Preprint volume - SIBM

Pre-print Volume –Oral presentations
Topic 4: THE ELASMOBRANCHS
archives. Overall trawl fishing effort indices were computed as days at sea per month
(Porto S. Stefano), and hours at sea per month by shelf and slope (Viareggio). Data
exploration and analysis were performed using the software package Brodgar 2.6.6
(www.brodgar.com). All the time series were standardized and explored by means of
the auto- and cross-correlation functions to highlight the presence of cyclic patterns in
time series and to highlight potential relationship between two variables. Cyclic
patterns were removed from the data through the seasonal decomposition by Loess
smoothing. The trends obtained were further analysed through Min/Max Autocorrelation
Factor Analysis (MAFA) (Solow, 1994), which extracts trends from
multiple time series. In MAFA, the first axis has the highest auto-correlation with time
lag 1, thus it represents the main pattern in the data: a trend is associated with high
auto-correlation with time lag 1. Correlations between the variables and the trends were
computed to evaluate the significance of their relationship.
Results – The time series of the elasmobranchs LPUEs and the explanatory variables
are summarized in Figs. 1 and 2. The fishing effort by the Porto S. Stefano fleet
highlighted a general decreasing pattern, while the effort by the Viareggio fleet showed
two scenarios: a decrease on the shelf and an increase on the slope. The presence of
cyclical components with time lag 12 was found using the auto-correlation function;
thus, the series were deseasonalized.
a
kg/day/vessel
b
kg/h
16
14
12
10
8
6
4
2
0
G. melastomus
S. canicula
D. oxyrinchus
Rays
Jan-91 Jan-93 Jan-95 Jan-97 Jan-99 Jan-01 Jan-03 Jan-05 Jan-07
4
months
G. melastomus
3
2
1
0
Jan-91 Jan-93 Jan-95 Jan-97 Jan-99 Jan-01 Jan-03 Jan-05 Jan-07
months
S. canicula
R. asterias
Fig. 1 – Porto S. Stefano (a) and Viareggio (b): time series plot of the mean monthly LPUE
(kg/day/vessel and kg/h, respectively).
Porto S. Stefano (a) e Viareggio (b): serie temporali di LPUE (kg/giorno/barca e kg/h,
rispettivamente).
41 st S.I.B.M. CONGRESS Rapallo (GE), 7-11 June 2010
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