Midwater Trawl Research Articles

Explaining technical change and its impacts over the very long term: The case of the Atlantic sardine fishery in France from 1900 to 2017

As an industry based upon the harvesting of wild resources, the fishery is often used in economics to showcase the effects of its specific nature on human behavior and the impacts of that behavior on the fish stock itself. The common-pool status usually encountered in the use of these resources makes them especially vulnerable to major shifts in the supporting ecosystems, but also to rapid technical change. In most other parts of the economy, technical change and the creative destruction that it brings along are viewed positively, and even given central role in recent theories of growth. Happily for the creatures of the ocean that are hunted, fishers do not fit the mould of Schumpetarian entrepreneurs bent on creative destruction. However, the technology that does drift onto boats has had concerning effects on the status of fish stocks with important economic and cultural consequences. We present a narrative of the French sardine fishery using the evolutionary theory of technical change. The wild binary segmentation approach was applied on a time-series of French sardine landings from 1900 to 2017. This analysis revealed three significant production change points associated with important technical changes in the fishery. The first change point, in 1927, is related to the introduction of the purse-seine in France. The introduction of the mid-water trawl is the second change point in the early 1970s. A third change point occurred in 1998, where we see a reswitch from mid-water trawl to purse-seine. Collectively, these results highlight the technological changes in sardine production that occurred, but more importantly, the impacts of these changes both on the structure of the population of sardines, and on the industry. The lesson we derive from this case study is that technical change should be considered as a succession of shifts rather than a unidirectional history.

A Baltic pelagic fish community revisited: Indications of profound changes in species composition in the Stockholm Archipelago

While the central Baltic herring (Clupea harengus) stock biomass is estimated to have been reduced by ∼80 per cent since the 1970 s, the local effects of this decline remain unassessed. Coastal commercial fishermen have witnessed a widespread depletion at the spawning grounds and generally low herring abundance in inshore areas. We carried out hydroacoustic surveys, supported by midwater trawling in August 2021, to test whether herring abundance and recruitment have changed over time in the inshore areas. We used assessed recruitment success, measured as young-of-the-year (YOY) abundance, as a proxy for herring abundance in several areas of the southeast Stockholm Archipelago and compared the results with previous investigations in this area (hydroacoustics 2000–2001 and midwater trawling 2002–2004 monitoring). We found that the composition of the fish community had undergone significant changes. While acoustic backscattering (sA) representing the whole fish community were similar to previous estimates, species composition was significantly changed. YOY herring, previously dominant, had declined from 55% to 4% in numbers, whereas three-spined stickleback (Gasterosteus aculeatus) increased from no representation to 76%. The observed changes makes it imperative that further investigation concerning herring population structure, biomass, and recruitment are conducted.

Spatial distribution of pacific herring <i>Clupea pallasii</i> on the northeastern shelf of Sakhalin Island in the modern period

Spatial distribution of pacific herring Clupea pallasii in the Okhotsk Sea waters at the northeastern coast of Sakhalin Island is described on the data of surveys conducted in June-October of 1989 and 1997-2020 using bottom and midwater trawls and Danish seine. The herring distribute widely, with the main aggregations in the area with depth up to 100 m between the Lunsky Bay and Piltun Bay. These aggregations are formed by the native herring of the North-East Sakhalin population, whereas the herring in the deeper waters belong to the Okhotsk population.

Exploring biophysical links of catch rate from mid-water trawl fishery in the north-west coast of India, Eastern Arabian Sea: A remote sensing approach

Fish frequently shift their distribution ranges as a result of changes in preferred environmental factors. Knowledge on distribution of fish in relation to their environmental optima is crucial for improving the understanding of fishing grounds and planning sustainable exploitation. This study investigated the monthly variability in environmental factors impacting the catch rate and the spatio-temporal distribution patterns of fish along northwest coast of India (NWCI) from 2017 to 2019. The time series images of sea surface temperature (SST), sea surface height anomaly (SSHa), chlorophyll-a concentration (CHL), and euphotic depth (Zeu) indicate close coupling between bio-physical variables in the region. Generalized Additive Model (GAM) applied to the catch rate of mid-water trawlers found that CHL and Zeu were the most influential variables for ribbonfish distribution, while CHL and SST influence squid distribution and SST and SSHa influence cuttlefish distribution. The total deviation explained 37.70 %, 10.70 %, and 22.28 % in the final model for simulating the spatio-temporal distribution of ribbonfish, squid, and cuttlefish, respectively. Environmental factors were significant in the final GAM model (p < 0.05). Spatio-temporal distribution patterns of fishery resources from mid-water trawlers showed that they were related to biophysical changes in the northwest coast of India. The high catch rate was observed in the northern latitudes during the post-monsoon (October), the major fishing season in the region. However, it shrank and shifted to southern latitudes during the winter (January). This study could be adapted to nearby countries in the Arabian Sea region for an effective and useful tool to monitor and manage the fishery resources. Furthermore, it could aid in decision-making for policymakers and resource managers to formulate strategies for holistic marine fishery management and sustainable resource exploitation.

북태평양 어장에서 명태 어획을 위한 소형 중층트롤 설계 및 성능 분석

In the North Pacific, trawl fishing is mainly carried out for pollack. Fishing gear of a size suitable for these trawler specifications is important in the operation of fishing gear. In addition, large-scale Russian shipping companies with capital power in the North Pacific are currently introducing large trawlers. The fishing gear used here is mainly from Europe and is very expensive. Therefore, it is necessary to design trawlers and fishing gear suitable for small shipping companies. The purpose of this study is to calculate the predictive towing forces by using the main specifications of small trawler and to present a midwater trawl design. For the price competitiveness of the completed fishing gear, it was designed using polypropylene ropes and polyethylene nets. In addition, we propose a trawl design method suitable for the specifications of a fishing boat and present the utility of a kite.

Turbulent flow interacting with flexible trawl net structure including simulation catch in flume tank

The interaction between fluid and the midwater trawl with stocked catches is extremely complex, but essential to improve the understanding of the drag force acting on the trawl, the behavior of the fishing structure during a trawling process, and to predict its selectivity process. The present study assesses the turbulent characteristics inside and around the midwater trawls with catch and without catch linked to its fluttering motion. The analysis is based on three-dimensional electromagnetic current velocity meter measurements performed in the multiple points inside and outside different parts of a 1/35 scaled midwater trawl model with the aim of access the main turbulent flow structure inside and around the gear. Time-averaged normalized flow velocity fields and turbulent flow parameters were analyzed from the measured flow data. Furthermore, Fourier analysis was conducted by watching the time–frequency Power spectrum content of instantaneous flow velocities fields, the fluttering trawl motions, turbulent kinetic energy, and momentum flux. Based on successive analyzes of mean flow characteristics and turbulent flow parameters, it has been demonstrated that the presence of catch inside the trawl net impacts the evolution of unsteady turbulent flow by creating large trawl fluttering motions that strongly affect the flow passage. The results showed that the time-averaged normalized streamwise and transverse flow velocities inside and around the trawl net with catch were 12.41% lower compared with that obtained inside and around the trawl without catch. The turbulent length scale and turbulent Reynolds number obtained in the different part of the trawl net with catch were about 33.05% greater than those obtained on the trawl net without catch, confirming that the unsteady turbulent flow developing inside and around the midwater trawl is influence by the catch and liner. It is observed that the motions of both the trawl without catch and the trawl with catch are mainly of a low-frequency activity and another component related to unsteady turbulent flow street. A complex fluid–structure interaction is then demonstrated where the fluttering motions of the trawl net affect the fluid flow inside and around trawl net, the fluid force, turbulent pattern, and simultaneously, the periodic unsteady turbulent flow influence the trawl motions.

A method to determine patterns for changing parameters of the rope-net shell of midwater trawl along its length

A technique for designing the rope-net shell of midwater trawl is described on example of the trawl Sirius Shelf project AS 11.012.000 designed by Aquaservice Ltd. Diagrams of the rope-net shell parameters distribution along trawl are used. The technique allows to analyze variations of these parameters for their optimization. A wide range of the trawl designing criteria can be taken into account using the diagram approach.

Developing a subseasonal ecological forecast to reduce fisheries bycatch in the Northeast U.S.

Over the past decade, substantial progress has been made in projecting and predicting the spatial distribution of many marine species at seasonal to multidecadal time scales. However, managers and fishers often need to make decisions at much shorter time scales. Subseasonal environmental forecasts, which generate predictions over one to several weeks, can now be combined with species-specific habitat preference data to create ecological forecasts that could facilitate dynamic spatial management. The development of such predictive tools could aid in identifying optimal times and areas for fishers to maximize target catch and avoid nontarget catch. Nontarget catch, or bycatch, can have numerous and potentially severe economic and ecological consequences. Here, we focus on a population of anadromous fish known collectively as river herring (alewife and blueback herring), as they are species of concern and are heavily impacted by bycatch. Using bottom trawl survey data from the Northeast US and subseasonal forecasts of sea surface temperature, we constructed a bycatch risk model to generate probabilistic predictions of river herring distributions in regions frequented by the US mid-water trawl fishery. Assessments of model skill showed that our ecological model performed well in predicting the distribution of river herring and that subseasonal forecasts were effective at 1-week timeframes. There was a clear seasonal effect on forecasted bycatch risk throughout the Northeast US, with particularly high risk in winter and spring months. Importantly, variability in risk was detectable at the weekly timescale and our model identified specific areas and times that fishers should avoid in order to decrease their likelihood of bycatch. The bycatch risk forecast developed in this study is a significant advance from near-real time forecasts and the foundation to build forecast systems by combining species co-occurrence models with subseasonal forecasts. As these subseasonal forecasts are available globally, this approach could be adapted to facilitate the management of other natural resource conflicts around the world.

Разработка физической модели процесса эксплуатации разноглубинного тралового комплекса

В статье авторы представляют смоделированную физическую траловую конструкцию и промысловую палубу рыболовного судна по теории мультифизического подобия траловых конструкций. Проведены экспериментальные исследования процессов эксплуатации модели тралового комплекса в гидроканале ООО «Фишеринг Сервис». Получена качественная картина динамических процессов тралового комплекса. Проведена сравнительная оценка модели промысловой машины, при смоделированном тралении с предоставленной моделью трала N-MWT-m2 1,65/6,0 м. Теоретически и экспериментально доказано обоснованность замены имитационной дели (число ниток, веревок и узлов в элементе дели). Проанализирована маневренность тралового комплекса и рыболовного судна для прицельности тралового лова и выполнения других промысловых операций с учетом ваера, качки и волнения. Все это выполнено для полномерной разработки физической модели процесса эксплуатации разноглубиного тралового комплекса. Все расчетные характеристики предварительно сделанные, были проверены во время экспериментальных исследований в гидроканале ООО «Фишеринг-Сервис». Экспериментальные исследования проводились с моделью разноглубиного трала методом: система «модель трала - голые концы - кабели - лапки - траловые доски - ваера». В ходе проведения экспериментов были получены геометрические и силовые параметры физической модели процесса эксплуатации разноглубинного тралового комплекса N-MWT-m2 1,65/6,0 м. Применено оборудование: пульт управления потоком воды в гидроканале; вертушка для измерения скорости потока воды: тензометрический комплекс MIC-200; миллиамперметр Ц4311; промысловая палуба (физическая модель) экспериментального рыболовного судна; эхолот Lowrance elite - 4xHDI; модель тралового комплекса N-MWT-m2 1,65/6,0 м. In the article, the authors present a simulated physical trawl structure and the fishing deck of a fishing vessel according to the theory of multiphysical similarity of trawl structures. Experimental studies of the operation processes of the model of the trawl complex in the hydrochannel of Fishering-Service LLC were carried out. A qualitative picture of the dynamic processes of the trawl complex has been obtained. A comparative evaluation of the model of a fishing machine was carried out during simulated trawling with the provided trawl model N-MWT-m2 1.65/6.0 m. Theoretically and experimentally proved the validity of replacing the imitation net (the number of threads, ropes and knots in the net element). Analyzed the maneuverability of the trawl complex and the fishing vessel was analyzed for the aiming of trawling and other fishing operations, taking into account warp, pitching and waves. All this was done for the full-scale development of a physical model of the process of operation of a mid-water trawl complex. All calculated characteristics, previously made, were verified during experimental studies in the hydraulic channel of Fishering-Service LLC. Experimental studies were carried out with a model of a mid-water trawl using the following method: the system "trawl model - bare ends - cables - legs - trawl doors - warps". In the course of the experiments, the geometric and power parameters of the physical model of the process of operation of the N-MWT-m2 1.65/6.0 m mid-water trawl complex were obtained. The following equipment was used: a water flow control panel in the hydrochannel; turntable for measuring the speed of water flow: strain gauge complex MIC-200; milliammeter Ts4311; fishing deck (physical model) of an experimental fishing vessel; echo sounder Lowrance elite - 4xHDI; model of the trawl complex N-MWT-m2 1.65/6.0 m.

Material properties and broadband backscatter measurements of individual euphausiids from the California coastal ecosystem

During the 14–24 May 2019 leg of the NOAA juvenile rockfish survey, zooplankton and small nekton were sampled off the coast of Northern California using a midwater trawl. This talk will focus on three euphausiid species (E. pacifica, T. spinifera, N. difficilis) which were present in most catches. Krill morphology (size and shape) and animal density relative to seawater were measured for 175 live (or very recently expired) individuals. Roughly 150 individual krill were frozen and brought back to land for mass measurements. Soundspeed relative to seawater was measured for 15 mixed-species aggregations of krill. Broadband (38–73 and 130–210 kHz) and narrowband (38, 50, 70, 120, 150, and 200 kHz) backscatter measurements were made on 32 live (or very recently expired) individual krill. These empirical measurements were then compared to predictive backscatter measurements using scattering model parameters measured for those individuals (when available). Differences between predicted and measured backscatter were generally small, although there were individual krill where measurements at some frequencies varied significantly (i.e. more than 3dB). Understanding the variability in scattering model inputs among and within these species, and the accuracy of scattering model predictions provides context (i.e. uncertainty estimates) for acoustic measures (or indices) of krill biomass.

Methods of Bidirectional Interpolation and Finite Volume Element for Surface Reconstruction and Volume Calculation of Midwater Trawl

The spatial surface of the midwater trawl was reconstructed with combined interpolation methods of cubic spline, cubic Heimite and polar-coordinate periodic spline interpolation. The results show that the trawl surface obtained only by the ring section method or the vertical section method has a certain deviation, especially when cubic spline is used only in the X direction. When the ring section is combined with the vertical section, and the cubic spline is combined with the cubic Heimite in the X direction, the smoothest feasible trawl surface can be obtained. Two different trawl volume algorithms (volume element method and numerical integration method) were proposed. The finite volume element method can accurately calculate the volume inside the trawl, and the approximation calculation result of the numerical integration method is also very ideal. The numerical integration method is easier to calculate and suitable for the calculation of the trawl volume under any circumstances.

Organisation of Fish Assemblage in the Mekong Delta as a Potential Indicator of Salt Water Intrusion

The problem of salt water intrusion into the Mekong Delta is a research topic involving various branches of science. Among the causes of this phenomenon are global sea level rise and regulation of the natural river discharge due to hydropower dam construction. The scale of the delta and its water dynamics complicate instrument-based observations. Meanwhile, communities of living organisms can serve as indicators of environmental heterogeneity. This study aimed to analyze the spatial variability of fish assemblage organisation in the Mekong Delta for use as an indicator of salt water intrusion. The composition of fish assemblages in different parts of the delta were determined by midwater trawl catches during January and April 2021. A total of ~15 thousand individual fish from 74 trawl hauls were captured and analyzed. The null hypothesis was that the fish assemblages in the Mekong Delta are represented by three taxonomic complexes that are localized in the upper course (freshwater), the lower course (brackish) and the middle course (zone of the contact of the fresh and brackish waters) of the delta. The taxonomic composition of the freshwater complex is the most poor and is represented by 9 families. Meanwhile, marginal (the contact zone) and brackish complexes include 26 and 23 families, respectively. Specimens of families Cobitidae, Eleotridae, Plotosidae and Siluridae that are only found in the zone of contact of fresh and brackish waters could be considered as indicator species of its position. The actual boundaries of the brackish and freshwater complexes were determined and the distance from them to the marine delta edge equals 34 and 78 km, respectively. Assessment of the salinity values that correspond to the complex boundaries is a task for future investigation. The position of the boundaries of freshwater and brackish fish taxonomic complexes has a biological basis and could be used as an indicator of salt water intrusion into the Mekong Delta.

Применение системы автоматизированного проектирования при разработке тралов

The system of computer-aided design of industrial fishing gear (CAD-FG), developed by the small innovative enterprise Digital Technologies Laboratory LLC with the support of FASIE, is an import-substituting product for manufacturers and designers of bottom and mid-water trawls. At the present stage of trawl operation, the use of CAD-FG on fishing vessels will ensure the correct setting of the trawl system (trawl doors, hydrodynamic shield, loading chain, etc.) for efficient operation of the trawl. The CAD-FG system allows you to save drawings of trawls in electronic format, which creates a database necessary for analyzing designs and transferring drawings to the relevant departments and divisions. An integral part of the competencies of highly qualified personnel is their ability to work with the CAD-FG software, with which it is possible to develop drawings of bottom and mid-water trawls, as well as to model their operation processes in 3D.

VERDA: A Multisampler Tool for Mesopelagic Nets

Different types and systems to discriminate plankton samples at different strata in the water column have been developed in recent decades. For sampling at sufficient depth, opening and closing zooplankton multinets are ideal because there is no contamination of one sample with organisms of the previous one. However, for bigger nets, such as those used to collect micronektonic organism, it is difficult to use multiple net units, and multiple cod ends are preferred because of their simplicity, but with the problem of sample contamination from having a common net passageway. We present here a cod-end Multisampler design, VERDA, that uses a carrousel-like system. Similar to some sediment traps, the system works like a revolver with six or eight compartments whose turning mechanism is triggered when the net arrives to a programmed depth level. This prototype was built with inexpensive and recycled components and electronics similar to Arduino® and Teensy PCB to carry out electronic control. The net we used for testing the equipment was a mid-size midwater trawl of ca. 30 m2 and total length of 58 m that works with a single towing cable and no doors. The overall system is useful for all type of ships, due to the relatively easy deployment operations and because the Multisampler does not need electrical cable or acoustics. In our case, we used a Marport® (Reykjavik Iceland) and Scanmar (Åsgårdstrand, Norway) sensors for real-time depth monitoring and opening distance.

ПРОМЫСЕЛ ТЮЛЬКИ В АЗОВСКОМ МОРЕ: ПРИМЕНЯЕМЫЕ ОРУДИЯ И СПОСОБЫ ЛОВА, ТЕНДЕНЦИИ СУДОВОГО ПРОМЫСЛА В СОВРЕМЕННЫЙ ПЕРИОД

This paper presents the main fishing methods and gears previously used or appraised and currently applied in the course of Black Sea-Caspian sprat (tyulka) fishing in the Azov Sea: their performance, merits and demerits. Black Sea-Caspian sprat (tyulka) fishing comprises the coastal fishing with pound nets and the fishing off the board of fishing vessels with purse seines ans midwater trawls. Recently, sprat catches with pound nets have been on the decline; meanwhile, exploitation of fishing vessels, albeit more resource- consuming and dependent on the multitude of factors, can prospectively be more productive than the coastal fishing, even though currently its shows lower catches. A brief characterization of the Black Sea-Caspian sprat (tyulka) industrial fishing in the recent years is given, its trends and challenges are discussed. Daily vessel reports have been processed for 2015-2021, and the comparative data on the operation of the vessels of various types are presented; the trends in the changing industrial fishing parameters during the investigated period show a considerable decrease in all the parameters. Another crucial issue is the necessity to reduce the by-catch of non-target fish species (sturgeons, Black Sea turbot) in long-continued depressed state with midwater trawls. The presented data will allow for the current status assessment of the Black Sea-Caspian sprat (tyulka) industrial fishing and envisioning the efficient, yet selective and environmentally sound fishing methods to be developed and revised.

Diversity of freshwater fish and water quality parameters associated threats of commercially important freshwater species caught by bottom trawl and mid-water trawl from the various habitat

Diversity of freshwater fish and water quality parameters associated threats of commercially important freshwater species caught by bottom trawl and mid-water trawl from the various habitat

Abundance and Distribution of Large Calcareous Thecosome Pteropods in the Northern Gulf of Mexico

The ecological role of large thecosome pteropods in the pelagic ecosystem of the northern Gulf of Mexico (GoM) may be substantial, both in the food web and biogeochemical cycling. We analyzed species abundances, vertical and horizontal distributions of large species with calcareous shells (those collected in 3-mm mesh nets). Pteropod samples were collected following the 2010 Deepwater Horizon oil (DWH) spill by two midwater sampling programs: the Offshore Nekton Sampling and Analysis Program (ONSAP 2011) and the Deep Pelagic Nekton Dynamics of the Gulf of Mexico (DEEPEND 2015) projects. All samples were collected using a 10-m2 Multiple Opening/Closing Net and Environmental Sensing System (MOC10) midwater trawl, with 3-mm mesh. This gear sampled five discrete depths between 0–1500 m. Over 13,000 pteropod specimens were examined, with 25 species identified. Clio pyramidata Linnaeus 1767 was the most abundant species during both collection periods. Five genera (Diacria, Clio, Styliola, Cuvierina, Cavolinia) demonstrated diel vertical migration from the mesoto epipelagic zone.

Can an invader support commercial fishing? A case study of walleye pollock Gadus chalcogrammus in the western Chukchi Sea

Based on research of bottom and mid-water trawl surveys conducted onboard Russian research vessels in 2003–2020, data on the spatial and vertical distributions of walleye pollock Gadus chalcogrammus in the western Chukchi Sea are presented. The response of walleye pollock distribution related to water temperature, as well as the features of its distribution are also presented. In the water column, schools were mainly comprised of juveniles, while medium-sized and large individuals were scarce. Approximately 98% of the total number of juveniles and 100% of the large pollock caught by bottom trawls were observed in the southern part of the study area, while most of medium-sized fish were found in the northern part. In the water column, walleye pollock was found within the upper 65 m layer. Near the bottom, the majority of small-sized fish and all large individuals were observed at depths shallower 100 m, while over 70% of medium-sized walleye pollock were found at depths than 100 m. In the water column, juveniles were observed at temperatures from −0.4 to 3.3 °C with largest concentrations within a temperature ranges of 0.9–1.8 °C) and 3.0 °C. Near the bottom, walleye pollock were recorded at a temperature from −1.2 to 4.3 °C with maximum number within the ranges 2–3 °C, > 3 °C, and 1–2 °C. A few individuals were found at bottom temperatures below 0 °C. In the water column, the abundance was low during the entire study period. Acoustic estimates were notably higher compared to those based on the results of bottom trawl surveys. The recent dramatic increase of walleye pollock abundance in the western Chukchi Sea appears to reflect a complex of factors, including warming of bottom temperatures, decreasing of ice cover, increasing of northward currents, better foraging conditions, and increasing abundance and biomass in the Bering Sea. An attempt is made to determine the prospects for commercial fishing of walleye pollock in the western Chukchi Sea. Bottom trawl surveys in 2019–2020 indicated an estimate of total allowable catch of 37,200 t for the 2021 and 2022 fishing seasons; in 2021 over 4000 t were caught. However, this kind of fishing cannot be considered sustainable due to several reasons, including the inability to forecast the number of fish migrating from the Bering Sea, the limited fishing season due to harsh climatic conditions, the remoteness of main ports, and the uncertain population status of its stocks.

Variation in mesopelagic fish community composition and structure between Mediterranean and Atlantic waters around the Iberian Peninsula

Mesopelagic fish populations are characterised by high species richness and abundance, and have been identified as important contributors to the active carbon fluxes in the open ocean. We report variability in communities of mesopelagic fish between five zones around the Iberian Peninsula, i.e. Balears and Alboran in the Mediterranean, and Cadiz, Lisboa and Galicia in the Atlantic. Day and night samples were collected from 7 layers of the water column with a midwater trawl fitted with a multisampler. Temperature and salinity regimes were very different on the Mediterranean and Atlantic sides of the peninsula, with much higher values through the entire water column in the Mediterranean, characterized by a strong pycnocline. The highest productivity was observed off Lisboa, where Chlorophyll a concentrations were two orders of magnitude higher than in any other zone. Samples from the western Mediterranean held 22 fish species, while 67 were found in the Atlantic. The lowest diversity and the highest dominance were observed in Balears, and the highest diversity in Cadiz zone. In all zones, but particularly in those in the Mediterranean, mesopelagic populations were dominated by a high number of small fish with low individual biomasses. The species Benthosema glaciale, Cyclothone pygmaea and Ceratoscopelus maderensis were common in the Mediterranean populations, whereas in the Atlantic, Cyclothone microdon/livida, Valenciennellus tripunctulatus, Ceratoscopelus warmingii and Benthosema suborbitale were the most common species. Temperature and salinity (both at surface and in the mesopelagic zone) were the main environmental factors explaining variability in assemblage composition. A persistent (day-night) deep scattering layer was observed using the vessel-based echosounder in all zones, and was comprised primarily of the gonostomatid Cyclothone spp. Night-time echosounder observations of scattering layers near the surface were observed in Balears, Alboran, Cadiz and Lisboa, where night surface net collections indicated that Myctophidae, Stomiidae and Phosichthyidae migration extended to the upper 100 m. Sternoptychids and the gonostomatid Sigmops elongatus seldom reached the upper 100 m in their night vertical migrations. Night stratified hauls of 30 m resolution carried out in the epipelagic zone showed that abundances maxima of migratory fish coincided with the location of the Chlorophyll a maxima.

Incorporating expanded sampling into an alternative abundance index for the Fall Midwater Trawl survey

The Fall Midwater Trawl (FMWT) Survey has been conducted near continuously since 1967 to assess the abundance and distribution of pelagic fish species throughout the San Francisco Bay/Sacramento-San Joaquin estuary (Bay Delta). For most of this period, sampling 100 core stations provided data for abundance and distribution analyses. Another 22 (non-core) stations were added to the FMWT 8 to 28 years ago to supplement the original 100 (core) stations. However, relative abundance indices are published annually from only the data collected at the core stations. Here we incorporate data from non-core stations along with core station data to calculate an alternative index that also integrates modern estimates of water volume within the Bay Delta into an index calculation. The use of data from non-core stations in calculating the alternative index was particularly useful for American Shad (Alosa sapidissima) and Threadfin Shad (Dorosoma petenense). Consistently high catches at non-core stations for a couple species and modest catches for a couple additional species highlight the value of these additional catch data for our understanding of how fishes are distributed in the estuary.