Retained school variables were: corrected school length (m), school height (m), school area (m 2 ... more Retained school variables were: corrected school length (m), school height (m), school area (m 2 ), school perimeter (m), school energy (Nautical area scattering coefficent, S A ), school density (energy vs. area), school depth (m) and bottom-depth-under-school (m). STATIS (Structuration de Tableux à Trois Indices de la Statisque), a multi-table analysis based on the Principal Component Analysis was used to analyse the correlation in different schools, parameters and intra-and interstock relationships. In all stocks, the first three axes extracted explained between 86.89% (Sete) and 91.84% (Aberdeen) of the total variance, because the annual variability present in the correlation structure of each stock is not widely dispersed. In all stocks, the first factorial axis was typified by the morphological variables (school length, school height, school area and school perimeter), which showed a very clear pattern with all of them being closely related. There were major differences between all stocks in regard to the general correlation relationship of the energetic variables analysed i.e. school energy and school density. Energy and density showed less variability than the morphological variables in all stocks with the exception of one survey. There were some differences between stocks in relation to the correlation of bathymetric variables. In all cases, school density and, to a lesser extent, school energy were opposed to the bathymetric variables, so that schools with higher densities would be located near the surface and in shallower areas. A complete analysis of all the surveys of each stock showed a homogeneous pattern which was very similar to most of the stocks considered separately. All morphological variables are well grouped and show a strong positive correlation. In general, therefore, all the schools analysed increased and decreased equally in all their dimensions. Both bathymetric variables are strongly correlated.
ABSTRACT As part of the EU funded project CLUSTER, databases were constructed of pelagic fish sch... more ABSTRACT As part of the EU funded project CLUSTER, databases were constructed of pelagic fish schools identified during a series of acoustic surveys in the NW North Sea, Bay of Biscay, western Mediterranean and Agean Seas and off Senegal. Among other descriptors, the databases usually included the height, length and energy (S,,) of each school. The number of schoo!.s per 1 nmi EDSU was also recorded. The relationship between these descriptors and a range of external variables (eg bottom depth, time of day and location) were examined using a suite of multiple regression models. The results indicate strong non-linear dependencies in some of the surveys on time of day and water depth. School count per EDSU tended to be high during the middle part of the day and lower at dawn and dusk. Furthermore, the ‘shape’ of this dependence on time of day is non-constant and changes with location and year. Possible explanations for such patterns and the differences and similarities between the survey areas will be discussed, as well as the impact of these findings on the conduct and analysis of acoustic surveys. In addition, we have examined the spatio-temporal pattern of sampling in each of the survey series and we will present an analysis of the impact of survey design on the potential for such spatio-temporal modelling studies.
The identification of nursery grounds and other essential fish habitats of exploited stocks is a ... more The identification of nursery grounds and other essential fish habitats of exploited stocks is a key requirement for the development of spatial conservation planning aimed at reducing the adverse impact of fishing on the exploited populations and ecosystems. The reduction in juvenile mortality is particularly relevant in the Mediterranean and is considered as one of the main prerequisites for the future sustainability of trawl fisheries. The distribution of nursery areas of 11 important commercial species of demersal fish and shellfish was analysed in the European Union Mediterranean waters using time series of bottom trawl survey data with the aim of identifying the most persistent recruitment areas. A high interspecific spatial overlap between nursery areas was mainly found along the shelf break of many different sectors of the Northern Mediterranean indicating a high potential for the implementation of conservation measures. Overlap of the nursery grounds with existing spatial fi...
We investigate the effect of strong meteorological perturbations in early spring on the success o... more We investigate the effect of strong meteorological perturbations in early spring on the success of mackerel (Scomber scombrus) recruitment in the N ⁄ NW Iberian area (southern Bay of Biscay) for the period 1999-2008. In 2000, the year of the most pronounced recruitment failure on record, two consecutive multidisciplinary surveys sampled hydrographic conditions and mackerel eggs, larvae and post-larvae over the main mackerel spawning grounds of the north and northwest coast of the Iberian Peninsula. Analysis of egg and larval abundance and birthdates based on the otoliths of mackerel juveniles caught between July and October 2000 showed that there were no survivors from the early spring spawns, indicating a massive loss of early spawning effort. Moreover, the abundance of 1-year-old mackerel estimated from an acoustic survey carried out in 2001 was the lowest observed within the 1999-2008 time series. This low or null survival from the early spawns in 2000 could be due to the meteorological and oceanographic conditions of that spring, in particular two storm events in April after a relatively calm March. The first storm event from the north caused strong local wind in the southern Bay of Biscay but a weak oceanographic response. The second storm event from the southwest was mainly felt west of Galicia and caused a notable increase in shelf currents and a shift of the hydrographical structure along the shelf. Detailed analysis of strong wind pulses in early spring within the historical recruitment record suggests that strong local turbulence generated by high wind speeds and advection of larvae caused by the enhancement of shelf currents can contribute to reduced recruitment. Our observations indicate that, in 2000, both mechanisms were present.
Acoustic surveys are being used increasingly to estimate the abundance of fish resources. One of ... more Acoustic surveys are being used increasingly to estimate the abundance of fish resources. One of the major uncertainties associated with the estimates is the identification of echotraces to species. Significant progress has been made recently in developing algorithms which use echotrace and multifrequency backscattering characteristics to identify fish schools. These algorithms are developed and tested on echotraces which have been identified by trawling, a process akin to groundtruthing. This paper provides a description of an ORACLE online database which holds a digital echogram library of ground-truthed echograms for a number of pelagic species. The database is based on the SIRENO (Seguimiento Integrado de los REcursos Naturales Oceánicos) software, developed by the Instituto Español de Oceanografía. It was developed for an EC project (SIMFAMI) aiming to develop species identification algorithms. The database stores data from acoustic surveys as: echogram images at different frequencies; polygon coordinates identifying the area in the echogram from which the trawl sample was taken; catch data (position, depth , species composition, etc.); and environmental data. The raw acoustic data is not held in the database, but is accessed on request from the host institute by reference to the filename. All acoustic data files are saved in a standard format (HAC files) to enable the exchange of data across multiple software platforms. The database can be filtered to obtain subsets of data verifying certain requirements. The database serves as a model for similar such species identification studies and provides a centralised searchable store of ground-truthed echotraces of the major commercial pelagic species of northern European waters.
Small pelagic fish are known to aggregate into schools and clusters of schools. It is commonly as... more Small pelagic fish are known to aggregate into schools and clusters of schools. It is commonly assumed that the number of such schools and clusters, as well as their size and densities, will vary with the stock abundance. We have carried out a PCA based meta-analysis, using series of acoustic survey data from five different locations in Europe to examine this assumption. The study concluded that there was no discernible relationship between stock abundance and the number of schools seen, or on the clustering of those schools. The study also showed that the number and structure of the school clusters was strongly correlated with the number of schools seen. An increased number of schools in an area tended to be linked with denser clusters (more schools per kilometre) and a higher occupation of the survey area by those clusters. There was also a weaker tendency to find more clusters. It is not clear whether these relationships and the absence of a link to abundance are due to density independence in aggregation patterns or whether such density dependence is only functional at relatively low stock abundance levels. 2001 International Council for the Exploration of the Sea
Small pelagic fish are known to aggregate into schools and clusters of schools. It is commonly as... more Small pelagic fish are known to aggregate into schools and clusters of schools. It is commonly assumed that the number of such schools and clusters, as well as their size and densities, will vary with the stock abundance. We have carried out a PCA based meta-analysis, using series of acoustic survey data from five different locations in Europe to examine this assumption. The study concluded that there was no discernible relationship between stock abundance and the number of schools seen, or on the clustering of those schools. The study also showed that the number and structure of the school clusters was strongly correlated with the number of schools seen. An increased number of schools in an area tended to be linked with denser clusters (more schools per kilometre) and a higher occupation of the survey area by those clusters. There was also a weaker tendency to find more clusters. It is not clear whether these relationships and the absence of a link to abundance are due to density independence in aggregation patterns or whether such density dependence is only functional at relatively low stock abundance levels. 2001 International Council for the Exploration of the Sea
Retained school variables were: corrected school length (m), school height (m), school area (m 2 ... more Retained school variables were: corrected school length (m), school height (m), school area (m 2 ), school perimeter (m), school energy (Nautical area scattering coefficent, S A ), school density (energy vs. area), school depth (m) and bottom-depth-under-school (m). STATIS (Structuration de Tableux à Trois Indices de la Statisque), a multi-table analysis based on the Principal Component Analysis was used to analyse the correlation in different schools, parameters and intra-and interstock relationships. In all stocks, the first three axes extracted explained between 86.89% (Sete) and 91.84% (Aberdeen) of the total variance, because the annual variability present in the correlation structure of each stock is not widely dispersed. In all stocks, the first factorial axis was typified by the morphological variables (school length, school height, school area and school perimeter), which showed a very clear pattern with all of them being closely related. There were major differences between all stocks in regard to the general correlation relationship of the energetic variables analysed i.e. school energy and school density. Energy and density showed less variability than the morphological variables in all stocks with the exception of one survey. There were some differences between stocks in relation to the correlation of bathymetric variables. In all cases, school density and, to a lesser extent, school energy were opposed to the bathymetric variables, so that schools with higher densities would be located near the surface and in shallower areas. A complete analysis of all the surveys of each stock showed a homogeneous pattern which was very similar to most of the stocks considered separately. All morphological variables are well grouped and show a strong positive correlation. In general, therefore, all the schools analysed increased and decreased equally in all their dimensions. Both bathymetric variables are strongly correlated.
ABSTRACT As part of the EU funded project CLUSTER, databases were constructed of pelagic fish sch... more ABSTRACT As part of the EU funded project CLUSTER, databases were constructed of pelagic fish schools identified during a series of acoustic surveys in the NW North Sea, Bay of Biscay, western Mediterranean and Agean Seas and off Senegal. Among other descriptors, the databases usually included the height, length and energy (S,,) of each school. The number of schoo!.s per 1 nmi EDSU was also recorded. The relationship between these descriptors and a range of external variables (eg bottom depth, time of day and location) were examined using a suite of multiple regression models. The results indicate strong non-linear dependencies in some of the surveys on time of day and water depth. School count per EDSU tended to be high during the middle part of the day and lower at dawn and dusk. Furthermore, the ‘shape’ of this dependence on time of day is non-constant and changes with location and year. Possible explanations for such patterns and the differences and similarities between the survey areas will be discussed, as well as the impact of these findings on the conduct and analysis of acoustic surveys. In addition, we have examined the spatio-temporal pattern of sampling in each of the survey series and we will present an analysis of the impact of survey design on the potential for such spatio-temporal modelling studies.
The identification of nursery grounds and other essential fish habitats of exploited stocks is a ... more The identification of nursery grounds and other essential fish habitats of exploited stocks is a key requirement for the development of spatial conservation planning aimed at reducing the adverse impact of fishing on the exploited populations and ecosystems. The reduction in juvenile mortality is particularly relevant in the Mediterranean and is considered as one of the main prerequisites for the future sustainability of trawl fisheries. The distribution of nursery areas of 11 important commercial species of demersal fish and shellfish was analysed in the European Union Mediterranean waters using time series of bottom trawl survey data with the aim of identifying the most persistent recruitment areas. A high interspecific spatial overlap between nursery areas was mainly found along the shelf break of many different sectors of the Northern Mediterranean indicating a high potential for the implementation of conservation measures. Overlap of the nursery grounds with existing spatial fi...
We investigate the effect of strong meteorological perturbations in early spring on the success o... more We investigate the effect of strong meteorological perturbations in early spring on the success of mackerel (Scomber scombrus) recruitment in the N ⁄ NW Iberian area (southern Bay of Biscay) for the period 1999-2008. In 2000, the year of the most pronounced recruitment failure on record, two consecutive multidisciplinary surveys sampled hydrographic conditions and mackerel eggs, larvae and post-larvae over the main mackerel spawning grounds of the north and northwest coast of the Iberian Peninsula. Analysis of egg and larval abundance and birthdates based on the otoliths of mackerel juveniles caught between July and October 2000 showed that there were no survivors from the early spring spawns, indicating a massive loss of early spawning effort. Moreover, the abundance of 1-year-old mackerel estimated from an acoustic survey carried out in 2001 was the lowest observed within the 1999-2008 time series. This low or null survival from the early spawns in 2000 could be due to the meteorological and oceanographic conditions of that spring, in particular two storm events in April after a relatively calm March. The first storm event from the north caused strong local wind in the southern Bay of Biscay but a weak oceanographic response. The second storm event from the southwest was mainly felt west of Galicia and caused a notable increase in shelf currents and a shift of the hydrographical structure along the shelf. Detailed analysis of strong wind pulses in early spring within the historical recruitment record suggests that strong local turbulence generated by high wind speeds and advection of larvae caused by the enhancement of shelf currents can contribute to reduced recruitment. Our observations indicate that, in 2000, both mechanisms were present.
Acoustic surveys are being used increasingly to estimate the abundance of fish resources. One of ... more Acoustic surveys are being used increasingly to estimate the abundance of fish resources. One of the major uncertainties associated with the estimates is the identification of echotraces to species. Significant progress has been made recently in developing algorithms which use echotrace and multifrequency backscattering characteristics to identify fish schools. These algorithms are developed and tested on echotraces which have been identified by trawling, a process akin to groundtruthing. This paper provides a description of an ORACLE online database which holds a digital echogram library of ground-truthed echograms for a number of pelagic species. The database is based on the SIRENO (Seguimiento Integrado de los REcursos Naturales Oceánicos) software, developed by the Instituto Español de Oceanografía. It was developed for an EC project (SIMFAMI) aiming to develop species identification algorithms. The database stores data from acoustic surveys as: echogram images at different frequencies; polygon coordinates identifying the area in the echogram from which the trawl sample was taken; catch data (position, depth , species composition, etc.); and environmental data. The raw acoustic data is not held in the database, but is accessed on request from the host institute by reference to the filename. All acoustic data files are saved in a standard format (HAC files) to enable the exchange of data across multiple software platforms. The database can be filtered to obtain subsets of data verifying certain requirements. The database serves as a model for similar such species identification studies and provides a centralised searchable store of ground-truthed echotraces of the major commercial pelagic species of northern European waters.
Small pelagic fish are known to aggregate into schools and clusters of schools. It is commonly as... more Small pelagic fish are known to aggregate into schools and clusters of schools. It is commonly assumed that the number of such schools and clusters, as well as their size and densities, will vary with the stock abundance. We have carried out a PCA based meta-analysis, using series of acoustic survey data from five different locations in Europe to examine this assumption. The study concluded that there was no discernible relationship between stock abundance and the number of schools seen, or on the clustering of those schools. The study also showed that the number and structure of the school clusters was strongly correlated with the number of schools seen. An increased number of schools in an area tended to be linked with denser clusters (more schools per kilometre) and a higher occupation of the survey area by those clusters. There was also a weaker tendency to find more clusters. It is not clear whether these relationships and the absence of a link to abundance are due to density independence in aggregation patterns or whether such density dependence is only functional at relatively low stock abundance levels. 2001 International Council for the Exploration of the Sea
Small pelagic fish are known to aggregate into schools and clusters of schools. It is commonly as... more Small pelagic fish are known to aggregate into schools and clusters of schools. It is commonly assumed that the number of such schools and clusters, as well as their size and densities, will vary with the stock abundance. We have carried out a PCA based meta-analysis, using series of acoustic survey data from five different locations in Europe to examine this assumption. The study concluded that there was no discernible relationship between stock abundance and the number of schools seen, or on the clustering of those schools. The study also showed that the number and structure of the school clusters was strongly correlated with the number of schools seen. An increased number of schools in an area tended to be linked with denser clusters (more schools per kilometre) and a higher occupation of the survey area by those clusters. There was also a weaker tendency to find more clusters. It is not clear whether these relationships and the absence of a link to abundance are due to density independence in aggregation patterns or whether such density dependence is only functional at relatively low stock abundance levels. 2001 International Council for the Exploration of the Sea
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