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Most types of fishery produce discards and offal in considerable quantities which are fed upon by seabirds. This paper demonstrates the importance to seabirds of fishery waste in the North Sea. The total amount of fishery waste in the North Sea region is estimated at 62800 t of offal, 262200 t of roundfish, 299300 t of flatfish, 15000 t of elasmobranchs and 149700 t of benthic invertebrates per year, representing 4 % of the total biomass of fish and 22% of the total landings This equals an energy value of about 3.4 X 10" kJ. Beam trawl f~sheries discharge discards at the highest rates of all fishing fleets. Their discard fraction is dominated by flatfish which are less favoured by seabirds because of their shape. In contrast, the amounts of discards from pelagic and gadid fisheries are less, but fish species and lengths are more appropriate as food for seabirds. The number of seabirds potentially supported by fishery waste in the North Sea is estimated to be roughly 5.9 million individuals in an average scavenger community (composition in proportion to the seasonal abundance of scavenging species). During expenmental discard studies, the proportions of fishery waste consumed by seabirds was calculated. We estimated that the mass of discards and offal consumed by birds during our study amounted to 55000 t of offal, 206000 t of roundfish, 38000 t of flatfish, 2000 t of elasmobranchs and 9000 t of benthic invertebrates.

This paper reviews the use and the effects of marks and devices on birds. Although most papers reporting research on birds make use of marks or devices, very few studies test for harmful effects or data biases caused by these. Many research projects have used marks and devices without encountering any harmful effects of these on the birds being studied. However, where assessments have been made it is clear that all of the methods of marking can have adverse effects, while most devices attached to birds do alter their behaviour to some extent. We conclude that more attention should be given to these effects before the normal assumption of no influence on the biological parameters being studied can be made with confidence. There is a need for careful tests of the effects of marking methods to be undertaken. We hope that this review will assist people studying birds to plan their use of marks and devices in such a way as to minimize problems caused by these research tools.

The detection of high concentrations of methylmercury in the sub-thermocline low oxygen seawater indicates a potential for enhanced bioaccumulation of mercury in such environments not yet explored. Here we present mercury concentrations in 8 fish species of low trophic level in relation to their vertical distribution. Fish species were selected to cover a wide range of vertical distributions, from epipelayic (<200 m) to mesopelagic (>300 m) environments in the sub-tropical mid-North Atlantic. Mean mercury concentrations in the study species ranged from 57 to 377 ppb dry \vt and were significantly and positively correlated with median daytime depth. Concentrations increased by 4-fold from epipelagic to mesopelagic species with no further increases with depth among mesopelagic species down to about 1200 m. Such enhanced mercury bioaccumulation in the marine mesopelagic compartment seems to be determined proximately by levels in food and ultimately by water chemistry that controls mercury speciation and uptake at the base of the food chain. We conclude that this is the best explanation for high and yet poorly understood mercury concentrations found in deep-sea predators.

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