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Maturation and gill-net selectivity of two small sea breams (genus Diplodus) from the Algarve coast (south Portugal)
Fisheries Research 36 (1998) 185±194
Maturation and gill-net selectivity of two small sea breams (genus Diplodus) from the Algarve coast (south Portugal) Miguel Neves Santosa,*, Carlos Costa Monteiroa, Karim Erzinib,1 Gerard Lasserrec,2 a
Instituto de Investigac,aÄo das Pescas e do Mar, Centro Regional de Investigac,aÄo Pesqueira do Sul, Av. 5 de Outubro s/n, 8700 OlhaÄo, Portugal b Universidade do Algarve, UCTRA, Campus de Gambelas, 8000 Faro, Portugal c Laboratoire d'Hydroliologie Marine, Unite de Recherche AssocieÂe au CNRS, Universite de Montpellier II, 34095 Montpellier Cedex 05, France Received 7 October 1997; accepted 19 February 1998
Abstract Diplodus annularis and Diplodus bellottii, two species of small sea breams from the Algarve coast (south Portugal), were studied to obtain information on the spawning period, ®rst maturity, weight±length relationship and gill-net selectivity curves. These species are important in local gill-net catches, and there is no information on their biology and selectivity in the Algarve waters. Spawning lasts for 6 months. Total length (TL) at ®rst maturity for females was 13.4 cm in case of D. annularis and 11.4 cm in case of D. bellottii. The relationships between weight and length were W0.0132 TL3.096 for D. annularis and W0.0093 TL3.193 for D. bellottii. The Sechin method was used to estimate gill-net selectivity curves. The estimated selectivity curves correspond closely to the catch curves of the legal mesh size (60 mm), but were narrower for the other meshes. D. annularis and D. bellottii will be caught using the legal mesh size in the following length ranges: 13±19 cm and 11±19 cm. # 1998 Elsevier Science B.V. All rights reserved. Keywords: Gill-net selectivity; Biology; D. annularis; D. bellottii; Maturation
1. Introduction The Senegal sea bream (Diplodus bellottii, Steindachner, 1882) and the annular sea bream (Diplodus annularis, Linnaeus, 1758) are two small demersal species of the Sparidae family. These two similar species are characterised by the total absence of black *Corresponding author. Tel.: 351 89 700504; fax: 351 897 00535. 1 Tel.: 351 89 800100; fax: 351 89 818353; e-mail: [email protected]. 2 Tel.: 33 467 526660; fax: 33 467 143719. 0165-7836/98/$19.00 # 1998 Elsevier Science B.V. All rights reserved. PII S0165-7836(98)00100-3
bands on the body. However, the Senegal sea bream has a distinguishing small dark spot at the origin of the lateral line and a dark longitudinal stripe along the middle of the sides, whereas the annular sea bream has an almost annular dark band around the caudal peduncle just behind the dorsal and anal ®ns (Whitehead et al., 1986). The Senegal sea bream was initially described only for the western coast of Africa, but is also present along the north coast of Morocco, southern Portugal (the Algarve), and the Gulf of Cadiz. The geographic distribution of the annular sea bream extends from the Mediterranean to the Iberian Atlantic coast, the
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Canaries and Madeira. While both species are most common at 10±30 m depths, their bathymetric range extends from the shallow subtidal to depths of 90 m. Several aspects of the life history of D. annularis and D. bellottii (Paz, 1975), the reproductive cycle, ecology and gill-net selectivity have been studied in different areas: Gulf of Lion (Girardin, 1978; Rabenevanana, 1985), Gulf of Cadiz (Rodriguez and Rodriguez, 1984), South Euboikos Gulf (Petrakis and Stergiou, 1995), and the Algarve (Monteiro, 1989; Martins et al., 1992). In the Algarve these species are found in the lagoon systems ``Ria de Alvor'' and ``Ria Formosa'', and the Guadiana Estuary. These highly productive ecosystems which act as nurseries, supply some of the most important ®sh stocks of coastal waters of south Portugal (Monteiro et al., 1987, 1990). Off the Algarve coast these species are caught by traps and long-lines, and especially by gill nets and purse seine. According to Martins et al. (1992), the catch of these small sea bream represents almost 9% of the gill-net landings in this region. Because they require little investment in manpower and equipment, and are ef®cient in catching widely scattered ®sh, gill nets are widely used in artisanal ®sheries. This ®shing gear plays an important role in local ®sheries, due to the large volume of ®sh landed, high commercial value of most species and the number of ®sherman involved (Costa and Franca, 1982; Santos et al., 1995). Because of the selective nature of gill nets, mesh size can be controlled to restrict the size of ®sh captured, and selection curves can be used to calculate the mesh size required. A minimum mesh size can be used to limit the catch of immature ®sh or to reinforce minimum landing size regulations (Potter and Pawson, 1991). The abundance of these species off the Algarve justi®es this study, the objectives of which were to determine gill-net selectivity curves, weight-length relationships and lengths at ®rst maturity. This information will allow a better understanding of the biology of these species in the Algarve and provide a scienti®c basis for ®sheries management. 2. Materials and methods We used a set net with mesh sizes of 40, 50, 60 and 70 mm (measured between opposite knots when fully
stretched), that included the legal mesh size (60 mm). The hanging ratio of these nets was 0.50 on the ¯oat rope and 0.55 on the lead rope. In all, 60 panels (15 of each mesh size) were used, randomly distributed along the net. Each panel was 50 m long and 2.5 m high. The ®shing strategy consisted of setting the nets 3±4 h before sunrise and pulling them immediately after sunrise. The ®sh were sorted by species and mesh size and the total length (TL) was measured to the nearest lower centimetre. The results of experimental ®shing are compiled in terms of species composition per mesh size Martins et al. (1992). Data on total length (Tables 1 and 2), head girth (Gh) and maximum girth (Gmax) for both species were collected from March 1993 to February 1994, corresponding to a total of 62 experimental catches. Gh and Gmax were measured to the nearest millimetre, and were collected from our experiments as well as from commercial catches. Data on weight and the state of gonad development were collected from April 1990 to August 1993, off the Algarve (Fig. 1). An eight-stage scale of gonad maturation was used (BuÈchmann, 1929), and all individuals from stages 4±8 were considered mature. The maturity stage 4 is characterised by the following: opaque orange coloured eggs that can be clearly observed, and white/reddish male gonads that do not drip when under pressure; both male and female gonads occupying two-third parts of the abdominal cavity. The parameters of weight±length relationship (WaLb), a and b, and the coef®cient of determination r2, were estimated by least-squares regression using the log transformed weights and sizes. Standard errors (SE) and the 95% con®dence intervals were calculated for the slopes to check if these were statistically different from 3. Aptness of the model was studied by plots of residuals and standardised residuals (Neter et al., 1983). The logistic function (1) was ®tted to the maturity data: pi
1 1 expÿr TLi ÿ L50
(1)
where p is the proportion mature in size class i, TLi the mean total length of class i, r a parameter determining the slope of the ogive, and L50 the TL at 50% maturity. The parameters were estimated for each species and sex by ®tting a general linear model with binomial
M.N. Santos et al. / Fisheries Research 36 (1998) 185±194
187
Table 1 Total-length-frequency distributions of Diplodus annularis caught in the Algarve waters from March 1993 to February 1994, percentage of mature fish, mean weight (g), mean of maximum girth and head girth (cm) Length class (cm) 10.0 11.0 12.0 13.0 14.0 15.0 16.0 17.0 18.0 19.0 20.0 Total
Mesh size (mm) 40
50
33 40 37 15 10 1
136
2 20 72 28 7 3
132
60
4 28 36 13 4 1 86
70
1 2 4 9 7 3 1 27
Mature female (%)
Mature male (%)
Mean weight (g)
Mean max. girth (cm)
Mean head girth (cm)
0 0 33 54 80 66 100 100 100 100 100 77
0 0 50 70 88 100 100 100 100 100 100 136
18 30 34 40 52 62 80 88 112 125 151 310
9.47 9.84 9.91 10.66 11.46 12.26 12.94 14.25 15.13 15.50 16.20 150
8.30 9.02 9.19 9.79 10.90 10.97 12.29 12.49 13.63 13.97 15.10 187
Table 2 Total-length-frequency distributions of Diplodus bellottii caught in the Algarve waters from March 1993 to February 1994, percentage of mature fish, mean weight (g), mean of maximum girth and head girth (cm) Length class (cm) 9.0 10.0 11.0 12.0 13.0 14.0 15.0 16.0 17.0 18.0 19.0 20.0 21.0 22.0 Total
Mesh size (mm) 40 16 283 746 533 209 79 16
1882
50 9 14 87 432 323 132 48 19 8
1072
60
6 15 36 113 207 115 70 15 5
582
70
2 9 15 24 44 28 6 1 129
Mature female (%)
Mature male (%)
0 43 68 77 80 87 89 93 90 100 100 100 100 263
0 50 64 75 83 89 97 100 100 100 100 100
variance function and the logit link using S-PLUS 4 (MathSoft, 1997; Chambers and Hastie, 1993). Log(pi/(1ÿpi)) is modelled as a function of TLi. The parameter r is equal to the slope and L50 is equal to the intercept divided by (ÿr). 2.1. Estimating gill-net selectivity Inference of selectivity from girth measurements differs from other methods in that the size distributions
216
Mean weight (g)
Mean max. girth (cm)
Mean head girth (cm)
12 17 24 29 36 47 57 70 86 102 122 149 166 193 2144
8.0 8.6 9.4 9.7 10.2 11.1 11.9 12.6 13.4 14.3 15.2 16.2 16.9 17.9 1748
7.6 8.1 8.5 9.1 9.6 10.5 10.9 11.6 12.5 13.1 13.9 14.1 14.6 15.4 848
of gill-net catches are not examined. Girth measurements allow selectivity to be estimated assuming that a ®sh, upon swimming into the net, is caught if its head girth is smaller but maximum girth larger than the mesh perimeter. On this basis and, assuming that girth among any particular length class of ®sh is normally distributed, with a common variance for all length classes, Sechin (1969) and Kawamura (1972) derived theoretical selectivity curves. In this study we adopted the Sechin model, as modi®ed by Reis and Pawson
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M.N. Santos et al. / Fisheries Research 36 (1998) 185±194
Fig. 1. Areas sampled for D. annularis and D. bellotii off south Algarve by gill nets.
(1992), which has the following expression for the selectivity: 2M ÿ Ghj 2M ÿ Gmaxj 1ÿ (2) Sj h max where Sj is the selectivity in the jth length class, Ghj the mean head girth in the jth length class, h the mean standard deviation of head girth, Gmaxj the mean maximum girth in the jth length class, max the mean standard deviation of maximum girth, 2 M the mesh perimeter and the cumulative standardised normal distribution function.
is allometric. The positive allometry found is statistically different from 3, with a 95% con®dence interval of 3.001b3.191. The relationship found for D. bellottii also indicates positive allometry, with the parameter b signi®cantly different from 3, with a 95% con®dence interval of 3.166b3.220. Plots of the residuals and the standardised residuals against TL and predicted weights showed no signi®cant departure from the simple linear model for the two weight±length data sets. In particular, there was no indication that the error terms did not have constant variance or were not independent. Also, error terms appeared to be normally distributed in all cases.
3. Results
3.2. Spawning period and length at first maturity
3.1. Weight±length relationship
The observed percentages of mature ®sh per size class, for the spawning period are given in Tables 1 and 2. The spawning period of D. annularis ranges from April to September (Fig. 2). The maturity ogive (Fig. 3) shows that 50% of females and males are sexually mature (L50) at total lengths of 13.4 cm and 12.9 cm, respectively. Off the Algarve, D. bellottii spawn between January and June (Fig. 2). Sexual maturity is reached at total lengths of 11.4 cm and 12.0 cm, respectively, for females and males (Fig. 3). The results of the generalised linear model ®tting are given in Table 3.
The results of the weight±length relationship are summarised in Tables 1 and 2. The determined weight (g)±total length (cm) relationships were: Diplodus annularis W 0:0132 TL3:096
(3)
2
(n310, r 0.93, SETL1.72, SEb0.048, mean TL14.3 cm, mean W52 g) Diplodus bellottii W 0:0093 TL3:193 2
(4)
(n2144, r 0.96, SETL2.07, SEb0.014, mean TL14.5 cm, mean W51 g) The relationship between weight and length for D. annularis shows that growth in the Algarve waters
3.3. Girth vs. length relationship The ®tted linear regressions for maximum girth (Gmax) and total length (TL) data (in cm) for the
M.N. Santos et al. / Fisheries Research 36 (1998) 185±194
189
Fig. 2. Percentage of mature (black) and immature (grey) fish for (left) D. annularis and (right) D. bellottii caught off the south coast of the Algarve.
D. annularis (Eq. (5)) and D. bellottii (Eq. (6)) were (see Tables 1 and 2): Gmax 0:888 0:747 TL n 150; r 2 0:96; max 0:51
(5)
Gmax 1:07 0:785 TL 2
n 1748; r 0:99; max 0:60
The relationship between head girth (Gh) and total length, obtained from ®tting the linear regression (in cm) for D. annularis (Eq. (7)) and D. bellottii (Eq. (8)) were as follows: Gh 0:25 0:72 TL
(6)
n 187; r2 0:97; h 0:81
(7)
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M.N. Santos et al. / Fisheries Research 36 (1998) 185±194
Fig. 3. Ogives of first maturity for (left) D. annularis and (right) D. bellotii in the Algarve: ( ) for males; and (ÐÐÐ) for females. MLS represents the minimum landing size.
Gh 1:00 0:65 TL 2
n 424; r 0:99; h 0:71
(8)
Plots of the residuals and the standardised residuals against TL and predicted girths showed no signi®cant departure from the simple linear model for the four girth±total length data. In particular, there was no indication that the error terms did not have constant
variance or were not independent. Also, error terms appeared to be normally distributed in all cases. 3.4. Gill-net selectivity Girth±length relationships indicated that maximum girth increased more with length than did head girth. This resulted in a slight increase in selection range as
M.N. Santos et al. / Fisheries Research 36 (1998) 185±194
191
Table 3 Results of estimation of parameters of maturity ogives for Diplodus annularis and D. bellottii using the generalised linear model approach Species
Sex
Intercept
(SE)
L
(Std. Error)
Ogive parameters r
L50
Diplodus annularis
female male female male
ÿ13.227 ÿ19.374 ÿ5.269 ÿ8.790
(3.459) (3.975) (1.347) (1.862)
0.984 1.505 0.462 0.731
(0.250) (0.300) (0.096) (0.135)
0.984 1.505 0.462 0.731
13.44 12.87 11.41 12.02
Diplodus bellottii
mesh size increased. The greatest probability of retention, which corresponds to the peak in ef®ciency, also increased with the mesh size. In the case of the Senegal sea bream, the maximum girth increased with length faster than in the annular sea bream. However, for the latter species the head girth increased more strongly with length than for the Senegal sea bream. The calculated normal selectivity curves are similar for both species. However, the estimated selectivity curves for D. bellottii were slightly higher and wider than those for D. annularis (Figs. 5 and 6). The probability of catching the annular sea bream and the Senegal sea bream is greatest in length classes 9 and 10 (40 mm mesh), 12 and 13 (50 mm mesh), 15 (60 mm mesh), and 18 (70 mm mesh). For the annular sea bream, the modes of the length-frequency distributions of the corresponding catch were 1.0 cm and 1.5 cm greater than those of the estimated curves for the 40 and 50 mm mesh size; equal for the 60 mm mesh; and 1 cm smaller for the 70 mm mesh. For the Senegal sea bream, the modes of the length-frequency distributions of the catches were 2.75 and 2.0 cm greater than those of the estimated curves for the 40 and 50 mm meshes; equal for the 60 mm mesh; and 0.5 cm smaller for the 70 mm mesh. 4. Discussion 4.1. Weight±length relationship The relationship between weight and length found for D. annularis differs from the values reported by Rabenevanana (1985), but is similar to that reported by Rodriguez and Rodriguez (1984). However, this species showed equal growth in weight off the Algarve and in the Mediterranean Sea. The relationship for D. bellottii showed exactly the same value that was
Fig. 4. Weight±length relationship for D. annularis and D. bellotii in the Algarve.
reported by Rodriguez and Rodriguez (1984). Growth in weight of these two species in Algarve is similar (Fig. 4) and does not differs signi®cantly from that found for the Mediterranean Sea (Table 4). 4.2. Length at first maturity Off the Algarve, the D. annularis spawning period starts one month earlier than in the neighbouring Gulf of Cadiz and the Gulf of Lion, and ®nishes, respectively, one and two months later. Off the Algarve, males are sexually mature at a larger size than in the Gulf of Lion, while females are sexually mature at the same size. In the case of D. bellottii, off-the-Algarve spawning starts two months earlier than in the Gulf of Cadiz, but ®nishes at the same time. The spawning period for these species occurs earlier in Atlantic waters than in the Mediterranean Sea (Table 5). If minimum landing sizes for these two small species of the genus Diplodus are to be established based on their sizes at ®rst maturity, then, in any future
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M.N. Santos et al. / Fisheries Research 36 (1998) 185±194
Table 4 Weight±length relationship for D. annularis and D. bellotii obtained in different areas Species
Length range
Length±weight relationship 3.096
Area
Author
D. annularis
10±20 12±19 4±23 7±17
W13.2 Eÿ03 TL W6.84 Eÿ03 TL3.089 W6.09 Eÿ06 TL3.222 W1.36 Eÿ04 TL2.5742
Algrave (south Portugal) Gulg of Cadiz (Spain) Gulf of Lion (France) Egypt (Mediterranean)
Present study Rodriguez and Rodriguez, 1984 Rabenevanana, 1985 Wassef, 1985
D. bellottii
9±22 11±20
W9.3 Eÿ03 TL3.193 W5.41 Eÿ03 TL3.193
Algrave (Portugal) Gulf of Cadiz (Spain)
Present study Rodriguez and Rodriguez, 1984
Table 5 Spawning period and total length (cm) at 1st maturity in different areas Species
Spawning period
Female
Male
Area
Author
D. annularis
April±August May±July May±June
13 cm 13 cm Ð
12 cm 11 cm Ð
Algrave (south Portugal) Gulg of Lion (France) Gulf of Cadiz (Spain)
Present study Rabenevanana, 1985 Rodriguez and Rodriguez, 1984
D. bellottii
January±June March±June
11 cm Ð
11 cm Ð
Algrave (south Portugal) Gulf of Cadiz (Spain)
Present study Rodriguez and Rodriguez, 1984
revision of minimum landing sizes, it should be reduced from 15 to 13 cm. 4.3. Gill-net selectivity The estimated selectivity curves for D. annularis are similar to the length-frequency distributions of the corresponding catches, for the 60 mm mesh sizes. However, for the 40, 50 and 70 mm mesh sizes this is not the case (Fig. 5), which may be due to the absence of some ®sh length classes in inshore waters. The small ®sh (under 11±12 cm) use the estuarine± lagoon systems as nursery grounds. Monteiro (1989) reported that the annular sea bream migrate from the lagoon Ria Formosa at lengths of 11±12 cm, and Rodriguez and Rodriguez (1984) did not catch ®sh under 12 cm outside the Guadalquivir Estuary. On the other hand, very few specimens were caught with total length larger than 18 cm (see Tables 1 and 2). The estimated selectivity curves for D. bellottii are similar to the length-frequency distribution of the corresponding catches only for the 60 mm mesh size (Fig. 6). The differences for mesh sizes 40, 50 and 70 mm can be partially explained by the low abun-
dance of specimens under 12 cm in total length in the coastal waters. For both species, the estimated selectivity curves are normal in shape, which is the case for ®sh that are usually wedged, as we could con®rm during the ®shing trials. On the other hand, the estimated curves are narrower than those of the corresponding catch. This may be due to the absence of coef®cients to account for body compressibility at the retention point and elasticity of netting material, in the formula used to estimate the selectivity curves. The estimated curves became higher and the selection range wider as mesh size increased, as was also reported in other studies (Hamley, 1972; Rudstam et al., 1984; Wulff, 1986; Jensen, 1986; Saila and Erzini, 1988; Ehrhardt and Die, 1988). The selectivity of each mesh indicates only the proportion of ®sh which one mesh size will capture relative to other mesh sizes. It does not show directly what fraction of the available ®sh population each mesh will catch. For management purposes, D. annularis and D. bellottii will be caught in the 13±18 cm and 12±19 cm length ranges by the legal mesh size (60 mm), which are larger than their lengths at ®rst maturity (L50).
M.N. Santos et al. / Fisheries Research 36 (1998) 185±194
Fig. 5. Gill-net estimated selectivity curves (line) and observed D. annularis size-frequency distribution (*) for the 40, 50, 60 and 70 mm mesh size used in Algarve.
193
Fig. 6. Gill-net estimated selectivity curves (line) and observed D. bellotii size frequency distribution (*) for the 40, 50, 60 and 70 mm mesh size used in Algarve.
194
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Acknowledgements We thank the staff of IPIMAR±CIMSul (E. BraÂs, T. SimoÄes and J.L. So®a) and the crew of ``NI DONAX'' for catching and collecting these data. References BuÈchmann, A., 1929. Die Methodik fischereibiologischer Untersuchungen an Meeresfischen. Abder-halen, Handbuck der biologischen Arbeitsmethoden, 9(6,1). Ed. Urban and Schwarzenberg, pp. 194. Chambers, J.M., Hastie, T.J., 1993. Statistical Models. S. Chapman and Hall, p. 608. Costa, F.C., Franca, M.L.F., 1982. Pesca artesanal na costa algarvia. SubsõÂdio para o conhecimento do seu estado actual. Lisboa, INIP. Pub. avulsas, no. 1, 107 p. il.. Ehrhardt, N.M., Die, D.J., 1988. Selectivity of gill nets used in the commercial Spanish mackerel fishery of Florida. Trans. Am. Fish. Soc. 117, 574±580. Girardin, M., 1978. Les Sparidae (Pisces, Teleostei) du Golf du Lion ± ecologie et biogeographie. TeÁse 3eÁme Cycle, Univers. Montpellier II, p. 146. Hamley, J.M., 1972. Use of the DeLury method to estimate gillnet selectivity. J. Fish. Res. Board Can. 29, 1636±1638. Jensen, J.W., 1986. Gillnet selectivity and the efficiency of alternative combinations of mesh sizes for some freshwater fish. J. Fish. Biol. 28, 637±646. Kawamura, G., 1972. Gill-net mesh selectivity curve developed from length±girth relationship. Bull. Jap. Soc. Sci. 38, 1119±1127. Martins, R., Santos, M.N., Monteiro, C.C., Franca, M.L.P., 1992. Contribuic,aÄo para o estudo da selectividade das redes de emalhar de um pano fundeadas na costa portuguesa no bieÂnio 1990±1991. Relat. Tecn. Cient. INIP, Lisboa 62, 27. MathSoft, Inc., 1997. S-PLUS 4 Guide to Statistics, Data Analysis Products Division, MathSoft, Seattle: p. 87. Monteiro, C., 1989. La faune ichtyologique de la Ria Formosa (sud Portugal): ReÂpartition et organisation spatio-temporelle des communauteÂs: application aÁ l'ameÂnagement des ressources. TheÁse Doctorat, Universite Montpellier II, p. 219. Monteiro, C., Lam Hoi, T., Lasserre, G., 1987. Distribution chronologique des poissons dans deux station de la lagune Ria Formosa (Portugal). Oceanol. Acta 10(3), 359±371.
Monteiro, C., Lasserre, G., Lam Hoi, T., 1990. Organisation spatiale des communauteÂs ichtyologiques de la lagune Ria Formosa (Portugal). Oceanol. Acta 13(1), 79±96. Neter, J., Wasserman, W., Kutner, M.H., 1983. Applied Linear Regression Models. Richard D. Irwin, Inc., pp. 547. Paz R., 1975. Systematique et phylogeneÁse des Sparidae du genre Diplodus Raf., (Pisces Teleostei), Travaux et Documents de l'O.R.S.T.O.M. 45, 96 p. Petrakis, G., Stergiou, K.I., 1995. Gill net selectivity for Diplodus annularis and Mullus surmuletus in Greek waters. Fish. Res. 21, 455±464. Potter, E.C.E, Pawson, M.G., 1991. Gill Netting. Lab. Leaflet, MAFF Direct. Fish. Res., Lowestoft 69, 34. Rabenevanana, M.-W., 1985. Les sars du golf du Lion ± Diplodus sargus, D. vulgaris, D. annularis (Pisces, Sparidae) ± Ecobiologie, Peche. TeÁse 3eÁme Cycle, Univ. Montpellier II, p. 361. Reis, E.G., Pawson, M.G., 1992. Determination of gill-net selectivity for bass (Dicentrarchus labrax L.) using commercial catch data. Fish. Res. 13, 173±187. Rodriguez, R.B., Rodriguez, A., 1984. El solapamiento de caracteres y su utilizacioÂn en taxonomõÂa numeÂrica. AplicacioÂn a Diplodus bellottii y Diplodus annularis (Pisces Sparidae) del golfo de Cadiz. Invest. Pesq. 48(2), 319±336. Rudstam, L.G., Magnuson, J.J., Tonn, W.M., 1984. Size selectivity of passive gear: A correction for encounter probability applied to gill nets. Can. J. Fish. Aquat. Sci. 41, 1252±1255. Saila, S.B., Erzini, K., 1988. An improved method for fitting gillnet selectivity curves to predetermined distributions. Fish. Stock Assess. CRSP, Work. Pap. 43, p. 27. Santos, M.N., Monteiro, C.C., Erzini, K., 1995. Aspects of the biology and gillnet selectivity of the axillary sea bream (Pagellus acarne, Risso) and common pandora (Pagellus erythrinus Linnaeus) from Algarve (south Portugal). Fish. Res. 23, 223±236. Sechin, Yu.T., 1969. A mathematical model for the selectivity curve of a gill net. Rybn. Khoz., 45 (9), 56-58. (Transl. from Russian by Int. Pac. Halibut Comm. Seatle, Wash.). Wassef, E.A., 1985. Comparative studies of four Diplodus species (Pisces, Sparidae). Cybium 9(2), 203±215. Whitehead, P.J.P., Bauchot, M.-L., Hureau, J.-C., Nielsen, J., Tortonese, E.; (Ed.), 1986 ± Fish of the North-eastern Atlantic and the Mediterranean. UNESCO, Paris, p. 1473. Wulff, A., 1986. Mathematical model for selectivity of gill nets. Arch. Fish Wiss. 37, 101±106.
Maturation and gill-net selectivity of two small sea breams (genus Diplodus) from the Algarve coast (south Portugal) Miguel Neves Santosa,*, Carlos Costa Monteiroa, Karim Erzinib,1 Gerard Lasserrec,2 a
Instituto de Investigac,aÄo das Pescas e do Mar, Centro Regional de Investigac,aÄo Pesqueira do Sul, Av. 5 de Outubro s/n, 8700 OlhaÄo, Portugal b Universidade do Algarve, UCTRA, Campus de Gambelas, 8000 Faro, Portugal c Laboratoire d'Hydroliologie Marine, Unite de Recherche AssocieÂe au CNRS, Universite de Montpellier II, 34095 Montpellier Cedex 05, France Received 7 October 1997; accepted 19 February 1998
Abstract Diplodus annularis and Diplodus bellottii, two species of small sea breams from the Algarve coast (south Portugal), were studied to obtain information on the spawning period, ®rst maturity, weight±length relationship and gill-net selectivity curves. These species are important in local gill-net catches, and there is no information on their biology and selectivity in the Algarve waters. Spawning lasts for 6 months. Total length (TL) at ®rst maturity for females was 13.4 cm in case of D. annularis and 11.4 cm in case of D. bellottii. The relationships between weight and length were W0.0132 TL3.096 for D. annularis and W0.0093 TL3.193 for D. bellottii. The Sechin method was used to estimate gill-net selectivity curves. The estimated selectivity curves correspond closely to the catch curves of the legal mesh size (60 mm), but were narrower for the other meshes. D. annularis and D. bellottii will be caught using the legal mesh size in the following length ranges: 13±19 cm and 11±19 cm. # 1998 Elsevier Science B.V. All rights reserved. Keywords: Gill-net selectivity; Biology; D. annularis; D. bellottii; Maturation
1. Introduction The Senegal sea bream (Diplodus bellottii, Steindachner, 1882) and the annular sea bream (Diplodus annularis, Linnaeus, 1758) are two small demersal species of the Sparidae family. These two similar species are characterised by the total absence of black *Corresponding author. Tel.: 351 89 700504; fax: 351 897 00535. 1 Tel.: 351 89 800100; fax: 351 89 818353; e-mail: [email protected]. 2 Tel.: 33 467 526660; fax: 33 467 143719. 0165-7836/98/$19.00 # 1998 Elsevier Science B.V. All rights reserved. PII S0165-7836(98)00100-3
bands on the body. However, the Senegal sea bream has a distinguishing small dark spot at the origin of the lateral line and a dark longitudinal stripe along the middle of the sides, whereas the annular sea bream has an almost annular dark band around the caudal peduncle just behind the dorsal and anal ®ns (Whitehead et al., 1986). The Senegal sea bream was initially described only for the western coast of Africa, but is also present along the north coast of Morocco, southern Portugal (the Algarve), and the Gulf of Cadiz. The geographic distribution of the annular sea bream extends from the Mediterranean to the Iberian Atlantic coast, the
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Canaries and Madeira. While both species are most common at 10±30 m depths, their bathymetric range extends from the shallow subtidal to depths of 90 m. Several aspects of the life history of D. annularis and D. bellottii (Paz, 1975), the reproductive cycle, ecology and gill-net selectivity have been studied in different areas: Gulf of Lion (Girardin, 1978; Rabenevanana, 1985), Gulf of Cadiz (Rodriguez and Rodriguez, 1984), South Euboikos Gulf (Petrakis and Stergiou, 1995), and the Algarve (Monteiro, 1989; Martins et al., 1992). In the Algarve these species are found in the lagoon systems ``Ria de Alvor'' and ``Ria Formosa'', and the Guadiana Estuary. These highly productive ecosystems which act as nurseries, supply some of the most important ®sh stocks of coastal waters of south Portugal (Monteiro et al., 1987, 1990). Off the Algarve coast these species are caught by traps and long-lines, and especially by gill nets and purse seine. According to Martins et al. (1992), the catch of these small sea bream represents almost 9% of the gill-net landings in this region. Because they require little investment in manpower and equipment, and are ef®cient in catching widely scattered ®sh, gill nets are widely used in artisanal ®sheries. This ®shing gear plays an important role in local ®sheries, due to the large volume of ®sh landed, high commercial value of most species and the number of ®sherman involved (Costa and Franca, 1982; Santos et al., 1995). Because of the selective nature of gill nets, mesh size can be controlled to restrict the size of ®sh captured, and selection curves can be used to calculate the mesh size required. A minimum mesh size can be used to limit the catch of immature ®sh or to reinforce minimum landing size regulations (Potter and Pawson, 1991). The abundance of these species off the Algarve justi®es this study, the objectives of which were to determine gill-net selectivity curves, weight-length relationships and lengths at ®rst maturity. This information will allow a better understanding of the biology of these species in the Algarve and provide a scienti®c basis for ®sheries management. 2. Materials and methods We used a set net with mesh sizes of 40, 50, 60 and 70 mm (measured between opposite knots when fully
stretched), that included the legal mesh size (60 mm). The hanging ratio of these nets was 0.50 on the ¯oat rope and 0.55 on the lead rope. In all, 60 panels (15 of each mesh size) were used, randomly distributed along the net. Each panel was 50 m long and 2.5 m high. The ®shing strategy consisted of setting the nets 3±4 h before sunrise and pulling them immediately after sunrise. The ®sh were sorted by species and mesh size and the total length (TL) was measured to the nearest lower centimetre. The results of experimental ®shing are compiled in terms of species composition per mesh size Martins et al. (1992). Data on total length (Tables 1 and 2), head girth (Gh) and maximum girth (Gmax) for both species were collected from March 1993 to February 1994, corresponding to a total of 62 experimental catches. Gh and Gmax were measured to the nearest millimetre, and were collected from our experiments as well as from commercial catches. Data on weight and the state of gonad development were collected from April 1990 to August 1993, off the Algarve (Fig. 1). An eight-stage scale of gonad maturation was used (BuÈchmann, 1929), and all individuals from stages 4±8 were considered mature. The maturity stage 4 is characterised by the following: opaque orange coloured eggs that can be clearly observed, and white/reddish male gonads that do not drip when under pressure; both male and female gonads occupying two-third parts of the abdominal cavity. The parameters of weight±length relationship (WaLb), a and b, and the coef®cient of determination r2, were estimated by least-squares regression using the log transformed weights and sizes. Standard errors (SE) and the 95% con®dence intervals were calculated for the slopes to check if these were statistically different from 3. Aptness of the model was studied by plots of residuals and standardised residuals (Neter et al., 1983). The logistic function (1) was ®tted to the maturity data: pi
1 1 expÿr TLi ÿ L50
(1)
where p is the proportion mature in size class i, TLi the mean total length of class i, r a parameter determining the slope of the ogive, and L50 the TL at 50% maturity. The parameters were estimated for each species and sex by ®tting a general linear model with binomial
M.N. Santos et al. / Fisheries Research 36 (1998) 185±194
187
Table 1 Total-length-frequency distributions of Diplodus annularis caught in the Algarve waters from March 1993 to February 1994, percentage of mature fish, mean weight (g), mean of maximum girth and head girth (cm) Length class (cm) 10.0 11.0 12.0 13.0 14.0 15.0 16.0 17.0 18.0 19.0 20.0 Total
Mesh size (mm) 40
50
33 40 37 15 10 1
136
2 20 72 28 7 3
132
60
4 28 36 13 4 1 86
70
1 2 4 9 7 3 1 27
Mature female (%)
Mature male (%)
Mean weight (g)
Mean max. girth (cm)
Mean head girth (cm)
0 0 33 54 80 66 100 100 100 100 100 77
0 0 50 70 88 100 100 100 100 100 100 136
18 30 34 40 52 62 80 88 112 125 151 310
9.47 9.84 9.91 10.66 11.46 12.26 12.94 14.25 15.13 15.50 16.20 150
8.30 9.02 9.19 9.79 10.90 10.97 12.29 12.49 13.63 13.97 15.10 187
Table 2 Total-length-frequency distributions of Diplodus bellottii caught in the Algarve waters from March 1993 to February 1994, percentage of mature fish, mean weight (g), mean of maximum girth and head girth (cm) Length class (cm) 9.0 10.0 11.0 12.0 13.0 14.0 15.0 16.0 17.0 18.0 19.0 20.0 21.0 22.0 Total
Mesh size (mm) 40 16 283 746 533 209 79 16
1882
50 9 14 87 432 323 132 48 19 8
1072
60
6 15 36 113 207 115 70 15 5
582
70
2 9 15 24 44 28 6 1 129
Mature female (%)
Mature male (%)
0 43 68 77 80 87 89 93 90 100 100 100 100 263
0 50 64 75 83 89 97 100 100 100 100 100
variance function and the logit link using S-PLUS 4 (MathSoft, 1997; Chambers and Hastie, 1993). Log(pi/(1ÿpi)) is modelled as a function of TLi. The parameter r is equal to the slope and L50 is equal to the intercept divided by (ÿr). 2.1. Estimating gill-net selectivity Inference of selectivity from girth measurements differs from other methods in that the size distributions
216
Mean weight (g)
Mean max. girth (cm)
Mean head girth (cm)
12 17 24 29 36 47 57 70 86 102 122 149 166 193 2144
8.0 8.6 9.4 9.7 10.2 11.1 11.9 12.6 13.4 14.3 15.2 16.2 16.9 17.9 1748
7.6 8.1 8.5 9.1 9.6 10.5 10.9 11.6 12.5 13.1 13.9 14.1 14.6 15.4 848
of gill-net catches are not examined. Girth measurements allow selectivity to be estimated assuming that a ®sh, upon swimming into the net, is caught if its head girth is smaller but maximum girth larger than the mesh perimeter. On this basis and, assuming that girth among any particular length class of ®sh is normally distributed, with a common variance for all length classes, Sechin (1969) and Kawamura (1972) derived theoretical selectivity curves. In this study we adopted the Sechin model, as modi®ed by Reis and Pawson
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Fig. 1. Areas sampled for D. annularis and D. bellotii off south Algarve by gill nets.
(1992), which has the following expression for the selectivity: 2M ÿ Ghj 2M ÿ Gmaxj 1ÿ (2) Sj h max where Sj is the selectivity in the jth length class, Ghj the mean head girth in the jth length class, h the mean standard deviation of head girth, Gmaxj the mean maximum girth in the jth length class, max the mean standard deviation of maximum girth, 2 M the mesh perimeter and the cumulative standardised normal distribution function.
is allometric. The positive allometry found is statistically different from 3, with a 95% con®dence interval of 3.001b3.191. The relationship found for D. bellottii also indicates positive allometry, with the parameter b signi®cantly different from 3, with a 95% con®dence interval of 3.166b3.220. Plots of the residuals and the standardised residuals against TL and predicted weights showed no signi®cant departure from the simple linear model for the two weight±length data sets. In particular, there was no indication that the error terms did not have constant variance or were not independent. Also, error terms appeared to be normally distributed in all cases.
3. Results
3.2. Spawning period and length at first maturity
3.1. Weight±length relationship
The observed percentages of mature ®sh per size class, for the spawning period are given in Tables 1 and 2. The spawning period of D. annularis ranges from April to September (Fig. 2). The maturity ogive (Fig. 3) shows that 50% of females and males are sexually mature (L50) at total lengths of 13.4 cm and 12.9 cm, respectively. Off the Algarve, D. bellottii spawn between January and June (Fig. 2). Sexual maturity is reached at total lengths of 11.4 cm and 12.0 cm, respectively, for females and males (Fig. 3). The results of the generalised linear model ®tting are given in Table 3.
The results of the weight±length relationship are summarised in Tables 1 and 2. The determined weight (g)±total length (cm) relationships were: Diplodus annularis W 0:0132 TL3:096
(3)
2
(n310, r 0.93, SETL1.72, SEb0.048, mean TL14.3 cm, mean W52 g) Diplodus bellottii W 0:0093 TL3:193 2
(4)
(n2144, r 0.96, SETL2.07, SEb0.014, mean TL14.5 cm, mean W51 g) The relationship between weight and length for D. annularis shows that growth in the Algarve waters
3.3. Girth vs. length relationship The ®tted linear regressions for maximum girth (Gmax) and total length (TL) data (in cm) for the
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189
Fig. 2. Percentage of mature (black) and immature (grey) fish for (left) D. annularis and (right) D. bellottii caught off the south coast of the Algarve.
D. annularis (Eq. (5)) and D. bellottii (Eq. (6)) were (see Tables 1 and 2): Gmax 0:888 0:747 TL n 150; r 2 0:96; max 0:51
(5)
Gmax 1:07 0:785 TL 2
n 1748; r 0:99; max 0:60
The relationship between head girth (Gh) and total length, obtained from ®tting the linear regression (in cm) for D. annularis (Eq. (7)) and D. bellottii (Eq. (8)) were as follows: Gh 0:25 0:72 TL
(6)
n 187; r2 0:97; h 0:81
(7)
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M.N. Santos et al. / Fisheries Research 36 (1998) 185±194
Fig. 3. Ogives of first maturity for (left) D. annularis and (right) D. bellotii in the Algarve: ( ) for males; and (ÐÐÐ) for females. MLS represents the minimum landing size.
Gh 1:00 0:65 TL 2
n 424; r 0:99; h 0:71
(8)
Plots of the residuals and the standardised residuals against TL and predicted girths showed no signi®cant departure from the simple linear model for the four girth±total length data. In particular, there was no indication that the error terms did not have constant
variance or were not independent. Also, error terms appeared to be normally distributed in all cases. 3.4. Gill-net selectivity Girth±length relationships indicated that maximum girth increased more with length than did head girth. This resulted in a slight increase in selection range as
M.N. Santos et al. / Fisheries Research 36 (1998) 185±194
191
Table 3 Results of estimation of parameters of maturity ogives for Diplodus annularis and D. bellottii using the generalised linear model approach Species
Sex
Intercept
(SE)
L
(Std. Error)
Ogive parameters r
L50
Diplodus annularis
female male female male
ÿ13.227 ÿ19.374 ÿ5.269 ÿ8.790
(3.459) (3.975) (1.347) (1.862)
0.984 1.505 0.462 0.731
(0.250) (0.300) (0.096) (0.135)
0.984 1.505 0.462 0.731
13.44 12.87 11.41 12.02
Diplodus bellottii
mesh size increased. The greatest probability of retention, which corresponds to the peak in ef®ciency, also increased with the mesh size. In the case of the Senegal sea bream, the maximum girth increased with length faster than in the annular sea bream. However, for the latter species the head girth increased more strongly with length than for the Senegal sea bream. The calculated normal selectivity curves are similar for both species. However, the estimated selectivity curves for D. bellottii were slightly higher and wider than those for D. annularis (Figs. 5 and 6). The probability of catching the annular sea bream and the Senegal sea bream is greatest in length classes 9 and 10 (40 mm mesh), 12 and 13 (50 mm mesh), 15 (60 mm mesh), and 18 (70 mm mesh). For the annular sea bream, the modes of the length-frequency distributions of the corresponding catch were 1.0 cm and 1.5 cm greater than those of the estimated curves for the 40 and 50 mm mesh size; equal for the 60 mm mesh; and 1 cm smaller for the 70 mm mesh. For the Senegal sea bream, the modes of the length-frequency distributions of the catches were 2.75 and 2.0 cm greater than those of the estimated curves for the 40 and 50 mm meshes; equal for the 60 mm mesh; and 0.5 cm smaller for the 70 mm mesh. 4. Discussion 4.1. Weight±length relationship The relationship between weight and length found for D. annularis differs from the values reported by Rabenevanana (1985), but is similar to that reported by Rodriguez and Rodriguez (1984). However, this species showed equal growth in weight off the Algarve and in the Mediterranean Sea. The relationship for D. bellottii showed exactly the same value that was
Fig. 4. Weight±length relationship for D. annularis and D. bellotii in the Algarve.
reported by Rodriguez and Rodriguez (1984). Growth in weight of these two species in Algarve is similar (Fig. 4) and does not differs signi®cantly from that found for the Mediterranean Sea (Table 4). 4.2. Length at first maturity Off the Algarve, the D. annularis spawning period starts one month earlier than in the neighbouring Gulf of Cadiz and the Gulf of Lion, and ®nishes, respectively, one and two months later. Off the Algarve, males are sexually mature at a larger size than in the Gulf of Lion, while females are sexually mature at the same size. In the case of D. bellottii, off-the-Algarve spawning starts two months earlier than in the Gulf of Cadiz, but ®nishes at the same time. The spawning period for these species occurs earlier in Atlantic waters than in the Mediterranean Sea (Table 5). If minimum landing sizes for these two small species of the genus Diplodus are to be established based on their sizes at ®rst maturity, then, in any future
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M.N. Santos et al. / Fisheries Research 36 (1998) 185±194
Table 4 Weight±length relationship for D. annularis and D. bellotii obtained in different areas Species
Length range
Length±weight relationship 3.096
Area
Author
D. annularis
10±20 12±19 4±23 7±17
W13.2 Eÿ03 TL W6.84 Eÿ03 TL3.089 W6.09 Eÿ06 TL3.222 W1.36 Eÿ04 TL2.5742
Algrave (south Portugal) Gulg of Cadiz (Spain) Gulf of Lion (France) Egypt (Mediterranean)
Present study Rodriguez and Rodriguez, 1984 Rabenevanana, 1985 Wassef, 1985
D. bellottii
9±22 11±20
W9.3 Eÿ03 TL3.193 W5.41 Eÿ03 TL3.193
Algrave (Portugal) Gulf of Cadiz (Spain)
Present study Rodriguez and Rodriguez, 1984
Table 5 Spawning period and total length (cm) at 1st maturity in different areas Species
Spawning period
Female
Male
Area
Author
D. annularis
April±August May±July May±June
13 cm 13 cm Ð
12 cm 11 cm Ð
Algrave (south Portugal) Gulg of Lion (France) Gulf of Cadiz (Spain)
Present study Rabenevanana, 1985 Rodriguez and Rodriguez, 1984
D. bellottii
January±June March±June
11 cm Ð
11 cm Ð
Algrave (south Portugal) Gulf of Cadiz (Spain)
Present study Rodriguez and Rodriguez, 1984
revision of minimum landing sizes, it should be reduced from 15 to 13 cm. 4.3. Gill-net selectivity The estimated selectivity curves for D. annularis are similar to the length-frequency distributions of the corresponding catches, for the 60 mm mesh sizes. However, for the 40, 50 and 70 mm mesh sizes this is not the case (Fig. 5), which may be due to the absence of some ®sh length classes in inshore waters. The small ®sh (under 11±12 cm) use the estuarine± lagoon systems as nursery grounds. Monteiro (1989) reported that the annular sea bream migrate from the lagoon Ria Formosa at lengths of 11±12 cm, and Rodriguez and Rodriguez (1984) did not catch ®sh under 12 cm outside the Guadalquivir Estuary. On the other hand, very few specimens were caught with total length larger than 18 cm (see Tables 1 and 2). The estimated selectivity curves for D. bellottii are similar to the length-frequency distribution of the corresponding catches only for the 60 mm mesh size (Fig. 6). The differences for mesh sizes 40, 50 and 70 mm can be partially explained by the low abun-
dance of specimens under 12 cm in total length in the coastal waters. For both species, the estimated selectivity curves are normal in shape, which is the case for ®sh that are usually wedged, as we could con®rm during the ®shing trials. On the other hand, the estimated curves are narrower than those of the corresponding catch. This may be due to the absence of coef®cients to account for body compressibility at the retention point and elasticity of netting material, in the formula used to estimate the selectivity curves. The estimated curves became higher and the selection range wider as mesh size increased, as was also reported in other studies (Hamley, 1972; Rudstam et al., 1984; Wulff, 1986; Jensen, 1986; Saila and Erzini, 1988; Ehrhardt and Die, 1988). The selectivity of each mesh indicates only the proportion of ®sh which one mesh size will capture relative to other mesh sizes. It does not show directly what fraction of the available ®sh population each mesh will catch. For management purposes, D. annularis and D. bellottii will be caught in the 13±18 cm and 12±19 cm length ranges by the legal mesh size (60 mm), which are larger than their lengths at ®rst maturity (L50).
M.N. Santos et al. / Fisheries Research 36 (1998) 185±194
Fig. 5. Gill-net estimated selectivity curves (line) and observed D. annularis size-frequency distribution (*) for the 40, 50, 60 and 70 mm mesh size used in Algarve.
193
Fig. 6. Gill-net estimated selectivity curves (line) and observed D. bellotii size frequency distribution (*) for the 40, 50, 60 and 70 mm mesh size used in Algarve.
194
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Acknowledgements We thank the staff of IPIMAR±CIMSul (E. BraÂs, T. SimoÄes and J.L. So®a) and the crew of ``NI DONAX'' for catching and collecting these data. References BuÈchmann, A., 1929. Die Methodik fischereibiologischer Untersuchungen an Meeresfischen. Abder-halen, Handbuck der biologischen Arbeitsmethoden, 9(6,1). Ed. Urban and Schwarzenberg, pp. 194. Chambers, J.M., Hastie, T.J., 1993. Statistical Models. S. Chapman and Hall, p. 608. Costa, F.C., Franca, M.L.F., 1982. Pesca artesanal na costa algarvia. SubsõÂdio para o conhecimento do seu estado actual. Lisboa, INIP. Pub. avulsas, no. 1, 107 p. il.. Ehrhardt, N.M., Die, D.J., 1988. Selectivity of gill nets used in the commercial Spanish mackerel fishery of Florida. Trans. Am. Fish. Soc. 117, 574±580. Girardin, M., 1978. Les Sparidae (Pisces, Teleostei) du Golf du Lion ± ecologie et biogeographie. TeÁse 3eÁme Cycle, Univers. Montpellier II, p. 146. Hamley, J.M., 1972. Use of the DeLury method to estimate gillnet selectivity. J. Fish. Res. Board Can. 29, 1636±1638. Jensen, J.W., 1986. Gillnet selectivity and the efficiency of alternative combinations of mesh sizes for some freshwater fish. J. Fish. Biol. 28, 637±646. Kawamura, G., 1972. Gill-net mesh selectivity curve developed from length±girth relationship. Bull. Jap. Soc. Sci. 38, 1119±1127. Martins, R., Santos, M.N., Monteiro, C.C., Franca, M.L.P., 1992. Contribuic,aÄo para o estudo da selectividade das redes de emalhar de um pano fundeadas na costa portuguesa no bieÂnio 1990±1991. Relat. Tecn. Cient. INIP, Lisboa 62, 27. MathSoft, Inc., 1997. S-PLUS 4 Guide to Statistics, Data Analysis Products Division, MathSoft, Seattle: p. 87. Monteiro, C., 1989. La faune ichtyologique de la Ria Formosa (sud Portugal): ReÂpartition et organisation spatio-temporelle des communauteÂs: application aÁ l'ameÂnagement des ressources. TheÁse Doctorat, Universite Montpellier II, p. 219. Monteiro, C., Lam Hoi, T., Lasserre, G., 1987. Distribution chronologique des poissons dans deux station de la lagune Ria Formosa (Portugal). Oceanol. Acta 10(3), 359±371.
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