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Seasonal abundance of the tropical tunas around fish aggregating devices anchored off the coast of Mauritius (2010-2012)

Year of publication 2013

Product type Meeting

Edition 1

Synopsis (short abstract) Catch rates of the pelagic species around the Fish Aggregating Devices (aFADs) anchored off the Western coast of Mauritius show that the seasol variation of the sea surface temperature (SST) has a significant influence on the abundance of the tropical tu. The results of the study carried out over the period 2010 to 2012 suggest that the season of relative abundance for the skipjack tu (Katsuwonus pelamis) is more pronounced during the summer months when SST is higher. The study on the other hand indicates that the yellow fin tu (Thunnus albacares) prefers the cooler waters which prevail both at start of winter and at the outset of the summer season. The broad annual migration of the yellow fin and skipjack tus around the anchored FADs is noticeably associated with the annual SST cycle. The results also indicated a marked scarcity in the presence of the big eye tu (Thunnus obesus) in the landings of the artisal FAD fishery. In contrast, data on landings from the licensed industrial fore ign long liners operating mostly in the EEZ of Mauritius illustrate that both the yellow fin and the skipjack tus abound during the summer season. In addition, the landings of the semi-industrial local fleet (long liners <24m) primarily targeting swordfish in Mauritius waters included the big eye tu (mostly caught in the winter season) and the yellow fin tu (all year round). This paper highlights the results based on the alyses on the fish catch data collected from the professiol artisal fisherm en operating around FADs off the Western coast of Mauritius with discussion on the relation between effort, SST and the seasol abundance of the tropical yellow fin and skipjack tu.

Statutory body / Meeting title Indian Ocean Tuna Commission (IOTC)

Meeting symbol IOTC-2013-WPTT15-11

Meeting Session Number 15

Language English

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    Movement behaviour of skipjack (Katsuwonus pelamis) and yellowfin (Thunnus albacares) tuna at anchored fish aggregating devices (FADs) in the Maldives investigated using acoustic telemetry 2013
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    The pole and line tu fishery in the Maldives relies heavily on an array of 45 anchored fish aggregat- ing devices (FADs), making it one of the largest anchored FAD-based tu fisheries in the world. We examined the behaviour of skipjack (Katsuwonus pelamis) and yellowfin (Thunnus albacares) tu around anchored FADs (1 000 to 2 000 m deep) in the Maldives using passive acoustic telemetry. Eight neighbouring FADs (distance range: 30 to 95 km, average: 50 km) were equipped with automated acoustic rece ivers in January 2009, for a period of 13 months. A total of 40 skipjack (37−54 cm FL) and 21 yellowfin (35−53 cm FL) tu were tagged with Vemco V13 transmitters in January (start of the northeast monsoon, dry season) and November (end of the southwest monsoon, wet season) 2009 and released at the two central FADs within this instrumented array. No movement between FADs was observed for any acoustically-tagged tu in the instrumented FAD array. These results suggest that FADs in the Maldives may a ct inde- pendently. The maximum time a tagged skipjack remained associated with a FAD was 12.8 days in January but only one day in November. In addition, residence times at FADs were found to differ with time (month) and space (FAD location) for skipjack tu, suggesting that exterl biotic factors (e.g., prey, conspecifics or predators) might influence the time this species spends at FADs. In November, the residence times of yellowfin tu (maximum observed time: 2.8 days) were three times greater t han those of skipjack tu at the same FADs. This specific difference could be explained either by the two species responding to different factors or by the species’ responses being dependent on the same factor but with different thresholds. No particular preference for time of departure from the FADs was observed. Some monospe- cific and multispecific pairs of acoustically-tagged individuals were observed leaving the FADs simultaneously. Thus, this study indicates a high degree of complexity in t he behavioural processes driving FAD associations.
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