SUMMARY:The reproductive biology of the European hake (Merluccius merluccius, Linnaeus, 1758) was studied on the eastern central Atlantic Moroccan coast from November 2001 to December 2003. The sex ratio was close to 1:1 for the specimens with a total length of less than 45 cm. European hake spawned all year round, though two spawning peaks were recorded with a certain interannual variability: the first and main peak in winter and a secondary intermittent peak in summer. Length at first maturity was estimated as 28.6 cm and 33.8 cm total length for males and females, respectively. These values are closer to Mediterranean than Atlantic hake values. The mean batch fecundity was 299872 eggs ind -1 . The relative batch fecundity was estimated as 228 eggs per gram of gutted weight. Batch fecundity increased proportionally with fish length and weight. The batch fecundity of European hake in the Moroccan Atlantic area is higher than that in northern Atlantic and northwestern Mediterranean areas.Keywords: Merluccius merluccius, sex ratio, reproductive cycle, length at first maturity, fecundity.RESUMEN: Madurez y fecundidad parcial de la merluza europea (Merluccius Merluccius, Linnaeus, 1758) en el Atlántico Oriental central. -Se ha estudiado la biología reproductiva de la merluza europea (Merluccius merluccius, Linnaeus, 1758) en la zona central del Atlántico Oriental de la costa de Marruecos durante el período de noviembre de 2001 a diciembre de 2003. La proporción sexual fue cercana a 1:1 para los individuos de talla total menor de 45 cm. La merluza europea se reproduce durante todo el año. Se han detectado dos picos de puesta con una cierta variabilidad interanual: el primero y más importante en invierno y otro pico secundario intermitente en verano. La talla de primera madurez se estimó en 28.6 cm y 33.8 cm de longitud total para machos y hembras respectivamente. Estos valores son más cercanos a los observados para la merluza en el Mediterráneo que a los del Atlántico. La fecundidad parcial media fue de 299872 huevos ind -1 . La fecundidad parcial relativa se estimó en 228 huevos por gramo de hembra referido al peso eviscerado. La fecundidad parcial se incrementó proporcionalmente a la longitud y peso del pez. La fecundidad parcial de la merluza europea en el área atlán-tica de Marruecos, muestra valores más altos que en las áreas del Atlántico situadas más al norte y del NW Mediterráneo.Palabras clave: Merluccius merluccius, proporción sexual, ciclo reproductivo, talla de primera madurez, fecundidad. 2011, 447-454, Barcelona (Spain) Scientia Marina75(3) September
The resistance of an east border upwelling system was investigated using relative index of marine pelagic biomass estimates under a changing environment spanning 20-years in the strongly exploited southern Canary Current Large marine Ecosystem (sCCLME). We divided the sCCLME in two parts (north and south of Cap Blanc), based on oceanographic regimes. We delineated two size-based groups (“plankton” and “pelagic fish”) corresponding to lower and higher trophic levels, respectively. Over the 20-year period, all spatial remote sensing environmental variables increased significantly, except in the area south of Cap Blanc where sea surface Chlorophyll-a concentrations declined and the upwelling favorable wind was stable. Relative index of marine pelagic abundance was higher in the south area compared to the north area of Cap Blanc. No significant latitudinal shift to the mass center was detected, regardless of trophic level. Relative pelagic abundance did not change, suggesting sCCLME pelagic organisms were able to adapt to changing environmental conditions. Despite strong annual variability and the presence of major stressors (overfishing, climate change), the marine pelagic ressources, mainly fish and plankton remained relatively stable over the two decades, advancing our understanding on the resistance of this east border upwelling system.
Abstract. Understanding the relationship between sound-scattering layers (SSLs) and pelagic habitat characteristics is a substantial step to apprehend ecosystem dynamics. SSLs are detected on echo sounders representing aggregated marine pelagic organisms. In this study, SSL characteristics of zooplankton and micronekton were identified during an upwelling event in two contrasting areas of the Senegalese continental shelf. Here a cold upwelling-influenced inshore area was sharply separated by a strong thermal boundary from a deeper, warmer, stratified offshore area. Mean SSL thickness and SSL vertical depth increased with the shelf depth. The thickest and deepest SSLs were observed in the offshore part of the shelf. Hence, zooplankton and micronekton seem to occur more frequently in stratified water conditions rather than in fresh upwelled water. Diel vertical and horizontal migrations of SSLs were observed in the study area. Diel period and physicochemical water characteristics influenced SSL depth and SSL thickness. Although chlorophyll-a concentration insignificantly affected SSL characteristics, the peak of chlorophyll a was always located above or in the middle of the SSLs, regularly matching with the peak of SSL biomass. Such observations indicate trophic relationships, suggesting SSLs to be mainly composed of phytoplanktivorous zooplankton and micronekton. Despite local hypoxia, below 30 m depth, distribution patterns of SSLs indicate no vertical migration boundary. The results increase the understanding of the spatial organization of mid-trophic species and migration patterns of zooplankton and micronekton, and they will also improve dispersal models for organisms in upwelling regions.
The target strength (TS) patterns of Sardina pilchardus and Sardinella aurita at 38 and 120 kHz were estimated by a prolate-spheroid model, using measurements of swimbladder length and width. The ratio of swimbladder length to total length (TL) was similar in both species, however the ratio of swimbladder width to TL was smaller and more variable for S. aurita. Assuming a normal distribution of fish swimming orientation angle (θfish) with mean ± standard deviation (SD) of 0 ± 10°, the normalized (by TL) average TS (b20) was estimated to be -64.0 dB (38 kHz) and -65.2 dB (120 kHz) for S. pilchardus, and -66.2 dB (38 kHz) and -67.2 dB (120 kHz) for S. aurita. Compared with currently applied b20 values at 38 kHz, our results under four different θfish assumptions (0 ± 10°, 0 ± 15°, -5 ± 10°, and -5 ± 15°) were 6-9 dB higher for S. pilchardus and 5-7 dB higher for S. aurita. This suggests four-to eightfold overestimation risk for S. pilchardus and three-to fivefold overestimation risk for S. aurita when using the currently applied b20 values.Keywords Sardina pilchardus·Sardinella aurita·Target strength·Theoretical approach·Prolate spheroid model ✉Tadanori Fujino
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