Evgeniy Sentyabov
Knipovich Polar Research Institute of Marine Fisheries and Oceanography, Murmansk, Russia Abstract
The results of the analysis of vertical profiles of the acoustic values of herring and blue whiting and temperature distribution are presented in paper. The combined data were obtained on transects during summer pelagic surveys in 2011–2017. The accumulated experience of such an analysis has made it possible to compare the spatial and vertical distribution of pelagic fish under different oceanographic conditions. Some climatic and inter-annual features of fish concentration of different age groups are revealed.
Keywords: temperature, acoustic values, transects, Norwegian Sea, herring, blue whiting Introduction
International investigations of pelagic fish stocks have been carried out in the Norwegian Sea for more than 20 years. They are part of the international ecosystem survey in the Nordic Seas (IESNS). Consideration of the main commercial fish of the Norwegian Sea, such as the Norwegian spring-spawning herring, the blue whiting of the North-East Atlantic, are performed in these trawl-acoustic surveys (ICES, 2015).
Asarule,aftercompletingthesurveys,thedistributionofjoinedacousticrecordsforvariousspecies of fish are integrated throughout the water column for farther estimate of stocks. To understand the fuller pattern of the fish distribution depending on the environmental conditions, the author of present work suggested a joint express analysis of the vertical distribution of both the acoustic records of pelagic fish and the temperature at the transects performed in ecosystem surveys.
The novelty of the presented approach is in the possibility of considering the quasi-3D distribution of fish acoustic records, in contrast to the traditional estimation of the spatial distribution of summarized acoustic values over the water column.
Material and methods
Oceanographic data collected on board RV ”G.O. Sars” and “Johan Hjort” (Institute of Marine Research/Havforskningsinstituttet, Norway) on transects in the Norwegian Sea during the ecosystem surveys of pelagic fish (May–June) 2011–2017 (ICES, 2011, 2013, 2014, 2015a, 2016, 2017) were used in the work. The collection of oceanographic information was carried out using a SBE 9Plus CTD profiler. Data of the water temperature from the surface to a depth of 500 m were used in the described work. The temperature at standard horizons was transformed by a Surfer package into grid nodes with a horizontal step of 0.5° in longitude and vertical step of 10 m in depth for plotting distribution graphs on transects.
The acoustic observation data obtained with the echosounders EK60, EK80 and expertly processed were divided into acoustic records for herring and blue whiting in the form of nautical area scattering coefficients (NASC, SA, m2/nm2, acoustic values) (ICES, 2015) in 1 nautical mile horizontally and 10 m in depth. The data obtained separately for herring and blue whiting were also included in grid nodes using a Surfer package.
Individual profiles of vertical temperature distribution, acoustic values for herring and blue whiting were combined on a general scale for each transect, spatially directed from west to east. As a result, from 8 to 10 combined transects were obtained for each survey in 2011–2017 (in which the author of the work participated).
Results and discussion
The obtained combined profiles of the vertical distribution of acoustic values for herring and blue whiting in relation to temperature made it possible to reveal some general patterns and features for each specific year.
The clearest inter-annual deviations in the distribution of pelagic fish and temperature were obtained on transects which were carried out in different years in close coordinates. The transect along 64°15' N in the southeastern part of the sea (Figure 1) has the most frequent repetition.
0° 1° 2° 3° 4° 5° 6° 7° 8°
Figure 1. Vertical distribution of water temperature (°C) in 0–500 m layer and acoustic values (SA, m2/nm2) for herring (red) and blue whiting (blue) in May 2012, 2014–2017 on the transect along 64°15' N.
Two rows of transects across the Norwegian Sea (between Iceland and Norway, carried out since 2015) were the most informative from the point of view of the distribution of fish in different parts of the sea in both warm and cold water masses with frontal zones between them (Figures 2 and 3).
2017
Figure 2. Vertical distribution of water temperature (°C) in 0–500 m layer and acoustic values (SA, m2/nm2) for herring (red) and blue whiting (blue) in May 2015 and 2017 on the transect along 64°50'–65°00' N.
D e p t h, m
Figure 3. Vertical distribution of water temperature (°C) in 0–500 m layer and acoustic values (SA, m2/nm2) for herring (red) and blue whiting (blue) in May 2015 and 2017 on the transect along 66°00'–66°15' N.
General pattern
It is shown that during the acoustic survey in May, the Atlantic-Scandinavian herring concentrates mainly in the western part of the investigated area over a wide range of depths and temperatures, reaching in some years in the west of the sea to depth of 450 m at a temperature below 0°C (Figure 3). Results of age analysis of the samples showed that the fish of the older age groups of the generation of 2004, 2006, 2009 prevails in the western part of the sea. Part of the fish, mostly younger (4–5 years old), is distributed in the eastern and northeastern parts of the sea in the upper 50-meter layer at a temperature of 6–8° C (Figures 1–3).
As well, it was revealed that during the whole observation period, the mass concentrations of blue whiting are always located in the eastern part of the sea above the Scandinavian shelf slope in layer 200–400 m under temperature 4–7°C. Also, blue whiting is often observed in the intermediate layer (200–300 m) in the warm side of the frontal zone between the Atlantic and subarctic waters under a temperature from 2 to 6°C sometimes under herring schools (Figures 1–3). More often, blue whiting in the spring-summer feeding period is generally observed mainly to the east of the zero
meridian. Schools of herring are often detected over the cold side of this frontal zone, preferring highly gradient areas. Relatively young herring, as mentioned earlier, is kept in the upper layer under a temperature above 7°C.
Inter-annual features
The series of observations, conducted in different years on transects in close positions, made it possible to find a number of features in the distribution of pelagic fish, depending on oceanographic conditions. From year to year, the frontal zone between warm and cold waters shifted in the east or west direction, and sharpness of this zone at different sites and in different layers was observed (see Figure 1). Most strongly, these differences in the frontal zone influent to the distribution of blue whiting, which is concentrated on the warm side of the front. Its distribution depends not only on the temperature conditions in the frontal area, but also on the slope angle of the zone.
The described features are not so important for the distribution of herring, as large-scale processes associated with environmental conditions, age structure and the availability of food. These processes mainly form herring migrating paths. So, for example, if in the summer 2015–2016 herring was distributed in the southern and central parts of the Norwegian Sea in large numbers, but in 2017 this fish was not found in these areas at all excluding only colder western sector (see Figures 1 and 2).
When a collapse of the blue whiting stock happened (2010–2011), fish recordings were concentrated only over the shelf slope in very small quantities. In some years (2012, 2015, 2016), on the contrary, blue whiting has been mixed with herring schools and has also extended in relatively cold waters in the central part of the sea. These schools are very difficult to differentiate and identify. This is probably due to close preferences in the environment and food. During the last 2 years, such mixing has not been observed. Fish is distributed in exactly bounded areas, differing in space and depth.
Last survey, carried out in spring 2018, confirmed obtained results on distribution of herring and blue whiting in depend from thermal conditions (Figure 4). Just like in all previous years, blue whiting was concentrated in the eastern part of the sea mainly over the continental slope and within the high-gradient temperature zone in the 200–300 m layer. Herring was distributed west of the zero meridian in a wide range of depths and temperatures, creating the largest concentrations in the upper layer over cold East-Icelandic waters at a temperature of 1 to 5–6°C. As in the past year, a pronounced separation of herring schools and blue whiting has been noted. In 2018 this boundary was located perpendicular to the frontal zone between 4°W and 1°E.
Summary
Based on the analysis of combined vertical profiles of water temperature and acoustic values, a number of general regularities and inter-annual features of the distribution of such pelagic species of the Norwegian Sea as blue whiting and Norwegian spring-spawning herring have been revealed.
The obtained combined vertical transects are currently used only in the express analysis of scientific surveys for scrutinizing of the distribution of fish in a given area in depend from environmental conditions. In the future, it is possible to use such combined transects in 3D distribution models not only in the Norwegian Sea, but also in other seas including high latitudes.
Acknowledgements
The author expresses great gratitude to Terje Haugland, Åge Høines and Are Salthaug (IMR) for their long-term tremendous assistance in the scrutinizing and interpretation of obtained acoustic data.
Longitude
Figure 4. Vertical distribution of water temperature (°C) in 0–500 m layer and acoustic values (SA, m2/nm2) for herring (red) and blue whiting (blue) on some transect in May 2018.
References
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