Report of the ICCAT GBYP international workshop on Atlantic bluefin tuna growth

In the last Atlantic bluefin tuna assessment, an age-length database coming from direct ageing was presented for the first time. It was observed that otolith age estimates for fish younger than 8 years old had a smaller size at age compared to spine (first dorsal fin radius) age estimates. This difference, although small, was enough to misallocate the year class. This misallocation was solved when introducing a vector of bias corrected aged otoliths based on paired otolithspine samples. We have identified two possible causes for over-estimating age in the otolith agelength data: the current age adjustment criterion (to convert the bands counting into ages) and a reading bias in age estimations from some laboratories. Otolith preparation and reading protocols have been reviewed. The edge type and marginal increment analysis showed that the formation of opaque zones would seem likely to occur primarily between December through to June, contrary to what was thought until now, for which a new criterion for age adjustment has been proposed

Calibration of the fish ageing services readings carried out in GBYP Phase 7, to estimate age of bluefin tuna from the eastern Atlantic stock

The Fish Ageing Services laboratory (FAS) was contracted by ICCAT GBYP in Phase 7 to provide age estimates from 2000 Atlantic bluefin tuna otolith samples. With the objective of ensuring that age readings provided by FAS follow the ICCAT reviewed reading protocol, a sub-sample calibration exercise was carried out. The findings show that band counts are similar between FAS and the group of laboratories involved in direct ageing. This is reflected by an acceptable precision between both readings. However, there is a one-year bias in the count of bands in older specimens, starting from 10-13 years of age, with a lower count by FAS compared to the rest of the laboratories. This bias seems to be due to the fact that FAS counts the bands in a different area of the otolith ventral arm than other laboratories. This counting discrepancy, although small, is significant and it would be necessary for FAS to reread the samples of specimens older than 10 years using the area close to the sulcus margin of the ventral arm

Evaluation of Atlantic bluefin tuna otolith ageing protocols

An otolith ageing bias for juvenile Atlantic bluefin tuna was detected at the 2017 population assessment. Consequently, revised reading criteria were proposed to reduce this ageing bias. To evaluate standardized and revised reading criteria, age estimates were compared to reading and radiocarbon age estimates. Age estimates showed a lack of bias and adequate level of precision for most readers, indicating that reading criteria are adequate. No bias was detected in the ageing of juveniles, contrary to the 2017 assessment findings. This may be due to the fact that one-year biases may not be noticeable given the wide age range and small sample size used in this study. Some readers showed lower age estimates for older individuals (20+) as compared to radiocarbon age estimates. Furthermore, age estimates were consistently higher than the Ailloud et al. (2017) growth curve for samples aged over 20 years, likely due to the lower mean asymptotic length of the growth curve using the Richards fitting. In terms of methodology, a potential section effect on identification of false annulus and edge type was noted

Natural geochemical markers reveal environmental history and population connectivity of common cuttlefish in the Atlantic Ocean and Mediterranean Sea.

Natural markers (δ13C and δ18O stable isotopes) in the cuttlebones of the European common cuttlefish (Sepia officinalis) were determined for individuals collected across a substantial portion of their range in the Northeast Atlantic Ocean (NEAO) and Mediterranean Sea. Cuttlebone δ13C and δ18O were quantified for core and edge material to characterize geochemical signatures associated with early ( juvenile) and recent (sub-adult/adult) life-history periods, respectively. Regional shifts in cuttlebone δ13C and δ18O values were detected across the 12 sites investigated. Individuals collected from sites in the NEAO displayed more enriched δ13C and δ18O values relative to sites in the Mediterranean Sea, with the latter also showing salient differences in both markers among western, central and eastern collection areas. Classification success based on cuttlebone δ13C and δ18O values to four geographical regions (NEAO, western, central and eastern Mediterranean Sea) was relatively high, suggesting that environmental conditions in each region were distinct and produced area-specific geochemical signatures on the cuttlebones of S. officinalis. A modified δ13C and δ18O baseline was developed from sites proximal to the Strait of Gibraltar in both the NEAO and Mediterranean Sea to assess potential mixing through this corridor. Nearly, all (95%) of δ13C and δ18O signatures of S. officinalis collected in the area of the NEAO closest to the Strait of Gibraltar (Gulf of Cadiz) matched the signatures of specimens collected in the western Mediterranean, signifying potential movement and mixing of individuals through this passageway. This study extends the current application of these geochemical markers for assessing the natal origin and population connectivity of this species and potentially other taxa that inhabit this geographical area

Juvenile Atlantic bluefin tuna otoliths exchange

Otolith reading bias for juvenile Atlantic bluefin tuna was detected at its 2017 assessment. To try to reduce this bias, the current standardized reading protocol was revised. The new protocol uses age estimates from the first dorsal fin ray (spine) to identify the growth increments in the otoliths removed from the same young specimen. An exchange involving 14 experienced otolith readers was conducted to verify if the new reading procedure minimized the difference between otolith and spine readings. The results showed that there is a good agreement in the first five years, but from age 6, otoliths ages tended to be higher than spine age. Some readers had difficulty following the new protocol despite being experts, which indicates that it is necessary to improve it. The use of annual band measurements has shown to be a good tool for the control of the quality of age estimates. The results indicate that we are progressing in the recognition of the deposition pattern of the first annuli. It is recommended to carry out an ageing workshop to achieve the tasks identified as necessary

Tracking the dispersion of a pathogen causing mass mortality in the pen shell Pinna nobilis

Peer reviewe

Tracking a mass mortality outbreak of pen shell Pinna nobilis populations: A collaborative effort of scientists and citizens

A mass mortality event is devastating the populations of the endemic bivalve Pinna nobilis in the Mediterranean Sea from early autumn 2016. A newly described Haplosporidian endoparasite (Haplosporidium pinnae) is the most probable cause of this ecological catastrophe placing one of the largest bivalves of the world on the brink of extinction. As a pivotal step towards Pinna nobilis conservation, this contribution combines scientists and citizens’ data to address the fast- and vastdispersion and prevalence outbreaks of the pathogen. Therefore, the potential role of currents on parasite expansion was addressed by means of drift simulations of virtual particles in a high-resolution regional currents model. A generalized additive model was implemented to test if environmental factors could modulate the infection of Pinna nobilis populations. The results strongly suggest that the parasite has probably dispersed regionally by surface currents, and that the disease expression seems to be closely related to temperatures above 13.5°C and to a salinity range between 36.5–39.7 psu. The most likely spread of the disease along the Mediterranean basin associated with scattered survival spots and very few survivors (potentially resistant individuals), point to a challenging scenario for conservation of the emblematic Pinna nobilis, which will require fast and strategic management measures and should make use of the essential role citizen science projects can play