Tidal Limit Research Articles

Environmental influences on the phenology of immigrating juvenile eels over weirs at the tidal limit of regulated rivers

Recruitment of the catadromous and critically endangered European eel Anguilla anguilla in Europe has declined substantially since the 1980s, with considerable knowledge gaps remaining in many aspects of their life cycle. The aim was to assess eel migration phenology in three regulated rivers in England between 2009 and 2019 through analyses of eel numbers using passes at their tidal limits, with calculation of the annual timings of migration initiation (10% of all eels passed, T10), peak (50%, T50) and conclusion (90%, T90). Across the three rivers, T10 varied between Julian Day (‘Day’) 94 and 173. Years of earlier T10 had significantly earlier T50, where T50 varied between Day 105 and 200. The considerable inter-annual variability in migration timings was associated with environmental variables; earlier T10 and T50 occurred in years of warmer river temperatures (RTs) and cooler sea surface temperatures (SST), and in years where RTs were higher than SSTs. No environmental variables were significant predictors of T90. These results indicate that whilst there is annual variability in the timing of eel migration initiation and peak into freshwaters, this variability is predictable according to differences in environmental conditions. As many of these conditions associated with annual variability in temperature and precipitation then climate change has the potential to shift these migration timings.

On the Fore-Shore Vegetation of Malacca Coast of the Car-Nicobar Island

In this paper, an account is given of the Fore-shore Vegetation of a part of E. Coast of Car-Nicobar along the Malacca Coast. The various types of plant species found on the shore above upper tidal limits are described, and the characteristic ones being Ipomoea pes-caprae (Linn.) Sweet., Panicum repens Linn., Vigna marina Mers., Ischaemum muticum Linn., Blainvillea acmella (Linn, f.) Philipson., Pandanus furcatus Roxb., Scaevola frutescens (Mill.) Krausse., Tournefortia argentea Linn., with Cocos nucifera Linn, occur behind, followed by Hibiscus tiliaceus Linn., Erythrina variegata Linn. var. orientalis (Linn.) Merr., Calophyllum inophyllum Linn., Thespesia populnea Corr., Guettarda speciosa Linn., Syzygium aqueum (Burm f.) Alston, and other trees in jungles. The various species met with are enumerated, in which 13 species and one variety would appear to be new records for the Andamans and Nicobars.

Climate control on the channel morphodynamics of the Sittaung River, Myanmar

The spatio–temporal development of a meandering river is controlled by its channel morphodynamics. In regions of rapid channel evolution, understanding the driving factors of meander migration is crucial in forecasting the rate and extent of morphological change. Sediment supply and fluvial discharge are the primary influences on migration rate, however climate oscillations are also integral in indirectly regulating migration rate through their control of regional precipitation, as well as the monsoon season of sub-tropical Asia. Despite this, an in-depth investigation into the impact of climate oscillations on meander bend migration remains undocumented. This study presents a satellite-based analysis of multi-decadal climatic forcing on the migration rate of the Sittaung River in Myanmar, through interpretation of the El Niño Southern Oscillation (ENSO). The mode of ENSO exerts significant climatic control on the migration rate of the meandering channels of the Sittaung River, with low-to-average migration rates recorded during dry El Niño events and peak migration rates observed during wet La Niña events. However, this climatic signal may have been obscured by certain local environmental conditions. In cases where meanders faced geological basement, the basement rock inhibited their migration through extension, forcing more rapid migration by way of seaward translation. Consequently, these translating meanders developed to be more elongate, with lower curvatures. Meanders downstream of the approximate tidal limit were less downstream skewed, indicative of tidal modulation, potentially obscuring the impact of fluvially driven climate forcing. Additionally, downstream of a major confluence, the input of sediment and fluvial discharge may have been regulated by upstream anthropogenic activities such as mining and dam construction, leading to greater variability in migration rate downstream of this confluence and further obfuscation of the climate signal.

Genetic data confirm the presence of juvenile Alosa alosa in the estuary of the River Tamar

AbstractWithin the UK, allis shad (Alosa alosa) are classified as Critically Endangered and are known to breed in only a single river, the Tamar. Despite evidence of spawning within the lower freshwater reaches of the river and at the tidal limit within the estuary, juvenile allis shad have never been found. Genetic analysis, based on mitochondrial DNA haplotype and nuclear Polymerase Chain Reaction ‐ Restriction Fragment Length Polymorphism (PCR‐RFLP) data, confirmed that juvenile shad found within the lower Tamar estuary in autumn 2022 were A. alosa.

Tidal range electricity generation into the twenty-second century

Tidal range electricity generation schemes are designed to have a minimum operational life of at least 120 years, making it important to plan for changes such as sea-level rise (SLR). Earlier studies have shown that schemes can maintain the existing tidal range within the impoundment and protect areas from flooding. Here it is demonstrated that tidal range technology can maintain the current tidal extent despite SLR and suggests the operational strategies to achieve it. The approach is the only way to safeguard existing intertidal habitats. Mechanical and electrical plant requires a major overall, upgrade or replacement every 40 years; the levelised cost of energy is structured in 40-year periods reducing after the first period. Increasing the capacity or efficiency of the plant during the refits allows the protection of low-lying areas to be maintained and more electricity to be generated. The strategy requires energy to be used in pumping to achieve the current low tidal limits and the incoming tide to be curtailed to maintain the high tide extent, but there is very little effect on annual electricity production. Flexible operation can offer some protection from riverine flooding and existing inundation cycles can be maintained.

Variability in the duration and timing of the estuarine to freshwater transition of critically endangered European eel Anguilla anguilla

The European eel (Anguilla anguilla L.) is a critically endangered catadromous fish. Their inshore and in-river arrival as glass eel and elvers is an important stage of their life cycle, marking the transition from marine to freshwater habitats. Considerable knowledge gaps remain on the temporal and spatial patterns of this transition period to freshwater residency. Stable isotope (SI) analysis (δ13C, δ15N) was used to assess the timing and duration of the marine to freshwater transition among glass eels and elvers migrating upstream of the weirs at, or just upstream of, the tidal limit of four English rivers. (Parrett, Frome, Piddle, Chelmer). Variability in SI was low in the Parrett and Frome, resulting in narrow isotopic niches, but was high in the Piddle and Chelmer, resulting in wider niches. The Parrett and Frome data were then used to train a discriminant function analysis (DFA) model to classify eels as ‘marine’, ‘freshwater-established’ and ‘transitioning’. When applied to the Piddle and Chelmer eel SI data, only a small proportion of eels were classified as marine and transitioning, with most being freshwater established. These results suggest that most eels present in the lower reaches rivers have been present for sufficient time for their SI values to represent feeding on local prey resources, with relatively few eels being newly arrived from the marine environment. The transition of eels from marine to freshwater in this species can therefore be prolonged, with many ascending rivers at least one winter after their initial arrival.

Warm Jupiters Beyond the Tidal Synchronization Limit may Exhibit a Wide Range of Secondary Eclipse Depths

With the JWST we can now characterize the atmospheres of longer-orbit planets, but this moves us into a regime where we cannot assume that tidal forces from the star have eroded the planets’ obliquities and synchronized their rotation rates. These rotation vectors may be tracers of formation and evolution histories and also enable a range of atmospheric circulation states. Here we delineate the orbital space over which tidal synchronization and alignment assumptions may no longer apply and present three-dimensional atmospheric models of a hypothetical warm Jupiter over a range of rotation rates and obliquities. We simulate the secondary eclipses of this planet for different possible viewing orientations and times during its orbital, seasonal cycle. We find that the eclipse depth can be strongly influenced by the rotation rate and obliquity through the timing of the eclipse relative to the planet’s seasonal cycle, and advise caution in attempting to derive properties such as albedo or day–night transport from this measurement. We predict that if warm Jupiters beyond the tidal limit have intrinsic diversity in their rotation vectors, then this will manifest itself as dispersion in their secondary eclipse depths. We explore eclipse mapping as a way to uniquely constrain the rotation vector of warm Jupiters but find that the associated signals are likely at the edge of the JWST performance. Nevertheless, as the JWST begins to measure the secondary eclipses of longer-orbital-period planets, we should expect to observe the consequences of a wider range of rotation states and circulation patterns.

Using acoustic tracking of an anadromous lamprey in a heavily fragmented river to assess current and historic passage opportunities and prioritise remediation

AbstractAnthropogenic structures extensively fragment riverine systems, reducing longitudinal connectivity, inhibiting migration and leading to severe declines in many fish populations, especially for diadromous species. This study investigated the upstream spawning migration of anadromous river lamprey (Lampetra fluviatilis) in a heavily fragmented tributary of the Humber Estuary, the location of one of the largest UK river lamprey populations. Overall, this study quantified river lamprey migration, spawning habitat distribution and historic river levels to develop a novel empirical index to understand the impact of man‐made barriers and prioritise their remediation. Passage at all weirs only occurred during episodic high river levels, often after prolonged delays with no lamprey passing below average levels for the time of year or utilising the fish pass at the first weir (T1) at the tidal limit. Barrier passage opportunities at the first four weirs were only possible for 30.3%, 38.7%, 52.1% and 6.7% of the migration period, but were lower and severely limited in 15 of the last 21 years. In addition, more lamprey (60%, n = 18) were last detected in reaches with no spawning habitat than in spawning habitat (40%, n = 12). Given the impassibility of, and lack of retreat from, T1 to other Humber tributaries, the River Trent is currently considered an ecological trap for a large proportion of lamprey that enter from the Humber Estuary. This passage should be urgently remediated, per the prioritisation index presented here, to aid river lamprey conservation, especially given their status as a designated feature of the Humber SAC.

Defining estuarine squeeze: The loss of upper estuarine transitional zones against in-channel barriers through saline intrusion

Here we define, for the first time, the concept of estuarine squeeze and lay out recommendations for the consistent use of terminology for this new but critical research area. Climate and catchment-driven reductions in river flow together with rising sea levels are increasing estuarine salinities and driving saltwater into upper estuarine zones. This saline intrusion is exacerbated in regions where land level is falling (i.e. relative sea level rise) and in catchments subject to high freshwater demand and water regulation, which reduces river flow. In unmodified systems, many estuaries would naturally migrate inland in response to sea level rise. However, estuaries are some of the most anthropogenically impacted ecosystems in the world, being settlement and development hubs due to the ecosystem services they provide. To protect these assets, many estuaries have man-made in-channel barriers (such as dams, weirs and sluices) at their inland tidal limits, a trend that is likely to continue in the future to protect against the impacts of climate change. As sea levels rise and river flows reduce, saltwater will move further inland. This increasing saline intrusion will be most detrimental for upper estuarine, low salinity (oligohaline) and tidal freshwater zones, which will progressively become ‘squeezed out’ against these barriers. We have termed this concept ‘estuarine squeeze’ and define this as ‘the progressive loss of extent of upper estuarine tidal freshwater and oligohaline zones against in-channel man-made barriers through saline intrusion and increasing salinities driven by relative sea level rise and/or reductions in river flow’. A lack of research into the structure and functioning of tidal freshwater zones in particular means that the impact of their reduction and/or loss on the wider estuary is unknown. However, there are indications that these zones may play a key role in estuarine biogeochemical cycling, habitat provision, primary and secondary production, food-web functioning, and the provision of trophic subsidies to the brackish estuary and coastal zone. Loss and/or reduction of these zones through estuarine squeeze may therefore result in a net loss of function, with critical implications for the ability of estuaries to continue to provide key ecosystem services into the future.

Observations of the incipient and penultimate stages of Holocene marine terrace development

AbstractFlights of Holocene marine terraces are useful for reconstructing past earthquakes, but coastal erosion can remove terraces from the landscape, potentially leading to incorrect estimates of earthquake magnitude and frequency. Relatively little effort has been afforded to studying terrace erosion processes, and this paper presents the first field evidence that we are aware of documenting terrace erosion rates. Two case studies from New Zealand provide a unique opportunity to observe the beginning and end phases of terrace development. We present downwear and backwear erosion measurements, showing that both sets of processes are important. Micro‐erosion meter measurements from Kaikōura Peninsula, South Island, confirm that downwear processes are modifying new marine terraces that were created when the peninsula was uplifted about 1 m during the 2016 earthquake. Erosion rates were high immediately following uplift as the relatively barren intertidal rock shore platform rapidly transformed into an incipient marine terrace with cover deposits. However, the Kaikōura earthquake uplifted shore platforms only a small distance above the upper tidal limit and ongoing downwear and backwear erosion may begin to remove parts of this terrace in future decades. We explored this prospect with a case study at Māhia Peninsula, North Island, where 100–300 years have elapsed since the last terrace‐forming earthquake. Historical photographs were used to document about 80 years of backwear erosion. Terrace erosion rates have been nearly constant through this period, and extrapolation implies that the terrace will be removed in places by 2030. The erosion data in this paper provide new insights into how terraces can be removed from the landscape, but there are many complicating factors. To help understand these factors we present a new conceptual model of marine terrace creation and destruction for soft‐rock coasts.

Study of Jingjiang Beach Morphodynamics in the Tidal Reach of the Yangtze River

Large marginal sandbanks in tidal rivers experience periodic splitting processes. In this paper, the morphodynamic evolution of Jingjiang Beach, a sandbank on the Yangtze tidal river, has been investigated based on measured data. The results show that the duration of the splitting process in the middle and lower sections of Jingjiang Beach is 4–6 years. The periodical evolution occurred both in flood season and dry season, with a slight difference in the initial stage of splitting. This paper focuses on the evolution characteristics related to strong human activities since 2003. Ever since the second stage of the 12.5 m Deepwater Channel Project (DCP), the volumes above the 10 m and 12.5 m isobaths of Jingjiang Beach have been generally decreasing. The elevation data in recent years have demonstrated that the upper section, and nearshore side of the middle section, of Jingjiang Beach have tended to be stable. Moreover, the migration distance for a splitting sand body at the tail of Jingjiang Beach appears to have shortened. With the operation of the Three Gorges Reservoir, the number of days with discharge less than 15,000 m3/s has shown a decreasing trend; thus, the development scale of Jingjiang Beach could decline in the future.

Human impacts on riverbed morphology and hydrology in the lower reaches of the West River, China

AbstractNatural and anthropogenic factors have led to a series of fluvial sedimentation and hydrological changes in the lower West River, China. To evaluate the influence of human activities on the channel's morphology and examine the hydrological responses, we analyzed channel adjustments and changes of low water levels in the main river channel between Wuzhou and Zhaoqing that occurred over the recent decades. Due to a remarkable decrease in the suspended sediment concentration, the suspended sediment outputs during this period were larger than the sediment inputs for 27 consecutive years from 1987 to 2013. Therefore, the riverbed became a sediment source, with a high scouring of riverbed material. Between 2004 and 2014, the channel capacity of the lower reaches increased by 302.02 × 106 m3, and the average riverbed elevation decreased by 2.21 m, approximately 11.8% of which was caused by bed sediment loss induced by natural scour through sediment depletion; the remainder was caused by waterway engineering and sand mining, accounting for 0.8 and 87.4%, respectively. Approximately 91.3% of 1,312 river cross‐sections exhibited downcutting, indicating significant adjustment of the riverbed. This resulted in a substantial decline in the low water level in the 44‐km upstream section above Ducheng, which is located at the end of the tidal limit during dry seasons. However, this change was not noticeable in the section below Ducheng owing to the small water surface gradient and tidal influence. These findings reveal that human activities are a major driving force in the evolution of large rivers and the complexity of the hydrological responses to riverbed downcutting in a tidal reach, providing useful information for future river sand mining management and navigable engineering design of the lower West River and other rivers.

Natural and reservoir-induced channel changes in the Yangtze River Tidal Reach

The Yangtze River Tidal Reach (YRTR) is one of the most important land-sea transition zones in China. The YRTR is 640 km long and the adjacent land area of 220,000 km2 has a population of ca. 150 million. Even though the effect of Three Gorges Dam (TGD) on its downstream has been extensively studied, a comprehensive understanding of natural and dam-induced changes in the YRTR is still a challenging task. Here we use a digital elevation model and a numerical model to identify the causes of channel changes in the YRTR. Our analysis highlighted three main points.1) Changes in the tidal range caused an increase in the ebb maximum discharge and led to an increase in channel width, depth, and cross-section area. However, the channel depth abruptly starts to dwindle at the transition zone where unidirectional ebb flow changes to bidirectional ebb-flood flows. 2) Dam interception has reduced the sediment supply into the YRTR causing a change from deposition to erosion in the river bed. An increase of one billion m3 in reservoir capacity cut down annual suspended sediment supply by 2.26 million tons and scoured 1.54 million tons of riverbed sediment annually from the YRTR into the estuary. 3) Dam storage caused an increase of ca. 3800 m3/s runoff into the YRTR in the spring season during 2001–2013 compared with the same period in 1959–1970. This caused an average increase of ca.10% in the water diversion rate into the branch and an increase in flow velocity in flood plains and branch channels of 2.9 and 2.5 times that in the trunk channels. Such hydrodynamic increases have accelerated erosion of the branches and flood plains in recent decades especially post TGD. The study presents original insights into the tidal channel longitudinal abrupt changes induced by shift of natural flow regime, and riverbed changes from deposition to erosion induced by reservoirs, and indicates the likely response of the YRTR to further decline in sediment load in the future.

Declines in suspended sediment concentration and their geomorphological and biological impacts in the Yangtze River Estuary and adjacent sea

Variations in suspended sediment concentrations (SSCs) in coastal areas affect both geomorphology and biological environment. In this study, more than 82,000 surface-water samples have been collected twice daily at 10 stations in the Yangtze River Estuary and adjacent sea (YRE–AS) between 1990 and 2020 to study how the SSC has varied in response to intense human activities in the Yangtze watershed. Between the periods 1990–1999 and 2000–2020, the annual averaged SSC at the Datong station (the tidal limit in the Yangtze River) decreased by 56% (from 0.36 to 0.16 g/l, p < 0.05), which was mainly owing to sediment retention by dam constructions and the implementation of soil conservation measures in the drainage basin. Meanwhile, the annual averaged SSC in the YRE–AS (10 stations) decreased by only 28% (from 0.56 to 0.40 g/l, p < 0.05), which was markedly smaller than the relative decrease in the SSC at Datong. This phenomenon is primarily because that the sediment resuspension induced by tides and waves can mitigate the influence of the fluvial suspended sediment discharge (SSD) decline on the SSC in the YRE–AS. There is a time lag of 0–12 years between the change in SSD from the Yangtze River and the subsequent response of the SSC levels in the YRE–AS. The time lag increased from 0 to 1 years in the tidal current limit to ∼12 years in the mouth bar area of the YRE, along with the marine force getting stronger and the riverbed sediments getting finer seaward. As the shading effect of suspended sediments in seawater can limit photosynthetically available radiation, the decline in the SSC might affect the growth of phytoplankton in the YRE–AS. Since 2000, the concentration of chlorophyll a (Chl-a) off the YRE–AS has increased by 61.4% (from 3.45 to 5.57 mg/m3, p < 0.05) and the annual frequency of algal blooms has increased by over 5 times (increased from 6.27 to 31.6, p < 0.05). The decreased SSCs in the YRE–AS and its impacts on the geomorphology and biological environment would aggravate because of increased human activities.

Coastal wetland responses to a century of climate change in northern Sahara, Morocco

AbstractCoastal wetlands are highly sensitive to changes occurring at the coastline. It is critically important to determine region‐specific projections for these areas due to their specificities and vulnerabilities to climate change. This work aimed to value the impacts of recent climate changes at West Africa Sahara coastland, southern Morocco, at Khnifiss Lagoon. We have applied a combined approach using remote sensing techniques and environmental reconstructions based on high‐resolution analysis of sediment cores, covering the current warm period. Remote sensing highlighted changes to the lagoon inlet, accompanied by a greater meandering character of the tidal channels. As a response, the sediment cores have recorded a predominant vegetation substitution due to changes in the tidal limit, and an increase in organic carbon accumulation was observed. For the current climatology, during positive phases of the North Atlantic Oscillation, winds reaching the coast strengthen in an east‐to‐west direction. In the Khnifiss Lagoon, whose inlet is dominated by the ebb tide, the intensity and direction of the winds on the coast at surface level modifies its connection to the ocean by increasing sediment transport toward the interior of the lagoon. Locally biological responses to wind intensification, and possibly sea‐level rise, exemplify the lagoon sensitivity to large‐scale processes. Coastal vegetated wetlands are considered to be highly dynamic environments. However, we expect a loss of the upper tidal vegetation due to boundary conditions limiting the accommodation space in this arid environment in a possible future scenario of continuously inland tidal line displacement.

Quantitative Relationships between the Tidal Current Limit, Tidal Level Limit and River Discharge in the Changjiang River Estuary

Estuaries are areas where runoff and tide interact. Tidal waves propagate upstream from river mouths and produce tidal currents and tidal level variations along rivers. Based on the hydrological frequency analysis of river discharge in the dry season and flood season at the Datong hydrological station over the past 70 years, a three-dimensional estuary numerical model was used to produce the quantitative relationships between the tidal current limit, tidal level limit and river discharge in the Changjiang River estuary. The positions of tidal current limit and tidal level limit depend not only on river discharge but also on river topography. When river discharge varies from a hydrological frequency of 95% to 5%, the relationship between the tidal current limit and river discharge is y=2×10−13x3+3 × 10−8x2− 0.0074x+359.35 in the flood season, with a variation range of 90 km, and y=−4×10−10x3−1 × 10−5x2−0.1937x − 1232.9 in the dry season, with a variation range of 200 km. The relationship between the tidal level limit and river discharge is y=6×10−8x2−0.0096x+775.94 in the flood season, with a variation range of 127 km, and y=0.3428x2−17.9x+777.55 in the dry season, with a variation range of 83 km, which is located far upstream of the Datong hydrological station.

Tropical cyclone-induced water and suspended sediment discharge delivered by mountainous rivers into the Beibu Gulf, South China

Small mountainous rivers can deliver considerable freshwater discharge and suspended sediment discharge (SSD) to the ocean during tropical cyclones (TCs), and this has received special attention worldwide. However, little information is available on how TCs impact variations in water discharge and SSD delivered by mountainous rivers into the ocean. Here, based on the daily water discharge and SSD at the gauging stations of the tidal limit in July and August from 1965 to 2017, the Qin River (QR) and Nanliu River (NR), 2 small mountainous rivers located in the north of the Beibu Gulf, South China, were selected to quantify variations in water discharge and SSD into the Beibu Gulf during the TC period. The main results show that the net TC-induced water discharges in the QR and NR during July and August are larger than those of non-TC days, respectively. The net SSD increases in the QR and NR induced by TCs are far larger than those of normal days, respectively. Meanwhile, the water discharge and SSD in the flood season can be dominated by TC-associated rainfall, but the impacts on water discharge and SSD are influenced by the TC characteristics and basin environment. Moreover, it is found that in these 2 mountainous rivers, high water discharge delivers high SSD, but extremely high water discharge caused by TCs fails to produce extremely high SSD. It is expected that TCs will produce great impacts on SSD and water discharge of the mountainous rivers in South China behind global climatic change with increasing TCs events.

The Circumnuclear Disk Revealed by ALMA. I. Dense Clouds and Tides in the Galactic Center

Abstract Utilizing the Atacama Large Millimeter/submillimeter Array, we present CS line maps in five rotational lines (J u = 7, 5, 4, 3, 2) toward the circumnuclear disk (CND) and streamers of the Galactic center. Our primary goal is to resolve the compact structures within the CND and the streamers, in order to understand the stability conditions of molecular cores in the vicinity of the supermassive black hole (SMBH) Sgr A*. Our data provide the first homogeneous high-resolution (1.″3 = 0.05 pc) observations aiming at resolving density and temperature structures. The CS clouds have sizes of 0.05–0.2 pc with a broad range of velocity dispersion (σ FWHM = 5–40 km s−1). The CS clouds are a mixture of warm (T k ≥ 50–500 K, n H 2 = 103–105 cm−3) and cold gas (T k ≤ 50 K, n H 2 = 106–108 cm−3). A stability analysis based on the unmagnetized virial theorem including tidal force shows that 84 − 37 + 16 % of the total gas mass is tidally stable, which accounts for the majority of gas mass. Turbulence dominates the internal energy and thereby sets the threshold densities 10–100 times higher than the tidal limit at distance ≥1.5 pc to Sgr A*, and therefore it inhibits the clouds from collapsing to form stars near the SMBH. However, within the central 1 pc, the tidal force overrides turbulence and the threshold densities for a gravitational collapse quickly grow to ≥ 108 cm−3.

From tides to nucleotides: Genomic signatures of adaptation to environmental heterogeneity in barnacles.

The northern acorn barnacle (Semibalanus balanoides) is a robust system to study the genetic basis of adaptations to highly heterogeneous environments. Adult barnacles may be exposed to highly dissimilar levels of thermal stress depending on where they settle in the intertidal (i.e., closer to the upper or lower tidal boundary). For instance, barnacles near the upper tidal limit experience episodic summer temperatures above recorded heat coma levels. This differential stress at the microhabitat level is also dependent on the aspect of sun exposure. In the present study, we used pool‐seq approaches to conduct a genome wide screen for loci responding to intertidal zonation across the North Atlantic basin (Maine, Rhode Island, and Norway). Our analysis discovered 382 genomic regions containing SNPs which are consistently zonated (i.e., SNPs whose frequencies vary depending on their position in the rocky intertidal) across all surveyed habitats. Notably, most zonated SNPs are young and private to the North Atlantic. These regions show high levels of genetic differentiation across ecologically extreme microhabitats concomitant with elevated levels of genetic variation and Tajima's D, suggesting the action of non‐neutral processes. Overall, these findings support the hypothesis that spatially heterogeneous selection is a general and repeatable feature for this species, and that natural selection can maintain functional genetic variation in heterogeneous environments.

Seasonal use of a macrotidal estuary by the endangered dwarf sawfish, Pristis clavata

Abstract Sawfishes (Family: Pristidae) are one of the most imperilled fish families worldwide. There is an increasingly urgent need to better understand the biology, ecology, and population status of the five sawfish species to develop more effective conservation measures. The dwarf sawfish, Pristis clavata, is one of the least researched members of the pristids, with literature limited to analysing disparate datasets or collations of rare encounters in northern Australia. This study examined the spatial ecology of dwarf sawfish using targeted surveys and acoustic telemetry to determine its habitat use in a macrotidal estuary in northern Australia. Seventeen dwarf sawfish were tagged with acoustic transmitters and monitored in the Fitzroy River estuary and adjacent King Sound (Kimberley, Western Australia) between August 2015 and November 2017. Dwarf sawfish observed within the Fitzroy River estuary and King Sound were juveniles, ranging between 740 and 2,540 mm in total length. Catch per unit effort of dwarf sawfish in the late dry season was relatively high in the estuary, with the catch rate in 2015 being one of the highest reported for any sawfish species. Acoustic detections revealed a distinct seasonal pattern in the use of different parts of the estuary and King Sound, which was found to be driven by salinity. Dwarf sawfish predominately occupied a single large pool near the terminus of the tidal limit in the late dry season (August–November), before transitioning to regions in closer proximity to the river mouth or in King Sound in the wet and early dry seasons (December–July). Given the high abundance and residency of dwarf sawfish in the Fitzroy River estuary, this area is an important nursery for the species during the late dry season and should be formally recognized as a habitat protection area for the species.