishing value chain experiences congestion due to the absence of alternative channels for fishers to directly sell their catch, hence relying solely on intermediaries. It also found that fishers have a heightened vulnerability as they lack the capacity to negotiate seafood pricing. Sangihe and Talaud islands are favourable locations for the direct export of fishery goods to General Santos in the Philippines, which provides a potential viable solution for the expansion of the fisheries markets. Currently, the Sangihe local administration is taking steps to implement the direct export of fishing products. The General Santos Fish Port Complex authority and other fishing firms in General Santos have responded favourably to this initiative. These initiates suggest that the outermost regions of Indonesia should be capable of influencing its geographical proximity to export destination countries for fostering economic development within the region.

Kelimpahan dan Biomassa Ikan Karang di Kawasan Konservasi Perairan Gili Balu, Kabupaten Sumbawa, Nusa Tenggara Barat

The Gili Balu Marine Conservation Area (KKP), located in Sumbawa Regency, West Nusa Tenggara, is ecologically and economically significant due to its rich coral reef ecosystems. This research aims to measure the abundance and biomass of reef fish in the KKP Gili Balu, which can be used as a basis for evaluating the effectiveness of conservation efforts in maintaining marine biodiversity. The Underwater Visual Census (UVC) method was used, recording types of reef fish at ten observation points. The results showed that the highest abundance was found on Mandiki Island, with functional fish abundance reaching 2973 ind/ha, followed by Pasareang Barat Island (1903 ind/ha) and South Kalong (1600 ind/ha). The biomass of functional fish also reflects their important role in maintaining the balance of the coral reef ecosystem. This research contributes to the scientific understanding of marine biodiversity in the KKP Gili Balu and provides a basis for sustainable conservation management efforts. Additionally, the research recorded the presence of several coralivorous and economically important fish species, although they tend to be smaller compared to functional fish. Economically important fish in the KKP Gili Balu waters were found only at four observation locations: East Belang Island, North Belang, Kenawa, and Southwest Namo. The highest value was recorded at Kenawa Island, with 93 ind/ha. These fish tend to be rare in this area, possibly due to past fishing practices.

Seasonal dynamics and habitat preferences of juvenile fish assemblages in the Wanquan Estuary, South China: Implications for conservation and management

Tropical estuaries encompass diverse habitats, which are crucial for supporting juvenile fish by providing nurseries, refuges, and migration routes. In the present study, we investigated the seasonal and spatial dynamics of juvenile fish assemblages in the Wanquan Estuary, Hainan Island, China, examining variations during the wet and dry seasons across four types of habitats. Our research aimed to test the hypotheses: (1) the wet season supports a greater abundance and species richness of juvenile fish compared to the dry season, and (2) juvenile fish showed varying habitat preferences across seasons. We recorded a total of 3617 individuals belonging to 33 families, 50 genera, and 78 taxa. Our findings revealed significant seasonal fluctuations in the assemblage structure, with peak abundance during the wet season in August, primarily consisting of diadromous species. A notable shift towards marine species was observed in March. Spatially, juvenile fish exhibited significant habitat preferences, with a strong preference for sand substrates for most species. Contrary to common expectations, mangrove fringe habitats were not identified as the primary nursery grounds for juvenile fish. Instead, our study highlights the importance of habitat diversity and connectivity in enhancing nursery functions. Water temperature and salinity were the key drivers influencing the seasonal and temporal patterns of juvenile fish assemblages in the Wanquan River estuary The pronounced temporal dynamics of these assemblages, driven by seasonal environmental alterations affecting water variations and habitat connectivity, were more significant than spatial differences. The findings emphasize the need for focused connectivity of various habitats to support the conservation of diverse fish communities in tropical estuarine ecosystems.

Study on Spatio-temporal Distribution of Chlorophyll-a on Pelagic Catch Productivity in Muara Bendera, West Java, Indonesia

Graphical Abstract Highlight Research The trend of chlorophyll-a in the pelagic fish catchment area of the Muara Bendera has increased. Trend of chlorophyll-a values around the waters of the Citarum and Cisadane River Estuaries increased. Chlorophyll-a and pelagic CPUE have a negative relationship. The waters around the mouth of the Citarum River are mesotrophic - eutrophic waters Abstract Chlorophyll-a has been considered an indicator of pelagic fish abundance in waters. Although a high nutrient load causes eutrophication, leading to fish mortality, global-scale climate anomalies will also influence the oceanographic conditions of the seas. This study aimed to investigate spatial patterns and trends of chlorophyll-a, the relationship between chlorophyll-a and pelagic fish catch productivity, and the effect of ENSO and IOD on pelagic fish catch productivity. The chlorophyll-a data were obtained from the SeaWiFS and Terra-Aqua MODIS time-series datasets of ocean color satellites. The results indicated that Jakarta Bay had the highest chlorophyll-A concentration. The Chlorophyll-A concentrations declined as the distance between the estuary and the coast grew. From 1997 to 2021, the regional pattern of increasing chlorophyll-a concentrations in the Citarum and Cisdane estuaries was continuous. Since 2001, the concentration of chlorophyll-a in Jakarta Bay has declined, whereas it has begun to climb in the seas of the North Seribu Islands. The increasing chlorophyll-A trend in the Seribu Islands is attributed to rising nitrate levels induced by human-driven coral degradation. Between 1997 and 2021, the concentration of chlorophyll-a in the Muara Bendera fishing region grew by +0.013 mg/m3/year, or 1.43 percent each year. The correlation between chlorophyll-a and pelagic fish catch productivity was -0.13. ENSO does not affect the productivity of pelagic catches in this region. However, it was discovered that IOD reduced the productivity of low-category pelagic catches.

Local hydrography and interactions with benthic megafauna explain fish assemblage structure on the shallow inner shelf of Paraná, southern Brazil

An important objective of fish ecology is to understand pure and shared effects of abiotic, biotic, and spatial predictors of fish assemblages’ structure on continental shelves. This study aims to assess the structure of the bottom-associated fish assemblage on the shallow inner continental shelf of Paraná, southern Brazil, in response to physicochemical, biological, and spatial factors. We hypothesize that this structure is associated with the benthic megafauna, particularly with the distribution of the Brazilian seabob shrimp Xiphopenaeus kroyeri, an abundant fishing resource on the shallow coast of Paraná. We used uni and multivariate analyses to examine the distribution of abundance, richness, and composition of ichthyofauna in the region. Using variance partitioning analysis, we determined the pure and shared effect of spatial (latitude and longitude), physicochemical, and spatiotemporal distribution patterns of crustaceans, echinoderms, and mollusks as potential predictors of the structure of the fish assemblage. The data were obtained monthly from a one-year bottom trawl survey, comprising five transects and four depths (six, nine, 12 and 15 m) in a 60 km long section of the inner continental shelf, under the influence of estuaries. We captured 57578 demersal fish, comprising 37 families and 88 species. The average abundance of fish per haul was 387 individuals at a depth of 6 m, decreasing to 114 individuals per haul at 15 m. Average richness also decreased along the bathymetric profile, from 15 to 10 species per haul. Fish abundance and richness responded positively to a seasonal estuarine discharge pattern that was stronger in summer and autumn. High biomass of juvenile seabob shrimp in shallower regions also explained the higher abundances and richness. The results also suggest the existence of two fish assemblages in the region, associated with the distribution of the megafauna and segregated by depth, water surface transparency, and sediment particle size composition. Despite significant seasonal variations, controlled by precipitation and bottom water temperature, the structure of fish assemblages responded to the pure and shared effects of the three predictive components in all seasons. In particular, local coastal hydrography and the prey-predator relationship between fish and seabob shrimp were ecologically relevant processes for structuring fish assemblages at the inner continental shelf of Paraná. This study highlights the complexity of ecological interactions within demersal fish assemblages and the importance of considering biotic and abiotic factors in understanding fish distribution patterns at the spatial scale of inner continental shelves that are influenced by estuaries.

Underwater Fish Object Detection with Degraded Prior Knowledge

Understanding fish distribution, behavior, and abundance is crucial for marine ecological research, fishery management, and environmental monitoring. However, the distinctive features of the underwater environment, including low visibility, light attenuation, water turbidity, and strong currents, significantly impact the quality of data gathered by underwater imaging systems, posing considerable challenges in accurately detecting fish objects. To address this challenge, our study proposes an innovative fish detection network based on prior knowledge of image degradation. In our research process, we first delved into the intrinsic relationship between visual image quality restoration and detection outcomes, elucidating the obstacles the underwater environment poses to object detection. Subsequently, we constructed a dataset optimized for object detection using image quality evaluation metrics. Building upon this foundation, we designed a fish object detection network that integrates a prompt-based degradation feature learning module and a two-stage training scheme, effectively incorporating prior knowledge of image degradation. To validate the efficacy of our approach, we develop a multi-scene Underwater Fish image Dataset (UFD2022). The experimental results demonstrate significant improvements of 2.4% and 2.5%, respectively, in the mAP index compared to the baseline methods ResNet50 and ResNetXT101. This outcome robustly confirms the effectiveness and superiority of our process in addressing the challenge of fish object detection in underwater environments.

Can juvenile supply predict future abundance of large-bodied reef fishes?

The extent to which juvenile abundance can predict future populations of lethrinids at Ningaloo Reef was assessed using size frequency data collected over 13 consecutive years. Annual abundance of juvenile lethrinids (<5 cm TL) was highest in northern Ningaloo during La Niña years, when seawater is warmer and oceanic currents stronger. Juvenile lethrinid abundance explained 35% of the variance in 1–2 year-old Lethrinus nebulosus abundance the following year, a steeper relationship in the north suggesting greater survival of juveniles. Juvenile lethrinid abundance was also positively correlated to abundance of 1–2 year-old L. atkinsoni in the southern region of Ningaloo. Abundance of juvenile lethrinids were however poor predictors of L. nebulosus and L. atkinsoni older than 2 years of age. Post settlement processes likely weaken the link between juvenile supply and abundance of lethrinids >2 years old making it difficult to accurately quantify the overall size of future lethrinid populations.

Spatio-Temporal Diversity and Abundance of Fish in the Little Ruaha River Catchments, Iringa Tanzania

Fish are an important component of Tanzania's aquatic biodiversity and contribute to the national economy. However, there has not been a sufficient evaluation of the spatial-temporal distribution of fish in the Little Ruaha River catchments. This study determined the spatiotemporal diversity and abundance of fish and generated information on the spatial distribution of fish across the catchments. Fish samples were collected during the dry and rainy seasons at three sampling sites (upper reach, middle reach, and lower reach) using gillnets of 76.2 mm mesh size. Using the Shannon diversity index, we computed the diversity of fish in different habitats and seasons. We used the Kruskal-Wallis and Mann-Whitney tests to determine the differences in fish diversity and abundance between habitats and seasons. We captured a total of 250 fish individuals from five different species. The fish species diversity and relative abundance were higher in the lower reach compared to the upper reach. The relative abundance of fish was statistically different (p = 0.01) between habitat and seasons. However, there was no significant (p = 0.3) difference in fish diversity between habitats and seasons. Oreochromis and Clarias were the most common species across all habitats. Management of river basins should integrate fisheries activities for integrated river basin management.

The impact of oceanographic factors on the composition and abundance of mesopelagic fish communities in the central and equatorial Atlantic

This paper presents an overview of the composition and abundance of pelagic fish communities collected during the 43rd cruise of the R/V ′Akademik Sergey Vavilov' on the transect in the central Atlantic, from the Azores to the Equator, in autumn 2016. Samples were taken only at night with a Samyshev-Aseev modified Isaacs-Kidd non-closing midwater pelagic trawl (SAMT) at 18 stations from two depth of 0–200 and 0–700 m. The study recorded 112 species of juvenile and adult mesopelagic fishes belonging to 20 families at the sampled stations. The statistical analysis indicated notable disparities in the fish communities that were caught at depths of 0–200 and 0–700 m. Our study suggests that there are notable differences in pelagic fish associations between the North Atlantic anticyclonic gyre zone and the equatorial jets region, at both depths analyzed. These differences are most likely the result of variations in hydro-physical and other environmental parameters. It has been observed that deep-sea fish communities in the equatorial and central North Atlantic are capable of inhabiting at the same time multiple habitats that exhibit varying physical and chemical structures of water masses. This phenomenon highlights the remarkable ecological adaptability of the species that make up mesopelagic deep-sea communities, as well as their wide geographical distribution. The higher abundance and diversity of mesopelagic fishes observed at stations located above the Mid-Atlantic Ridge may be due to a general decrease in depth over the ridge compared to the surrounding regions and/or local circulations occurring over its crests. Thus, the tops and slopes of seamounts may become unique concentrations of mesopelagic fishes for a limited time due to their topography.

Evaluation of extinction risk for stream fishes within an urban riverscape using population viability analysis

Abstract1. The Santa Ana River in the Los Angeles region of California demonstrates common habitat degradation symptoms that are characteristic of the urban stream syndrome. These impacts have altered the Santa Ana River community structure, with few species as impacted as the native Santa Ana sucker (sucker; Pantosteus santaanae). 2. Consequently, a recovery plan developed for sucker identified the need for a population viability analysis (PVA) to assess sucker extirpation risk. However, PVAs can be data‐intensive and are subject to several sources of bias when standardized protocols are absent. 3. More than 20 years of sucker and arroyo chub (chub; Gila orcuttii) surveys using different methods were compiled to build an integrated hierarchical multi‐population PVA to estimate trends in abundance and extirpation probability of these native fishes from the Santa Ana River. 4. PVA modelling indicated similar patterns in sucker and chub abundance along the Santa Ana River, with the highest abundance of both species in the upper regions of the river during the early 2000s and downstream in recent years (2018–2022). Extirpation risk was estimated to be greatest near wastewater treatment facilities, where native fish abundance estimates have been zero since 2018. Extirpation risk was lower downstream of the wastewater treatment facilities for both species, although extinction risk was higher for sucker than chub throughout the river. 5. As the model evolves and more data are collected, the PVA could be used to assess the effects of various management actions, such as non‐native predator removals and native fish re‐introductions, on sucker and chub persistence.

Artisanal fishing affects the local distribution of broad‐snouted caiman (Caiman latirostris) within the Atlantic Forest of Brazil

Abstract Artisanal fishing is an important subsistence practice in freshwater habitats worldwide, but overexploitation threatens the conservation of several nontarget species including crocodylians. We investigated the effects of artisanal fishing on the distribution of a population of broad‐snouted caiman (Caiman latirostris) inhabiting the Tapacurá reservoir, within the highly altered and threatened Atlantic Forest biome. We conducted spotlight surveys to detect caimans and gillnets deployed in the reservoir from April 2015 to June 2022. We evaluated temporal differences in gillnet encounter rates and the relationship between caimans and gillnet distribution. Gillnet encounter rates remained consistent year‐round, while caiman encounter rates were highest near gillnets, especially in the river channel and in forested margins. Caimans are opportunistic predators attracted by tangled fish in gillnets and likely prefer habitats with increased fish abundance. Future research should continue monitoring the interaction between caimans and fishing and include local communities in conservation efforts.

Fish biodiversity in modified ecosystems: a case study of Nigeria's Challawa and Tiga dams in the Challawa and Kano rivers

River impoundment is usually accompanied with consequences of alteration in the hydrological system and change in the biodiversity composition of the ecosystem. This study investigated the impact of the dams built on Challawa (that is, Challawa George-Dam) and Kano (that is, Tiga Dam) on the fish biodiversity of the reservoir and below-dam sections of the two rivers. Fish sampling was done using long lines, Malian traps, cast and gill nets from stations on the reservoir and below-dam sections of the rivers. Ten fish species were identified from the reservoir and the below-dam sections of Challawa River belonging to nine families and four orders. Fish communities of the reservoir and below-dam sections of Kano River consisted of 28 species belonging to 12 families and 7 orders. Results of the Shannon diversity index (H^') indicated that the two reservoirs have relatively higher fish species diversity measures (that is, Challawa=1.948 &amp; Kano=2.294) than the below-dam parts (that is, Challawa=1.833 &amp; Kano=2.247) of their respective rivers. Non-metric Multidimensional Scaling (NMDS) analysis produced two-dimensional maps with stress values of 0.0554 and 0.0537, an indication that the model provided good representation of the original data in reduced-dimensional space. The ordination map indicated wide dissimilarities between fish communities of the impounded and below-dam sections. Furthermore, a one-way analysis of similarity indicated significant difference (P=0.0001) and high dissimilarities (R=0.643) between fish communities in the two sections of Challawa River and also in Kano River (P=0.001, R=0.929). Analysis of Similarity Percentage indicated 74.45% of the dissmilarities between fish communities in the two sections were cummulatively contributed by four species in Challawa River and 72.18% by five species in Kano River. The distinct differences in fish abundance and richness between the two sections is ascribed to increased ecosytem productivity often associated with the inundated portion of dammed river. It is recommended that more water flow across the dams should be encouraged as a mitigating measure to boost basin sizes of the rivers at the below-dam sections. Key words: Diversity, dam, impound, reservoir, community, freshwater and fish species

Temporal integration analysis of environmental behavior and fish abundance in selected coastal systems of the southern Gulf of Mexico

Temporal integration analysis of environmental behavior and fish abundance in selected coastal systems of the southern Gulf of Mexico

Quantifying uncertainty when extrapolating the relationship between snorkel counts and mark-recapture estimates of juvenile salmonids

Snorkel surveys are frequently used to monitor stream-dwelling fish. Inferring local abundance from snorkel surveys is complicated by two primary factors: variable fish detection probabilities and the relative abundance of fish in habitat types below the recommended minimum depth for snorkeling. We examine these factors across three salmonid species ( Oncorhynchus spp .), 4 years, and 113 location-years in Oregon coastal streams. We calibrate snorkel counts to mark-recapture estimates and develop mathematically explicit expressions that convert a new snorkel count into a probability density of abundance for streams containing mixed habitat types that were and were not snorkeled. Snorkelers detected 63%, 47%, and 39% of juvenile coho salmon ( Oncorhynchus kitsutch), steelhead trout ( Oncorhynchus mykiss), and cutthroat trout ( Oncorhynchus clarkii clarkia) estimated by mark-recapture, respectively, but uncertainty within and among sampling units was high. Further, analytics developed here can be used to infer abundance and uncertainty for habitat types that were not snorkeled. Our quantification of uncertainty arising from using snorkel counts as a proxy for abundance will help managers balance biological risks with available resources.

Flood effects on estuarine fish are mediated by seascape composition and context

Estuaries are crucial feeding, nursery and resting sites for fish but can also be subject to the impacts of severe flooding. The environmental features of estuaries can mediate how they respond to these impacts. For example, the size, configuration, and context of estuarine habitats across seascapes affects the value of patches for fish, and so fish assemblages at sites with a greater habitat extent or closer to the mouth of an estuary may rebound more quickly from flooding. We investigated how a once in 100-year flood event affected fish assemblages at approximately 600 sites across 13 estuaries and six estuarine habitats (bare sediments, log snags, mangrove forests, rocky structures, saltmarsh and seagrass meadows) in southeast Queensland, Australia, and determined whether flood impacts were mediated by the position of sites within the broader estuarine seascape. Sites were surveyed annually in 2020/2021 (pre-flood) and 2022 (6 months post-flood) using underwater videography. Flooding modified the structure of the fish community and reduced the abundance of fish targeted by local fisheries in all six habitats. Crucially, flood effects on fish were greater at sites near more expansive urbanisation in some ecosystems, but lower at sites nearer to the estuary mouth. Maximising the extent of natural habitats across estuaries can mediate the effects of floods and should be priorities for restoration and management plans seeking to maintain biodiversity and fisheries productivity in the face of increasing climate-related disturbances.

Spatial Dynamics of Demersal Fish Assemblages off the Korean Coast in the East Sea.

This study assessed variations in demersal fish assemblages with respect to the study site and water depth. Seasonal samplings from May 2018 to March 2019 were conducted along the northern (Ayajin, Goseong) and southern (Hupo, Uljin) sites of the East Sea off the Korean coast, using commercial gill nets. Samples were collected at depths of ~50, ~80, ~150 m across the study sites, with concurrent monitoring of water column structures. A total of 73 species and 6250 specimens were collected. Distinctive fish species compositions were observed according to the study site and depth. Although Glyptocephalus stelleri was the most abundant fish species in both Ayajin and Hupo, Gadus macrocephalus, Icelus cataphractus, and Alcichthys elongatus were most predominant in Ayajin, whereas Cleisthenes pinetorum, Hippoglossoides dubius, and Gymnocanthus herzensteini were more prevalent in Hupo. In terms of depth layer, in Ayajin, G. stelleri dominated in both intermediate and deeper layers, with Hemilepidotus gilberti, A. elongatus, Enophrys diceraus common in shallower depths. Conversely, in Hupo, G. stelleri, C. pinetorum, and A. nadeshnyi dominated across all depth layers, whereas Dasycottus setiger and G. herzensteini dominated in deeper and shallower depths, respectively. Significant influences of the study site and water depth on fish assemblage structures were observed due to variations in water temperature at the seasonal thermocline boundary.

Microplastic pollution in the Thumbprint emperor (Lethrinus harak) from Tanzanian coastal waters: Occurrence, abundance, characterization, and relevance as a monitoring species

In the present study we collected the Thumbprint emperor (Lethrinus harak) from seven landing sites from the coastal waters around Dar es Salaam and Zanzibar (Tanzania) to (i) quantify and characterize microplastics (MPs) in their digestive tracts and (ii) use previously assessed environmental levels in nearshore surface waters and seabed sediments to determine whether L. harak could be a relevant biomonitor for MP pollution in the region. L. harak (n=387) had an overall frequency of occurrence (FO%) of 48 % and displayed spatial variation between sites with Kunduchi (FO=66.7 %) and Mijimwena (FO=17.1 %) having the highest and lowest FO%, respectively. Fish from Mjimwema had a mean MP content of 0.17 ± 0.38 MPs individual-1 whilst fish from Kizimkazi had the highest MP abundance (1.75 ± 2.33 MPs individual-1). Fibers (overall 64.7 %, range across sites 48–86 %) and fragments (17.9 %, 5–25 %) were the most dominant MP types whilst black (46.9 %, 40–58 %) and blue (22.5%, 7–36 %) MPs were the most common colours. Fish length (ρ= -0.09, p=0.09) or weight (ρ=0.07, p=0.18) did not significantly correlate to MP abundance in fish (Spearman rank correlations). Neither MP occurrence nor abundance was linked to MP concentrations in either surface waters or seabed sediments (Spearman rank correlation), but MPs in the fish better reflected MPs in the sediment compared to surface water (two-way ANOVA on ranked data). Whilst L. harak presents as a promising candidate to monitor MP pollution along the East African coast due to its ecology, overall, it lacks reliability. Nonetheless, the present study fills important knowledge gaps both geographically on the East African Coast and with an underrepresented taxonomic family (Lethrinidae ‘Emporer fishes’).