Trends in tuna carbon isotopes suggest global changes in pelagic phytoplankton communities

Considerable uncertainty remains over how increasing atmospheric CO₂ and anthropogenic climate changes are affecting open‐ocean marine ecosystems from phytoplankton to top predators. Biological time series data are thus urgently needed for the world's oceans. Here, we use the carbon stable isotope composition of tuna to provide a first insight into the existence of global trends in complex ecosystem dynamics and changes in the oceanic carbon cycle. From 2000 to 2015, considerable declines in δ¹³C values of 0.8‰–2.5‰ were observed across three tuna species sampled globally, with more substantial changes in the Pacific Ocean compared to the Atlantic and Indian Oceans. Tuna recorded not only the Suess effect, that is, fossil fuel‐derived and isotopically light carbon being incorporated into marine ecosystems, but also recorded profound changes at the base of marine food webs. We suggest a global shift in phytoplankton community structure, for example, a reduction in ¹³C‐rich phytoplankton such as diatoms, and/or a change in phytoplankton physiology during this period, although this does not rule out other concomitant changes at higher levels in the food webs. Our study establishes tuna δ¹³C values as a candidate essential ocean variable to assess complex ecosystem responses to climate change at regional to global scales and over decadal timescales. Finally, this time series will be invaluable in calibrating and validating global earth system models to project changes in marine biota.     

Genetic isolation between Atlantic and Mediterranean albacore populations inferred from mitochondrial and nuclear DNA markers

Genetic population structure of Atlantic and Mediterranean albacore Thunnus alalunga was investigated using nucleotide sequence variations of the glucose‐6‐phosphate dehydrogenase gene intron ( G6PD ) and the mitochondrial DNA (mtDNA) D‐loop region ( Dloop ). Restriction analysis using Ase I digestion detected two major restriction types ( A and B ) at the Dloop locus with strong frequency differences between Atlantic and Mediterranean samples. Thirty‐six individuals of 100 Mediterranean albacore were of the B type whereas no B type individuals were found in the Atlantic samples ( n  = 102). Phylogenetic analysis using nucleotide sequence data of the Dloop locus indicated that the B type lineage recently arose from the ancestral A lineage in the Mediterranean Sea and has not dispersed into the Atlantic Ocean. The frequencies of two alleles ( L and S ) at the G6PD locus were significantly different between the samples from the Atlantic ( L  = 0·495) and the Mediterranean ( L  = 0·725), but no significant heterogeneity was observed between mtDNA‐ A and ‐ B types of the Mediterranean sample. These molecular data indicate that gene flow between the Atlantic and Mediterranean albacore populations have been considerably restricted and strongly suggest these populations should continue to be treated as two distinct management units.     

Age and growth of Mediterranean albacore

Estimated ages of 1136 individual albacore Thunnus alalunga (57–92 cm L F) from the Aegean and Ionian Sea ranged from 1⁺ to 9 years. Males grew faster and reached a greater size and age than females. No significant differences were found in the mean lengths at estimated ages between the two sampling areas. The von Bertalanffy growth model was fitted to mean lengths of estimated ages of individual fish and estimated growth parameters for the combined sexes were: L _∞=94·7 cm, K =0·258, to=–1·354 years. Significant differences were found when the Mediterranean albacore growth parameters were compared with those determined for Atlantic Ocean albacore. It is not possible to determine if the differences in growth rates for the two populations are phenotypic or genotypic at the present time.