Tanga Fisheries Analysis - IUCN

• Data for spatial analysis over the period 1995-2004 was restricted to the principle ground (i.e.
from where the majority of the fishing effort was recorded pre-2001) of Majivike. Analyses
related to wider geographical coverage of course sought to utilise all available (post-2001) data;
• No zero-catch fishing trips were recorded under the 1 st protocol, as noted in 4.1, and therefore
in a further attempt to standardize the data across the years, zero-catches were excluded from
the CPUE data derived from the 2 nd and 3 rd protocols;
• Because effort was only recorded as a ‘trip’, with length of trip unknown, pre-2001 the only
comparable CPUE index that could be used was catch per gear/trip, even though true effort
data does exist in post-2001 data. It might be argued that one could take the mean trip length
(for each boat/gear combination) from periods that included such information in the monitoring
profile and apply this to periods when no such data was collected. This was in fact investigated
as an option. Figure 7 presents data on the mean trip length for ngalawas (outrigger canoes)
using mishipi (handlines). The graph would seem to indicate that trip length varies between
years, and also seasonally (within year), and an ANOVA (single-factor) revealed a significant
difference between the data (F = 13.23, Fcrit 0.05, 354 1.747). On the basis that it would be very
subjective to allocate annual mean trip lengths (would one take data from 2002 or 2003 for
example?) let alone seasonal mean trip lengths it was felt that allocating mean trip length was
not a sensible option and would just add further caveats to the analyses.
Figure 7 - Mean Trip Length (Hrs) for Ngalawa/Mishipi
Mean Trip Length NG/Mishipi by Year Month
TCZCDP Data 2001-2004
11
10
9
8
7
6
5
4
3
2
1
0
2001 8
2001 10
2001 12
2002 2
2002 4
2002 6
2002 8
2002 10
2002 12
2003 2
2003 4
2003 6
2003 8
2003 10
2003 12
2004 2
Mean Trip Length (Hrs)
2004 4
2004 6
Year/Month
Perhaps the gear most likely to remain deployed for a set period of time (e.g. from dusk to dawn) is
the fish trap (madema). However, analysis of data from post-2001 for mtumbwi (dug-out canoe)
using madema in fact indicates that the trip length of this gear also varies both seasonally and
inter-annually (see Figure 8). Similar seasonal and inter-annual variations can be observed for
other vessel/gear combinations thus making any attempt to establish a mean trip length to apply to
pre-2001 data very difficult and statistically dubious.
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