Diet of albacore Thunnus alalunga from the waters of Mauritius (western Indian Ocean) inferred from stomach contents and fatty acid analysis

The diet of albacore Thunnus alalunga from their spawning ground in the waters of Mauritius was investigated. The stomach contents of 249 albacore individuals, caught by industrial longliners and artisanal fishers, was analysed, and the dominant prey taxa were selected for lipid and fatty acid (FA) analysis. The FA profiles of prey were compared with those of liver tissue of spawning-capable and post-spawning female albacore, using multivariate analysis. Whereas stomach content analysis identified cephalopods and crustaceans as the dominant prey items in number, FA-profile analysis identified crustaceans and fishes as the most-frequently consumed prey of post-spawning female albacore. In contrast, the FA profiles of spawning-capable albacore and those of prey showed very low similarity. Analysis of the prey suggests that although cephalopods, crustaceans and fishes could all provide albacore with the required lipids and FAs, cephalopod prey seem to be less desirable owing to their lower energy content (i.e. low in total lipid, triacylglycerol, 16:0, 18:0 and 18:1ω9). Instead, the most beneficial food appeared to be locally available fishes, which are more energy-rich and have a higher docosahexaenoic acid/ eicosapentaenoic acid ratio. These results provide new information on the trophic ecology of albacore, promote our understanding of the importance of prey type for successful reproduction of albacore, and highlight the advantages of FA profiling to study diet.     

A Study of the Relationships between the Characteristic Acceleration Frequencies and Earthquake Magnitude: The Baikal Rift Zone

This study is concerned with characteristic frequencies of acceleration spectra as functions of the moment magnitude М _w for crustal earthquakes in the 3 ≤ М _w ≤ 6.4 range that have occurred in the Baikal Rift Zone (BRZ). The characteristic frequencies were defined as those at which maxima occur in the spectra: f_Sm, f₁, and f₂, at the 0.7S_m and 0.5S_m levels, as well as the f_c2 and f_c3 frequencies, which characterize the transition from the ascending branch of the spectrum (f_c2) to the gently sloping branch, and the f_c3 frequency, which shows the transition from the gently sloping part to the descending branch. All the characteristic frequencies move toward lower values with increasing earthquake magnitude. The resulting relationships for f_c2 and f_c3 were used to estimate the scaling, with the result that no similarity is present. As well, we have found no wellpronounced differences in earthquake spectra between different focal mechanism events. It was found that the slope of the low-frequency branch decreases and the high-frequency descending branch becomes flattened with increasing magnitude.     

Acceleration response spectra for the earthquakes of the southwestern flank of the Baikal Rift Zone

The acceleration response spectra of earthquakes with M = 4–6.5 in the southwestern part of the Baikal Rift Zone have been studied. The absorption properties of the medium and the attenuation of seismic signals in the study area were determined. Average acceleration response spectra were obtained for regional earthquakes. A comparative analysis of the acceleration response spectra was made for earthquake focal mechanisms with different senses of motion: reverse fault, reverse slip, strike slip, and oblique slip. The effect of the sense of fault motion in the seismic source on acceleration response spectra was determined.