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- Abundance and population structure of Plesionika narval (Fabricius, 1787) in the Northeastern AtlanticPublication . Sousa, R.; Pinho, M. R.; Delgado, J.; Pinto, A. R.; Biscoito, M.; Dellinger, T.; Henriques, P.Analysis of abundance and population structure of Plesionika narval was performed on data concerning 5,255 specimens obtained from 62 fishing sets carried out off the Madeira archipelago (Northeastern Atlantic) between 2004 and 2008 in a depth range from 101 to 350 m. Abundance ranged from 0.01 to 19.74 specimens-per-trap and significant differences were found between seasons, probably as a result of an increment of population in the spring during the recruitment season. The analysis of size distribution revealed that the carapace length (CL) ranged from 2.45 to 28.61 mm and that mean female size consistently exceeded that of males. Differences in mean CL were statistically significant between depth strata and seasons. Of the specimens sampled, 57.00% were males, 41.88% females and 1.42% undetermined. Sex ratio also differed significantly between seasons according to depth strata, consolidating the hypothesis of the existence of seasonal migrations related with the reproductive cycle of this species. Ovigerous females showed larger sizes and occurred all year around and remain in shallow waters in winter, summer and autumn and move to deeper waters in spring. The highest frequency of ovigerous females was recorded in summer, between 151 and 200 m deep supporting the hypothesis that spawning of this species occurs in shallow waters, especially in late summer.
- Estimating diet composition from scat analysis in otariid seals (Otariidae): is it reliable?Publication . Dellinger, T.; Trillmich, F.Analysis of teleost sagittal otoliths contained in scats has been widely used to determine the diet of seals. This method is based on the assumption that relative frequencies of otoliths in scats faithfully reflect those of fish in the diet. This assumption has rarely been tested experimentally. We compared the ratios of herring (Clupea harengus) to sprat (Sprattus sprattus) oto liths in faeces (output) of captive California sea lions (Zalophus calijornianus) and South American fur seals (Arctocephalus australis) with the ratios at feeding (input). Sea lions and fur seals showed no consistent differences in recovery rates and partial digestion of otoliths. Output ratios deviated only slightly from input ratios, the smaller sprat otoliths being underrepre sented in the output by 8%. Only about 40% of the otoliths fed to the seals were found in the scats. For both species partial digestion of otoliths led to a 16% underestimation of fish length and a 35% underestimation of fish mass.
- Foraging behavior of juvenile loggerhead sea turtles in the open ocean: from Lévy exploration to area-restricted searchPublication . Freitas, C.; Caldeira, R.; Reis, J.; Dellinger, T.Most sea turtle species spend part of, or their entire juvenile stage in pelagic habitats. A key question is how pelagic turtles exploit their environment to optimize prey intake and max imize fitness. This study combined animal telemetry with remote-sensed environmental data to quantify the drivers and patterns of foraging behavior of juvenile loggerhead sea turtles in the pelagic eastern North Atlantic. Juveniles ranged in size from 34 to 58 cm straight carapace length. First-passage time (FPT) analysis, used to quantify search effort, indicated that turtles performed area-restricted searches at nested spatial scales of 10 and 50 to 200 km. High-usage areas, as quantified by FPT, were associated with increased dive activity and weak surface currents, as well as with oceanographic features (high chlorophyll a and shallower bathymetry) thought to stimu late prey availability. Conversely, low-usage areas (i.e. transit areas) were associated with deep, probably exploratory dives, typical from Lévy movement patterns. Further interpretation of dive data indicates greater dive activity in shallow depths (0 to 10 m) during the night and during tran sit. Conversely, greater activity at intermediate depths (10 to 50 m) was observed during daytime, under strong lunar illumination and in high-usage areas, suggesting these depths are major day time foraging layers. This study clarifies the foraging ecology of sea turtles during their develop ment phase in the open sea, providing evidence that these pelagic predators can adjust their for aging strategies and effort in response to the local conditions of their dynamic environment.