Ichthyoplankton Surveys Research Articles

Effects of Sampling Design on Population Abundance Estimation of Ichthyoplankton in Coastal Waters

The abundance, spatial distribution of and dynamic changes in ichthyoplankton species affect the recruitment and fish population dynamics, which are fundamental for stock assessment and fisheries management. An evaluation of alternative sampling designs needs to be carried out to determine the optimal scheme that is cost-effective in collecting high-quality ichthyoplankton data. A simulation study was conducted to evaluate the performances and consistency of six potential sampling designs for an ichthyoplankton survey in the coastal waters of the central and southern Yellow Sea. Relative estimation error (REE) and relative bias (RB) were used to measure the performances in estimating the population abundances of five target ichthyoplankton species in different sampling designs. In general, the two systematic sampling (SYS) designs had high precision and accuracy estimation and remained stable over years for estimating the population abundance of ichthyoplankton species compared with the other four sampling designs. SYS did not result in the overestimation or underestimation of the mean population abundance. Most sampling designs showed relatively high accuracy in abundance estimation when sample sizes were higher than medium levels. This study could improve the performances of sampling designs of ichthyoplankton surveys and provide a reference for the further optimization of sampling designs.

Shifting spawning phenology in Hudson River American Shad

AbstractObjectiveThe objectives of this study were to investigate potential shift in the phenology of spawning for the Hudson River American Shad Alosa sapidissima and to identify factors contributing to the decline of the Hudson River shad population.MethodsThis study utilized American Shad eggs collected from the Long River Ichthyoplankton Survey. Logistic models were employed to estimate spawning phenology metrics, including the onset, peak, cessation, and duration of the spawning season. Additionally, we investigated the effects of biotic and abiotic variables on spawning timing.ResultThis study suggests that temporal changes in the spawning onset exhibited high variation and were not significant. However, a significantly delayed spawning peak and cessation, along with an extended spawning season, were observed. Additionally, the findings of this study revealed a correlation between the timing of spawning onset and water temperature, indicating that a 1°C increase in water temperature was associated with a 3.66‐day earlier initiation of spawning. Smaller female spawner size was associated with delayed peak and cessation of the spawning season. With a 1‐cm decrease in the average female spawner size, the peak of the spawning season was delayed by 1.79 days and the cessation of the spawning season was delayed by 2.87 days.ConclusionThe findings indicate that water temperature may be a crucial trigger for initiating spawning in Hudson River American Shad, with higher water temperatures associated with earlier spawning onset. Moreover, the findings indicate that the timing of spawning peaks and cessations has been progressively delayed over time, and this is linked to the size of female spawners.

A method for estimating biomass of age-0 fish in case of protracted spawning leading to multiple larval cohorts in the population

We propose a new method for estimating the density and biomass of fish larvae and early juveniles when spawning is extended over time and the egg production in spawning groups varies widely with time. As a proof of concept, the method is applied to catch data for age–0 polar cod (Boreogadus saida), collected by the ichthyoplankton survey in the Kara Sea in 2019. The biomass among different size classes of age–0 fish was calculated based on the selectivity of a Bongo net with 500-μm mesh size. The age of larvae and juveniles in each group and average length at hatch (5.5 mm) were determined used for assessing average growth rate (0.18 mm/day) and hatching date for each age–0 fish assemblage. By fitting the population curve to the samples of age–0 polar cod collected in a small area sequentially over ten days, the instantaneous rate of natural mortality was estimated as ∼ 0.05 day-1. Changes in abundance and biomass over time were modeled by the Baranov's equation and power growth function. The average biomass for the time step of interest was calculated by projecting each cohort abundance to a selected point in time and taking a weighted average of all cohort biomasses.

Spatio-temporal larval abundance dynamics of a depleted Baltic Sea herring ecotype

Abstract Fish stocks composed of several populations are considered to have more stable productivity than stocks containing only one or few populations. This stability is attributed to complementary or independent dynamics among the populations within the stock—the so-called portfolio effect. Declines of populations within stocks that have delayed recovery potentially increase risk to productivity and local extinctions. Portfolio effects may be relevant in the Baltic Sea, where autumn-spawning herring was the dominant herring ecotype until the early/mid-20th century. The current fishery assessment and management in the area assumes that the abundance of autumn spawning herring has been negligible since the 1970s; the share of spring spawning herring has become dominant and is considered to be the only ecotype. Data from a multi-annual ichthyoplankton survey in the southern Baltic Sea (Bornholm Basin) during autumn 2002–2019 show that abundance and spatial distribution of the smallest larval stages have increased significantly. This pattern could be due to an increase in autumn spawners in the area. Genetic studies confirm that these larval herring are offspring from true autumn spawners. Preserving intraspecific diversity is critical for a species’ future abilities to adapt to and survive in changing environmental conditions. Thus, our results not only provide new insights to the temporal dynamics of herring ecotypes and challenge the current understanding of central Baltic herring stock processes in general, but are also important in the context of monitoring, assessment, and spatial management of herring in the Baltic Sea.

Drag force and filtration performance of square-frame ichthyoplankton sampling net

A constant-depth ichthyoplankton sampler (CDIS) is widely employed for quantitative sampling surveys, yet challenges related to sampling distortion due to unstable tows of the CDIS persist. The drag force of the square-frame sampling net is pivotal in maintaining the dynamic stability of the CDIS during horizontal towing. This study delves into the hydrodynamics and filtration performance of square-frame sampling nets through flume-tank experiments and numerical simulations. The results show that the drag coefficient for the square-frame ichthyoplankton sampling net initially decreases and then increases as the solidity ratio rises from 0.3 to 0.7. It becomes greater as the net length becomes shorter and the net mouth area becomes larger. A response model for the drag force for the square-frame sampling net is proposed that is capable of accurate predictions. The results also reveal that the flow decelerates at the net mouth and gradually accelerates in the posterior section of the net, and the filtration efficiency increases as the net length and mouth area increase but decreases slightly as the solidity ratio increases. Our study provides vital scientific data for reference and methodologies for designing or modifying a square-frame CDIS, fostering enhanced stability and accuracy in ichthyoplankton surveys.

Predator biomass affects west–east shifts in Barents Sea capelin (Mallotus villosus) spawning ground use

AbstractSpawning locations of migratory fish influence the environmental and oceanographic fate of eggs and larvae. However, we have an incomplete understanding of how and why realized spawning locations vary. We quantified the yearly variation in spawning location for Barents Sea capelin (Mallotus villosus) within the recognized spawning areas along the coast of northern Norway and Murman. Furthermore, we tested whether water temperature, sea ice cover, predation and capelin fisheries are associated with spawning locations. Estimated spatial variation in spawning longitude used data from Soviet‐Russian ichthyoplankton surveys from 1959 to 1993 and a bootstrap procedure. The mean spawning area along the coast was then calculated by combining larvae observations with backtracking a larvae drift model from potential spawning areas. We fitted a generalized additive model (GAM) to assess the effects of environmental conditions on these drift‐corrected spawning longitudes. Capelin's mean yearly spawning location varied substantially across the study period. An increase in immature cod biomass, a major predator of capelin, during the spawning migration period in winter and early spring corresponds to more eastward capelin spawning. The eastward shift in mean spawning longitude seems to reach an eastern limit at high immature cod biomass. Sea temperature, sea ice cover and capelin fisheries were not associated with mean capelin spawning longitude. We show that the realized spawning areas for capelin are related to cod biomass, potentially through cod predation depleting and/or acting as a migration barrier for mature capelin.

Winter Time Ichthyoplankton Assemblages Following Dense Mucilage Event in the Sea Of Marmara, Türkiye

The community structure of ichthyoplankton after mucilage was investigated. For this purpose, three ichthyoplankton survey were conducted from 34 stations between December 2021 and March 2022. A total of 19 fish egg and 16 fish larvae species were identified. It was detected that the biodiversity of winter time ichthyoplankton was higher than previous studies. European Sprat, Sprattus sprattus had the highest mean biomass for both fish eggs and fish larvae, whereas Sardine, Sardina pilchardus had relatively low biomass. Karacabey Floodplain area, Büyükçekmece estuarine area and Erdek Bay were the most important spawning areas for winter spawners. It was found that ecosystem health of the Marmara Sea was suitable for fertilization, hatching and larval growth soon after the mucilage and hydrological regime recover itself immidiatelly.

Interspecific and intraspecific difference in egg size of two mackerel (Scomber spp.) species in relation to sea surface temperature in the western North Pacific: A new approach to species identification

AbstractChub mackerel (Scomber japonicus, Scombridae) and blue mackerel (Scomber australasicus) are two important fishery resources in the western North Pacific that spawn eggs during the same season. Although estimating the total egg production (TEP) is important for evaluating the spawning stock biomass of mackerel species, it is difficult to accurately identify formalin‐preserved eggs collected during ichthyoplankton surveys. Hence, a new identification criterion that incorporates the effects of water temperature on egg size was developed in this study. The diameter of 37,304 mackerel eggs collected over 16 years (2006–2021) was measured, and frequency distributions of egg size across sea surface temperature (SST) ranges at 1°C interval were constructed. The frequency distributions were classified into two groups using a Gaussian mixture model; based on the results of DNA analysis, the small group was chub mackerel, while the large group was blue mackerel. The SST at the sampling stations and the mean egg size of both groups were negatively correlated. The new identification criterion, incorporating the relationship between SST and egg size, provided reasonable estimates of the TEP of the two mackerel species compared with the conventional criterion. The new species identification approach is applicable to other fish taxa in the western North Pacific.

РАСПРЕДЕЛЕНИЕ И ДИНАМИКА ЧИСЛЕННОСТИ ИХТИОПЛАНКТОНА И РАННЕЙ МОЛОДИ ТЮЛЬКИ И ХАМСЫ В АЗОВСКОМ МОРЕ В 1993–2023 ГГ.

Background. Natural and anthropogenic factors have a major influence on the fish population status. In the Azov Sea, for the last 30 years, substantial changes in the environment have been observed and new exotic species of aquatic living organisms have been introduced, which affected reproduction of fish species. Relevance. To identify the driving forces behind the present dynamics of population abundance of European anchovy and Black and Caspian Sea sprat (tyulka), it is crucial to summarize the data collected during the annual ichthyoplankton surveys of the Azov Sea. This work is aimed at the assessment of the distribution of the ichthyoplankton and early juveniles of Black and Caspian Sea sprat (tyulka) and European anchovy in the Azov Sea area over time with regard to various environmental factors. Methods. Ichthyoplankton surveys are customarily conducted over the entirety of the Azov Sea in the middle of June, when eggs and larvae of Black and Caspian Sea sprat (tyulka) and European anchovy are caught at the highest quantities. Results. The dynamics of distribution and abundance variations for the early developmental stages of Black and Caspian Sea sprat (tyulka) and European anchovy under the influence of environmental factors, including ctenophores and scyphozoan jellyfishes, has been identified; a decrease in the survival rates of their jellyfishes juveniles in the present period of the Azov Sea salinization has been observed. Conclusion. In the context of increasing salinity of the Azov Sea waters, the main factors contributing to the reduced survival rates of the Black and Caspian Sea sprat (tyulka) larvae and fingerlings are the decrease in spawning ground area and competition for food resources with scyphozoan jellyfishes and ctenophores. In the case of the European anchovy larvae and fingerlings, the main factor affecting their abundance is a small number of breeders; food competition and predation of the scyphozoan jellyfishes and ctenophores are of secondary importance.

Assessment of spawning stock for walleye pollock <i>Gadus chalcogrammus</i> Pallas, 1814 in the northern Okhotsk Sea by ichthyoplankton method taking into account the eggs mortality

A new approach for assessment of the spawning stock of walleye pollock in the northern Okhotsk Sea on the data of ichthyoplankton surveys is proposed. To estimate the actual number of spawned eggs, average age of eggs at each stage of development was determined and the eggs mortality was estimated in dependence on age. For the spring ichthyoplankton surveys in 2019–2023, the eggs mortality was evaluated as 27 % for stage I, 57 % for the stage II, 74 % for the stage III, and 86 % for the stage IV – these portions of the spawned eggs were not caught and counted. So, the number of counted eggs was in 1.7–2.1 times lower that the actual number of spawned eggs. The spawning stock biomass was amounted as 12.9 . 106 t in 2019–2023, on average, on the base of the actual number of eggs determined taking unto account the eggs mortality (while the biomass calculated from the counted number of eggs only would be 6.9 . 106 t). Results of the spawning stock estimation with this new method are comparable with the stock assessments by trawl surveys. The eggs number correction through mortality is recommended for calculation of the spawning stock for walleye pollock in the Okhotsk Sea using the ichthyoplankton method.

Climate effects on the timing of Maryland Striped Bass spawning runs

AbstractObjectiveChesapeake Bay water temperatures have increased through time, and under various climate change scenarios they are projected to increase by an additional 2–6°C by the end of the 21st century. Previous work has shown that water temperatures are the primary trigger for Striped Bass Morone saxatilis spawning.MethodsUsing spawning stock survey data (1985–2020) from the Potomac River and upper Chesapeake Bay and temperature data from an ichthyoplankton survey (sampled non‐continuously between 1954 and 2021) in the Nanticoke and Choptank rivers, this study examined how water temperature milestones that are important for Striped Bass spawning have changed over time as well as how selectivity‐corrected total female catch per unit effort has shifted with changes in water temperature and the age structure of the stock.ResultWater temperatures observed in the spawning stock survey were lower in the 1980s and 1990s and have been higher since the 2000s. A significant change in timing of the start of spawning was not detected in the spawning stock survey data, but a significant change was found in the timing of the end of spawning, suggesting that the spawning season has shortened. In addition, the date on which the last prespawn female was observed on the spawning grounds has also occurred earlier in the year since the 2000s. Results from the long‐term temperature data collected in the Choptank and Nanticoke rivers only showed a significant change through time for the start of spawning in the Nanticoke River.ConclusionWhile these changes could affect egg and larval Striped Bass survival either through either direct mortality due to water temperature or changes in the timing of zooplankton blooms, the effects of climate change could be mitigated by having a broad range of spawning ages to maximize the chances that larval Striped Bass matching with prey availability.

Tidal Variations of Fish Larvae Measured Using a 15-Day Continuous Ichthyoplankton Survey in Subei Shoal: Management Implications for the Red-Crowned Crane (Grus japonensis) Population in Yancheng Nature Reserve.

The National Yancheng Rare Birds Nature Reserve is a vitally important staging habitat for the wild population of red-crowned cranes (Grus japonensis) in China. The population relies on local high-protein food sources, such as fish juveniles, to fuel their migratory journeys. However, little is known about the ecology of the fish larvae and juveniles that migrate to the inshore area via tidal rhythm in Subei Shoal, which is adjacent to the reserve. Therefore, we used a fixed study station (32°55'1.2″ N, 121°19'58.8″ E) to conduct a continuous 15-day ichthyoplankton survey at 2 h intervals beginning at 05:00 on 25 April and ending at 03:00 on 10 May 2019. We identified the tidal variations in the number of fish larvae and juveniles and the number at various developmental stages and assessed how they were related to environmental variables such as sea surface temperature, salinity, turbidity, and tidal height in the Dafeng Sea area of Subei Shoal. We found that the number of species and larval individuals were highest and lowest, respectively, at the highest and lowest tidal height, and they obviously increased and decreased with the rising and ebb tide, respectively. Our findings indicate that the variation in numbers of the larvae and juveniles depends on species and developmental stage. The species Acanthogobius ommaturus, Pholis fangi, Cynoglossus joyneri, Liza haematocheila, and Lateolabrax japonicus and the total number of larvae were most influenced by tidal height. These results provide a better understanding of the habitat of prey species of the red-crowned crane wild population as well as scientific data that can be applied to manage the wild population in the reserve sustainably.

Spatial and seasonal fluctuations of Ichthyoplankton assemblage in the Mediterranean coast of Morocco (Southwestern Alboran Sea)

Ichthyoplankton represent the first life stages of fish. The study of ichthyoplankton is crucial to understanding marine ecosystems and plays an important role in the management and durability of fisheries resources. During March and October of 2019, two oceanographic ichthyoplankton surveys were conducted in the Mediterranean Sea of Morocco from Tanger to Saadia by studying the horizontal structure of the ichthyoplankton species assemblage and its relation to environmental parameters. The average surface water temperature was (15.8°C in spring and 16.4°C in autumn). The fish eggs and larvae were more abundant in March than in October (21268 eggs/10m² and 14084 larvae/10m² in spring and 10094 eggs/10m² and 13796 larvae/10m²). In both seasons, fish eggs from the families Sternoptychidae and Sparidae were dominant (10101 eggs/10m² and 7527 eggs/10m² in spring and 4422 eggs/10m² and 3928 eggs/10m² in fall, respectively). However, Myctophidae larvae were the most abundant in the study area, reaching 7601 larvae/10m² in spring and 11021 larvae/10m² in autumn. The environmental parameters: temperature, salinity and chlorophyll-a (surface) seem to directly influence the spatial distribution of ichtyoplancton. On the other hand, it seems that predation by jellyfish (Pelagia noctiluca)was a very important factor that added to the factors that influenced the distribution of the species of fish eggs and larvae. This work represents the first survey conducted in the southwestern Alboran Sea, which studies the horizontal structure of the ichthyoplankton species assemblage and its relation to environmental factors in the spring and autumn of 2019.

Gulf of Mexico larval dispersal: Combining concurrent sampling, behavioral, and hydrodynamic data to inform end-to-end modeling efforts through a Lagrangian dispersal model

We developed a Lagrangian larval dispersal model to estimate trajectories for eleven fish taxa inhabiting the Gulf of Mexico (GOM). Dispersal models are at family level resolution for Scaridae, Lutjanidae, Scombridae, Labridae, Ophichthidae, and Ophidiidae, at genus level resolution for Hemanthias, and at species level resolution for Trachurus lathami, Decapterus punctatus, Katsuwonus pelamis, and Euthynnus alleteratus. Hydrodynamics are provided by the West Florida Coastal Ocean Model (WFCOM). Larval samples are from the spring and fall SEAMAP ichthyoplankton surveys from 2007 to 2011. The Lagrangian model was run backwards/forwards in time from the sampling event to estimate spawning/settlement locations. Results were used to update larval dispersal dynamics in the GOM Atlantis ‘end-to-end’ ecosystem model for twelve functional groups. We compare dispersal and non-dispersal scenarios in the Gulf of Mexico Atlantis model and find differences in stock abundance and distribution of fish. This highlights that the abundance and distribution of fishery resources are sensitive to changing circulation patterns. This work takes an interdisciplinary approach to understanding larval dynamics and their impacts on ecosystems at the intersection of predictive statistical modeling, hydrodynamic modeling, and ecosystem modeling.

Large sardine resource discovered off south-eastern Australia: Potential risks, challenges and benefits of establishing a new fishery

This paper documents the discovery of a large sardine (Sardinops sagax) resource off South-eastern Australia. We use information from three ichthyoplankton surveys for jack mackerel to apply the Daily Egg Production Method (DEPM) to the South-eastern sardine stock. Mean daily egg production was estimated from survey data. Spawning area was estimated from the surveys and using a habitat suitability model to predict presence/absence of eggs in unsurveyed areas. Spawning biomass was calculated using adult parameters from the adjacent Southern stock. The spawning biomass in the area surveyed was 262,564 t (95% CI 125,670–326–438 t). Habitat modelling using a conservative threshold suggested that the total spawning biomass was 346,388 t (95% CI 143,936–548,840 t). These results suggest this stock has the capacity to sustain an annual catch of several tens of thousands of tonnes, which is considerably larger than the recent catch of < 2500 t. Our findings provide further evidence that opportunities exist to increase Australia’s fisheries production. Risks and challenges associated with establishing a new large-scale sardine fishery, including bycatch and ecological impacts, and opposition from conservation groups and recreational fishers are discussed. We also outline the potential socio-economic and environmental benefits, including the use of marine protein and oil from a sustainable local source with low environmental impacts as an input to the AU$1.3B Tasmanian salmon aquaculture industry. We emphasize the need to adopt a precautionary approach to developing a new fishery and the critical importance of conducting comprehensive surveys to confirm our findings and inform ongoing management.

Results of monitoring of microalgae of harmful algal bloom (HAB) complex in the Avachisky Gulf in 2022

The object of the research is microalgae of HAB complex (harmful algal “bloom”). The purpose of the work is to provide monitoring data on HABs and the microalgae habitat conditions near the eastern coast of Kamchatka, which is currently the focus of attention of the Kamchatka government in terms of development of recreational and tourist activities, one of the priority economic directions. Suface water samples were collected at the coastal transect, stretching along the whole coast of the Avachinsky Gulf in the process of the work from April to October. Hydrological data were collected with Rinko- Profi lerASTD-102 and YCI CastAway probes. In June, the data were collected as part of planned ichthyoplankton survey. As a result, a real database on taxonomic composition, quantitative vales and distribution of microalgae of the HAB complex was formed. Data on water temperature and salinity, biogenic background of the Avachinsky Gulf coastal waters were obtained.

New Finding of Eggs and Leptocephalus for Muraenesox cinereus off Jeju Island, Korea and Its Relation to Environmental Variables

The daggertooth pike conger, Muraenesox cinereus, is an important demersal fish species in East Asia but the catch amount has declined in recent years. Spawning areas of M. cinereus have not yet been determined; identifying these have serious implications for resource management. Ichthyoplankton surveys are an effective method of distinguishing fish spawning areas and periods. Fish eggs were sampled from the waters adjacent to South Korea in August 2020 and 2022 using a Multiple Opening/Closing Net and Environmental Sensing System or a bongo net. In 2021, M. cinereus eggs were not collected. Three unidentified eggs (2.0–2.2 mm in diameter) were collected from the southeastern sea of Jeju Island at a seawater depth of 20–30 m and temperature of 20–22 °C. Muraenesox cinereus preleptocephali were gathered at 10–20 m depths from Jeju Island’s southernmost and eastern sea areas. The eggs and preleptocephali were identified as M. cinereus by their mitochondrial DNA 16S rRNA sequences. This is a new finding of eggs and leptocephalus of Muraenesox cinereus off Jeju Island, South Korea, which increases our understanding of the recruitment process of M. cinereus to facilitate resource management and species conservation.

Seasonal distribution of the early life stages of the small yellow croaker (Larimichthys polyactis) and its dynamic controls adjacent to the Changjiang River Estuary

AbstractThe distribution of the early life stages of Larimichthys polyactis indicates the availability of suitable spawning and nursing grounds and enables a better understanding of the distribution of its geographic subpopulations and isolation. Large‐scale quantitative ichthyoplankton surveys of L. polyactis were performed from March to July 2018, and a numerical hydrodynamic model was applied to explore the spatiotemporal distribution and habitats of L. polyactis early life stages adjacent to the Changjiang River Estuary. Two distinct early life stage distribution patterns were found. One occurred during March and April in the deep waters seaward of the Changjiang River bottom plume front, to the east and southeast of the Changjiang River Estuary. The other occurred in May and June throughout the entire water column in coastal waters to the east and north of the Changjiang River Estuary. Rising seasonal temperatures, and the local dynamic processes such as the Changjiang River plume, determined the transitions between the two distribution patterns. A potential fish larval migration route from the southern deep shelf waters to the northern coastal waters within the motor‐trawl prohibition line was suggested. This study contributes to our understanding of the mixture of habitats used by the two L. polyactis subpopulations in the southern Yellow Sea and the East China Sea.

Variability of hydrological conditions at spawning sites of East Kamchatka walleye pollock (Gadus chalcogrammus) в 2012–2022

Analysis of inter-annual variability of temperature and dynamic conditions at sites of mass spawning of East Kamchatka walleye pollock is made based on the data obtained in the course of spring ichthyoplankton surveys for the period from 2012 to 2022 and used data of remote (satellite) monitoring. Tight correlation is demonstrated between the temperatures in the water column in spring and “severity” of previous autumnwinter period in the south-western part of the Bering Sea. According to the results of remote monitoring, there is a noticeable intensification of the Kamchatka current in the last decade in the area of the research. At the same time, at the sites of mass spawning, the water transfer rate is much lower, especially in the area of Avachinsky Gulf. But even here an insignificant increase of current velocity with time can be traced.

A long-term ichthyoplankton monitoring program suggests climate-induced environmental variabilities changed fish communities in the Hudson River estuary

Climate change is known to have significant impacts on coastal marine ecosystems. However, a good understanding of the exact impacts requires consistent long-term monitoring programs, which are not available for many coastal marine ecosystems. The Hudson River Estuary (HRE) provides critical habitats for many estuarine, freshwater, and diadromous species. The Longitudinal River Ichthyoplankton Survey and Water Quality Survey in the Hudson River Biological Monitoring Program provided a long-term time series (1974-2017) for examining the changes in salinity and other environmental variables and identifying the temporal variability of fish diversity. In this study, we examined the environmental data and found a 2-phase pattern in the salinity time series data with a change point of 1984. We calculated a Margalef index and a Simpson index to examine species richness and evenness for early and older life stages in the upper and lower HRE, and explored the potential effects of rising sea level and salinity on fish biodiversity. This study reveals that sea level and salinity may play an important part in explaining increased species richness and evenness. The increased occurrence, establishment and range expansion of marine species as well as possible displacement of freshwater species may be associated with sea level-induced salinity change observed in the HRE. This study highlights a need for a long-term consistent monitoring program and a better understanding of climate change effects on the biota in coastal estuary ecosystems such as the HRE in order to enhance management, conservation, and restoration plans.