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Inland Water Biology

, Volume 11, Issue 4, pp 407–416 | Cite as

New Subspecies of Twaite Shad Alosa fallax balticus (Clupeidae)

  • E. I. Kukuev
  • A. M. OrlovEmail author
BIOLOGY, MORPHOLOGY, AND SYSTEMATICS OF HYDROBIONTS
  • 19 Downloads

Abstract

We have carried out a comparative analysis of the morphological features of Twaite shad Alosa fallax based on our own data (Curonian Lagoon, Baltic Sea (2008 and 2009)) and comparative materials (collections of Russian and foreign natural history museums, including published data). The results of this study allow us to assign the Baltic Sea population of Twaite shad to the taxonomic status of subspecies Alosa fallax balticus subsp. nov. The Baltic Sea subspecies differs from other subspecies of Atlantic Twaite shad A. fallax by a lower number of gill rakers (mean 36.7) and vertebrae (53.6) and larger pre dorsal distance (46.2% SL or 41.6–42.7% TL). A. fallax balticus is one of the subspecies that have the lowest number of gill rakers on the first gill arch among subspecies of the species Alosa fallax in the northeastern Atlantic and seas of its basins.

Keywords:

Baltic Sea Twaite shad Alosa fallax balticus subsp. nov. new subspecies morphology description Curonian Lagoon Baltic Sea 

INTRODUCTION

The species Alosa fallax was described based on single specimen from the Seine River (France) in 1803 as Clupea fallax Lacépède, 1803 [39]. Later (in 1829), it was included in the genus Alosa [25]. On the whole, Twaite shad Alosa fallax has quite a wide range in the northeastern Atlantic, covering the area from Morocco to Iceland and southern Norway, including the Mediterranean, North, and Baltic seas (up to the gulfs of Finland and Bothnia in the Baltic Sea) [2, 13, 14, 15, 19, 23, 32, 36, 43, 46, 48, 49] (Fig. 1). A. fallax is represented by different poorly studied geographical morphs throughout a range covering different climatic zones [14].

Fig. 1.

Range of Twaite shad Alosa fallax (1) and locations of captures of the studied individuals (2).

Twaite shad of the population of the Baltic Sea basin has historically served as an object of fishery for people inhabiting its coasts [5, 6, 7, 8, 9, 10, 16, 17, 30, 37, 40, 47]. Twaite shad in the gulfs of Finland and Bothnia was initially described by L.S. Berg [1] and then by A.N. Svetovidov [14] as A. fallax fallax. The recent reviews on northeastern Atlantic fisheries [46, 48] and reports on clupeids in the World Ocean [49] include the Baltic Sea population of Twaite shad in the species Alosa fallax.

Twaite shad A. fallax is listed in the Red Book of Russia, the Red List of the International Union for Conservation of Nature (IUCN), and the Annex 3 to the Berne Convention [4, 24, 28]. The increased number of catches of Twaite shad after 1990 [3, 5, 8, 37, 47] and unusually high commercial catches in 2008 and 2009 in southern Baltic waters, especially in the Curonian Lagoon [5, 6, 7], caused problems between fish and nature conservation bodies, which were unwilling to violate their duties, and fishermen who caught Twaite shad as additional catch while catching other commercial fish [5]. Members of the Atlantic Research Institute of Fisheries and Oceanography attempted to solve the acute problem and exclude Twaite shad from the Red Book of Russia. For this purpose, a number of studies based on the assumption that Twaite shad in the Baltic Sea basin is a particular subspecies that is not included in the Red Book due to its high abundance were prepared and published using a large volume of materials [3, 5, 8]. However, the amount of comparative material from other sites of its vast range, especially from the basin of the Atlantic Ocean itself, was not sufficient for the final solution.

According to the reports on clupeids of the World Ocean [14, 49], as well as the world ichthyological database (FishBase) [32] and review articles on East Atlantic shads of the genus Alosa [19, 20], Twaite shad Alosa fallax belongs to the genus Alosa, subfamily Alosinae (family Clupeidae). The subfamily Alosinae combines 18 species of anadromous herrings. Among them, seven inhabit the Black Sea, the Sea of Azov, and the Caspian Sea; six inhabit the northwestern Atlantic (waters of North America), and five species (Alosa alosa, A. fallax, A. agone, A. algeriensis, and A. killarnensis) live in the northeastern Atlantic and its seas. The northeastern Atlantic species of the subgenus Alosa differ from Ponto-Caspian herrings of the subgenus Caspialosa by the absence of teeth on their palatines. Twaite shad A. fallax and its subspecies and closely related species (which previously ranked as a subspecies of Alosa fallax) had a lower number of gill rakers than allis shad A. alosa (30–80 versus 85–130). As a more thermophilic species, allis shad occurs from Mauritania to the British Isles and southern Norway, including the Mediterranean and North seas and the western part of the Baltic Sea. Allis shad occurs very rarely in waters of the Southern Baltic and its bays [16], which actually excludes its hybridization with Twaite shad here. However, allis shad and Twaite shad can form hybrids in the southern sites of the range, namely, in the Atlantic and Mediterranean [14, 19, 20, 44].

It should be noted that researchers have no unambiguous opinion on the number of Alosa fallax subspecies. According to the data of Svetovidov [14], Twaite shad falls into several poorly studied geographical morphs throughout its range: A. fallax fallax (the eastern coast of the Atlantic), A. fallax killarnensis (southwestern Ireland), A. fallax algeriensis (described based on single specimen from Algerian waters), A. fallax nilotica (the Mediterranean and Black seas), A. fallax lacustris (Lake Maggiore, Italy), A. fallax gracilis (Lake Lago di Garda, Italy), and A. fallax rhodenensis (the northwestern Mediterranean). According to the author [14], the differences are insignificant between these morphs, and many of them were studied based on a few specimens. According to the report on A. alosa and A. fallax [19], the latter includes six subspecies: A. fallax algeriensis (Regan, 1916), which inhabits the waters of North Africa (the number of gill rakers is 40–60); A. fallax lacustris (Fatio, 1890) from waters of the Mediterranean (Italy and Sardinia) (38–72 gill rakers); A. fallax killarnensis (Regan, 1916) from fresh waters of Ireland (43–53 gill rakers); A. fallax rhodanensis (Roule, 1924) from the northwestern Mediterranean (30–49 gill rakers); A. fallax nilotica (Geoffroy St. Hilaire, 1809) from the Adriatic and eastern Mediterranean (28–40 gill rakers) [33]; and A. fallax fallax (Lacépède, 1803) from the eastern part of the Atlantic and the North and Baltic seas (28–60 gill rakers). According to the data of different authors, the number of gill rakers in A. fallax fallax is 37–43 [45] or 40–60 [26, 27, 29]. However, the subspecies of Alosa fallax were differentiated using a low volume of material in the early 20th century [41, 42]; therefore, additional studies are required. It should be noted that a Mediterranean subspecies, A. fallax nilotica, has been relatively recently differentiated from A. fallax and this subspecies was assigned the status of the species A. agone (Scopoli, 1786) as a result of resurrection [21, 22, 34, 35]. This species is widespread in the eastern part of the Mediterranean Sea and in the Adriatic. However, based on the morphological studies of individuals from the Adriatic and Lake Shkodra isolated from the Adriatic, Albanian ichthyologists believe that there are sympatric populations of both the subspecies A. fallax nilotica in the Adriatic and the species A. agone in Lake Shkodra [43]. The subspecies A. fallax algeriensis and A. fallax killarnensis are also assigned the status of valid species, Alosa algeriensis and A. killarnensis [34, 35]. Relatively recently, it was proposed to combine the subspecies A. fallax algeriensis, A. fallax lacustris, A. fallax nilotica, and A. fallax rhodanensis into an independent Mediterranean species, A. agone (the number of gill rakers is 37–72) [21, 32].

Therefore, it is necessary to carry out a revision for specifying the taxonomic status of the species A. fallax with a vast range, including different climatic zones of the northeastern Atlantic.

The objective of this research was to reveal the morphological features of Twaite shad in the basin of the southern Baltic and adjacent waters (the northern boundary of the species range) for ranking it as a subspecies of the species Alosa fallax.

MATERIAL AND METHODS

Type Material. The new subspecies is described based on three specimens (April 15, 2008, Curonian Lagoon, collected by V.P. Shopov): one holotype (ZIN 56152, TL 450 mm) and two paratypes (ZIN 56153, TL 390 and 395 mm).

Main Material. The main material included Twaite shad individuals that were caught in the Curonian Lagoon in April–June 2008 and 2009. Morphometric studies were carried out under cameral conditions for 80 fish specimens with the total length (TL) of 26–54 cm from the Curonian Lagoon and two individuals (TL 12.5 and 13.5 cm) from the open part of the Baltic Sea. Gill rakers were counted for all 82 specimens and vertebrae were counted for 69 specimens only from the Curonian Lagoon. The data on the morphometric and meristic features of 547 fish specimens were obtained by Zapbaltrybvod specialists under field conditions in May to July 2008.

As comparative material, we used the results of our own calculations of gill rakers and X-ray images of vertebrae from the Zoological Institute of the Russian Academy of Sciences, as well as calculations that were kindly provided by French, German, Norwegian, and Spanish colleagues (Table 1).

Table 1.  

Number of gill rakers on the first gill arch number of vertebrae and length of Twaite shad Alosa fallax

Area and subspecies

Life-cycle stage

TL, mm

sp. br

vt

Source of data

Arcachon Bay

Alosa fallax fallax

Adults

295–360

\(\frac{{39-41}}{{40\left( 2 \right)}}\)

\(\frac{{57-58}}{{57.5\left( 2 \right)}}\)

ZIN 14405

Lake Lago di Garda (Italy)

A. fallax lacustris

Juveniles

210

56 (1)

58 (1)

ZIN 45991

Lake Lago di Garda (Italy)

A. fallax lacustris

Adults

290

65 (1)

59 (1)

Same

Rhein River (Germany)

A. fallax fallax

Same

330–390

\(\frac{{40-41}}{{40.5\left( 4 \right)}}\)

\(\frac{{55-60}}{{57.3\left( 4 \right)}}\)

ZIN 9223

North Sea

A. fallax fallax

Juveniles

230

34 (1)

54 (1)

ZIN 4894

Baltic

A. fallax balticus

Same

125–135

\(\frac{{33-34}}{{33.5\left( 2 \right)}}\)

Authors’ collections

Baltic (Curonian Lagoon)

A. fallax balticus

''

290–480

\(\frac{{34-41}}{{36.7\left( {111} \right)}}\)

\(\frac{{50-56}}{{53.6\left( {69} \right)}}\)

Same

Minho River (Spain)

A. fallax fallax

Juveniles

\(\frac{{25-36}}{{30.3}}\)

According to the data of David Nachón, Fernando Cobo, and Sergio Silva (University of Santiago de Compostela (USC)

Minho River (Spain)

A. fallax fallax

Adults

\(\frac{{34-59}}{{45.4}}\)

Same

Ulla River (Spain)

A. fallax algeriensis?

Juveniles

\(\frac{{30-40}}{{34.5}}\)

′′

Ulla River (Spain)

A. fallax algeriensis

Adults

\(\frac{{47-58}}{{52.9}}\)

′′

Goulette Bay (Tunis)

A. fallax algeriensis

Same

354

49 (1)

54

ZMUB 00653

North Sea (Great Britain)

A. fallax fallax

Adults

406

40 (1)

54

ZMUB 6372

North Sea (Norway)

A. fallax fallax

Same

450

44 (1)

54

ZMUB 048000

North Sea (Germany)

A. fallax fallax

Juveniles

\(\frac{{29-34}}{{30.8\left( 9 \right)}}\)

SMNH

North Sea (Germany)

A. fallax fallax

Adults

\(\frac{{35-41}}{{38.0\left( 5 \right)}}\)

SMNS 9850

France (coast)

A. fallax fallax

Juveniles

73–125

\(\frac{{26-37}}{{31.5\left( 6 \right)}}\)

MNHN

France (coast)

A. fallax fallax

Adults

168–305

\(\frac{{37-43}}{{38.1\left( {17} \right)}}\)

Same

TL, total fish length; sp. br, number of gill rakers on the first gill arch; and vt, number of vertebrae. The limit of feature fluctuations is above the line, the mean value of features is below the line, and the number of specimens is in brackets. Here and elsewhere: – indicates no data.

Zoological Institute (ZIN), Russian Academy of Sciences (St. Petersburg): ZIN 9223 Alosa fallax fallax, 4 spec., TL 325–392 mm (mouth of the Rhein River); ZIN 4894 A. fallax fallax, TL 230 mm (North Sea); ZIN 14405 A. fallax fallax, 2 spec., TL 303, 360 mm (Arcachon Bay); and ZIN 45991 A. fallax lacustris, 2 spec., TL 210, 290 mm (Lake Lago di Garda, Italy). Gill rakers and vertebrae were counted.

National Natural History Museum (Muséum National d’Histoire Naturelle, MNHN), France: MNHN 0000-3163 A. fallax fallax, TL 128 mm (the coast of France); MNHN 0000-3188 A. fallax fallax, TL 350 mm (the coast of France); MNHN 0000-3395 A. fallax fallax, 7 spec., the size is unknown (the coast of France); MNHN 0000-3396 A. fallax fallax, SL 125.7 mm (the coast of France); MNHN 3397 A. fallax fallax, 3 spec., TL 73, 89.8, and 98 mm (the coast of France); MNHN 2000-0356 A. fallax fallax TL 364 mm (the coast of France); MNHN 2000-0357 A. fallax fallax, TL 307 mm (the coast of France); MNHN 2004-0671 A. fallax fallax, TL 283 mm (the coast of France); MNHN 2004-0672 A. fallax fallax, TL 277 mm (the coast of France); MNHN 2004-0674 A. fallax fallax, TL 302 mm (the coast of France); MNHN 2004-0675 A. fallax fallax, TL 168 mm (the coast of France); MNHN 1959-0004 A. fallax fallax, TL 231 mm (Bay of Biscay); MNHN 2005-1504 A. fallax fallax, TL 335 mm (the coast of Netherlands); MNHN 0000-3170 A. fallax fallax, TL 236 mm (the coast of France); MNHN 0000-3399 A. fallax fallax, TL 87.5 mm (the coast of France); MNHN 1898-1066 A. fallax fallax, TL 156 mm (the coast of France); MNHN B-2242 A. fallax fallax, 2 spec., TL 309, 305 mm (the coast of France); MNHN B-2244, A. fallax fallax, TL 350 mm (the coast of France); and MNHN B-2246 A. fallax fallax, 2 spec., TL 355, 275 mm (the coast of France). The number of gill rakers was counted.

State Museum of Natural History (SMNS), Germany: SMNS 9850 A. fallax fallax, 4 spec., adult (North Sea); SMNS 25225 A. fallax fallax, 1 spec., juvenile (Germany); SMNS 19122 A. fallax fallax, 5 spec., juvenile (Germany); SMNS 2421 A. fallax fallax, 2 spec., juvenile (North Sea); SMNS 24600 A. fallax fallax, 3 spec., juvenile (North Sea); and SMNS 26973 A. fallax fallax, 5 spec., juvenile (North Sea). The number of gill rakers was counted. The data on the length are not provided.

Zoological Museum of the University of Bergen (Zoological Museum, University of Bergen, ZMUB), Norway: ZMUB 048000 A. fallax fallax, TL 450 mm (Norway); ZMUB 00653 A. algeriensis, TL 354 mm (Tunis); ZMUB 6372 A. fallax fallax, TL 406 mm (North Sea). The number of gill rakers and vertebrae was counted.

University of Santiago de Compostela (USC): see the unpublished data provided by David Nachón, Fernando Cobo, and Sergio Silva on juvenile and adult individuals from the Ulla and Minho rivers (Spain) in Table 1.

The measurement scheme was taken from the works of [1] and [14] with some modifications. We measured the total or absolute fish length (TL) from the tip of lower jaw when mouth closed to the perpendicular erected from the end of the longest paddle of caudal fin; the standard length (SL) from the tip of upper jaw when mouth closed to the origin of the middle rays of caudal fin; head length (c) from the tip of lower jaw when mouth closed to the end of gill cover; the predorsal distance (aD) from the tip of lower jaw when mouth closed to the origin of the base of dorsal fin; the largest body depth (H) to the origin of dorsal fin; and head depth (h) at the level of the posterior margin of gill cover. The total number of rays in dorsal and anal fins and number of gill rakers on the first gill arch were also counted. If it was impossible to count the number of gill rakers on the first gill arch under field conditions, the first gill arch was cut out in each of the studied specimens and delivered to the coast with the indication of TL and SL for counting. The measured body proportions were calculated in % SL and % TL, which made it possible to compare this data with the previously published ones. The description used the generally accepted abbreviations: D, dorsal fin; A, anal fin; sp. br, gill rakers; and vt, vertebrae.

RESULTS

Alosa fallax balticus subsp. nov.—Baltic Sea Twaite shad (Fig. 2).

Fig. 2.

Baltic Twaite shad Alosa fallax balticus (holotype) (spots are poorly defined on body).

Clupea fallax [39]; Alosa finta (non [25]); A. fallax [1]; A. fallax fallax [14, 16, 46]; and A. fallax [8, 9, 13, 48, 49].

Diagnosis. The number of gill rakers in adult fish is 34–41 (mean 36.7). The total number of vertebrae is 50–56 (53.6). The length of gill rakers is approximately equal to the length of gill filaments. The average predorsal distance is 46.2% SL or 42.0% TL.

Description. Dorsal fin (D) with 15–19 rays (mean 17.6): 17 rays for the holotype and 17 and 18 rays for the paratypes; anal fin (A) with 19–23 rays (mean 21.2): 19 for the holotype and 20 and 22 for the paratypes; the total number of gill rakers on the first gill arch (sp. br.) is 34–41 (mean 36.7) (15 + 23 (total 38) for the holotype and 15 + 23 (38) and 14 + 24 (38) for paratypes; number of vertebrae (vt): total 50–56 (mean 53.6). Gill rakers are rigid, with spinules; their length is slightly larger or equal to the length of gill filaments (Fig. 3). The ratio of their lengths varies from 1 to 1.1. The number of gill rakers is 34–41 (mean 36.7) for adult individuals of Baltic Sea Twaite shad. Like other closely related species and subspecies, the number of gill rakers in Baltic Twaite shad is lower for juvenile individuals than for adult individuals (Tables 1, 2). Head length is 19.3–27.0% SL (mean 23%) (21.5 for the holotype and 21.7 and 22.3% for paratypes; head depth is 17.6–25.8% SL (mean 20%) (18.7 for the holotype and 20.2 and 18.5% for paratypes). The mean length of head is slightly lower than its mean depth. Mouth large; posterior margin of maxillary bone extends beyond eye orbit. Diameter of eye is lower than interorbital space and is approximately one-sixth of head length. Fat eyelid is well defined and lenticularly covers eye orbit (the horizontal diameter of eye with fat eyelid is 3.5–4.8 times lower than head length.

Fig. 3.

Gill arche of adult individual of Baltic Twaite shad (Curonian Lagoon).

Table 2.  

Number of Alosa fallax balticus individuals from the Curonian Lagoon with a certain length and different numbers of gill rakers

Total fish length, cm

Number of individuals (spec.) with a number of gill rakers

Total

34

35

36

37

38

39

40

41

29

1

1

2

30

1

1

33

1

1

34

1

1

1

3

35

1

1

2

1

5

36

1

1

10

3

1

1

17

37

1

1

6

38

1

4

1

3

1

10

39

4

1

5

1

2

13

40

1

4

5

3

13

41

1

1

42

1

1

43

1

1

45

1

1

1

46

1

1

-

2

47

2

2

48

1

1

54

1

1

Total, spec.

9

10

17

9

19

11

6

1

82

%

11.0

12.2

20.7

11.0

23.2

13.4

7.3

1.2

100.0

The largest body depth is 21.3–31.3% SL (mean 28%) (21.5% for the holotype and 27.1 and 26.0% for the paratypes). It should be noted that, according to the data of Zapbaltrybvod specialists, catches of Twaite shad from the Curonian Lagoon were dominated by large (males and females) deep-bodied (Table 3) individuals from June 4 to June 14 in 2008. Their data also show that catches in May 2009 contained relatively slender individuals of both sexes; the latter is confirmed by materials delivered for cameral treatment (Table 3). Catches in spring 2008 contained both deep-bodied and slender individuals (Fig. 4).

Table 3.  

Mean values of morphometric features of Alosa fallax balticus from the Curonian Lagoon in April to June 2008 and 2009

N

aD

H

c

h

% SL

% TL

% SL

% TL

% SL

% TL

% SL

% TL

\(\frac{{540}}{{80}}\)

\(\frac{{48.5}}{{46.8}}\)

\(\frac{{42.7}}{{41.6}}\)

\(\frac{{31.0}}{{25.3}}\)

\(\frac{{27.1}}{{22.5}}\)

\(\frac{{23.6}}{{21.8}}\)

\(\frac{{20.2}}{{19.4}}\)

\(\frac{{25.4}}{{18.7}}\)

\(\frac{{21.6}}{{16.6}}\)

aD, predorsal distance; H, body depth; c, head length; and h, head depth. Above the line: 2008 (according to the data of Zapbaltrybvod specialists); Below the line: 2009 (according to cameral treatment materials).

Fig. 4.

Alosa fallax balticus from Curonian Lagoon: (a) deep-bodied individual, TL 39 cm, with clearly defined row of spots on body; (b) slender individual, TL 54 cm, with a double rows of spots.

The predorsal distance covers 43.0–48.7% SL or 42.0% TL (43.75% SL for the holotype and 49.3 and 45.7% SL for the paratypes). The origin of ventral fins is posterior to the vertical of the dorsal fin origin (at the level of two to three rays). As a rule (sometimes, only after removing scales), five to 13 dark spots are well defined on body (Fig. 4). In Fig. 4b, a slender individual with TL 54 cm has two rows of well-defined spots on body. There are 31 to 36 keel scales along the lower margin of venter. According to calculations, the number of vertebrae varied from 50 to 60 for 73 specimens (Tables 1, 4, 5). The largest specimen had a TL of 54 cm and the number of its gill rakers was 41.

Table 4. 

Mean values of some meristic and morphometric features of Twaite shad Alosa fallax from different parts of the range

     Study areas

     Feature

     Source

sp. br

aD (% TL)

vt

Baltic

     36.7

     42.1

     53.6

Data of the authors

Germany (North Sea)

     40.67

     –

     54.85

[19]

Ireland

     40.53

36.81–39.25

     55.87

Same

France

     42.0–48.5

38.2–39.2

     55.9–56.3

''

Spain

     41.9–47.5

36.9–38.2

     54.93

''

Morocco

     41.7–41.97

34.9–36.6

     53.74–55.61

''

Table 5.

Number of Alosa fallax balticus individuals with a certain length and different numbers of vertebrae from the Curonian Lagoon

Total fish length, cm

     Number of individuals (spec.) with a number of vertebrae

Total

50

51

52

53

54

55

56

35

4

1

5

36

1

1

2

2

6

37

1

2

5

1

9

38

5

1

3

3

12

39

2

6

1

9

40

1

2

3

41

1

1

2

2

1

7

42

1

2

3

43

 

45

3

3

46

4

4

47

1

1

48

1

1

2

49

 

50

3

3

Total, spec.

1

6

2

10

29

16

3

69

%

1.4

8.7

2.9

14.5

42.0

23.1

4.3

100

DISCUSSION

Comparative notes. In accordance with the key to determining the subspecies of Twaite shad Alosa fallax [14, 19], the Mediterranean morph of Alosa fallax nilotica was considered a subspecies with a lowest number of gill rakers (sp. br. 34–37, mean 35). However, this subspecies was later assigned the status of an independent species, A. agone, in which the number of gill rakers is 37–72 [35]. After the differentiation of this subspecies from A. fallax and after the resurrection of the species A. agone, Twaite shad from the Baltic Sea population became a morph with the lowest number of gill rakers within the species A. fallax; according to our data, adult individuals of this morph have 34–41 gill rakers (mean 36.7), which is lower than that for individuals from other areas of the northeastern Atlantic [19] (Tables 1, 2, 4). According to the results of the extensive research into Twaite shad A. fallax and allis shad A. alosa in Portugal and French rivers [20], Twaite shad had 40–60 gill rakers (mean 42–48) and its hybrids with allis shad had 65–100 (75) gill rakers. Cases of hybridization of these species were not observed in the Baltic Sea. The number of vertebrae is 55–59 for Atlantic Twaite shad [19]. Their average number is 54.85–56.30 for Twaite shad from different areas of the northeastern Atlantic [19]. According to our data, the number of vertebrae of Baltic Twaite shad from the Curonian Lagoon varies from 50 to 56 (mean 53.6) (Tables 1, 4, 5). On the basis of X-ray images and published data on the number of vertebrae in Twaite shad from other sites of its range, it can be concluded that the Baltic Sea population of Twaite shad is characterized by the minimum number of vertebrae and gill rakers among all subspecies of A. fallax (Tables 1, 4, 5).

The habitat conditions for A. fallax significantly differ throughout its range in the northeastern Atlantic (from Morocco subtropics and the Mediterranean Sea to boreal latitudes (the North Sea, the coast of southern Norway, the Baltic to the gulfs of Finland and Bothnia)). Unlike the Atlantic and Mediterranean and North seas, the Baltic Sea is characterized by a low salinity and lower water temperatures in all seasons [67, 18]. Fish species that once penetrated into this geologically young water body from the Atlantic through the North Sea have adapted to its features and formed Baltic Sea subspecies [6, 7]. Thus, North Atlantic sprat with the same range as Twaite shad, i.e., from Morocco to Norway, is represented by the subspecies Sprattus sprattus balticus (Schneider, 1904), Atlantic herring Clupea harengus is represented by Baltic herring Clupea harengus membras (Linnaeus, 1761), and Atlantic cod Gadus morhua is represented by Baltic cod Gadus morhua callarias (Linnaeus, 1758) in the Baltic. Presumably, after penetrating from the North Sea into the Baltic and reaching the gulfs of Finland and Bothnia, Twaite shad Alosa fallax might also form its own Baltic Sea population over several millennia, to which the authors assign the status of a Baltic Sea subspecies on the basis of their own and comparative data. The hypothesis of Twaite shad penetration from southern areas into the Baltic Sea basin along the river network at later stages of waterbody formation is also plausible. The subspecific status of the Baltic Sea population of Twaite shad can be confirmed by the results of genetic studies of Twaite shad in the Baltic and North seas [12, 38], which indicate genetic isolation between them, although the validity or conspecificity of the two closely related North Atlantic herring species, A. fallax and A. alosa, was not been confirmed until recently [11].

CONCLUSIONS

Unlike Atlantic Twaite shad, Baltic Twaite shad A. fallax balticus is characterized by a lower number of gill rakers (mean 36.7) and vertebrae (53.6) and a larger predorsal distance (46.2% SL, or 41.6–42.7% TL). In the northeastern Atlantic and seas of its basin, Baltic Twaite shad among the species and subspecies of the genus Alosa have the lowest number of gill rakers on the first gill arch.

ACKNOWLEDGMENTS

We are grateful to V.S. Sukhorukaya and V.V. Sukhovershin (AtlantNIRO, Kaliningrad) for assistance in cameral treatment of the material and A.V. Gushchin, E.V. Gerasimenko, T.Yu. Volovaya, L.G. Garmashev, E.M. Ovcharova, and Yu.V. Kopeikin (West Baltic Basin Board for the Conservation and Regeneration of Aquatic Biological Resources and Regulation of Fishery (Zapbaltrybvod), Kaliningrad) for providing the data on Twaite shad treatment under field conditions. We are also grateful to A.D. Shul’ga, Yu.G. Sazonov, V.P. Shopov, V.V. Konstantinov, and S.V. Sergeev (West Baltic Territorial Administration of the Federal Fishery Agency and AtlantNIRO, Kaliningrad) for providing materials from Curonian Lagoon and A.V. Balushkin, M.V. Nazarkin, V.G. Sideleva, and N.V. Chernova for assistance in studying Twaite shad from ZIN collections (St. Petersburg). We are particularly grateful to our foreign colleagues Stefan Merker (Staatliches Museum für Naturkunde Stuttgart, Stuttgart, Germany), Romain Causse (Muséum National d’Histoire Naturelle, Paris, France), Hsuan-Ching (Hans) Ho (Research Centre for Biodiversity, Academia Sinica, Taiwan), Ingvar Byrkjedal (Zoological Museum, University of Bergen, Bergen Norway), David Nachón, Fernando Cobo, Sergio Silva (University of Santiago de Compostela, Santiago de Compostela, Spain) for providing information on Twaite shad from North Atlantic and North Sea areas, as well as to other members who contributed to this research.

COMPLIANCE WITH ETHICAL STANDARDS

Сonflict of interests. The authors declare that they have no conflict of interest.

Statement on the welfare of animals. All applicable international, national, and/or institutional guidelines for the care and use of animals were followed.

Notes

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Copyright information

© Pleiades Publishing, Ltd. 2018

Authors and Affiliations

  1. 1.Atlantic Research Institute of Fisheries and OceanographyKaliningradRussia
  2. 2.Russian Federal Research Institute of Fisheries and OceanographyMoscowRussia
  3. 3.Severtsov Institute of Ecology and Evolution, Russian Academy of SciencesMoscowRussia
  4. 4.Dagestan State UniversityMakhachkalaRussia
  5. 5.Tomsk State UniversityTomskRussia
  6. 6.Caspian Institute of Biological Resources, Dagestan Scientific Center, Russian Academy of SciencesMakhachkalaRussia

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