SIZE AT FIRST MATURITY OF THE NW MEDITERRANEAN ANCHOVY
by I. Palomera(1), B. Tejeiro(1) and X. Alemany(2)
(1)
Institut de Ciències del Mar. P. Marítim, 37-49. Barcelona Spain.
Instituto Español de Oceanografía. Centro de Baleares. Palma de Mallorca.
Spain
(2)
Introduction
During the 2001 SCSA meeting in Rome, it was proposed to revise the
existing information on the size at first maturity of the Mediterranean anchovy.
Giraldez (2001) presented the results of that revision. The majority of studies
were performed by the maturity method, that means, visual classification of the
maturity stage of the gonads. Only one of the analysis was made by the
histological method (Ochoa Baez, 1998) at the anchovy population of the
Catalan sea. This revision showed important differences between some of the
Mediterranean populations. For that reason, during the last meeting of the
Working Group of Small Pelagics of the SCSA (Rome, 2002), it was decided to
perform a simultaneous analysis of this parameter of the anchovy populations in
the whole Mediterranean sea.
In this context we have analysed the size at first maturity of the anchovy,
Engraulis encrasicolus, at management unit 6. This parameter has been studied
before by Pertierra (1992) and Ochoa Baez (1998) at the same area.
Methods
We have analysed anchovy samples coming from the landings of Roses,
Port de la Selva and Tarragona harbours. We have considered two different
regions for our analysis: the northern one in the Gulf of Lions that includes the
two former harbours and the southern one represented by the sampling at
Tarragona (Fig.1).
At the Gulf of Lions area, we only obtained 4 samples since June, due to
bad weather conditions. At the Tarragona area, we analysed 10 samples, from
April to September.
We analysed 657 anchovy individuals being 338 males and 319 females
(see Table 1).
After measuring the individuals to the nearest 0.5 cm and weighting at a
precision of 0,01 gr, we dissected and stored the ovaries of the females. In the
case of small or undetermined individuals, the gonad was also stored in order to
analyse the sex and maturity stage. All the ovaries were analysed under a
dissecting microscope to check the presence of mature oocytes and in some
cases histological analysis and observation under light microscope was also
performed to accurately analyse the degree of development of the ovaries. We
followed the methodology used by Hunter and Macewicz (1985) and Hunter et
al. (1992).
Table 1: Data concerning number of individuals analysed (N), percent of sexes
and average total length (TL) by sex and by areas.
AREA
SEX
N
%
TL
SD
Gulf of Lions
Males
87
41.88
13.96
1.92
Females
121
58.22
14.14
1.47
Males
251
55.90
13.73
1.46
Females
198
44.10
13.45
1.92
Tarragona
The gonosomatic index of the females was also calculated as the relation
between the weight of the ovary (W O) and the weight of the individual (W T),
where
GSI = W O * 100/ WT - WO
The mean size at sexual maturity was estimated as the length at which
50 per cent of all individuals are sexually mature (Lm). A logistic curve was fitted
to the proportion (P) of sexually mature individuals by length (L). The equation
is:
P = 1/(1 + exp[-r(L-Lm)])
Results
We present the results separately for the two areas considered, Gulf of
Lions and continental shelf off Tarragona, taking into account that two different
spawning grounds have been defined in those regions (Fig.1). Also, the
spawning period of anchovy in those areas is different, being from May to
September at the Gulf of Lions and from April to October at Tarragona area
(Palomera, 1992).
Gulf of Lions
We analysed 208 anchovy individuals being 87 males and 121 females.
The length range was 9 – 17 cm TL, being the biggest ones from June and July
samples and the smallest from September.
The size frequency distributions obtained are shown in figure 2. During
the peak spawning, June – July, the smallest female obtained had a length of
13 cm. On the contrary, in September the range was between 9 and 13 cm.
The gonosomatic index (IGS) was also calculated (Table 2). The
maximum values were obtained in June and July (6.8) with an average of 3.9
and 3.3, respectively. In August it decreased, with an average of 1.70.
We analysed the size at first maturity with all the females obtained. The
result obtained was 12.78 cm. (Fig.3) This result is very closed to that obtained
before for the Catalan coast by Pertierra (1992; 12.51 cm) and Ochoa-Baez
(1998; 12.71 cm).
Table 2. Values of gonosomatic index of anchovy obtained in the NW
Mediterranean during the spawning period. Results are shown separately for
the two regions considered.
IGS
Gulf of Lions
Tarragona
Range
April
May
-----
-----
June
July
August September
2.36-6.81 0.87-6.42 2.48-4.77
Average
3.90
3.29
1.70
(SD)
(1.33)
(0.90)
(0.89)
Range
1.01-7.82 1.01-7.67 0.73-7.24 0.73-5.28 2.48-4.77
Average
4.62
3.29
3.37
3.07
3.46
(SD)
(1.42)
(1.02)
(1.17)
(0.74)
(0.54)
0.08
0.08
0.10
0.10
Tarragona
We analysed 684 anchovy individuals being 375 males and 309 females.
The total length range was 9.5 – 17 cm TL (Fig. 2). In April the range was
between 12 and 17 cm and from May to July few smaller individuals appeared.
In September the range was from 9.5 to 12 cm.
The maximum gonosomatic index values were obtained between April an
June (Table 2), although the average values were very similar between April
and August.
Females from all the samples obtained were analysed to get the size at
first maturity. The result obtained was 11.08 cm (Fig. 3). This result is lower
from the one obtained for the Gulf of Lions region and also from those reported
before Pertierra (1992) and Ochoa-Baez (1998).
Conclusion
The results on size at first maturity for the two regions analysed are quite
different. In the case of the Gulf of Lions we consider that, although they are
very similar to those reported before, they should be taken with caution, due to
the absence of individuals below 12 cm during the peak spawning period. In the
Tarragona samples some individuals between 11 and 12 cm appeared during
the peak spawning giving us a more accurate value of that parameter.
Nevertheless, with previous studies and the present one, we can conclude that
at the NW Mediterranean, the size at first maturity for anchovy is over 11 cm.
That means a size at least two centimetres over the legal minimum size
established for this species (Council Regulation (EC) No 1626/94, 1994).
REFERENCES
Giraldez, A., 2001. The length at first maturity of anchovy Engraulis
encrasicolus in the Mediterranean. GFCFM, SAC, Athens, 3 pp.
Hunter, J.R. and B.J. Macewicz, 1985. Measurement of spawning frequency in
multiple spawning fishes. In: An egg production method for spawning biomass
of pelagic fish: Application to the Northern anchovy, Engraulis mordax. NOAA
Tech. Rep. NMFS, 36, pp: 79-94
Hunter, J.R., B.J. Macewicz, N.C. Lo and C.A. Kimbrell, 1992. Fecundity,
spawning, and maturity of female Dover sole Microstomus pacificus, with an
evaluation of assumptions and precision. Fish. Bull, U.S. 90: 101-128.
Ochoa-baez. R.I., 1998. Estacionalidad reproductiva y producción ovárica de la
anchoa europea E. encrasicolus en el mar catalán. Ph.D. Thesis. Univ.
Politécnica de Catalunya.
Palomera, I. 1992. Spawning of anchovy, Engraulis encrasicolus, in the
Nortwestern Mediterranean relative to hydrographic features in the region.
Marine Ecology Prog. Ser., 79:215-223.
Pertierra, J.P. 1992. Biología pesquera de la anchoa, E. encrasicolus del litoral
catalan. Ph.D. Thesis. Univ. Barcelona.
Figures
Anchovy egg distribution
1992
Port de la Selva
Roses
Tarragona
Gulf of Lions
M.U. 7
Northern Spain
M.U. 6
Figure 1. Spawning ground of anchovy in the NW Mediterranean (M.U. 6 and
7). The harbours from which the samples were obtained are indicated.
Tarragona
40.00
n=60
% 20.00
0.00
9 10 11 12 13 14 15 16 17
40.00
May 2002
n=235
% 20.00
Gulf of Lions
0.00
9
June 2002
40.00
n=131
June 2002
40.00
10 11 12 13 14 15 16 17
n=46
% 20.00
% 20.00
0.00
0.00
9
9
10 11 12 13 14 15 16 17
n=65
%
July 2002
40.00
July 2002
40.00
20.00
% 20.00
0.00
0.00
n=156
9 10 11 12 13 14 15 16 17
9
10 11 12 13 14 15 16 17
August 2002
40.00
n=47
August 2002
40.00
10 11 12 13 14 15 16 17
% 20.00
n=68
% 20.00
0.00
0.00
9
9
10 11 12 13 14 15 16 17
10 11 12 13 14 15 16 17
September 2002
September 2002
40.00
n=29
40.00
n=55
% 20.00
% 20.00
0.00
0.00
9
10 11
12
13
14
Length (cm)
15 16
17
9
10 11 12 13 14 15 16 17
Length (cm)
Figure 2. Length frequency distributions of anchovies analysed in this study.
Males
Females
GULF OF LIONS
1
Percent
0.75
0.5
0.25
P = 1/1 + e--2.41*(x-12.8)
0
9
10
11
12
13
14
15
16
17
Total length (cm)
TARRAGONA
1
Percent
0.8
0.6
0.4
P = 1/1 + e-1.70*(x-11.1)
0.2
0
8
9
10
11
12
13
14
15
16
17
18
fitted model
Total length (cm)
observed
Figure 3. Percent of mature anchovy (E. encraicolus) by length. Equation and
length at 50% are shown.