Mitochondrial DNA locus cytochrome oxidase I was used to asses intraspecific genetic diversity of a didymozoid species Didymosulcus katsuwonicola. Adult forms of this species live encapsulated in pairs in the gills of the reared Atlantic bluefin tuna (Thunnus thynnus). The life cycle of this food-borne parasites and its migration in the host tissues after releasing from the digestive tract to the definitive site in the gills are unknown. Our goal was to assess whether two encysted didymozoids share the same haplotype, indicative of a common maternal origin, as well as the extent of cross- in respect to self-fertilization strategy. Intraspecific comparison showed high haplotype diversity, while the presence of two matching haplotypes within a single cyst encompassed only 17% of sampled individuals. This infers that cross-fertilization between paired individuals within the cyst is more common mechanism than self-fertilization. Such hermaphroditic parasite's trait suggests the existence of intricate infection and reproduction mechanisms, presumably as an adaptation for successful fulfillment of their indirect life cycle through dissemination of genetically more diverse and consequently more fit offspring.

This paper is based on recently recovered daily catch and effort data from the Algarvetraps (south of Portugal) during the 1898−1900 period. These 18 traps targeted the twomigrations of bluefin tuna, entering and leaving the Mediterranean Sea, each year during aperiod between mid April and the end of August. Their bluefin catches were significant,corresponding to average yearly catches of 61 000 bluefin, i.e., approximately 7900 t.Migration dates and durations towards and away from the Mediterranean Sea were estimatedbased on the daily catch per unit effort (CPUE) by the traps. Migration patterns were verystable during these three years. The entry migration took place over an average of 42days, and the return lasted 53 days. Daily catches show large waves of tunas passingduring the two migratory phases. It can be hypothesised that each of these waves of tunaspent about 52 days in the Mediterranean Sea, and that each group was stable andconsistent during their arrival and return migrations. Migration trajectories along theAlgarve coast can be estimated from the daily CPUEs of the different traps. They appear tobe different between the arrival and return migrations. The daily data per trap show thatmost traps caught entering bluefin at quite low CPUE, whereas only a few traps wereefficient for catching the returning bluefin, but with much larger catches and CPUEs. Theanalysis of catch and CPUEs during the entering and leaving phases allowed us to concludethat the catchability and fishing mortality of the bluefin stock in the Algarve trapfishery was much larger during the return migration. Additional daily data from othertraps, both historical and contemporary, should be collected and analyzed in order toprovide more comprehensive results on the patterns and changes in migration, stockstructure and stock exploitation rate.

The swimming path of a reared Pacific bluefin tuna, Thunnus orientalis, was measured in a submerged aquaculture net cage to understand how reared fish use the space in such a cage. A bluefin tuna (fork length, FL, 0.51 m) was captured by angling in the cage, and two micro data loggers (PD3GT, Little Leonardo; DST Comp-Tilt, Star-Oddi) were attached to its body. The fish was then released back into the net cage. The PD3GT measured its swimming speed and depth at 1-s intervals and recorded these in flash memory. The DST Comp-Tilt measured the magnetic field strength at 1-s intervals and recorded the heading estimated from the magnetic field strength in flash memory. The fish moved through the water in the cage at speeds of 0.7–0.8 m s-1 and attained a maximum speed of 3.6 m s-1. Burst swims exceeding 2 m s-1 were confirmed only after dark and a significant difference was found between the daytime and night-time swimming speeds (p < 0.001). The fish moved at depths between 2 and 22 m, swimming near the bottom during the day and at 10–15 m at night, with a significant difference in swimming depth between day and night (p < 0.001). The swimming path reconstructed by dead reckoning was visualised using night-time data. For this period, the absolute speed was corrected from 0.75  ±  0.09 m s-1 to 0.71  ±  0.15 m s-1 by removing the accumulated error from the reconstruction vector. This study allowed us to examine the behaviour of a tagged tuna in three dimensions and is the first to monitor the behaviour of a bluefin tuna in a submerged net cage. Although only one fish was analysed, this study provides useful information on the space use of reared fish in aquaculture net cages. Future studies must obtain sufficient data to understand the underlying generalities of tuna behaviour.

Sampling operations are not routinely conducted in France to monitor the landings of bluefin tuna caught by purse seiners in the Mediterranean. The length, weight and age frequencies of bluefin caught by this fleet are predicted based on sale records, length-at-age and weight-length relations, trends in fishing and natural mortality rates, and other ancillary data. The resulting length frequencies are more accurate than those predicted by Liorzou and Bigot (1992) using a simpler algorithm, and are spread more evenly across adjacent size classes. Catch-per-unit-effort (CPUE) was expressed as the numbers of age 2 bluefin caught per successful seine fishing trip in the Gulf of Lions fall fishery. CPUE indices were computed by several methods, and compared to that of the spanish baitboat fishery in the Bay of Biscay. The index trends of the french and spanish fleets exhibit a general increase during 1970–95, but the results do not demonstrate whichof these is the best indicator of juvenile abundance. Complementary surveys are required to determine the temporal and spatial distributions of surface schools and fishing effort, and identify the most reliable index of abundance.