Characterisation of the dietary relationships of two sympatric hake species,Merluccius capensis and M. paradoxus,in the northern Benguela region using fatty acid profiles

The two sympatric species of Cape hake, Merluccius capensis and M. paradoxus, have been the main targets of bottom-trawl fisheries off Namibia for several decades. The feeding ecology of these hakes has been studied mainly using stomach content analyses and thus there remain some gaps in our knowledge about food assimilated over the longer term. In this study, we used fatty acid (FA) profiles to characterise the dietary relationships of M. capensis and M. paradoxus. Muscle samples from hake (n=110) and their known prey (n=68) were collected during trawl surveys off Namibia during 2011. Significant differences between the neutral FA profiles of the hake populations were detected in December 2011 but not in January 2011, an indication of temporal variations in diet and resource partitioning. Comparisons of the neutral FAs in hake and the total FAs of potential prey showed no clear trophic connections, with the exception of flying squid Todarodes sagittatus, which had FA profiles very similar to those of M. paradoxus in December 2011. Our results highlight the complex and temporally shifting relationships that exist between hake and the large pool of prey available to them, and between the two hake species that overlap in their feeding habits and distribution within the highly productive Benguela Current region.     

西太平洋羊鱼科(Mullidae)鱼类多样性及其分布的研究

文中整理记述了西太平洋羊鱼科鱼类31种,分属于Upeneus、Parupeneus、Mulloidichthys属,列写了种属检索表;并报告其各个种的地理分布区。在此基础上,讨论了西太平洋羊鱼科鱼类生物多样性特征、动物地理学特点及部分名称订正,资源与渔业上的意义。     

Population abundance and seasonal migration patterns indicated by commercial catch-per-unit-effort of hakes (Merluccius capensis and M. paradoxus) in the northern Benguela Current Large Marine Ecosystem

We developed generalised additive models (GAMs) to estimate standardised time-series of population abundance indices for assessment purposes and to infer ecological and behavioural information on northern Benguela hakes, Merluccius capensis and M. paradoxus, using haul-by-haul commercial trawl catch-rate data as proxies for hake densities. The modelling indicated that individual ship identifiers should be used rather than general vessel characteristics, such as vessel size. The final models explained 79% and 68% of the variability in the commercial catch rates of M. capensis and M. paradoxus, respectively. The spatial density patterns were consistent and confirmed existing knowledge about these species in the northern Benguela system. Furthermore, seasonal migration patterns were described for the first time and were found to correspond to the known spawning areas and seasons for M. capensis and M. paradoxus. Spatial density patterns were validated using the geostatistical modelling results of fisheries-independent trawl survey data. Improved understanding of the relationships between fleet dynamics and fish movement can be achieved by taking into consideration the present catch-rate model and spatial and seasonal distribution maps. We conclude that the yearly standardised CPUE time-series are problematic as proxies for total stock abundance because of spatial coverage issues. Consequently, such CPUE data should not be used for stock-size assessments and fisheries advice concerning northern Benguela hakes until this is solved. We generally recommend the exclusion of standardised CPUE time-series from stock assessments when important and changing parts of the stock distribution cannot be targeted by the fishery, such as due to closed areas or seasons.     

In Situ Correction of Porosity and Velocity of Pelagic Carbonate Sediments in the Eastern Equatorial Pacific (Sites 846 and 850, ODP Leg 138)

During Ocean Drilling Program Leg 138, eleven sites were drilled. The sediments recovered are mainly composed of carbonate-rich pelagic ooze including minor diatom and radiolarian ooze. Average carbonate contents of Sites 846 and 850 are about 59% and 75%, respectively. The porosity and velocity measured in laboratory were corrected to in situ condition using empirical equation (Hamilton, 1976) and delta velocity (log value-laboratory value). The corrected laboratory porosity matches well with logging data, but it is deviating in the intervals of low carbonate content. The correlation between log porosity and the corrected porosity shows a similar trend. Depth profiles between the corrected velocity and logging data do not agree in the intervals of low carbonate content, but the intervals characterized by high carbonate content are relatively matched. This suggests that Hamilton's equation is not appropriate for diatom-rich sediments. The correlation between log velocity and the corrected velocity is also a much better fit than that of laboratory velocity. In addition, the relationship between velocity and porosity is a better fit using corrected data than uncorrected data. Thus, mechanical rebound correction needs to interpret laboratory data to in situ condition. However, the magnitude of rebound by the removal of overburden pressure should be carefully considered because the rebound depends on sediment types.