Journal of
Applied Ichthyology
J. Appl. Ichthyol. 27 (Suppl. 3) (2011), 56–59
2011 Blackwell Verlag, Berlin
ISSN 0175–8659
Received: November 21, 2010
Accepted: April 17, 2011
doi: 10.1111/j.1439-0426.2011.01853.x
Short communication
First report on the status of Allis shad (Alosa alosa) in the Minho River
(Northwestern Iberian Peninsula)
By M. Mota1,2,3 and C. Antunes2,3,4
1
ICBAS – Institute of Biomedical Sciences of Abel Salazar, University of Porto, Porto, Portugal; 2CIMAR ⁄ CIIMAR –
Interdisciplinary Centre of Marine and Environmental Research, University of Porto, Porto, Portugal; 3Aquamuseu do Rio Minho,
Parque do Castelinho, Vila Nova de Cerveira, Portugal; 4ESG - Escola Superior Gallaecia, Largo das Oliveiras, Vila Nova de
Cerveira, Portugal
Introduction
The Allis shad (Alosa alosa Linnaeus 1758) is an anadromous
member of the family Clupeidae. Like other diadromous fish
(see Limburg and Waldman, 2009), this species has been
progressively disappearing from European rivers due to river
fragmentation, increasing sediment load and pollution. In
Portugal a marked decline in the shad populations was noticed
since the middle of the 20th century, particularly in the rivers
Minho and Lima, while in the rivers Tagus and Guadiana only
a residual population still exists (Faria, 2007). The anthropogenic pressures that seem to cause the gradual disappearance
of A. alosa in several European basins, has resulted in its
listing in (i) the Red List of the International Union for the
Conservation of the Nature (IUCN), (ii) in Annex III of the
Bern Convention, and in the Habitats Directive. The drastic
decline in the spawning stock biomass and the restrictions of
usable habitat area prompted its classification at national level
as a species in danger (Cabral et al., 2006), now being under
legal protection. As other Portuguese rivers (Lima, Cávado,
Douro, Vouga, Mondego, Tejo, Sado and Guadiana), the
Minho was one of the rivers with the highest catches of the
species (Baldaque da Silva, 1892) and, in the Minho River
basin, A. alosa still has a great commercial and cultural value.
Portuguese official statistics of the small-scale fishery confirm
mean catches of 200 tonnes during the first half of the 20th
century, with peaks of 300 tonnes. After the 1950s catches
decreased by about 90% (Fig. 1), coinciding with the construction of the first dam (Alexandrino, 1996; Baglinière et al.,
2003). In the last 20 years mean annual catches reached about
four tonnes, while in 1980 the catch peaked with about 18
tonnes. However, these values seem to be underestimates due
to the lack of good official statistics and unofficial Portuguese
and Spanish data over the last eight years indicates that yields
may have been twice as high, pointing to the existence of a
noticeable population worth studying to develop conservation
and restoration strategies. Furthermore, this species has
become rare in Southern Europe (Limburg and Waldman,
2009). The objectives of this study, therefore, were to identify
key parameters of the general biology of the species in the
Minho River and assess the actual state of the Alosa alosa
population in this specific river.
Material and methods
Specimens were sampled by experimental fishing and from
fishermen of the international part (Spain and Portugal) of the
Minho River between April and August 2009. Two sampling
stations (estuary and spawning area) were selected (Fig. 2). A
typical trammel net (140 m long; 140 mm loose inner layer
mesh size) was used.
Fig. 1. Time series data for half a century of A. alosa catches (kg and numbers) in the Minho River reported by Portuguese fishermen to the local
Maritime AuthoritiesAuthority of Fishing Port of Caminha
Status of Allis shad (Alosa alosa) in the Minho River
57
Results and discussion
Fig. 2. Map of the study area showing the location of the two
experimental fishing sites (w)
Total length (TL) was measured to the nearest 0.1 cm
(measuring board) and total weight (TW) was determined to
the nearest 5 g (digital hanging Kern HDB balance). The TL
and TW statistics were expressed as means and standard
deviations, separately according to sex, as well as the respective
maximum and minimum values.
Gonads were removed from both sexes and weighed using a
digital A&D EK-610i balance with a precision of 0.01 g. The
gonadosomatic index (GSI) was calculated for each sex in
order to document changes in fish maturity and spawning state
during the upstream migration, using the equation
GSI = 100*(Wg ⁄ Wt) where Wg is the gonad weight (g) and
Wt is the fish total weight (g).
Condition factors with gonad weight (K) and without gonad
weight (K¢) and gonadal factor condition (DK) were estimated
as K ¼ 100 ðWt =Lbt Þ; K 0 ¼ 100 ðWc =Lbt Þ; DK = K)K¢ where
Wt is the fish total weight (g), Wg is the gonad weight (g),
Wc = Wt)Wg; Lt is the total length (cm) and b is the
allometric coefficient obtained through the length-weight
relationship.
Scales of 111 specimens were removed and treated according
to Baglinière et al. (2001) for ageing and identification of
spawning marks.
The available habitat for the A. alosa in the Minho River
during freshwater migration covers a stretch of approximately
80 km in the international Minho River section (from the
estuary to the first dam). Their estuary upstream migration
began in March and ended in June. The earlier spring
migration is related to the latitude: south populations migrate
earlier in the year than further north populations (MennessonBoisneau et al., 2000a; Aprahamian et al., 2002; Baglinière
et al., 2003).
The highest percentage of males occurred in March and
April (63.6 and 69.6% respectively), while females dominated
May and June (61.2 and 80%, respectively). This migratory
behaviour is in accordance with the life history described for
this species (Baglinière et al., 2003; Maitland and Hatton-Ellis,
2003), where males migrate upstream first, followed by
females. The presence of post-spawner shads in the upper
international Minho River, in early August, may suggest the
spawning season to extent until this month, as reported for
other A. alosa populations (Baglinière et al., 2003).
From the total samples, 49.5% were females and 50.5%
were males, indicating no significant difference (P > 0.05)
from a 1 : 1 sex ratio as also described for the Douro River
A. alosa population (Eiras, 1981). However, other A. alosa
populations showed different sex ratios (Table 1) and it seems
difficult to explain whether annual fluctuations of this parameter reflects a biological principle or represent a bias in the
sampling methods, periods and ⁄ or locations (MennessonBoisneau et al., 2000b).
The results on biometrical and demographical structure of
the 200 specimens sampled are given in Table 1 and Fig. 3.
Comparing with northern populations, the Minho River
A. alosa population presents the highest growth rate. This
pattern was also demonstrated by Lassalle et al. (2008), where
comparisons of Portuguese and Moroccan populations with
northern population showed larger lengths to the first ones
(negative latitudinal gradient). In contrast, allometric coefficient b was lowest than in the Portuguese and Moroccan
populations considered by Mennesson-Boisneau et al. (2000b).
However, the Douro River population (Froese and Pauly,
2005) showed an allometric coefficient close to the one given
for the Minho River population. Although the Minho and
Table 1
Total length and weight results for the Minho River A. alosa L. sampled between March and July 2009 and comparison with European and
Moroccan populations (adapted to Mennesson-Boisneau et al., 2000b)
Minho River
(present study)
Females
Males
b
2.81 (mixed)
N
99
101
%
49.5
50.50
Length (mm)
X
664
607
SD
34
25
Min
540
530
Max
740
670
Weight (g)
X
2678
2208
SD
819
408
Min
915
890
Max
4355
3450
Lima River
(Alexandrino,
1996)
Females
Males
3.1139 (mixed)
86
97
47
53
Loire River
(MennessonBoisneau, 1990)
Gironde River
(Taverny, 1991)
Adour River
(Prouzet,
unpublished data)
Sebou River
(Sabatié 1993)
Females
Males
Females
Males
Females
Males
Females
Males
3.019
1041
47
3.114
1172
53
3.3429
191
56
3.2252
152
44
2.671
101
46
3.116
118
54
3.4105
317
40
3.2587
482
60
636
3.8
490
720
549
4.4
445
670
634
44
480
733
537
61
400
666
556
39
414
680
484
39
339
635
585
26
509
650
533
29
383
597
539
25
475
600
492
32
415
605
2864
611
1237
4400
1663
575
668
3215
1712
489
500
3800
1067
342
174
2487
2185
391
1245
3250
1572
299
610
2775
2066
329
1400
2900
1411
324
800
2800
3019
58
1250
4525
N, n of individuals; b, allometric coefficient; %, percentage; X , average; SD, standard deviation; Min, minimum and Max, maximum.
1800
47
850
3275
58
M. Mota and C. Antunes
Fig. 5. Variation in the Gonadal factor condition (DK) of males and
females of Alosa alosa during the months of the spawning migration in
the Minho River
Fig. 3. Length-weight relationship for the A. alosa population migrating for spawning into the Minho River. The allometric equation is
given as: W = total weight (g), L = total length (cm); a = 0.0221
and b = 2.8147. Data are combined for both sexes. The low b value
may have been caused by spent fish
Table 2
Age structure of Minho River A. alosa population by sex and length
Males
Douro populations are larger in length, they present lower
weights compared to length-weight relationships to the rest of
the populations elsewhere and thus may explain the difference.
GSI revealed highest values for females and, with the
exception in April when we observed a slight decrease in the
GSI values. GSI increased until June and strongly decreased in
July for both sexes, indicating an increase of gonadal weight
during the upstream migration and the presence of postspawners in July (Fig. 4).
Condition factor (K) decreased along the five migratory
months for both sexes. In contrast, gonadal condition factor
(DK) was the same from March to May and slightly decreasing
in June and July for males and, for the females, slightly
decreased from March to April, rose from May to June, and
fell again in July (Fig. 5). The high female (DK) observed in
June indicates fish are ready to spawn.
The age structure of migratory adults is given in Table 2.
The scales reading revealed an average age of 6.7 years for the
total fish sampled. For males, 57.4% of the specimens were
6 years old and 58.1% of females were 7 years old. As seen in
other studies (Mennesson-Boisneau et al., 2000b; Aprahamian
et al., 2002; Lassalle et al., 2008) the majority of females
matured one year later than males, however, the majority of
the Minho River adults spawner population seems to be one
Females
Length (mm)
Age
N
%
5
6
7
8
7
58
36
–
6.93
57.4
35.66
–
X
604,3
604.7
614.6
–
Length (mm)
SD
N
%
35.64
23.53
21.66
–
2
9
54
28
2.15
9.68
58.06
30.11
X
620.0
655.6
663.1
667.3
SD
84.85
17.40
35.89
33.95
N, sample size; %, percentage; X , average; SD, standard deviation.
year older than those of other European and Mediterranean
populations. Further, 5.4% of the spawners had spawning
marks on their scales. These results show the evidence of a few
fish returning for a second spawning migration and thus, as
other European populations (see Mennesson-Boisneau et al.,
2000b; Aprahamiam et al., 2002; Maitland and Hatton-Ellis,
2003), this population is considered semelparous.
In the last 30 years two peaks were revealed concerning the
data of Portuguese fishermen (1980 and 2009 spawning
seasons). Although fluctuations in the abundance of Alosa
populations are a reality (Baglinière et al., 2003), future
investigations will be carried out in the way of enhance
knowledge of the status and dynamic of the Minho River
A. alosa population.
Acknowledgements
Micaela Mota is supported by a PhD grant from the
Portuguese Foundation for Science and Technology – FCT
(SFRH ⁄ BD ⁄ 44892 ⁄ 2008). Special thanks to Eduardo Martins, Eugénio Pereira and Dr. Ronaldo Sousa for the field
assistance, Vânia Gonçalves, Cátia Patrão and Rita Carvalho
for the laboratory assistance and to the fishermen concerning
biological samples and to Eng. Claudio Ramos for the study
area map arrangement.
Conflict of interests
Fig. 4. Changes of the Gonadossomatic Index (%) (GSI) in A. alosa
specimens during the spawning upstream migration in the Minho
River, shown separately for males and females. Total n = 200
specimens
The authors have no conflict of interest to declare and note
that the sponsors of the issue had no role in the study design,
data collection and analysis, decision to publish, or preparation of the manuscript.
Status of Allis shad (Alosa alosa) in the Minho River
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AuthorÕs address: Micaela Mota, Várzea – Parada, 4940-441 Paredes de
Coura, Portugal.
E-mail: [email protected]