Sensitivity analysis of school parameters to compare schools from different surveys: a review of the standardisation task of the EC-FAIR programme CLUSTER

Echo traces seen on echo grams contain a lot of information about the aggregation of fish in schools. But the acosutic image obtained with a vertical biomass assessment echosounder contains distorsions mainly due to the beam angle, the equipment settings and the school depth. When the acoustic image of aggregation patterns changes over the years or varies between stocks, it is important to know up to what extent biological interpretation is meaningful!. The present paper reviews the work performed by a group of scientist within the EC FAIR programme CLUSTER. Simulations were performed to correct school parameters. Digital data were replayed to assess the importance of these corrections. Charts were derived to limit biological interpretation of changes on the school acoustic images

Vulnerability of the North Water ecosystem to climate change

High Arctic ecosystems and Indigenous livelihoods are tightly linked and exposed to climate change, yet assessing their sensitivity requires a long-term perspective. Here, we assess the vulnerability of the North Water polynya, a unique seaice ecosystem that sustains the world’s northernmost Inuit communities and several keystone Arctic species. We reconstruct mid-to-late Holocene changes in sea ice, marine primary production, and little auk colony dynamics through multi-proxy analysis of marine and lake sediment cores. Our results suggest a productive ecosystem by 4400–4200 cal yrs b2k coincident with the arrival of the first humans in Greenland. Climate forcing during the late Holocene, leading to periods of polynya instability and marine productivity decline, is strikingly coeval with the human abandonment of Greenland from c. 2200–1200 cal yrs b2k. Our long-term perspective highlights the future decline of the North Water ecosystem, due to climate warming and changing sea-ice conditions, as an important climate change risk

Understanding the coupling between small pelagic fish and marine top predators using data collected on ecosystem-based surveys

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On the relation between schools, clusters of schools and abundance in pelagic fish stocks

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.Publicado

Holocene polynya dynamics and their interaction with oceanic heat transport in northernmost Baffin Bay

Baffin Bay hosts the largest and most productive of the Arctic polynyas: the North Water (NOW). Despite its significance and active role in water mass formation, the history of the NOW beyond the observational era remains poorly known. We reconcile the previously unassessed relationship between long-term NOW dynamics and ocean conditions by applying a multiproxy approach to two marine sediment cores from the region that, together, span the Holocene. Declining influence of Atlantic Water in the NOW is coeval with regional records that indicate the inception of a strong and recurrent polynya from ~ 4400 yrs BP, in line with Neoglacial cooling. During warmer Holocene intervals such as the Roman Warm Period, a weaker NOW is evident, and its reduced capacity to influence bottom ocean conditions facilitated northward penetration of Atlantic Water. Future warming in the Arctic may have negative consequences for this vital biological oasis, with the potential knock-on effect of warm water penetration further north and intensified melt of the marine-terminating glaciers that flank the coast of northwest Greenland

Consistency in the correlation of school parameters across years and stocks

Acoustic surveys have been undertaken in different parts of the waters around Europe. In this paper we consider those taking place in NE Atlantic waters – off the NE Scottish coast, the Bay of Biscay and off the Spanish Atlantic coast – and in the Mediterranean Sea – the Catalonian Sea, the Gulf of Lion and the Aegean Sea. Retained school variables were: corrected school length (m), school height (m), school area (m2), school perimeter (m), school energy (Nautical area scattering coefficent, SA), 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.Publicado

Geographic variability of sardine growth across the northeastern Atlantic and the Mediterranean Sea

This study describes broad-scale spatial variations in sardine growth across the northeastern Atlantic and Mediterranean waters using opportunistic samples collected in recent years. More detailed information on spatial, decadal and seasonal growth variations is provided for the Iberian-Biscay region using data collected in acoustic surveys since the mid-1980s. Growth curves are fitted to annual or monthly length-at-age data using a robust Von Bertalanffy model; parameters for recent samples are compared with literature information using an auximetric plot while differences between areas within the Iberian-Biscay region are tested by log-likelihood ratio tests. Sardine growth performance is generally lower in the Mediterranean and declines across the northeastern Atlantic from the English Channel to north Morocco but increases sharply off Mauritania. Lower growth of Mediterranean sardines is possibly associated to the overall oligotrophy of this Sea while differentiation from the Atlantic is likely sustained by reproductive isolation between populations from the two areas. Within the northeastern Atlantic, size- and age related migrations may partly explain differences in maximum length/age and mean length-at-age between neighbouring areas but the broad-scale latitudinal decline in growth is consistent with adaptation to the north–south decline in seasonal temperature gradients and to the annual cycles of plankton production. Within the Atlantic Iberian waters, sardine grows and improves in condition during spring and summer when the allocation of energetic resources for gonad development cease, temperature is close to the annual maxima and plankton production is high. Variation in sardine length-at-age and growth within the Atlanto-Iberian stock area has implications for stock structure and needs to be taken into account in the calculation of weight and maturity-at-age for assessment purposes. No evidence of broad temporal changes in sardine growth within the Iberian-Biscay region is obtained.Versión del editor1,586