Assessment of natal homing and mixing of Atlantic bluefin tuna using d13C and d18O signatures in otoliths 
PI: Jay Rooker and David H. Secor
PROJECT DESCRIPTION:
This project represents the application phase of previous research funded through NOAA. In previous projects/publications, we have
evaluated alkaline earth, trace, and heavy metals in otoliths. Discrimination of yearling bluefin tuna from eastern and western
natal regions approached 90% but the mixture of discriminating metals varied year to year, which ultimately could limit application.
We have recently concentrated on stable isotopes ( d13C, d18O), which should vary more consistently year to year due to differing
physical conditions between the Mediterranean and western Atlantic nurseries. Indeed, otolith stable isotopes showed substantially
stronger inter-annual stability, and the discriminatory power of stable isotopes in otoliths was high with > 90% cross-validated
classification success of yearlings to eastern and western nurseries over 5 year - classes (1999-2003). Our findings clearly indicate
that these natural tags can predict the natal origin of Atlantic bluefin tuna. We have also verified that we can effectively mill
internal material of adult bluefin tuna otoliths that gives similar isotopic signatures to whole otoliths of yearling bluefin tuna.
In preliminary work, we milled otolith core material from 100+ medium and giant category bluefin tuna and observed compelling evidence
of mixed stock fisheries. Most notably, a large fraction of the medium and giant category bluefin tuna collected in the western
Atlantic (northeastern US) originated from nurseries in the east, while adults in the Mediterranean were largely (>90%) of
Mediterranean origin. Here we take the next logical step and build on previous research to assess natal homing and mixing rate of
Atlantic bluefin tuna. Objectives of the current investigation are threefold: 1) quantify stable ? d13C and d18O isotopes in whole
otoliths of yearling Atlantic bluefin tuna from eastern and western nurseries to complement the current set of reference samples or
birth certificates; 2) determine trans-Atlantic mixing and natal homing tendencies of ? adult bluefin tuna collected from the two
known principal spawning regions (Gulf of Mexico and Mediterranean Sea) by comparing d13C and d18O isotopic signatures in otolith
cores of adults to yearling signatures (reference samples); 3) predict the natal origin of adult bluefin tuna sampled from mixed
stock fisheries in the central North Atlantic, Canada (Gulf of St. Lawrence ), and New England (U.S.A).
Progress during the first six months:
Efforts during the first 6 months of the project centered on the collection of otoliths from yearling and adult bluefin tuna.
Drs. Rooker and Secor traveled to the Mediterranean in mid October to sample from tuna pens as well as process yearling samples
collected by colleagues (Dr. Gregorio DeMetrio, University of Bari ) in the region over the past 1-2 years. Samples of yearlings
and adults bluefin tuna collected on this trip include 40 medium/giants as well as over 50 yearlings (see Preliminary Data section
or details). In addition, Texas A&M University entered into a cooperative agreement with the primary tuna farming operation in
the region (Fuentes Group), and the head biologist form the company (Antonio Belmonte) will assist with collection this year as well
as in 2007. In the central North Atlantic and Gulf of St. Lawrence, Dr. Neilson has been working with IVY Fisheries and local
contractors to procure bluefin tuna heads (otoliths) for the project. To date, otoliths from 29 bluefin tuna were in 2006. Otoliths
from the central North Atlantic have been more challenging. An initial batch of heads was provided by IVY Fisheries; however, cuts
were too far anterior to obtain otoliths. In the western Atlantic, Dr. Secor has continued to sample yearlings and giants in 2006,
albeit numbers are lower than previous year (note: otoliths samples from over 60 giants collected earlier by Dr. Secor will be used
for the present study).
Preliminary Data:
Collected material (note: O = otoliths, T = tissue)
Region |
Year |
Size Class |
No. Samples |
Type |
| Mediterranean | 06 |
Giant |
20 |
O,T |
| E. Atlantic | 06 |
Medium |
20 |
O,T |
| Ionian Sea | 06* |
Yearling |
56 |
O,T |
| Gulf of Mexico | 05 |
Giant |
12 |
O |
| W. Atlantic | 06 |
Yearling |
12 |
O, T |
| C. N. Atlantic | 06 |
Giant |
9 |
O |
| Gulf of St. Lawrence | 06 |
Giant |
29 |
O |
*Collections were obtained in 06 but include samples of yearling from 04 and 05 being saved by colleagues in Italy .
Plans for the next six months to year:
High-resolution milling will be used over the next 6-month period to isolate core material from otoliths of adult bluefin tuna
collected in 2006. Prior to milling, all sagittal otoliths will be embedded in Stuers epoxy resin and sectioned using a low
speed ISOMET saw to obtain transverse sections through the core. We will then isolate the portion of the otolith corresponding
to the first year of life (based on measurements from sectioned otoliths of yearling bluefin tuna ) using a drill path programmed
into the New Wave © MicroMill System. All core material will be carefully removed and cleaned prior to further analysis.
Otolith cores from adults will then be pulverized (powdered) using an acid - washed mortar and pestle. Carbon and oxygen stable
isotopes will be measured, and isotopic signatures of adults from all areas will be matched to our library of yearling signatures
to predict natal origin and mixing rates. In addition to otolith cores, we are also in the process of running otoliths from
yearling bluefin tuna collected in 2005 and 2006 to expand our reference set of signatures from both the east and west. Also,
we will compare different methods of obtaining core material (prism from drill path versus power from multiple point estimates)
over the next few months to fine tune our core isolation procedure. When all of this is completed, we will use mixed stock
analysis to retrospectively determine natal origin, trans-Atlantic mixing, and natal homing behaviors of bluefin tuna from our
2006 collections.
Conferences:
Invited talk entitled “Blue travelers: is the sustainability of Atlantic bluefin tuna and blue marlin stocks linked to the
Gulf of Mexico.” 59 th Annual Gulf and Caribbean Fisheries Institute meeting, Belize City , Belize (Nov 2006)
