Modeling the effects of fish migration on bias and variance in area-swept estimates of biomass: a vector-based approach

1998 ◽  
Vol 55 (12) ◽  
pp. 2622-2641 ◽  
Author(s):  
Murdoch K McAllister
Keyword(s):  
Interannual Variability ◽  
Fish Abundance ◽  
Fish Migration ◽  
Fish Biomass ◽  
Trawl Survey ◽  
Migration Rates ◽  
Acoustic Surveys ◽  
Survey Designs ◽  
Migration Velocities

Estimates of fish abundance from trawl and acoustic surveys are typically imprecise and may often be biased because of uncontrollable factors such as fish migration. In this paper, I present a model for the bias in area-swept biomass estimates when fish migration occurs as the survey is conducted. Bias in acoustic and trawl survey biomass estimates is treated as a function of the direction and speed of the vessel and the fish in each survey transect and results mainly from distortions in the observed area across fish aggregations. I use the model to evaluate the magnitude and direction of bias and interannual variability in biomass estimates from fish migration in systematic transect survey designs. For even relatively low fish migration velocities (<0.5 m/s), bias in estimated fish biomass can be very large (>500%). Furthermore, relatively small interannual variability in fish migration rates and vessel speeds (e.g., CV = 0.1 for each) can result in very large interannual error variability in biomass estimates (e.g., CV > 0.5). Designs with the least bias and variance in biomass estimates had the fastest vessel speeds along transects, more survey vessels, shorter transects, and transects aligned roughly parallel to the direction of fish migration.

Towards a balanced presentation and objective interpretation of acoustic and trawl survey data, with specific reference to the eastern Scotian Shelf

2016 ◽  
Vol 73 (12) ◽  
pp. 1914-1921 ◽  
Author(s):  
J. Michael Jech ◽  
Ian H. McQuinn
Keyword(s):  
Atlantic Cod ◽  
Management Strategies ◽  
Pelagic Fish ◽  
Trophic Levels ◽  
Fish Abundance ◽  
Forage Fish ◽  
Specific Reference ◽  
Trawl Survey ◽  
Scotian Shelf ◽  
Eastern Canada

A debate has developed over the ecosystem consequences following the collapse of Atlantic cod throughout the coastal waters of eastern Canada. The explosive increase in pelagic fish abundance in scientific bottom-trawl catches on the eastern Scotian Shelf has been interpreted as being due to either (i) a “pelagic outburst” of forage fish abundance resulting from predator release or conversely (ii) a change in pelagic fish vertical distribution leading to a “suprabenthic habitat occupation” thereby increasing their availability to bottom trawls. These two interpretations have diametrically opposing ecological consequences and suggest different management strategies for these important forage fish species. We argue that an objective evaluation of the available evidence supports the hypothesis that the abundance of forage fish has not increased in response to the demise of cod and other top predators, and the reliance on a single sampling gear with low catchability has biased and will continue to bias the interpretation of demographic trends of pelagic fish populations. We advocate that multiple sampling technologies providing alternative perspectives are needed for the monitoring and management of the various trophic levels if we are to achieve a balanced and objective understanding of marine ecosystems.

The effects of salinity, turbidity and flow on fish biomass estimated acoustically in two tidal rivers

2014 ◽  
Vol 65 (3) ◽  
pp. 267 ◽  
Author(s):  
V. F. Matveev ◽  
A. D. L. Steven
Keyword(s):  
Large Scale ◽  
River System ◽  
Fish Abundance ◽  
Practical Significance ◽  
Small Scale ◽  
Fish Biomass ◽  
Strongly Correlated ◽  
Commercial Catch ◽  
Four Seasons ◽  
Annual Means

Establishing drivers of fish abundance in estuaries is an important task of both theoretical and practical significance. Commercial catch data help explain large-scale variation in fish productivity; however, there is insufficient understanding of small-scale changes. We analysed correlations between acoustically estimated fish biomass (FB) and environmental variables, which included indices of primary productivity and physio-chemistry in a coastal river system during four seasons. Spatial series of FB were obtained for the Logan River (main estuary) and the Albert River (tributary) located in South East Queensland, Australia. Most of the year, FB was significantly higher in the Albert River. Annual means for discharge, salinity and pH were significantly lower, whereas phosphorus concentrations were higher in the Albert River. Out of 15 hydrological variables tested, FB was strongly correlated only with salinity, conductivity and turbidity. In the Albert River, where fish were larger, as indicated by greater target strengths, FB was positively correlated with river discharge. Our results suggest that salinity and turbidity can be important seasonal drivers of fish abundance in communities dominated by Mugil spp. and Nematalosa erebi and that the flow-biomass relationship may appear independent of the effects of primary production.

Ecological effects of trawling fisheries on the eastern Australian continental shelf: a modelling study

2013 ◽  
Vol 64 (11) ◽  
pp. 1068 ◽  
Author(s):  
Marie Savina ◽  
Robyn E Forrest ◽  
Elizabeth A Fulton ◽  
Scott A Condie
Keyword(s):  
Continental Shelf ◽  
New South ◽  
Ecosystem Model ◽  
Fish Biomass ◽  
Trawl Survey ◽  
Fishing Pressure ◽  
Survey Area ◽  
Trawl Fishery ◽  
South Wales ◽  
Survey Results

The New South Wales Offshore Trawl Fishery began to expand in 1976, following a large exploratory trawl survey carried out on the fishing grounds of the upper continental slope. This survey was repeated 20 years later with the same vessel and using similar protocols. Comparison of the survey results suggested that the overall fish biomass in the survey area had substantially decreased after 20 years. We have implemented an ecosystem model using the Atlantis framework to (1) emulate the evolution of the shelf ecosystems from 1976 to 1996 and (2) explore the effects of alternative fishing pressures on those ecosystems. We have been able to emulate the observed decline of most of the commercial groups of fish species in the Offshore Trawl Fishery, including sharks, and our results confirmed that fishing pressure was the most important cause of these observed changes. Fourteen fishing scenarios highlight the competing nature of some of the ecosystem-based sustainable fishing objectives.

A combined acoustic and trawl survey for efficiently estimating fish abundance

1996 ◽  
Vol 26 (1-2) ◽  
pp. 75-91 ◽  
Author(s):  
Inigo Everson ◽  
Mark Bravington ◽  
Catherine Goss
Keyword(s):  
Fish Abundance ◽  
Trawl Survey

Seasonal and diel effects on acoustic fish biomass estimates: application to a shallow reservoir with untargeted common carp (Cyprinus carpio)

2017 ◽  
Vol 68 (3) ◽  
pp. 528 ◽  
Author(s):  
Imed Djemali ◽  
Jean Guillard ◽  
Daniel L. Yule
Keyword(s):  
Cyprinus Carpio ◽  
Methane Flux ◽  
Cold Season ◽  
Fish Biomass ◽  
Night Time ◽  
Shallow Reservoir ◽  
Four Seasons ◽  
Carp Cyprinus Carpio ◽  
Fishery Exploitation ◽  
Acoustic Surveys

The aim of the present study was to understand how seasonal fish distributions affect acoustically derived fish biomass estimates in a shallow reservoir in a semi-arid country (Tunisia). To that end, sampling events were performed during four seasons (spring (June), summer (September), autumn (December) and winter (March)) that included day and night surveys. A Simrad EK60 echosounder, equipped with two 120-kHz split-beam transducers for simultaneous horizontal and vertical beaming, was used to sample the entire water column. Surveys during spring and summer and daytime hours of winter were deemed unusable owing to high methane flux from the sediment, and during the day survey of autumn, fish were close to the reservoir bottom leading to low detectability. It follows that acoustic surveys should be conducted only at night during the cold season (December–March) for shallow reservoirs having carp Cyprinus carpio (L.) as the dominant species. Further, night-time biomass estimates during the cold season declined significantly (P<0.001) from autumn to winter. Based on our autumn night-time survey, overall fish biomass in the Bir-Mcherga Reservoir was high (mean (±s.d.) 185±98 tonnes (Mg)), but annual fishery exploitation is low (19.3–24.1Mg) because the fish biomass is likely dominated by invasive carp not targeted by fishers. The results suggest that controlling carp would help improve the fishery.

scholarly journals Empirical comparisons of survey designs in acoustic surveys of Gulf of Maine-Georges Bank Atlantic herring

2006 ◽  
Vol 36 ◽  
pp. 127-144 ◽  
Author(s):  
W J Overholtz ◽  
J M Jech ◽  
W L Michaels ◽  
L D Jacobson ◽  
P J Sullivan
Keyword(s):  
Gulf Of Maine ◽  
Atlantic Herring ◽  
Georges Bank ◽  
Acoustic Surveys ◽  
Survey Designs

Gas Migration in Wellbores During Pressurized Mud Cap Drilling PMCD

2021 ◽  
Author(s):  
Ganesh Arunkumar Samdani ◽  
Sai Sashankh Rao ◽  
Vishwas Paul Gupta
Keyword(s):  
Multiphase Flow ◽  
Field Data ◽  
Gas Migration ◽  
Field Scale ◽  
Cfd Simulations ◽  
Migration Rates ◽  
Parametric Effects ◽  
Slow Moving ◽  
Gas Volume ◽  
Migration Velocities

Abstract In PMCD operations, reservoir gas is expected to migrate uphole, and the uncertainty in gas migration rates under downhole conditions leads to challenges in planning logistics and fluid requirements. Estimates of migration velocities based on current methods (e.g. Taylor-bubble correlation) are highly conservative and involves simplifying assumptions. This paper presents a systematic approach to understanding the fundamentals of gas migration in wellbores, relates it to field data, and provides recommendations to improve PMCD design and planning. Our approach includes analysis of PMCD field data, multiphase flow literature and computational flow simulations. The field data on gas migration is used to establish the field-scale parametric effects and observed trends. Multiphase flow literature is used to qualitatively understand some of these parametric effects at downhole conditions. A comparison between multiphase flow literature and field data overwhelmingly demonstrates the gaps in understanding of underlying physics. 3-dimensional multiphase CFD simulations for a representative well geometry and downhole conditions are used to understand gas migration physics at downhole conditions and the reasons for its sensitivity to different conditions. CFD simulations showed a strong impact of pressure on bubble breakup. As a result, the gas migrates as a slow-moving swarm of smaller bubbles. The formation of smaller bubbles from a given gas volume is a rate dependent process and requires a finite time to reach to an equilibrium/steady-state. The field conditions provide both high downhole pressure and sufficient length-scale for formation of smaller slow-moving bubbles. For the same reason, small scale-experiments are limited in their application for field-scale designs due to use of low pressure and/or insufficient length-scales. The CFD results also compare well with field data in showing ~30% holdup of migrating gas at low migration rates and negligible effect of rotation and wellbore geometry i.e. annulus vs openhole. The extent and rate of disintegration of gas volume (bubble) has a negative correlation with well inclination, liquid viscosity, and surface tension. The rheology and liquid viscosity also affect the ability of liquid to sweep the gas back into the reservoir and therefore it is expected to have an optimum range for a given PMCD application. Use of high viscosity fluids for typical downhole well conditions is counterproductive and results in higher gas migration rates and therefore not recommended. The understanding of downhole physics is expected to improve logistics/storage/ planning/fluid choice and lead to lower gas migration rates and reliable operation. The same approach can be applied to other operations and scenarios where gas migration velocities are a key design factor.

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