Exploitation of small tunas by a purse-seine fishery with fish aggregating devices and their feeding ecology in an eastern tropical Atlantic ecosystem

F. Ménard, A. Fonteneau, D. Gaertner, V. Nordstrom, B. Stéquert, and E. Marchal

ICES Journal of Marine Science, 57: 525-530. 2000

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

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/M. Taquet)

(FADIO/IRD-Ifremer/D. Itano)(FADIO/IRD-Ifremer/D. Itano)

(FADIO/IRD-Ifremer/D. Itano)

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

Impacts on fisheries

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.

From:FADIO

Outline

Background

Objectives

Material & Method

Result

Discussion and Conclusions

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

　　

Background

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.

Objectives

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.

Material & Method

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 FAD-associated 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.

Results

Discussion and Conclusions

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.

Discussion

Thank You!Thank You!