Bottom Set Research Articles

Observation of shear‐wave splitting in the multicomponent node data from Atlantis field, Gulf of Mexico

ABSTRACTIn 2005, a multicomponent ocean bottom node data set was collected by BP and BHP Billiton in the Atlantis field in the Gulf of Mexico. Our results are based on data from a few sparse nodes with millions of shots that were analysed as common receiver azimuthal gathers. A first‐order look at P‐wave arrivals on a common receiver gather at a constant offset reveals variation of P‐wave arrival time as a function of azimuth indicating the presence of azimuthal anisotropy at the top few layers. This prompted us to investigate shear arrivals on the horizontal component data. After preliminary processing, including a static correction, the data were optimally rotated to radial (R) and transverse (T) components. The R component shows azimuthal variation of traveltime indicating variation of velocity with azimuth; the corresponding T component shows azimuthal variation of amplitude and phase (polarity reversal). The observed shear‐wave (S‐wave) splitting, previously observed azimuthal P‐wave velocity variation and azimuthal P‐wave amplitude variation, all indicate the occurrence of anisotropy in the shallow (just below the seafloor) subsea sediment in the area. From the radial component azimuthal gather, we analysed the PP‐ and PS‐wave amplitude variation for the first few layers and determined corresponding anisotropy parameter and VP/VS values. Since fracture at this depth is not likely to occur, we attribute the observed azimuthal anisotropy to the presence of microcracks and grain boundary orientation due to stress. The evidence of anisotropy is ubiquitous in this data set and thus it argues strongly in favour of considering anisotropy in depth imaging for obtaining realistic subsurface images, at the least.

Protection of the coelacanth, a primitive fish in the coastal waters of Tanzania

Since the first coelacanth ( Latimeria chalumnae) capture in Tanzania in 2003 there have been about 35 other captures. With increasing call for protection, imposing ban on gill net fishery, which has resulted in the accidental captures is being considered off Kigombe, northern Tanzania. There is however, a dilemma because the gill net fishery is just recent, having been introduced towards attaining poverty eradication and food security goals. On the basis of the assessment of the environmental conditions of the coelacanth it is suggested that a total ban on bottom set shark nets may not be necessary except during the cooler SE monsoon period when coelacanth can easily venture into the cool shallow waters in search for food.

The artisanal elasmobranch fishery on the east coast of Baja California, Mexico: Characteristics and management considerations

The Mexican elasmobranch fishery ranks among the most important in the world, but details associated with fishing effort and species composition are generally unavailable, particularly for the extensive artisanal fishery. Seasonal surveys were conducted during 1998–1999 in the state of Baja California to determine the extent and activities of the artisanal elasmobranch fishery along its Gulf of California coast. Artisanal fishermen were highly opportunistic, and temporally switched targets depending on the local abundance of teleost, invertebrate, and elasmobranch fisheries resources. At least 17 species of shark, 13 species of ray, and 2 species of skate were identified among the 4495 specimens observed. Small, coastal sharks and rays were typically landed with bottom set gillnets during all surveyed seasons (spring–autumn) and large sharks were targeted with surface set (drift) gillnets during summer and autumn. Sharks and rays contributed similarly to elasmobranch landings, comprising 50.9% and 48.3%, respectively, of the total recorded catch. Overall landings were dominated by mustelid sharks (Mustelus henlei, Mustelus spp.) and the shovelnose guitarfish (Rhinobatos productus), with Alopias pelagicus, Gymnura spp., Heterodontus mexicanus, Mobula munkiana, Myliobatis californica, and Squatina californica also representing important components of the landings. Alopias vulpinus had previously been unreported for the Gulf of California and a new maximum size record was documented for Myliobatis longirostris. Catch rates (mean elasmobranchs/vessel/trip ± standard error) were lowest during autumn (17.4 ± 3.9) and highest in spring (35.3 ± 14.1). Adult, commonly gravid, A. pelagicus, Carcharhinus falciformis, and R. productus females comprised a disproportionately greater component of the landings than did their male counterparts. Given the decades of largely unrestricted exploitation of elasmobranchs in Mexican waters, population declines and shifts in size structure are likely to have occurred among those species with the lowest fecundity and latest ages at maturity.

Nutrient spatial variation during intrabasinal upwelling at the Cenomanian–Turonian oceanic anoxic event in the westernmost Tethys: An ichnological and facies approach

Facies and ichnofabrics of two sections of the Cenomanian–Turonian boundary interval are correlated and compared to evaluate changes in palaeoceanographic conditions controlled by palaeogeography in the distal zone of the westernmost Tethys (southern Iberian continental palaeomargin). The oceanic anoxic event (OAE-2) was interrupted by short aerobic or dysaerobic sub-events with slight differences in facies and ichnofabrics. These sub-events display a tendency to expansion from the more distal section (Hedionda) towards the proximal one (El Chorro). The anoxia spread out from the open sea, probably from slope upwellings. Higher diversity and abundance of trace fossils in comparatively distal position during the pre- and post-event intervals can be caused by a higher abundance of food available for tracemakers. Shape of the correlation bands of the OAE-2 event suggests that the event started earlier by about a few tens of thousands years in the most distal areas than in proximal, slightly higher bottom settings.

Temporal variation in the artisanal elasmobranch fishery of Sonora, Mexico

Baseline, species-specific information is largely unavailable for artisanal elasmobranch fisheries, but is essential for the monitoring of exploited populations and the development of effective management plans. Seasonal surveys were conducted during 1998–1999 in Sonora, Mexico to determine the extent and activities of artisanal elasmobranch fisheries operating in the eastern Gulf of California. Nineteen fishing sites were documented, the majority of which (84.2%) targeted elasmobranchs during some part of the year. Most small demersal sharks and rays were landed in bottom set gillnet fisheries that also targeted demersal teleosts, whereas large sharks were usually taken in directed surface longline or, to a lesser extent, drift gillnet fisheries. Rays numerically dominated sampled landings in Sonora (63.4%, n = 100,136), and catch rates exceeded those of sharks during spring and summer months. The shovelnose guitarfish, Rhinobatos productus, was the primary fishery target during these seasons. During autumn, small sharks, especially mustelids ( Mustelus spp.) were numerically dominant, but rays (e.g., Dasyatis dipterura) were also caught in large numbers. Winter landings in Sonora were principally composed of mustelid sharks, which represented the greatest seasonal catch rates of all elasmobranch taxa during this study. Large sharks were of comparably minor importance, with a limited summer fishery operating in the southern part of the state. Variation in catch composition was evident in association with differential interannual environmental conditions (El Niño and La Niña) and seasonal temperature fluctuations. Size composition of landings varied greatly by species, but relatively small size classes of sharks and rays were abundant and large, often gravid females of several ray species (e.g., R. productus and Narcine entemedor) supported spring and summer fisheries in nearshore waters. Populations of many large shark species (e.g., Carcharhinus leucas, Carcharhinus limbatus, Carcharhinus obscurus and Galeocerdo cuvier) have likely been overfished, prompting a shift in effort towards coastal populations of smaller elasmobranchs.

Factors controlling physico-chemical characteristics in the coastal waters off Mangalore—A multivariate approach

Water quality parameters (temperature, pH, salinity, DO, BOD, suspended solids, nutrients, PHc, phenols, trace metals—Pb, Cd and Hg, chlorophyll- a (chl- a) and phaeopigments) and the sediment quality parameters (total phosphorous, total nitrogen, organic carbon and trace metals) were analysed from samples collected at 15 stations along 3 transects off Karnataka coast (Mangalore harbour in the south to Suratkal in the north), west coast of India during 2007. The analyses showed high ammonia off Suratkal, high nitrite (NO 2-N) and nitrate (NO 3-N) in the nearshore waters off Kulai and high nitrite (NO 2-N) and ammonia (NH 3-N) in the harbour area. Similarly, high petroleum hydrocarbon (PHc) values were observed near the harbour, while phenols remained high in the nearshore waters of Kulai and Suratkal. Significantly, high concentrations of cadmium and mercury with respect to the earlier studies were observed off Kulai and harbour regions, respectively. R-mode varimax factor analyses were applied separately to surface and bottom water data sets due to existing stratification in the water column caused by riverine inflow and to sediment data. This helped to understand the interrelationships between the variables and to identify probable source components for explaining the environmental status of the area. Six factors (each for surface and bottom waters) were found responsible for variance (86.9% in surface and 82.4% in bottom) in the coastal waters between Mangalore and Suratkal. In sediments, 4 factors explained 86.8% of the observed total variance. The variances indicated addition of nutrients and suspended solids to the coastal waters due to weathering and riverine transport and are categorized as natural sources. The observed contamination of coastal waters indicated anthropogenic inputs of Cd and phenol from industrial effluent sources at Kulai and Suratkal, ammonia from wastewater discharges off Kulai and harbour, PHc and Hg from boat traffic and harbour activities of New Mangalore harbour. However, the strong seasonal currents and the seasonal winds keep the coastal waters well mixed and aerated, which help to disperse the contaminants, without significantly affecting chlorophyll- a concentrations. The interrelationship between the stations as shown by cluster analyses and depicted in dendograms, categorize the contamination levels sector-wise.

Comparative Analysis of Bottom Set Long Line and Boat Seine Fishing Effort Productivity In Juwana, Pati Regency

Bottom set long line and Boat seine are the divece which more used by fisherman at Juwana - Pati. This research is used for learn and comparison of productivity rate and many factors and profit rate rate of catch applied of Bottom set long line and Boat seine in Juwana - Pati. This research is used descriptive method. The output of this research is the productivity average per unit of Bottom set long line is 39,09 ton/unit/trip. Productivity per ABK is 0,89 ton/people/trip and the productivity average per trip is 12,27 ton/trip. The productivity average of boat seine is 260,10 ton/unit/trip. Productivity per ABK is 0,92 ton/people/trip and productivity average per trip is 12,60 ton/trip. Factor which influence the fishery product of Bottom set long line and Boat seine is the operasional value.F test analysis is known that all free variable can influence unfree variable significantly, which coefisien rate of determination (R 2 =R square) for bottom set long line is 96,6% and coefisien rate 97,7% for boat seine with the fault probability T tab Key word : Bottom set long line, Boat seine, Productivity, Pati

IMPACT OF A HYDROPOWER PLANT ON THE DOWNSTREAM REACH OF A RIVER/HIDROELEKTRINĖS POVEIKIS NEMUNO UPEI ŽEMUTINIAME BJEFE/ВОЗДЕЙСТВИЕ ГИДРОСТАНЦИИ НА РЕКУ НЯМУНАС В НИЖНЕМ БЬЕФЕ

The impact of a hydropower plant (HPP) on the environment, first of all on the riverbed of downstream reach as well as on fish communities is analysed in the paper. Frequent switching on and off of turbines has been determined to be the reason of intensive and long‐lasting riverbed scour, also significant reduction of fish communities. Each switching on and off of HPP turbines is found to cause a sudden change of water discharge and level in the downstream reach. Water level suddenly drops down after the turbine switches off. Uplift force of ground water flowing from a riverbed destructs a reinforcing layer of large ground particles formed during the self‐lining process. Scour of small particles from the bottom sets in. The riverbed deepens significantly until a new reinforcing layer forms. Suggestions are given to slow down turbine switching within technical possibilities. This simple measure allows to increase the length of a reflux wave, to reduce the speed of water level drop and the length of river reach under the scour danger. Santrauka Nagrinėjamas hidroelektrinės (HE) poveikis aplinkai, pirmiausia upės vagai žemutiniame bjefe, taip pat ir žuvims. Buvo nustatyta, kad dažnas HE turbinų įjungimas ir išjungimas yra pagrindinė intensyvaus ir ilgai trunkančio vagos plovimo, taip pat žymaus žuvų skaičiaus sumažėjimo priežastis. Žinoma, kad kiekvienas HE turbinos įjungimas sukelia staigų vandens debito ir lygio kitimą žemutiniame bjefe. Išjungiant turbiną vandens lygis staiga mažėja. Iš upės dugno ištekančio gruntinio vandens slėgio jėga sulaužo iš didžiausių grunto dalelių susidedantį apsauginį sluoksnelį, susidariusį savigrindos proceso metu. Prasideda smulkių grunto dalelių plovimas iš dugno. Upės dugnas žemėja, kol susidaro naujas apsauginis sluoksnis. Siūloma turbinų stabdymo procesą lėtinti kiek įmanoma iki techniškai priimtino mažiausio greičio. Ši paprasta priemonė leidžia padidinti atoslūgio bangos ilgį, sumažinti vandens lygio kritimo greitį ir sutrumpinti upės ruožo, kuriam grėstų plovimas, ilgį. Резюме Рассматривается воздействие гидростанции (ГС) на окружающую среду, в первую очередь на русло реки Нямунас в нижнем бьефе, а также на рыб. Было установлено, что частое включение и выключение турбин ГС является основной причиной интенсивного и длительного размыва русла реки, а также сокращения численности рыб. Известно, что каждое включение турбины ГС вызывает резкое колебание расхода и уровня реки в нижнем бьефе. После выключения турбины уровень воды резко понижается. Сила давления воды, вытекающей со дна реки, ломает защитный слой крупных частиц грунта, сформированный во время процесса самоотмостки. Начинается размыв мелких частиц грунта со дна. Дно реки понижается, пока не образуется новый защитный слой. Предлагается замедлить до технически возможной минимальную скорость остановки турбин. Этот простой способ позволяет увеличить длину волны отлива, уменьшить скорость падения уровня воды и сократить длину участка реки, которому грозит опасность размыва.

Pupping areas and mortality rates of young tiger sharks Galeocerdo cuvier in the western North Atlantic Ocean

From 1995 through 2006, 2577 bottom longline sets were conducted in the western North Atlantic Ocean from ~36° N, 75° W to 26° N, 97° W, an area that encompasses both the coastal waters off the southeastern USA (Atlantic) and the northern Gulf of Mexico (Gulf). Over the course of the study, 335 young-of-the-year (YOY) and 219 juvenile tiger sharks Galeocerdo cuvier were cap- tured. In the Gulf, YOY catch per unit effort (CPUE) was highest from ~88 to 83° W. In the Atlantic, the area of highest YOY CPUE occurred from 31 to 33° N. Distribution of juvenile tiger sharks was more uniform than observed for YOY throughout the range of the study. Annual survival rates of YOY and Age 1+ tiger sharks were estimated to be 51 and 62% in the Gulf and 39 and 27% in the Atlantic. Total instantaneous mortality rates of YOY and Age 1+ tiger sharks were, respectively, estimated to be 0.67 and 0.47 in the Gulf and 0.93 and 1.32 in the Atlantic. In the Atlantic, age-specific natural mortality (M) estimates ranged from 0.12 to 0.50 and from 0.08 to 0.36 for YOY and juvenile tigers sharks, respectively. Estimates of age-specific M were more variable in the Gulf, with values ranging from 0.11 to 0.98 for YOY and from 0.08 to 0.57 for juveniles. In the absence of nursery areas, the pro- duction of numerous fast-growing offspring presumably helps the species survive in the face of high total mortality.

The Occurrence and Distribution of Dolphins in Zanzibar, Tanzania, with Comments on the Differences Between Two Species of <i>Tursiops</i>

Incidental catches (bycatch) in gillnet fisheries off Zanzibar (Unguja Island), as a source of mortality among several species of dolphins, were reported in a questionnaire survey conducted in 1999. As a follow-up to that survey, from January 2000 to August 2003, we monitored the incidental catches of dolphins collected from 12 fish landing sites. Six species of dolphins were recorded from 143 specimens retrieved from bycatches in drift- and bottom set gillnets. Of these, 68 (48%) were Indo-Pacific bottlenose dolphins (Tursiops aduncus), 44 (31%) spinner dolphins (Stenella longirostris), 12 (8%) Risso's dolphins (Grampus griseus), 11 (8%) Indo-Pacific humpback dolphins (Sousa chinensis), 6 (4%) Pan-tropical spotted dolphins (Stenella attenuata) and 2 (1%) common bottlenose dolphins (Tursiops truncatus). Most of the bycatches (71%) were in nets set off the north coast of Unguja Island. In this paper, bycatch records are examined to describe the occurrence and distribution of dolphin species in Unguja Island coastal waters. The relatively large numbers of bycatch dolphins recorded indicate that bycatch may be a potential threat to local populations that need to be addressed in future conservation and management efforts in the region. Keywords: incidental catch, occurrence, distribution, dolphins, Unguja Island, Zanzibar West Indian Ocean Journal of Marine Science Vol. 4 (1) 2005: pp. 85-94

Development of Chutney Powder from Spider Conch, Lambis lambis (Linne, 1758)

The spider conch, Lambis lambis is one of the important fishery resources in the Vellapatti Village, Tuticorin, Southeast Coast of India. These conches land as by catches from crab nets (a modified bottom set gill net). The meat of gastropods is considered as a delicacy, rich in protein and the fat content is very low. Only a small section of the fisher folk consume its flesh, as it is less popular among the coastal fisher folk. For the utilization of this meat, chutney powder (a side dish for Idli or Dhosai in South India) was prepared from the meat. Biochemical, microbiological and sensory analyses were made to assess the shelf life for a period of four months. The meat had a protein content of 5.2%, 0.5% of carbohydrate and lipid content of 1%. The shelf life assessment of the chutney powder for a period of four months was carried out and it was found out that the product was good until the end of the assessment period. Even the TMA and TVB levels were found to be within the limits (10.8 mg per 100g and 18.36 mg per 100g, respectively). The present work reveals that the chutney powder is microbiologically and organoleptically good until the end of the storage period and safe for human consumption.

Reduction of harbour porpoise ( Phocoena phocoena) bycatch by iron-oxide gillnets

Reduction of harbour porpoise bycatch by use of high-density iron-oxide (IO) gillnets was tested in sea trials in the Danish North Sea bottom set gillnet fishery in September–October 2000. The trials were conducted as a controlled experiment with conventional gillnets as the control group. Eight porpoises were caught in the control nets and none in the IO nets, a highly significant reduction ( P < 0.01). Of the four fish species analysed only catch rates of cod ( Gadus morhua) were significantly ( P < 0.01) different between the two net types, with CPUE in the IO nets being ca. 70% of the CPUE in the control nets. Subsequent investigations in seawater tanks revealed that the difference in acoustic target strength of the two net types was not significant and that the nets behaved similarly under various water flow conditions. Based on laboratory tests of twine samples and analyses of catch composition we conclude that it is the mechanical properties of the IO nets, primarily the measured increase in stiffness, that are the main reasons for the differences in catch rates for cod and for porpoises between IO and conventional nets.

Carbon burial on river‐dominated continental shelves: Impact of historical changes in sediment loading adjacent to the Mississippi River

Seabed cores collected on the continental shelf adjacent to the Mississippi River show a direct temporal correlation between decreases in mass accumulation rate (factor of 2–3) and suspended sediment loads in the river. This mid 20th century decline is not apparent shelf‐wide due to sediment focusing and biological seabed mixing. Total organic carbon diagenetic loss rate across this sediment age interval is relatively uninterrupted when corrected for the non‐steady state mass flux. This suggests that organic carbon burial rates in oxic bottom water settings on river‐dominated continental margins are directly proportional to lithogenic flux. Variations in OM remineralization rates due to changes in the composition (marine vs. terrestrial) of the particulate OM flux at the sediment surface are a second‐order effect that cannot be distinguished in the bulk carbon sediment record at these oxic sites; although they may significantly alter the OM degradation‐induced CO2 flux to the overlying water column.

Escaping from the bottom set: finding a voice for school improvement

The author is the headteacher of a one form entry primary school near London. She describes the improvement journey that brought her school out of the Ofsted category of ‘special measures’ to become ‘outstanding’. Her view is that schools improve most radically when children and staff are encouraged to find a voice to express their learning needs. She argues that ability labelling, whether at an institutional or individual level, does nothing to further the process of school improvement.

Crustal seismic imaging from multifold ocean bottom seismometer data by frequency domain full waveform tomography: Application to the eastern Nankai trough

In offshore surveys, the deep crust is generally investigated by traveltime tomography applied to sparse ocean bottom seismometer data. The inferred velocity models are of limited resolution precluding a quantitative analysis of deep tectonic discontinuities. If dense arrays of ocean bottom seismometers can be deployed, the resulting data sets become amenable to full waveform processing, which should lead to a significant improvement in the resolution of structures. Such acquisition and processing were achieved on the eastern Nankai subduction zone. Full waveform inversion is entirely implemented in the frequency domain, enabling efficient finite difference wave modeling and allowing to limit the inversion to a few discrete frequencies of increasing value. Such a hierarchical procedure defines a multiresolution approach to seismic imaging. The data set was recorded by 93 instruments deployed along a 105‐km‐long profile. Thirteen frequencies ranging from 3 to 15 Hz were inverted sequentially. Wavelengths imaged by the full waveform inversion typically range between 0.5 and 8 km. Full waveform tomography reveals an intense compression in the upper prism and underlying backstop, as evidenced by several thrusts, the frontal ones still active, corresponding to negative velocity anomalies. The subducting Paleo‐Zenisu ridge is also reconstructed, above which stands the décollement. Velocities in the upper mantle beneath the ridge are rather low (7.5 km s−1), suggesting partially serpentinized mantle beneath the ridge. This paper demonstrates the feasibility of full waveform tomography based on dense ocean bottom seismometer data sets and its ability to quantitatively image the entire crust with a significantly improved resolution compared to what is usually achieved through traveltime tomography.

수조 실험에 의한 삼중자망의 뜸줄 높이에 대한 수치해석

An estimation of the headline height of a bottom trammel net set across under uniform current was achieved numerically from a differential equations describing the forces of the net and compared with the measured value in a flume tank experiment. The analysis on the shape of the bottom trammel net with the headline free was based on the equilibrium equation of the bottom gill net which was modified and slack of the trammel net was varied with net depth as shown in the tank experiment. The differential equations were solved by a forth-order Runge-Kutta method. The estimated headline heights with varied slack was found to be closer than that with constant slack when compared with the actual values.

‘Archeoegadi’: A GIS for the marine archaeological survey in the Egadi Islands

An important step in successfully locating an archaeological target is the definition of the search box. Then, the establishment of a target area involves aggregation and comparison of information from multiple sources. ‘ArcheoEgadi’ was a feasibility study to verify the application of geophysical and magnetic equipment and GIS system in marine archaeology. The main results of ‘ArcheoEgadi’ were a detailed map of the sea bottom and help setting up the equipment parameters for detecting operations on archaeological sites hidden by sediments and marine flora on the sea bottom. In particular, a relevant result was the discovery of the wreck of an 11th-century Arabian vessel.

Movement heterogeneity of dugongs, Dugong dugon (Müller), over large spatial scales

Seventy dugongs were fitted with satellite PTTs and/or GPS transmitters in sub-tropical and tropical waters of Queensland and the Northern Territory, Australia. Twenty-eight of the 70 dugongs were also fitted with time-depth recorders. The dugongs were tracked for periods ranging from 15 to 551 days and exhibited a large range of individualistic movement behaviours; 26 individuals were relatively sedentary (moving < 15 km) while 44 made large-scale movements (> 15 km) of up to 560 km from their capture sites. Male and female animals, including cows with calves, exhibited large-scale movements (LSM; > 15 km). Body length of travelling dugongs ranged from 1.9 to 3 m. At least some of the movements were return movements to the capture location, suggesting that such movements were ranging rather than dispersal movements. LSMs included macro-scale regional movements (> 100 km) and meso-scale inter-patch local movements (15 to < 100 km) and were qualitatively different from tidally-driven micro-scale commuting movements between and within seagrass beds (< 15 km). The mean ± S.E. macro-scale movement distance per individual was 243.8 ± 35.4 km ( N = 14 individuals that travelled > 100 km), with a mean ± S.E. travel time of 179.8 ± 29.0 h. The mean ± S.E. meso-scale movement distance per individual was 49.7 ± 3.3 km ( N = 28 individuals that made movements of 15–100 km), with a mean ± S.E. travel time of 52.3 ± 7.1 h. LSMs were rapid and apparently directed (mean ± S.E. travel speeds for GPS tagged animals; meso-scale movements = 1.3 ± 0.11 km/h, min = 0.3, max = 3.0; macro-scale movements = 1.6 ± 0.16 km/h, min = 0.8, max = 1.3). Tracked dugongs rarely travelled far from the coast (mean ± S.E. max distance = 12.8 ± 1.3 km). Dive profiles from the time-depth recorders suggest that dugongs make repeated deep dives while travelling rather than remaining at the surface, increasing their likelihood of capture in bottom set gill nets. Some animals caught in the high latitude limits of the dugongs' range on the Australian east coast in winter apparently undertook long distance movements in response to low water temperatures, similar to migrational movements by Florida manatees. Our findings that dugongs frequently undertake macro-scale movements have implications for management at a range of scales, and strengthen the aerial survey and genetic evidence for management and monitoring at ecological scales that cross jurisdictions.

Evaluation of Sampling Techniques for Age‐0 Lake Sturgeon in a Lake Michigan Tributary

AbstractRehabilitation of lake sturgeon Acipenser fulvescens in the Great Lakes would benefit from a greater understanding of early life stage attributes. The objectives of this study were to determine the efficacy of active and passive sampling gears for capturing age‐0 lake sturgeon and evaluate whether the most effective gear types were size selective. Fish were collected by means of active (i.e., wading, haul‐seine, backpack electrofishing, snorkeling surveys, and bottom trawls) and passive (i.e., gill nets, set lines, and fyke nets) sampling gears from June to November of 2002 and 2003 in the lower Peshtigo River, Wisconsin, and nearshore waters of Green Bay. Mean catch per unit effort (CPUE) was greatest for day wading (0.036 fish/min), followed by day haul‐seine surveys (0.029 fish/min), night wading (0.020 fish/min), and night haul‐seine surveys (0.015 fish/min). Night wading surveys captured the greatest number of fish (N = 118). Day and night electrofishing (CPUE = 0.008 and 0.013 fish/min, respectively), snorkeling (CPUE = 0.010 fish/ min), and gill nets (CPUE = 0.026 fish/d) were less effective gear types, while bottom trawls, set lines, and fyke nets did not capture fish. Smaller fish were captured during day wading and haul‐seine surveys (median = 71 and 68 mm, respectively) than during night wading and haul‐seine surveys (median = 120 and 133 mm, respectively). Based on these results, we recommend the use of wading and haul‐seine surveys to collect age‐0 lake sturgeon in shallow lotic and lentic systems, such as the lower Peshtigo River and Green Bay, that are characterized by low current velocity and high water clarity.

Data assimilative hindcast of the Gulf of Maine coastal circulation

A data assimilative model hindcast of the Gulf of Maine (GOM) coastal circulation during an 11 day field survey in early summer 2003 is presented. In situ observations include surface winds, coastal sea levels, and shelf hydrography as well as moored and shipboard acoustic Doppler D current profiler (ADCP) currents. The hindcast system consists of both forward and inverse models. The forward model is a three‐dimensional, nonlinear finite element ocean circulation model, and the inverse models are its linearized frequency domain and time domain counterparts. The model hindcast assimilates both coastal sea levels and ADCP current measurements via the inversion for the unknown sea level open boundary conditions. Model skill is evaluated by the divergence of the observed and modeled drifter trajectories. A mean drifter divergence rate (1.78 km d−1) is found, demonstrating the utility of the inverse data assimilation modeling system in the coastal ocean setting. Model hindcast also reveals complicated hydrodynamic structures and synoptic variability in the GOM coastal circulation and their influences on coastal water material property transport. The complex bottom bathymetric setting offshore of Penobscot and Casco bays is shown to be able to generate local upwelling and downwelling that may be important in local plankton dynamics.