WELL OVER HALFWAY THROUGH THE MACKEREL ECOSYSTEM EXPEDITION IN THE NORWEGIAN SEA

Well over halfway through the mackerel ecosystem expedition in the Norwegian Sea. So far the Institute of Marine Research have completed 62 stations in the Norwegian Sea with ‘Eros’ and ‘Vendla’.

Author: Leif Nøttestad and Hector Peña, Institute of Marine Research

The chartered pelagic trawlers ‘Eros’ and ‘Vendla’ have so far found relatively low concentrations of mackerel with the exception of a few stations. 

Adult Norwegian spring-spawning herring have so far been recorded acoustically and caught in trawls in the northern and northwestern parts of the Norwegian Sea, while juvenile herring have dominated in the northeastern areas.

The acoustic recordings of blue whiting have been good, especially for younger blue whiting, so far on the cruise. We have also had significant trawl catches over large areas of the Norwegian Sea of salmon and especially postsmolt, but also larger individuals.

A good number of pods of killer whales have been observed scattered throughout the eastern, central and western parts of the Norwegian Sea, in addition to observations of minke whales, sperm whales, fin whales, humpback whales, springers and porpoises far offshore. 

As standard for this mackerel ecosystem cruise, 30-minute pelagic trawling from the surface down to 30 metres depth has been carried out at predefined stations, along with plankton sampling (0-200 metres depth) and CTD measurements (0-500 metres depth).

Continuous echo sounder recordings were made along the transects down to 500 meters depth for acoustic quantification of herring and blue whiting.

‘Vendla’ started from Bergen on June 30, covering the southern part of the area and moving northward together with the research vessel ‘Jakup Sverri’ from the Faroe Islands.

“Eros” started in Tromsø on July 10 and covered the area west and north of Tromsø, including the areas around Jan Mayen. “Eros” will continue north and west of Bjørnøya after a crew change on July 21, while ‘Vendla’ will conclude its surveys in Tromsø on July 24.

Mackerel 

There have generally been low mackerel catches as far as Bodø and north with ‘Eros’ and ‘Vendla’.

The highest mackerel catches were observed in the central and eastern parts of the Norwegian Sea, with catches ranging from 0 to 3500 kg. Of the total of 62 stations included in this report, mackerel catches were below 50 kg in 31 stations.

There have mainly been larger and larger mackerel the further west and north we have surveyed.

The average size of the mackerel per station varied between 81 and 631 grammes, with an average value of 481 grammes.

Most of the larger fish, over 500 grammes, were observed north of 66°N, with homogeneous distribution from the eastern border of Jan Mayen.

Due to the great weather conditions with almost a view of the ocean on board, we have often observed schools of mackerel on the sonar and mackerel searching for prey right at the surface.

Herring

The highest catches of herring were recorded west of Vesterålen, east of Jan Mayen and northwest of Tromsø.

Catches ranged from 0 to 6238 kg, with a mean value of 310 kg. Similar to mackerel, most of the large herring (>300 grammes) were observed north of 62°N, with the younger fish in the northeastern part of the coverage area, where 3 and 4 year olds dominated.

The average weight of herring per station varied from 18 to 419 grammes, with an average of 262 grammes.

The acoustic recordings of Norwegian spring-spawning herring have been low so far on board ‘Vendla’ and ‘Eros’.

The first significant herring catches were made in the Jan Mayen zone with catches of several tons. While we registered very little on the echo sounder, there were significant shoals of herring registered on the sonar, close to the surface, primarily <15-20 m depth.

Herring shoals that stay close to the surface are rarely registered on the sonar. This is partly due to the herring avoiding the vessel and partly because the herring were in the acoustic blind zone shallower than the depth of the sonar transducers on the keel.

Blue whiting

Acoustic recordings of blue whiting have been solid, especially for younger blue whiting, based on pelagic trawling on recordings in deeper water layers, so far during this year’s mackerel ecosystem cruise.

Blue whiting has been largely found between 50 and 400 metres deep, with higher densities along the continental shelf decreasing towards deeper waters in the Norwegian Sea.

It will be exciting to see what the total acoustic abundance index for blue whiting from this cruise will be, when the acoustic results in combination with the trawl samples from all the vessels from the different countries that have participated are compiled.

Salmon

We have had high trawl catches of salmon on board ‘Vendla’ and ‘Eros’.

It is particularly post-smolts of about 100-200 grammes that have been caught in larger numbers in several trawl hauls.

In the transect and course line from southern Lofoten in and westward all the way to the southeast of Jan Mayen, post-smolt were caught on almost all trawl tails in addition to small salmon of 1-2.6 kg. Larger salmon were trawled on board ‘Eros’ further north in the Norwegian Sea.

It will be important to carry out thorough genetic analyses to find out and confirm whether all the individuals are actually Atlantic salmon, or whether they are also post-smolt individuals from humpback salmon that migrate out of the rivers and graze in the Norwegian Sea in the summer.

0- group fish

In several stations north of 73°N and up to 160 nautical miles west of Bjørnøya, the presence of 0-group fish was important in the trawl catches on board “Eros”, which covers the northernmost sea areas.

In varying proportions we got 0-group cod, haddock, sandeel, capelin, saithe, polar cod, plaice and whiting. In one trawl haul, up to 17 kg of 0-group fish were caught, of which cod accounted for 13 kg.

Marine mammals

There are four dedicated marine mammal observers on board ‘Vendla’ and ‘Eros’, with two taking turns on duty on the wheelhouse roof.

A large number of schools of killer whales have been observed scattered throughout the eastern, central and western parts of the Norwegian Sea.

The killer whales are most likely hunting schools of mackerel near the surface, since they overlap in time and space, during the second part of the mackerel ecosystem cruise.

We have even experienced a pod of killer whales swimming close to the stern of ‘Vendla’ after a trawl catch dominated by mackerel.

In addition, whale observers have observed minke whales, sperm whales, fin whales, humpback whales and porpoises far out to sea on board ‘Eros’ and ‘Vendla’.

The results from the marine mammal observations from the wheelhouse roof on board ‘Vendla’ will be compared with the results of the quantity and distribution of marine mammals from “Eros”. 

The final results will be made available and published later in the North Atlantic Marine Mammal Commission (NAMMCO).

Plankton samples

Plankton samples are taken with a WP2 hoe from 0-200 m depth at each predetermined station with surface trawling for mackerel.

After the survey, the samples will be processed, and then we will get more quantitative information about the state of zooplankton quantities and distribution in the Norwegian Sea, which represent important prey in the Norwegian Sea during the summer for the pelagic fish stocks of mackerel, herring, blue whiting, salmon and roach.

Oceanographic measurements of temperature, salinity and nutrients

The oceanographic measurements for temperature, salinity and nutrients, carried out by a Seabird CTD probe, are collected from the surface down to 500 m depth at each predetermined station with surface trawling for mackerel.

All oceanographic measurements and water sampling are stored on board, and comprehensive analyses of temperature trends and the depth of the thermocline for the various ocean and coastal areas only become available after the end of the cruise.

Satellite information indicates sea surface temperatures that are 1-3 degrees above the long-term average in parts of the Norwegian Sea for the month of July. These are values that will be compared with our in-situ CTD measurements on board ‘Eros’ and ‘Vendla’.

Weather conditions

There have mainly been exceptionally good weather and wind conditions during the mackerel ecosystem cruise so far on board ‘Vendla’ and ‘Eros’.

Mostly calm, bright sunshine, mild summer weather and fantastic visibility have made it very favourable for continuous acoustic fish registrations with sonar and echo sounder, during plankton sampling, CTD measurements and during trawling, in addition to conducting whale observations from the wheelhouse roof.

Image credit: Leif Nøttestad/Institute of Marine Research

Source