Tagging at sea

Tagging activity continued throughout September and October 2016 around the Canary Islands and Azores. Tagging in Zone B (Gulf of Guinea) began on 20th October 2016; starting with a trip to the Sierra Leone seamounts where more than 5,000 tuna were tagged. In early January 2017 work began again and the Aita Fraxku (AZTI chartered bait boat) is currently en route towards Sao Tome and Principe which will be used as a base for tagging in the eastern Gulf of Guinea.

Table 1. R-1 total releases by species (26-01-2017)

Species R-1 R-2 R-3
BET 9114 102 1
LTA 420 1 0
SKJ 11878 36 0
UNK 141 0 0
WAH 1 0 0
YFT 5912 12 0
Total 27466 151 1

To date more than 27,000 tropical tunas (Figure 1 and Table 1), across species and size-ranges, have been tagged and released in total using a mix of conventional, electronic and chemical marking techniques. By species, ca 12000 SKJ (ca 43%), ca 9000 BET (ca 33%), and ca 6000 YFT (ca 22%), have been tagged (R-1), together with a few neritic species (LTA and WAH). Progress can also be followed here, http://aottp.iccat.int/index.php?option=search.

Table 2. Releases (R-1) length-frequencies by species (26-01-2017)

  20-40 40-60 60-80 80-100 100-120 120-140 140-160 160-180
BET 651 4459 3534 391 60 7 1 5
LTA 4 416 0 0 0 0 0 0
SKJ 1705 9638 522 0 0 0 0 0
WAH 0 1 0 0 0 0 0 0
YFT 744 3862 1246 51 4 0 1 1

The balance between species and size-ranges tagged is satisfactory (Table 2) overall, although both large (>100cm) BET and YFT have been difficult to catch. Detailed analyses of the length-distributions by area also remain to be done. In the Azores, fishing during 2016 was unusually poor, very few YFT and BET appeared, and 2,766 tuna, mostly SKJ, were tagged (176 BET and only 1 YFT) in total. The remaining conventional tags from the Azores have been transferred for potential deployment in the Gulf of Guinea.

Figure 1. Distribution of tropical tuna tagged and released (conventional tags, R-1 and R-2) by ICCAT/AOTTP between July and December 2016 (26-01-2017)

Electronic tags

AOTTP has experienced problems with all the electronic tags purchased. The Wildlife Computers PSATs were re-called during August due to a technical fault and the Desert Star (DS) tags had to be sent out instead. In the Canary Islands 15 tuna were tagged with these DS Tags during August and September 2016, and 5 in the Azores in November. Seven (DS tags) popped up prematurely and are drifting steadily south-west, transmitting data to satellite which are currently being analysed. Very few fish large enough for carrying PSATs were caught at all in Zona A (Cabo Verde/Mauritania/Senegal) between July and August 2016.

Figure 2. Diving behaviour of 3 BET tagged with Wildlife Computers PSATs between 29 October and 1 November 2016 (26-01-2017)

The Wildlife Computers PSATs arrived back in Madrid in August and three were deployed during Aita Fraxku’s cruise to the Sierra Leone seamounts in late October. Unfortunately, however, all 3 fish were caught by a purse-seiner less than 3 days later. All 3 tags were recovered by the TROs in Abidjan and sent back to ICCAT HQ where the data were extracted, e.g. see Figure 2 which shows the diving behaviour of the 3 fish by hour. Tag 1220 was on a bigeye, and tags 1248 and 1261 were on yellowfin. Note that the bigeye (Figure 2) is represented by the red points and line and maintained a deeper average depth than the 2 yellowfin (green and blue).

Figure 3. Spatial distribution of ICCAT/AOTTP recoveries between July and December 2016 (26-01-2017)

Tag recoveries

More than 4,000 converntional tags have been recovered (Table 3) so far and uploaded the data to the ICCAT database. The tags found translate into recovery rates/percentages overall, of between 12% and 20% (see Table 4).

Table 3. RCF total recoveries by species (26-01-2017)

BET LTA SKJ WAH YFT Total
1316 81 1449 0 1258 4173

Table 4. R-1 recovery percentages by species (26-01-2017)

BET LTA SKJ WAH YFT
14.3 19.2 12.2 0 21.2

The locations of the recovered tags are shown by species in Figure 3, while the overall ‘history’ of releases and recoveries over time is summarised in Figure 4. Note: recoveries of the 376 little tunny tagged during July are still being made.

Figure 4. Total ICCAT/AOTTP releases (green) and recoveries (red) over time by species (BET=bigeye, LTA=little tunny, SKJ=skipjack, YFT=yellowfin). The numbers have been square-root transformed so they can be seen on the same axes (26-01-2017)

Figure 5. West Africa: migrations between release and recovery for BET, LTA, and YFT (26-01-2017)

It is now more than six months since AOTTP tagging began and there are now some reasonably long migrations and times at ‘liberty’ (Figure 5). A SKJ (tag ID = 8274) tagged on 15 July 2016, for example, was re-caught on 1 December having spent 139 days at liberty, growing from 38cm to 50cms in length. In the AOTTP database there are now more than 300 individuals which have spent more than 100 days at liberty. Long-distance migrations by skipjack between Azores and the West African coast also continue to be seen (Figure 6).

Figure 6. Skipjack migrations between release and recovery by size category. The maps show movement of several individuals between Azores and West Africa (26-01-2017)

2017 Activities