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Central Baltic Spring Spawning herring

There are two different assessments available for the Central Baltic Spring Spawning herring: One for the whole area (Sub-Divisions 25-29 including Gulf of Riga, and 32), and one for the Gulf of Riga only. Note that the total allowable catches (TACs) are set for a wider area (SDiv. 22-24 and 25-29(South), 32), which does not match the distribution area of this stock.

Note that herring in the Central Baltic consists of a number of stock components or even distinguishable stocks (see stock separation below). Most of these are spring spawners and spawn at the coast, others spawn in open water. At present, the number of different stocks or components in the Central Baltic remains unclear.

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Central Baltic Spring Spawning herring: whole area

Main characteristics and peculiarities
• herring and sprat are caught in a mixed fishery in the area. Species composition has significantly changed during the last decade (now: ca 80% sprat, formerly: ca 50%).
• spawning stock biomass is at an historic low level
• low productivity, declining weights at age. Increasing proportions of slow growing herring, especially in Sub-Divisions 25 and 26
• separate assessment available for the Gulf of Riga (this component is within safe biological limits).
• development of a recovery plan needed

Assessment Summary
year	2000 (WG 2001)
type	single species: Extended Survivors Analysis (XSA)
assessment quality	poor (high variability in assessed stock size)
main problems	(1) significant species misreporting between herring and sprat (2) high seasonal and spatial variability in mean weights at age (3) only one tuning fleet
fisheries independent information	(1) international acoustic survey on herring and sprat
catch	207'800 7 (1999: 178'300 t)
spawning stock biomass	491'000 t (1999: 490'000 t)
fishing mortality	F(adults [3-6])=0.47 (1999: 0.50)
reference points	Bpa: not defined, Fpa: 0.17, Flim: 0.33
state of the stock	outside safe biological limits (F above Fpa)
perspective	management plan needed to assure recovery of the stock

Stock parameters (under construction)

This standard figure can be downloaded as a printable pdf-file (requires Acrobat Reader)

Distribution
Central Baltic Spring Spawning herring is distributed in Sub-Divisions 25-29(South) (including Gulf of Riga) and 32. Separate assessment available for the Gulf of Riga.

Management advice
Fishing mortality should be reduced below Fpa=0.17, catches in 2002 should be less than 73'000 t. A recovery plan should be developed to allow the SSB to rebuild to historic size.

Spawning
Spring spawning at the Swedish coast (Sub-division 25) is concentrated to the northern archipelago of the Hanö Bight during April and May. Scuba diving studies indicate that spawning is confined to temperatures between 5.5 to 15 °C and occurs in very shallow waters from 0.5 to 5.5 m (Elmer 1982). Eggs are deposited mostly on Zostera marina but also on other phanerogams and benthic algae (e.g. Fucus vesiculosus). Samples from the fishery in recent years indicate a progressively lower length at first maturity and often malformed gonad development.
Further spawning grounds of spring spawning herring are accommodated along the whole Polish coast from the Pomerania Bay on the west to the Gulf of Gdansk (Sub-division 25 and 26), including the Vistula Lagoon. The spawning period continues from March (sometimes from the end of February depending on water temperature) mostly till the first half of May. In the western part of Polish coast it starts about two weeks earlier. The spawning fishes are caught mainly over 6 to 12 m of bottom depth. The roe is laid on the vegetation, sand, gravel, stones, and also on underwater artificial buildings and barriers. The maturation is reached in the second year of life (about 90 % of year class total number) with total fish length about 14 – 16 cm. The growth rate of these herrings decreases eastward.
The spawning grounds of autumn herring population are localised on the slopes of Bornholm Basin (including Slupsk Bank/Sub-division 25) and western part of Gulf of Gdansk. These herrings spawn in deeper waters up to 20 – 25 m depth with more gravely and stony bottom. The main spawning period continues from September to November.
The Latvian coast of the Gulf of Riga (Sub-division 28) is characterised by 10 spawning grounds with areas ranging from 0.1 – 2.35 km2. In Estonian part of the Gulf of Riga the most important herring spawning grounds are located in the Paernu Bay area. The spawning grounds are situated on stony grounds on which seaweeds are growing. The eggs are usually found on algae, but sometimes also on stones, sand and gravel.
Spawning takes place at a broad range of water temperature from 3.5 – 19 °C. In late spring the spawning begins at 3.5 – 4 °C. In normal terms the water temperature for spawning is reaching about 6 °C. On the average the spawning period is two months long – from the end of April till the beginning of July. The highest spawning intensity is observed in the end of May – beginning of June, by water temperatures around 9.5 – 16.9 °C (Kornilovs 1994).
Open sea herring, which differs from the gulf herring mainly by bigger length and weight at age, maturates for the first spawning usually by the second, sometimes by the third year of life. Compared to the gulf herring the open sea herring starts to spawn at lower temperatures. As temperature increases the gulf herring gradually joins the spawning. The spawning is finished by the youngest age groups of the gulf herring. During the spawning period the size and age of the herring diminish. The spawning in the Gulf of Riga is further characterised by following conditions/parameters (Kornilovs 1994):
• salinity of water at the south-eastern coast of 1.76 – 6.49,
• water depth range of 0.5 – 7.5 m,
• grounds with stony bottom covered by seaweeds (red, brown and green algae),
• usually the density of eggs are 10,000 – 300,000 per m2,
• 1.0 – 2.5 millions eggs per 1 m2 forming 1 – 1.5 cm thick carpets.
In the Asko archipelago (Sub-division 29) spawning dominates during May and June (Aneer 1989). The preferred temperatures range from 4 to 15 °C. Eggs are deposited on algae (typically Chorda filum, Pilayella littoralis and Ceramium sp.), on available phanerogams, on blue mussels and even on sand and gravel from the water surface down to 20 m depths. Egg mortality has been estimated to be high and even higher in the presence of filamentous algae (Aneer 1989). Egg density was low averaging 10,000 eggs per m2 or 200 g/m2. Spawning beds were restricted to shallow waters along the shores but could cover long distances (km). Only 10 % of the estimated suitable shallow waters were occupied.
Nearly all herring in the Gulf of Finland (Sub-division 32) are spring spawners. The spawning period is long. In early spring the spawning starts in the end of April, but usually in the first half of May. The main spawning months are May and June. The common length of herring is 15 – 18 cm. Fast-growing and old herring spawn first, slow growing and young herring later. Spawning takes place in shallow water along the whole coast. Usual spawning depth is 1 – 5 m. Spawning places are often in sounds or in underwater slopes with hard bottom covered by vegetation. Spawning begins in early spring in shallow water, even in the depth of 20 cm, and moves gradually deeper when water gets warmer. In summer spawning may take place even in the depth of 20 m. At the beginning of spawning period the temperature of the water is about 5 °C and at the end 15 °C.

Migration pattern
Tag recaptures indicated that feeding migration during autumn and winter is confined to the Bornholm basin (Sub-division 25). However, occasional recaptures has also been reported from Sub-division 24 and north of the island Öland, i.e. in Sub-division 27 (Otterlind 1978).
The spawning ground of the coastal herring (Sub-division 25 and 26) are situated near the coasts from Poland till Lithuania icncluding the Bay of Gdansk and the Vistula Bay. After spawning coastal spring spawning herring take the feeding migrations to the open waters of the southern Baltic where they mix woth open sea and autumn populations. A part of them migrate to the Danish straits and the North Sea. The most of these migrating part of herrings are naturally marked with nematode Anisakis simplex, which they infested there. After feeding period they migrate back to the traditional spawning grounds closing their biological cycle.
Results from tagging along the Swedish coast (Sub-division 27) in the 1960s revealed a distinct southbound migration towards the Bornholm basin where the Swedish spring spawning herring mix with other stocks (Otterlind 1978, Aro 1989).
Large part of the open sea herring (Sub-division 28) performs spawning migrations to the spawning grounds along the Lithuanian and Latvian coasts in March-April. A part of the open sea herring spawns in the Gulf of Riga. After spawning the herring returns to the open sea. The Gulf herring is wintering and spawning in the Gulf of Riga. After spawning some part of this herring migrates to the nearest parts of the open sea area for feeding.
The adult stock component in the Archipelago Sea (Sub-division 29) mainly migrates after spawning to the south into the Baltic sea proper and also to the north into the Bothnian Sea. Herring returns again for spawning inn the next year. Part of young herring stay in the Archipelago Sea also in autumn and winter.
A part of adult stock in the Gulf of Finland (Sub-division 32) migrates after spawning to the Baltic Sea proper, and returns in winter for spawning in the next spring. Young herring mainly stays in the Gulf during the whole year.

Stock separation
Results from comparative vertebrae counts (mean 55.0-55.15) and tagging experiments suggest that the spring spawners in the Hanö Bight (Sub-division 25) belong to a separate stock unit (Otterlind 1976, Aro 1989). The coastal spring spawning herring, open sea and autumn spawning herring in Sub-division 25 and 26 are separated using differences in the morphological structure of their otoliths.
Vertebrae counts from the herring along the Swedish east coast (Sub-division 27) in the 1960s (mean range 55.15-55.35) deviate only marginally and can not be used for stock separation (ICES 1998/H:2).

References
Aneer, G. 1989. Herring (Clupea harengus L.) spawning ground characteristics in the Baltic Sea. Fish. Res. 8: 169-195
Aro, E. 1989: A review of fish migration patterns in the Baltic. Rapp. P. –v. Reun. Cons. Int. Explor Mer 190: 72-96.
ICES 1998. Report of the Baltic Herring Age-Reading Study Group. ICES CM 1998/H:2. 86 pp.
Elmer, S. 1983. Undersökning av sillens reproduktionsomrade i Blekinge skärgard 1980-1982. Medd. fr. Havsfiskelab. Lysekil 292: 1-14
Kornilovs, G. 1994. The spawning grounds and embryonal development of Baltic herring in the Gulf of Riga. Thesis of Doctor dissertation, Latvian University, Riga. 1994: 40 pp.
Otterlind, G. 1976. Fish stocks and fishing migration in the Baltic Sea environment. Ambio Spec. Rep. 4:89-101.
Otterlind, G. 1978. Sill/strömming i svenska fiskezonen i Östersjön. Yrkesfiskaren 2 (6): 6-7 and 2(7): 4-6.

Source
ICES Baltic Fisheries Assessment Working Group 2000, ICES CM 2000/ACFM:14, and 2001, ICES CM 2001/ACFM:18

Data entered/updated by (Date)
Christopher Zimmermann (01-07-00)/Tomas Gröhsler (21-08-00/26-06-01)

Please contact clupea@clupea.de if you like to contribute more information on this or other stocks!

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Central Baltic Spring Spawning herring: Gulf of Riga

Main characteristics and peculiarities
• Herring catches include both Gulf herring and open-sea herring, which enter the Gulf of Riga from April to June for spawning
• about 70 % of the catches are taken by the trawl fishery and 30 % by the trapnet fishery on the spawning grounds

Assessment Summary
year	2000 (WG 2001)
type	single species: Extended Survivors Analysis (XSA)
assessment quality	medium
main problems	(1) migration of open sea herring in and out of the Gulf of Riga (2) misreporting
fisheries independent information	(1) number of trap-nets and corresponding abundance of herring
catch	34'100 t + 4'300 t open sea (1999: 31'400 t + 5'600 t)
spawning stock biomass	120'000 t (1999: 124'200 t)
fishing mortality	F(adults [3-7])=0.33 (1999: 0.26)
reference points	Bpa: 50'000, Blim: 36'500 t, Fpa: 0.4
state of the stock	within safe biological limits (SSB above Bpa, F below Fpa)
perspective	stock size seems to be stable

Distribution
Gulf of Riga herring is distributed in Sub-Division 28

Management advice
There are no explicit management objectives. At the current exploitation rate the stock component is forecast to remain within safe biological limits. The expected landings in the year 2002 corresponding to this rate are 33'200 t.

Migration pattern (see also Central Baltic herring)
A large part of the open sea herring performs spawning migrations to the spawning grounds along the Lithuanian and Latvian coasts in March-April. A part of the open sea herring spawns in the Gulf of Riga. After spawning the herring returns to the open sea. The Gulf herring is wintering and spawning in the Gulf of Riga. After spawning some part of this herring migrates to the nearest parts of the open sea area for feeding.

Spawning (see also Central Baltic herring)
The Latvian coast of the Gulf of Riga (Sub-division 28) is characterised by 10 spawning grounds with areas ranging from 0.1 – 2.35 km2. In Estonian part of the Gulf of Riga the most important herring spawning grounds are located in the Paernu Bay area. The spawning grounds are situated on stony grounds on which seaweeds are growing. The eggs are usually found on algae, but sometimes also on stones, sand and gravel.
Spawning takes place at a broad range of water temperature from 3.5 – 19 °C. In late spring the spawning begins at 3.5 – 4 °C. In normal terms the water temperature for spawning is reaching about 6 °C. On the average the spawning period is two months long – from the end of April till the beginning of July. The highest spawning intensity is observed in the end of May – beginning of June, by water temperatures around 9.5 – 16.9 °C (Kornilovs 1994).
Open sea herring, which differs from the gulf herring mainly by bigger length and weight at age, maturates for the first spawning usually by the second, sometimes by the third year of life. Compared to the gulf herring the open sea herring starts to spawn at lower temperatures. As temperature increases the gulf herring gradually joins the spawning. The spawning is finished by the youngest age groups of the gulf herring. During the spawning period the size and age of the herring diminish. The spawning in the Gulf of Riga is further characterised by following conditions/parameters (Kornilovs 1994):
• Salinity of water at the south-eastern coast of 1.76– 6.49 ,
• water depth range of 0.5 – 7.5 m,
• grounds with stony bottom covered by seaweeds (red, brown and green algae),
• usually the density of eggs are 10'000 – 300'000 per m2,
• 1.0 – 2.5 millions eggs per 1 m2 forming 1 – 1.5 cm thick carpets.

Stock separation (see also Central Baltic herring)
The open sea herring and the Gulf herring may be separated using differences in the morphological structure of their otoliths (ICES 1998/H:2)

References
Kornilovs, G. 1994. The spawning grounds and embryonal development of Baltic herring in the Gulf of Riga. Thesis of Doctor dissertation, Latvian University, Riga. 1994: 40 pp.
ICES 1998. Report of the Baltic Herring Age-Reading Study Group. ICES CM 1998/H:2. 86 pp.

Source
ICES Baltic Fisheries Assessment Working Group 2000, ICES CM 2000/ACFM:14, and 2001, ICES CM 2001/ACFM:18

Data entered/updated by (Date)
Tomas Gröhsler (21-08-00/26-06-01)

Please contact clupea@clupea.de if you like to contribute more information on this or other stocks!