Cadrin, S. X., Bernreuther, M., Daníelsdóttir, A. K., Hjörleifsson, E., Johansen, T., Kerr, L., Kristinsson, K., Mariani, S., Nedreaas, K., Pampoulie, C., Planque, B., Reinert, J., Saborido-Rey, F., Sigurðsson, T., and Stransky, C. 2010. Population structure of beaked redfish, Sebastes mentella: evidence of divergence associated with different habitats. – ICES Journal of Marine Science, 67: 1617–1630. Throughout their range, Sebastes spp. are adapted to a diversity of ecological niches, with overlapping spatial distributions of different species that have little or no morphological differences. Divergence of behavioural groups into depth-defined adult habitats has led to reproductive isolation, adaptive radiation, and speciation in the genus Sebastes. Recent genetic research, supported by life-history information, indicates four biological stocks of Sebastes mentella in the Irminger Sea and adjacent waters: a western stock, a deep-pelagic stock, a shallow-pelagic stock, and an Iceland slope stock. Congruent differences in fatty acids and parasites suggest that these genetically distinct populations are adapted to disparate trophic habitats in pelagic waters (shallower and deeper than the deep-scattering layer) and in demersal habitats on the continental slope. Morphology of pelagic forms is also more streamlined than demersal forms. Although genetic differences and evidence for reproductive isolation are clear, these populations appear to share common nursery habitats on the Greenland shelf. We propose a redefinition of practical management units near the Irminger Sea based on geographic proxies for biological stocks and minimizing mixed-stock catches according to the spatial patterns of the recent fishery.

-Beaked redfish inhabits North Atlantic waters in the depth range 100-950 m, over the continental shelf, slope and the open ocean. Individuals can live demersal or pelagic, at various stages of their life cycle. The geographical distribution of the species extends to most of the Atlantic waters from Newfoundland and the Labrador basin in the west to the Barents Sea in the east. Monitoring beaked redfish is challenging because of the species wide geographical distribution and large scale migrations; deep distribution, which complicates trawling and hydroacoustic surveys; difficulties with tagging; and persistent difficulties in taxonomic identification. These challenges make it a particularly problematic species to observe with conventional research methods. We review these key challenges and provide recommendations for the coordinated observation of Sebastes mentella in the North Atlantic that would best contribute to the assessment and ecological research on this species.

Gastric evacuation of groups of juvenile (mean 63 mm total length, LT, 0.283 g dry mass, MD) sprat Sprattus sprattus feeding on brine shrimp Artemia sp. nauplii was studied at six temperatures (7.5, 10, 13, 16, 19.5 and 21.5 degrees C) in the laboratory. Gastric evacuation was best described with a general model: St=S0(1-B)-R(1-B)t)1-B(-1), with St=stomach content at time t, S0=stomach content at time 0, t=time , R, B=constants. The shape parameter was estimated as B=0.668. For comparison with other studies, an exponential model was fitted also to the data. The evacuation constant (R) of the general gastric evacuation model increased exponentially with temperature between 7.5 and 16 degrees C. The slope of the increase was reduced between 16 and 19.5 degrees C and a slight decrease was observed between 19 and 21.5 degrees C. Additionally, the effect of mean MD (range 0.286-1.025 g) was examined. A simple power function (R=R'MDC) described the influence of predator mass on exponential evacuation constant with C=0.503. The results of this investigation were integrated into a consumption model for the calculation of daily rations of S. sprattus: C24=0.0177e0.0775T/S0.668 MD0.503 (1-{1[1+e(-0.659)(T-23.989)](-1)})24/S0.668, with T=ambient temperature (degrees C) and /S0.668= mean of field stomach contents (g dry) individually raised to the power of 0.668.

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