Swimming depth, ambient water temperature preference, and feeding frequency of young Pacific bluefin tuna (Thunnus orientalis) determined with archival tags

We investigated the migration and behavior of young Pacific Bluefin tuna (Thunnus orientalis) using archival tags. The archival tag measures environmental variables, records them in its memory, and estimates daily geographical locations based on measured light levels. Of 166 archival tags implanted in Pacific bluefin tuna that were released at the northeastern end of the East China Sea from 1995 to 1997, 30 tags were recovered, including one from a fish that migrated across the Pacific. This article describes swimming depth, ambient water temperature, and feeding frequency of young Pacific bluefin tuna based on retrieved data. Tag performance, effect of the tag on the fish, and horizontal movements of the species are described in another paper. Young Pacific bluefin tuna swim mainly in the mixed layer, usually near the sea surface, and swim in deeper water in daytime than at nighttime. They also exhibit a pattern of depth changes, corresponding to sunrise and sunset, apparently to avoid a specific low light level. The archival tags recorded temperature changes in viscera that appear to be caused by feeding, and those changes indicate that young Pacific bluefin tuna commonly feed at dawn and in the daytime, but rarely at dusk or at night. Water temperature restricts their distribution, as indicated by changes in their vertical distribution with the seasonal change in depth of the thermocline and by the fact that their horizontal distribution is in most cases confined to water in the temperature range of 14−20°C.

Vertical and horizontal movements of southern bluefin tuna (Thunnus maccoyii) in the Great Australian Bight observed with ultrasonic telemetry

The vertical and horizontal movements of southern bluefin tuna (SBT), Thunnus maccoyii, in the Great Australian Bight were investigated by ultrasonic telemetry. Between 1992 and 1994, sixteen tuna were tracked for up to 49 h with depth or combined temperature-depth transmitting tags. The average swimming speeds (measured over the ground) over entire tracks ranged from 0.5 to 1.4 m/s or 0.5 to 1.4 body lengths/s. The highest sustained swimming speed recorded was 2.5 m/s for 18 hours. Horizontal movements were often associated with topographical features such as lumps, reefs, islands and the shelf break. They spent long periods of time at the surface during the day (nearly 30%), which would facilitate abundance estimation by aerial survey. At night, they tended to remain just below the surface, but many remained in the upper 10 m throughout the night. SBT were often observed at the thermocline interface or at the surface while travelling. A characteristic feature of many tracks was sudden dives before dawn and after sunset during twilight, followed by a gradual return to their original depth. It is suggested that this is a behavior evolved to locate the scattering layer and its associated prey when SBT are in waters of sufficient depth. SBT maintained a difference between stomach and ambient temperature of up to 9°C.

Mechanisms of upper-ocean thermal variability in a 1970-1988 simulation and observations

EXTRACT (SEE PDF FOR FULL ABSTRACT): Variations in temperature that occurred in the North Pacific thermocline (250 to 400 meters) during the 1970s and 1980s are described in both a numerical simulation and XBT observations.

Record catch of rock perch, Epinephelus fasciatus by bottom trawling off Ratnagiri

A productive fishing ground for rock perch (Epinephelus fasciatus) was located off Ratnagiri between 17 degree 00' to 17 degree 10' N latitude and 72 degree 20' to 72 degree 30' E longitude, around a depth of 96m during November, 1983. An average catch of 2 ton/hr. was recorded. Length frequency studies of E. fasciatus indicate the probability of a nursery ground consisting of three brood stocks. Hydrographic studies of the ground revealed that the bottom water was oxygen deficient below a strong thermocline extending from 15-75 m depth with the recorded bottom temperature amounting to 20.5 degree C.

The zooplankton population structure and their trend of variations in the southern of Caspian Sea (Iranian waters)

Caspian Sea has gone under a lot of changes due to human influences and the unwanted presence of a ctenophora Menomiopsis leidyi which has greatly changed the structure of planktons in the last recent years. Therefore, this study was carried out in order to determine these changes in the zooplankton community. the Sampling was done in 8 transacts in Astara, Anzali, Sefidrood, Tonekaboun, Noushahr, Babolsar, Amirabad and Bandar Torkaman coastal waters at 5 different depths including 5, 10, 20, 50 and 100 m. Sampling was carried out in four seasons of spring, summer, autumn and winter during 2008, 2009 and 2010 on board of R/V Gilan. Altogether, 12 species of zooplankton were identified in 2008, 22 species in 2009 and 14 species in 2010. The zooplankton included four groups: copepoda (4 species), cladocera (8species), rotatoria (10 species) and protozoa (2 species).The increase of diversity in 2009 was due to cladocera and rotatoria groups. The abundance of zooplankton in the spring was 5074 + 7807 ind/m3 more than other season in 2008. The abundance of copepoda in the summer reached the highest value of 3332 ind/m3 and since autumn the abundance gradually decreases and in the winter reached to the lowest value. The most abundance of cladocera was 797 ind/m3 in winter and decreased in summer and autumn. The abundance of rotatoria was 2189 ind/m3 in winter. rotifera and copepoda consisted the main population of Zooplanktons in the winter. The results of 2009 and 2010 showed that the abundance of zooplankton in winter was 2.6 fold of autumn, 1.6 fold of summer and 1.1 fold (1/9 fold in 2010)of spring. After increasing increased of temperature, phytoplankton, and zooplankton in summer, M.leidyi increased too. In the autumn M. leidyi reached to the highest rate and decreased zooplankton. The maximum population of zooplankton was in the layer 0-20 m and in the layer more than 20 meters, the abundance of zooplankton decreased very much. In 216 2008, 2009 and 2010, the abundance of zooplankton was 87, 77 and 77 percent in the layer 0-20 m respectively. In this study, the thermocline was observed in the layer 10 – 20 meters in the spring, that formed a thin layer but in the summer it was in the layer 20 to 50 meters. Temperature decreased between 11 to 15 oC in this layer. The variation of temperature between surfaces to bottom was 10 to 13 oC in spring, 19 to 21 in summer, about 9 oC in autumn and maximum 3 oC in winter. The most biomass of zooplankton was in the west. The biomass of zooplankton in central west and east of Southern of Caspian Sea was 54 %, 22 % and 24 % respectively in 2008, in 2009 was 48%, 33% and 20% respectively and in 2010 was 54 %, 29 % and 16 % respectively .The biomass decreased from west to east. The model of zooplankton designed by principal component analysis (PCA)and linear regression for Southern of Caspian Sea.

An instance of cyclonic upwelling in the southern offshore waters of Lake Victoria

Evidence is presented that cyclonic upwelling took place in the southern offshore waters of Lake Victoria on the 22nd to the 23rd March, 1969. Before this, the main upper thermocline was tilted downwards in the leeward northern half of the lake in response to the accumulation of epilimnetic water. Cyclonic upwelling might be quite common because of the frequent occurrence of storms on Lake Victoria, and probably on other tropical African lakes. However, to my knowledge, it has never been reported from any lake.