Why we listen for fishes: Small pelagics in the North Sea (HERAS)

The internationally coordinated Herring Acoustic Survey in the North Sea (HERAS)

What is the status of small pelagic, schooling fish such as herring and sprat in the North Sea? Are southern species such as sardine and anchovy spreading further north as the climate warms? Answers to these and other questions are provided by the International Hydroacoustic Survey, which is carried out simultaneously by several research vessels each summer.

Walther Herwig III on its journey into the sunset

Background and objectives

Map overview of the study area to be sampled in the central and southern North Sea — Survey area of the German contribution to the Herring Acoustic Survey (HERAS) in the central and southern North Sea. The sub-areas S51, S61, S71 and S131 are marked in red. Blue lines show the hydroacoustic courses (= transects) planned for 2024, each with random start points (diamonds) and end points (crosses). The fixed orientation of the transects allows coverage of both shallow and deep areas of the survey strata. This is necessary for a statistically robust evaluation of fish abundance and distribution.

The beginning of the International Herring Acoustic Survey in the North Sea (HERAS), coordinated by the International Council for the Exploration of the Sea (ICES), dates back to 1979. Every summer, we set sail and use scientific echosounders and pelagic trawls to collect standardised data on the distribution and abundance of pelagic schooling fish, i.e. fish that do not live on or near the seafloor but in the water column.

The target species of HERAS are clupeid fish: herring, sprat, sardine and anchovy.

The surveys provide the most important fishery-independent stock index for these fish, i.e. the basis for estimating the abundance and trends of these commercially and ecologically very important species and their populations in the North Sea.

The annual survey is coordinated by WGIPS - the ICES Working Group on International Pelagic Surveys. In addition to the German research vessel, other ships from Denmark, the Netherlands, Norway, Scotland and Ireland also collect hydroacoustic and biological data at the same time in their survey strata.

We compile the German data as part of the EU Data Collection Framework (DCF) as a contribution to the International Herring Acoustic Survey (HERAS).

In addition, this information is used within ICES to calculate scientific advice on fishing opportunities, e.g. by the Herring Assessment Working Group (HAWG).

Approach

The abundance and biomass inidces from HERAS are based on hydroacoustic data collected continuously by scientific echosounders. Echo sounders emit sound pulses into the water, which are reflected by the seafloor and various marine organisms. The distribution and overall strength of the measured echoes from schooling fish can be used to calculate their abundance. Hydroacoustic data provide the most suitable basis for a quantitative analysis of schooling fish.

View of the hydroacoustics laboratory, which is located in the area of the bridge of the RV "Solea" — The hydroacoustics laboratory in a compartment of the bridge on RV "Solea" - the HERAS data centre: this is where the scientific echosounders are operated and monitored, the fishing hauls are coordinated and the hydrographic probes are deployed and controlled.

Display of the echo sounder, where the colour scale on it serves as a measure of the echo volume of the measured reflection values — Echo sounder display in the hydroacoustics laboratory: The colour scale of the display is a measure of the echo intensity of the measured backscatter. The seafloor can be seen as a thick brown line, the current water depth is 40.5 m. Schools of small pelagics can be seen as red 'pillars' a few metres above the seafloor in the open water.

Activity on the displays of the echo sounder and sonar during a fishing haul — Displays of the echosounder (left) and sonar (right) during a fishing haul: The pelagic trawl is currently in the water to collect targeted biological samples from the fish school visible on the echosounder and utilizing the omnidirectional fishing sonar.

In addition, biological data on species composition, length, weight and fecundity of individual species, as well as the age structure of the stocks, will be collected through targeted pelagic trawl surveys. These data, in combination with the acoustic data, can then be used to calculate, among other things, the spawning stock biomass for all target species in the survey area.

The codend of the pelagic trawl is hauled out of the water after a targeted haul...

The fish from the net is emptied into the so-called Hock, which is directly connected to the laboratory via a conveyor belt — … and emptied into the hock, which is connected to the fish lab by a conveyor belt.

Fish catch, which is transported via conveyor belts to the fish laboratory of the FV "Solea", where it is scientifically analysed — The catch is transported into the fish lab of the RV "Solea" for scientific processing. The catch is first sorted by species, then length measurements are conducted for each species, including determination of sample and total catch weight. Additional data on age, fecundity and individual weight are collected for the target species of the survey.

Herring, Sprat, Sardine, Anchovy — The target species of the Herring Acoustic Survey (HERAS) - clupeid fishes. From top to bottom: Herring (Clupea harengus), sprat (Sprattus sprattus), sardine (Sardina pilchardus), anchovy (Engraulis encrasicolus).

The HERAS study area is divided into different sub-areas (strata), which are assigned to the participating countries. Germany, allocatred the southern strata in the German Bight, the central North Sea and the English Channel, covers the main distribution area of the North Sea sprat as well as important habitats for juvenile herring.

Hydroacoustic measurements:

map overview of the distribution of hydroacoustic data recorded with scientific echosounders and fishing hauls based on echosounder data — Distribution of hydroacoustic data recorded with scientific echosounders (blue circles) and fishing hauls based on echosounder data (red dots with haul number). The blue circles represent the summed backscatter (NASC - Nautical Scattering Coefficient) of the measured echoes - the larger the circle area, the stronger the measured backscatter. On the map, the circles can be interpreted as an indicator of the distribution and and abundance of small pelagic fishes.

Catch composition of small pelagics intrawl catches:

ap overview of the catch composition of small pelagic fish in pelagic trawl catches — Illustration of the catch composition of small pelagics in pelagic trawl catches. The hauls were targeted on schools recorded by the echosounder. The diameter of the circles indicates the standardised bulk catch. ANE = anchovy, HER = herring, HOM = horse mackerel, MAC = mackerel, PIL = sardine, SPR = sprat.

During data collection at sea, our research vessel "Solea" navigates at a constant speed of 10 knots on pre-determined courses (= transects). Hydroacoustic data are continuously recorded with echosounders. Biological samples are collected on each transect for species classification and analysis of the acoustic data. Based on the echosounder readings, a representative fish school is selected, from which a sample is taken with a targeted pelagic trawl haul. Additional sampling includes vertical profiles of environmental parameters such as temperature, salinity and oxygen. These are measured using a hydrographic CTD-probe after each fishing station and at regular intervals along the transects in the study area.

Once the data collection at sea has been completed and all the hydroacoustic and catch data have been analysed, a post-cruise meeting to collate and analyse the individual results is held by the cruise-leaders of all participating nations. This results in a combined abundance and biomass index for the entire study area for all target species.