ICES CM 2005/N:30
Depth distribution of the velvet belly lantern shark,
Etmopterus spinax, in southern Portugal
1
2
2
Rui Coelho , Ivone Figueiredo , Pedro Bordalo Machado
1
and Karim Erzini
1: Universidade do Algarve, CCMAR/FCMA, Campus de Gambelas, 8005-139 Faro [Tel: +351289800900 ext.7242,
Fax: +351289818353]; Web: http://www.ualg.pt/fcma/cfrg/; e-mail: [email protected]
2: INIAP – IPIMAR, Av. de Brasília, 1449 – 006, Lisboa, Portugal. [Tel: +351213027000, Fax: +351213015948].
INTRODUCTION
MATERIAL AND METHODS
¾ The velvet belly lantern shark, Etmopterus spinax (Figure 1) is a small sized deep-water squalid
commonly captured as by-catch and discarded by deep water trawlers and deep water longliners that
operate in the South of Portugal (Coelho et al., 2003).
¾ The sampling was carried out aboard an INIAP - IPIMAR scientific vessel in the southwestern and southern Portuguese
coasts during the summer of 2003 (Figure 2). A total of 67 bottom trawls were carried out at depths that ranged in average,
from 84 to 786m. All Etmopterus spinax specimens captured were sexed, measured and weighed.
¾ Previous studies have established relationships between depth of capture and size of specimens for
deep water sharks (e.g.: Jakobsdóttir (2001) in Iceland, Clarke et al. (2002) in Northern Ireland and
Massuti & Moranta (2003) in Western Mediterranean).
¾ Trawls were grouped in 100 m depth intervals and relationships between total size of specimens and depth class
established for both males and females. Kruskal-Wallis test followed by Dunn pairwise tests were carried out in order to
access if the differences found at each depth class were significant both on males and females. T-student tests were carried
out to access if there were significant differences at each depth class between males and females.
¾ The objective of this study was to determine if there was a relationship between depth of capture and
specimen total length for both males and females of Etmopterus spinax.
STUDY AREA
Figure 1 – Scientific drawing (above) and Photo (below) of an Etmopterus spinax.
Depth
Class
0
100
200
300
400
500
600
700
Total
Hours
Trawling
1
6
6
11
10
16
9
8
67
Males
captures
0
0
0
21
40
123
8
0
192
Females
captures
0
0
1
20
31
129
21
2
204
Total
captures
0
0
1
41
71
252
29
2
396
Males
CPUE
0.00
0.00
0.00
1.91
4.00
7.69
0.89
0.00
Females
CPUE
0.00
0.00
0.17
1.82
3.10
8.06
2.33
0.25
Total
CPUE
0.00
0.00
0.17
3.73
7.10
15.75
3.22
0.25
Table III - Number of specimens captured and CPUE (number of captures per 1 hour of
trawling) for both males and females in each 100m depth class.
Figure 2 - Map of Southern Portugal with the location of the fishing trials. Portuguese
mainland and batimetric lines were obtained from the “Atlas do Ambiente Digital –
Instituto do Ambiente”
300
1
Depth class (m)
400
500
1
1
4
5
3
1
1
2
1
1
1
1
1
1
4
2
3
6
4
2
2
1
1
1
1
2
2
3
3
7
1
9
6
6
7
13
7
5
2
7
7
2
2
6
3
4
5
4
9
4
2
600
700
1
3
3
8
9
10
3
11
12
9
11
19
11
7
4
8
7
3
2
7
1
1
1
1
3
3
5
2
2
1
1
1
1
1
32.0
20
13.9
2.6
31
19.5
4.3
1
129
23.0
7.3
21
34.4
2.8
Total
2
34.0
1.6
4
5
7
9
12
10
4
2
1
1
1
204
Length class
(cm)
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
Total
Av. size (cm)
St. dev (cm)
200
300
Depth class (m)
400
500
2
2
7
5
2
3
1
2
1
2
3
5
5
7
5
3
2
2
1
1
2
4
1
1
5
2
3
10
11
10
19
9
4
8
3
2
3
3
7
5
7
1
3
600
1
1
4
1
1
700
Total
4
6
9
8
8
7
3
13
16
15
26
14
7
10
4
4
4
3
8
9
8
3
3
400
500
600
-
-
-
-
400
YES
-
-
500
YES
NO
-
-
600
YES
YES
YES
-
-
Females
Depth class (m)
400
500
600
300
-
-
-
-
400
YES
300
-
-
-
500
YES
NO
-
-
600
YES
YES
YES
-
Table V - Differences found at each
depth class between males and
females with t-student tests. “YES”
refers to depth classes where
differences were found (t-student:
p<0.05) and “NO” to depth classes
where no differences were detected (tstudent: p>0.05).
Depth
class (m)
Females
200
300
300
Table IV - Differences found
between pairs of depth classes
for both males and females
separately with a Kruskal-Wallis
test followed by Dunn pairwise
tests. “YES” refers to pairs of
classes where differences were
found (Dunn: p<0.05) and “NO”
to pairs of classes where no
differences
were
detected
(Dunn: p>0.05).
Males
300
400
500
600
300
NO
-
-
-
400
-
NO
-
500
-
-
NO
-
600
-
-
-
YES
Females
80%
21
12.6
1.6
40
19.6
3.8
123
21.1
5.4
8
28.4
2.3
192
Tables I and II - Count, average size and standard deviation of females (left) and males (right) of each total length class captured at each depth class. Total length classes are in 1cm
units and depth classes in 100m units.
RESULTS AND DISCUSSION
-
100%
Captures (%)
Length class
(cm)
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
Total
Av. size (cm)
St. dev (cm)
Males
Depth class (m)
Males
60%
40%
300
400
500
600
20%
0%
Depth class (m)
Figure 3 - Males and females captured in percentage, for each 100m depth class
¾ A total of 396 specimens were captured (192 males and 204 females), with total lengths ranging from 9.8 to 41.1 cm (Table I and II). The depth class where most specimens occurred was the 500 m depth class (252 specimens
representing 63.6% of the total capture), with a CPUE of 15.75 specimens per 1 hour of trawling, and with most of the length classes present (Table III). Females were captured in a wider depth range (from 200 to 700 m) than males
(from 300 to 600m), but in the marginal depth classes very few specimens were captured, respectively 1 and 2 at the 200 and 700 m depth. Males and females captures in percentage were similar for the shallower depth classes but at
the 600 m depth class sex ratio favored females (Figure 3).
¾ Significant differences were found in the sizes of both males and females found at the different depth classes (ANOVA: p<0.001; all pairwise Dunn test: p<0.05), except for the 400m and 500m depth classes were no differences were
found neither for males nor females (Dunn test: p>0.05) (Table IV). There were no significant differences between males and females at each depth class (t-student: p>0.05) except at the depth class of 600m (t-student: p<0.05) (Table
V).
¾ There seems to be a relationship between depth of occurrence and total size of specimens in Etmopterus spinax, with the larger specimens occurring at deeper waters and the smaller ones at shallower waters. Massuti & Moranta
(2003) also observed a clear segregation of sizes in the Mediterranean, with the juveniles of Galeus melastomus and Etmopterus spinax occurring especially in shallower waters and adults in deeper waters. This situation is opposite of
what had been observed by Jakobsdóttir (2003) for Etmopterus princeps and Centroscyllium fabricii where there was a trend for a decrease in mean length with increasing depth. In terms of sex ratio, there were no clear differences
between males and females at the shallower depths, but at deeper waters, the females of E. spinax were significantly more numerous than males. A similar situation was observed by Jakobsdóttir (2003) for E. princeps and C. fabricii.
We must consider however, the fact that we only sampled during the summer period and that the depth distribution for E. spinax during the winter may be different than the one described in this work.
ACKNOWLEDGMENTS
¾ This study was in part funded by FCT (Foundation for Science and Technology) through a PhD grant (Ref. SFRH/BD/10357/2002).
¾ We would like to thank Patrícia Tello for the scientific drawing of Etmopterus spinax.
REFERENCES
¾ Clarke, M.W., Connolly, P.L. & Bracken, J.J. 2002. Catch, discarding, age estimation, growth and maturity of the squalid shark Deania calceus west and north of Ireland. Fisheries Research, 56 (2): 139-153.
¾ Coelho, R., Bentes, L., Gonçalves, J.M.S., Lino, P.G., Ribeiro, J. & Erzini, K. 2003. Reduction of elasmobranch by-catch in the hake semi-pelagic near bottom longline fishery in the Algarve (Southern Portugal). Fisheries Science, 69 (2): 293-299.
¾ Jakobsdottir, K.B. 2001. Biological aspects of two deep-water squalid sharks: Cetnroscyllium fabricii (Reinhardt, 1825) and Etmopterus princeps (Collett, 1904) in Icelandic waters. Fisheries Research, 51 (2-3): 247-265.
¾ Massuti, E. & Moranta, J. 2003. Demersal assemblages and depth distribution of elasmobranchs from the continental shelf and slope off the Balearic Islands (western Mediterranean). ICES Journal of Marine Science, 60: 753–766.