Preprint volume - SIBM

Pre-print Volume –Oral presentations
Topic 4: THE ELASMOBRANCHS
Observing trends obtained by the GRUND dataset we can see that they are clearly
negative (Fig.3), specifically abundance undergoes a half of values starting with 965
N/km 2 in 1994 to 400 N/km 2 in 2008. At the same time biomass undergoes a
significant decline until 2006 with a small increase in 2008 that is given by the biomass
of the mesobathyal layer. As a matter of fact both in abundance and biomass is the
epibathyal layer that shows the greatest decline with values that from 2003 are
negligible.
kg/km 2
160
140
120
100
80
60
40
20
0
y = -7,0791x + 122,9
R 2 = 0,5511
Fig. 3 – G. melastomus. Biomass and abundance trends in 200-800 m depth. GRUND (1994 -2008).
G. melastomus. Andamento della biomassa e dell’abbondanza tra 200-800 m di profondità.
GRUND (1994-2008).
On the contrary mean weight has a positive trend (Fig.4) with values that increase of
the double starting from 0.09 kg/N in 1994 to 0.18 kg/N in 2008, specifically the
increase is effective in the mesobathyal layer while the epibathyal layer is influenced
by the lack of organisms fished. Analyzing the abundance of juveniles we observed
that they are the part of the population that most suffer in decline, in fact starting from
2003 they almost disappear both in the epibathyal and mesobathyal layer. Looking at
box plot graphs, both for females (Fig.4) and males (not shown) there is a significant
increase in the median length of almost 10 cm from 1994 to 2008.
LT (cm)
70
60
50
40
30
20
10
1994 1995 1996 1997 1998 2000 2001 2002 2003 2004 2005 2006 2008
anni
- min - max(99%) - LT mediana
0
1994 1995 1996 1997 1998 2000 2001 2002 2003 2004 2005 2006 2008
anni
Fig. 4 – Galeus melastomus. Trends of median length of females and total mean weight trend in 200-
800 m depth. GRUND (1994-2008).
Galeus melastomus. Tendenza della mediana delle femmine e peso medio della popolazione
totale tra 200-800 m di profondità. GRUND (1994-2008).
All those parameters have been tested through the Spearman test resulting each one
significant in their trends. Moreover correlations between abundance and biomass, and
between median (males+females) and total mean weight have resulted positive with a
high statistical significance. Although since 2003 the number of GRUND hauls were
halved, the decrease in both abundance and biomass indexes started between 1996-
1997, meaning that the decrease seems not affected by the sampling scheme.
Concerning MEDITS results (not shown), any significant trend has been detected. As a
matter of fact we found out that the MEDITS net has a 3-4 times lower catch efficiency
41 st S.I.B.M. CONGRESS Rapallo (GE), 7-11 June 2010
208
kg/N
N/km 2
0,35
0,30
0,25
0,20
0,15
0,10
0,05
0,00
3500
3000
2500
2000
1500
1000
500
0
y = -111,88x + 1482,2
R 2 = 0,4645
1994 1995 1996 1997 1998 2000 2001 2002 2003 2004 2005 2006 2008
anni
y = 0,0117x + 0,0669
R 2 = 0,4466
1994 1995 1996 1997 1998 2000 2001 2002 2003 2004 2005 2006 2008
anni