Latitudinal variations in age and size at maturity ...

Journal of Fish Biology (2008) 73, 1799–1809 doi:10.1111/j.1095-8649.2008.02036.x, available online at http://www.blackwell-synergy.com

Latitudinal variations in age and size at maturity among allis shad Alosa alosa populations G. L ASSALLE *, T. T RANCART , P. L AMBERT

AND

E. R OCHARD

Cemagref, Estuarine Ecosystems and Diadromous Fishes Research Unit, 50 avenue de Verdun, 33612 Gazinet-Cestas Cedex, France (Received 31 March 2008, Accepted 17 July 2008) Age and total length (LT) at maturity of allis shad Alosa alosa exhibited a significant negative latitudinal gradient over the species’ distribution range. Particular thermal conditions experienced over the distribution area could be the key factor involved to explain this # 2008 The Authors negative trend. Journal compilation # 2008 The Fisheries Society of the British Isles

Key words: Alosa alosa; diadromous fishes; latitudinal gradient; life history; temperature.

Biogeographical trends in life-history variables of organisms often vary along a latitudinal gradient because of predictable changes in important environmental factors, as seen from studies on freshwater fishes (Blanck & Lamouroux, 2007), amphibians (Morrison & Hero, 2003) and birds (Chalfoun & Martin, 2007). Among anadromous fishes, geographical variations in life-history traits have been reported in many species and frequently occur in latitudinal clines (L’Ab
ee-Lund et al., 2004; Power et al., 2005). The life history of allis shad Alosa alosa (L.) is well known. The species has a pelagic marine existence but migrates to the higher middle watercourse of rivers to spawn. The spawning migration of adults takes place from December for the Moroccan populations to August for a few French populations when temperatures range from 12 to 20° C (Mennesson-Boisneau et al., 2000a). Generally, anadromous A. alosa shad are considered semelparous, since no more than 5–6% of the Atlantic stock spawn more than once (Mennesson-Boisneau et al., 2000b). Adult A. alosa do not feed during their spawning migration (Mennesson-Boisneau et al., 2000a). Alosa alosa spawn mainly at night between April and mid-August in habitat characterized by a coarse substratum and a water temperature between 16 and 18° C (Cassou-Leins et al., 2000). Downstream migration towards the sea occurs in schools, taking

*Author to whom correspondence should be addressed. Tel.: þ33 5 57 89 09 98; fax: þ33 5 57 89 08 01; email: [email protected]

1799 2008 The Authors Journal compilation # 2008 The Fisheries Society of the British Isles

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G. LASSALLE ET AL.

place in the summer, and lasts from 3 to 6 months according to juvenile captures (Taverny et al., 2000). A recent study based on spawners, however, showed an estuarine-phase duration of 11 days on average (Lochet et al., 2008). Most young of the year reach the sea at the beginning of winter and remain on the continental shelf feeding mainly on zooplankton (Taverny & Elie, 2001). Most life-history studies on A. alosa have focused on single populations (Douchement, 1981; Eiras, 1981; Mennesson-Boisneau, 1990; Taverny, 1991; Sabati
e, 1993; Prouzet et al., 1994; Alexandrino, 1996; V
eron, 1999; Lambert et al., 2001; Maitland & Lyle, 2005). Studies that have sought to compare populations have been restricted to a limited geographic area or have compared only a handful of populations (Sabati
e et al., 1996). Few studies have attempted a detailed zoogeographic analysis of population life-history patterns. The literature reviews by Baglinie`re & Elie (2000) and Aprahamian et al. (2003a) provided a useful compilation of the biological knowledge on the two European shad, A. alosa and twaite shad Alosa fallax (Lacepe`de), at the end of the 20th century. Neither of these two studies, however, analysed in a detailed way or demonstrated statistically the relationships between latitude and life-history variables for A. alosa. This detailed analysis was performed for A. fallax by Aprahamian et al. (2003b). The general aim of this study, based on published data, was to investigate patterns in A. alosa age and size at sexual maturity along a latitudinal gradient of c. 20° from the southern to the northern extent of the species’ distribution range. In order to test whether latitudinal clines were significant and consistent, linear models were built. It was hypothesized that the direction of the latitudinal variations for these life-history traits was related to the occurrence of particular thermal conditions experienced over the species’ distribution range. The life-history characteristics, mean size (mm) and age (years) at sexual maturity of anadromous A. alosa populations were obtained from the literature (Table I). Landlocked populations were excluded from the analysis (Furnestin, 1952; Eiras, 1983). Size corresponded to fish total length (LT). Fork length was converted to LT with the equations provided in Aprahamian et al. (2003a) except for the study of Prouzet et al. (1994) for which conversion was made with the equations given by the authors in their paper. Ages were systematically determined from scales (Mennesson-Boisneau & Baglinie`re, 1992; Baglinie`re et al., 2001). The mean age and size at maturity for each river population were calculated using all the data weighted by the number of years where samples occurred. The magnitude of variation in traits was quantified by calculating, for each trait, the coefficient of variation (C.V.). First, the assumption of normality of the data was checked with a Shapiro–Wilk test particularly relevant for small sample sizes. Then, linear models were built to study the relationship between latitude and both mean LT and age at sexual maturity. Males and females were treated separately in the statistical analysis. The statistical software R 2.5.1 (R Foundation Core Team, 2005) was used. The Cook’s distance was examined as the measure of the global influence of an observation on all the predicted values of the model (Cook & Weinsberg, 1982). Observations with a Cook’s distance >1 were considered as outliers and were excluded

Journal compilation

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# 2008 The Authors 2008 The Fisheries Society of the British Isles, Journal of Fish Biology 2008, 73, 1799–1809

Latitude (° N)

34
25

40
15

41
15

41
67

43
40

43
53

45
58

45
94

47
20

Basin, country

Sebou, Morocco

Mondego, Portugal

Douro, Portugal

Lima, Portugal

Nivelle, France

Adour, France

Garonne, France

Charente, France

Loire, France LT (mm)

Age (years) LT (mm) Age (years)

LT (mm)

Age (years)

LT (mm)

Age (years) LT (mm) Age (years) LT (mm) Age (years) LT (mm) Age (years) LT (mm) Age (years) LT (mm) Age (years)

Life-history traits 729 729 20 20 509 509 158 183 257 257 219 487 89 219 487 141 1646 929 82 929 109 60 60 2213 99 2213 136

Sample size 1978–1988 1978–1988 1991–1994 1991–1994 1977–1979 1977–1979 1991–1994 1991–1994 1984–1992 1984–1992 1994 1987–1991 1977–1979 1994 1987–1991 1977–1979 1989–1991 1986–1988 1977–1979 1986–1988 1977–1979 1997–1998 1997–1998 1984–1989 1977–1979 1984–1989 1977–1979

Sample years 5
00 549
0 5
57 570
1 4
29 565
6 4
98 537
5 4
77 504
3 5
02 5
1 4
32 492
0 501
0 480
7 4
86 5
07 5
00 517
0 498
4 4
88 527
1 4
37 4
01 493
0 498
3

Male 5
80 636
0 5
55 633
9 4
42 634
3 5
94 634
5 5
26 566
6 5
44 5
8 6
41 539
0 531
0 543
0 5
02 5
42 5
61 569
3 561
7 5
25 570
2 4
96 5
39 564
4 540
0

Female

Sabati
e (1993) Sabati
e (1993) Alexandrino (1996) Alexandrino (1996) Eiras (1981) Eiras (1981) Sabati
e et al. (1996) Alexandrino (1996) Goni (2002) Goni (2002) Baglinie`re & Elie (2000) Prouzet et al. (1994) Douchement (1981) Baglinie`re & Elie (2000) Prouzet et al. (1994) Douchement (1981) Lambert et al. (2001) Taverny (1991) Douchement (1981) Taverny (1991) Douchement (1981) V
eron et al. (2001) V
eron et al. (2001) Mennesson-Boisneau (1990) Douchement (1981) Mennesson-Boisneau (1990) Douchement (1981)

Sources

TABLE I. Run-mean size and age at maturity by location and literature source. To designate the latitude of each river, the location of the river mouth was used and expressed in decimals. Table I is ordered by increasing degrees of latitude

LIFE-HISTORY TRAITS OF ALOSA ALOSA

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# 2008 The Authors Journal compilation # 2008 The Fisheries Society of the British Isles, Journal of Fish Biology 2008, 73, 1799–1809

Journal compilation

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49
27 49
33

54
95

Orne, France Vire, France

Solway Firth, U.K.

LT, total length.

LT (mm) LT (mm) Age (years) Age (years)

47
50 47
74 47
74 48
28

Vilaine, France Blavet, France Scorff, France Aulne, France

LT (mm) Age (years) LT (mm) LT (mm)

LT (mm)

Life-history traits

Latitude (° N)

Basin, country 11