Trisopterus

Abstract: Application of digestion correction factors to measurements and counts of fish otoliths and cephalopod beaks recovered from seal scats is required before the size or quantity of prey consumed can be accurately estimated. We carried out 86 feeding trials with seven grey seals (Halichoerus grypus) and 18 prey species to derive estimates of digestion coefficients (to account for partial digestion), recovery rates (to account for complete digestion), and passage rates (to estimate the time between consumption and excretion of an item). Mean digestion coefficients were greatest for sandeel (Ammodytes marinus) and then less for large gadoid, flatfish, and Trisopterus spp. otoliths; and finally squid (Loligo forbesii) beaks. Recovery rates were greatest for squid beaks and then less for large gadoid, Trisopterus spp., flatfish, and sandeel otoliths. Greater than 95% of otoliths and beaks recovered were passed within 4 days (~88 h) of consumption. The large differences in partial and complete digestion rates foun...

Abstract: We applied PCR–RFLP (Polymerase Chain Reaction – Restriction Fragment Length Polymorphism) analysis to identify seven gadoid species of different biogeographical origin and commercial relevance, namely Gadus morhua (Atlantic ocean); Trisopterus minutus capelanus, Trisopterus minutus minutus, Molva elongata, Phycis blennoides, Micromesistius poutassou (Atlantic ocean and Mediterranean sea); Theragra chalcogramma (Pacific ocean). Two DNA fragments belonging to mitochondrial 12S and 16S rRNA genes of about 430 and 630 bp, respectively, were isolated by PCR amplification. Their direct sequencing showed a significant genotypic diversity among gadoid species, useful for species identification. Digestion of 16S rRNA gene PCR fragment with MvaI or Bsh1285I restriction enzymes, followed by agarose gel electrophoresis of the cleaved products, yielded specific restriction profiles that enabled direct, visual identification of the species analyzed. This PCR–RFLP method allowed a clear and rapid discrimination of the gadoid species studied.

Abstract: The spatial and temporal structure of predator-prey relationships in the Celtic Sea was investigated for 4 commercially important predator species (cod Gadus morhua, hake Merluccius merluccius, megrim Lepidorhombus whiffiagonis, and whiting Merlangius merlangus) using stomach- content and bottom-trawl survey data for the period 1982 to 1995. Blue whiting Micromesistius poutassou were consumed more often during the summer months, whereas mackerel Scomber scombrus and Trisopterus spp. (poor cod T. minutus, Norway pout T. esmarkii, and bib T. luscus) were found more often in predator stomachs during the winter half-year. On a spatial scale, blue whiting was consumed over the shelf edge, in accordance with their higher densities in the environ- ment, while mackerel, horse mackerel Trachurus trachurus and Trisopterus spp. were eaten more often on the continental shelf, again in agreement with their depth-related density-distribution patterns. The spatial distribution of whiting closely matched that of a key prey, Trisopterus spp. This might indicate an 'aggregative response' by the predator. The results of this study suggest spatial and seasonal prey-switching behaviour by cod, hake and whiting. Overall, the Celtic Sea fish community is characterised by opportunistic predators and general independence between predator and prey distributions. Interspecific predator interactions are reduced by size-, space- and time-dependent feeding behaviours.