Most of the world's really large fish populations are pelagic. They include the highly migratory species of tuna, herring ( Clupea harengus) and mackerel ( Scomber scombrus), and smaller species such sardines and anchovy.
Pelagic fish often occupy the open waters between the coast and the edge of the continental shelf in depths of 20-400 metres. These areas are highly productive and supply nutrients for the growth of plankton which forms the food for the smaller pelagic species.
Huge pelagic fish populations can be supported by these productive areas, for example the Peruvian anchovetta which is an important stock living in the Humboldt current off the western coast of South America. These populations provide an important source of food for other fish species, marine mammals, seabirds and man.
In northern waters around Scotland the main pelagic fish species are herring, mackerel and blue whiting ( Micromesistius poutassou), with smaller populations of sprat ( Sprattus sprattus) and horse mackerel ( Trachurus trachurus).
The main characteristic of these pelagic fish populations is that they are mobile or migratory. Mackerel migrate from the spawning areas west of Ireland that they occupy from March through to July to the Viking bank area of the North Sea for the winter. Herring stocks are less mobile, but migrate within areas such as the west of Scotland or the North Sea.
Life cycle strategies are quite different. Herring lay demersal eggs once a year in restricted areas on gravel beds and there are spring and autumn spawning herring in Scottish waters. Mackerel spawn several batches of eggs over a five month period (March to July) in an extensive area along the shelf edge from the Bay of Biscay to the Outer Hebrides. Blue whiting spawn in a similar area in March and migrate northwards to the Norwegian Sea for the summer.
How are Pelagic Fish Caught?
Scottish fisheries for herring and mackerel are carried out by a highly specialised fishing
fleet. Herring are fished primarily to the north and west of Scotland whereas mackerel are fished over a wider area, reflecting their broader geographical distribution.
Currently the Scottish pelagic fleet operates as single or pair trawlers. Fish are caught using pelagic trawl nets, generally targeting pelagic fish marks identified on echosounders. The nets are towed until net sensors indicate that a good haul has been caught. Towing speeds for pelagic fish are generally faster than those for demersal fish. Pelagic fish can be caught in mid-water or close to the seabed.
How are Pelagic Fish Managed?
Pelagic fisheries in the North Sea and to the west of Scotland are managed under the Common Fisheries Policy (CFP) predominantly using a system of catch quotas known as Total Allowable Catch (TAC), and with some area restrictions. Stock assessment of the pelagic species exploited by Scottish vessels is carried out under the auspices of the International Council for the Exploration of the Sea ( ICES), in conjunction with scientists from other countries that fish those resources. Additonal background information can be found in the published articles Managing Scotland's herring stocks and North eastern Atlantic mackerel.
Fishery-dependent biological data on these species are collected within Marine Scotland Science Fish Market Sampling Programme and a programme of observer trips where data are collected on discarding and by-catch.
Fishery-independent information on herring distribution and abundance is obtained from acoustic surveys, deep-water surveys and methot net surveys. Mackerel population information is obtained through egg surveys.
In addition to the stock assessment activities, Marine Scotland Science participate in two European projects on herring genetics and life history:
- Marine Scotland Science is the scientific overall lead in WESTHER
- Marine Scotland Science provides input on herring sampling and management advice for HERGEN