Transcript What is a FAD?

Exploitation of small tunas by a purse-seine fishery with fish
aggregating devices and their feeding ecology in an eastern
tropical Atlantic ecosystem
圍網使用人工聚集器對於小型鮪類的開發及在熱帶東大西洋攝食食性之研究
ICES Journal of Marine Science, 57: 525-530. 2000
F. Ménard, A. Fonteneau, D. Gaertner, V. Nordstrom, B. Stéquert, and E. Marchal
Reporter: 謝佳樺
What is a FAD？
For centuries, men have noticed that fish tend to gather around
objects floating on the surface of the ocean, such as logs, debris,
etc. This natural phenomenon has been exploited by fishermen
for a long time to help them detect and catch fish. Fishermen
then started to build artificial floating objects (buoys, rafts, etc.).
WCPFC
WCPFC
WCPFC
(FADIO/IRD-Ifremer/D. Itano)
( FADIO/IRD-Ifremer/M. Taquet)
(FADIO/IRD-Ifremer/D. Itano)
(FADIO/IRD-Ifremer/D. Itano)
Impacts on fisheries
Western and Central Pacific Fisheries Commission
SCG2
a) FADs increase the efficiency of purse seine effort for tuna including skipjack.圍網漁業使用FAD作業
會提昇鮪類捕獲效率
b) The species composition of purse seine sets on floating objects (including FADs) is different to those
on unassociated tuna schools. In particular the proportion of bigeye tuna in the catch is much higher
for FAD-sets than unassociated sets圍網使用FAD補獲之漁獲物，其魚種組成及混獲種類較捕撈
浮水群複雜，特別是大目鮪幼魚混獲比率會提高
c) Purse seine and other fishing around floating objects results in juvenile bigeye tuna being fished by
multiple gears.大目鮪幼魚隨漂浮物洄游，造成多種漁法可捕獲大目鮪幼魚
d) The deployment of FADs has compounded the difficulties of assessing CPUE trends for purse seine
fisheries and hence undertaking stock assessment for skipjack, bigeye and yellowfin tuna.FAD的使
用導致評估圍網漁業CPUE變動趨勢的準確度，並致使無法準確估計正鰹、大目鮪、及黃鰭鮪
的資源
e) Purse seine sets around floating objects have much higher catches of non-target species (both in
terms of species diversity and total weight) than purse seine sets on unassociated tuna schools.圍網
使用漂浮物捕撈鮪類，其非目標魚的比例增加
f) FADs may alter the movements, feeding and other behaviours of tuna resulting in a range of either
negative impacts (e.g. tuna may become “trapped” in unproductive waters, on small and/or large
spatial and temporal scales, reducing growth and possibly increasing mortality) or positive impacts
(e.g. FADs may provide additional meeting points that assist in schooling and spawning). FAD可能會
改變鮪類洄游、攝食及其他行為，使在FAD周圍洄游魚群成長減緩及死亡率增加，而對資源產
生負面影響;但FAD的聚集效果使魚群容易成群並增加生殖機會
From:FADIO
Why do fish aggregate under FADs?
Hypotheses on FADs
1.Shelter from predator :The object can be used by the prey as a refuge.
2.Concentration of food supply :A floating object or a seamount aggregates prey in
its close vicinity on which large fish could feed.
3.Indicator log :Natural floating objects are often indicators of productive areas.
4.Spatial orientation :Floating objects, underwater structures or seamounts provide
spatial references around which fish can orient in the otherwise
unstructured pelagic environment.
5.Meeting point :Tuna can make use of floating objects to increase the encounter
rate between isolated individuals or small schools and other
schools, to form larger schools.
Outline
Background
Objectives
Material & Method
Result
Discussion and Conclusions
Background
Background
1. There had been no surface fishing activity in the area until 1975.
Since 1991, fishing operations on schools of tuna associated with
FADs has become widespread and this offshore area has developed
into a major fishing zone.
Background
Figure 1. Mean annual catch (t) associated with FADs per 1 square of yellowfin , skipjack , and
bigeye,1991–1996.
Background
Objectives
Objectives
To give an overview of the FAD tuna purse-seine fishery in the
SSA.
To estimate the by-catch using scientific observer data collected
during 1998.
To describe the feeding ecology of tunas associated with FADs
based on stomach content data.
Material & Method
Material & Method
1.Fishery data were obtained from logbooks of purse seiners
operating in the Eastern Atlantic Ocean from 1991 to 1997.
2. We used data obtained during the European Union bigeye research
program by French scientific observers to estimate by-catch and
discards generated by FAD fishing during 1998.
3. The length of each fish sampled was measured and the stomach
was preserved in formalin or deep-frozen. Stomach contents were
sorted, identified to the lowest possible taxa, counted, and weighed.
Results
Results
1.Purse-seine catches in South Sherbro area
Mean FAD catch per fishing season represented 27000 t of the total average
catch of 40000 t in the SSA (1991-1997)
Fig 2. Monthly mean purse-seine catches in the South Sherbro area (bars) and proportion of FADassociated catches (line).
Results
Fig 3. Size distribution of catches of skipjack and of yellowfin and bigeye tunas in 76
FAD-associated sets (FL: forklength, in 2-cm classes). Skipjack (71%) was the main
target followed by bigeye (15%) and yellowfin (14%).
Results
2. Discard and by-catch status
(a) The species composition of tuna discards (total 67 t) estimated from 18 sets
showed that skipjack, frigate, bigeye, yellowfin, and little tuna represented
44%, 28%, 12%, 11%, and 5%, respectively. Tuna discards consisted mainly
of small fish (mode: 38 cm).
(b) By-catches of other pelagic species (Table 1) were associated with 64 FAD
sets (84%).
Results
Results
Fig 4. Proportions of catches and discards of major and minor tuna species and of associated fauna.
Results
3. Tunas associated with FADs based on stomach content data
Table 2. Number of stomachs investigated (N; e: empty; non-e: non- empty),
minimum, maximum, and mean live weight of individuals sampled (Wt), and
of stomach content live weight (SCW) of different species of tuna in the SSA.
Stomach-content analysis was carried out for 572 tunas sampled, including
87 with non-zero stomach content weights
Results
Fig 5. Prey composition of stomach contents in weight percentage for skipjack (SKJ), bigeye
(BET), yellowfin (YFT; two size classes) sampled from FAD associated catches.
Discussion and Conclusions
Discussion
Fig 6. Variation in mean weight of skipjack in FAD-associated catches (for 1979–1990,
means are given for October–January and February–April; for 1991–1997, quarterly
means are given).
Discussion
1. The large number of FADs in use may have a negative
biological effect regardless of whether tunas are actually
caught or not.
2. Small tunas are concentrated under the objects during the night
and may form free swimming loose schools during the day to
feed , according to the stomach samples, mainly on V. nimbaria,
which are not associated with the FAD.
Discussion
3. The purise-seine fishery of discards and by-catch are low
compared with many other fisheries, the impact of this tuna
fishery on total mortality of associated species, some of which
may be considered sensitive (e.g. sharks or turtles), remains
unknown.
4. If the massive use of FADs in the area has led to a change in
migration and growth patterns, this fishery may have a much
greater impact on tuna productivity and on their geographical
distribution.
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