Observations made in the scientific and popular literature suggest that the characteristics of both marine and terrestrial ecosystems are changing rapidly due to increasing global air and sea temperatures. Here, we examine the hypothesis that fish species with more 'southern' distributions are increasing in the northern North Sea over time. In order to do this, 2 important databases on fish abundance collected by trawl on research cruises are interrogated. When combined, the databases cover both the entire North Sea and the Scottish west coast and span a period of 80 yr (1925 to 2004). The data take the form of length-frequencies for all species caught (> 300 different species), while additional information (e.g. age, sex, weight and stage of sexual maturity) is available for the commercially important component (e.g. cod). The trawl data suggest that the North Sea is experiencing waves of immigration by exotic, southern species (e.g. red mullet, anchovy and pilchard). The purpose of this paper is to describe and document these changes.KEY WORDS: Anchovy · Sardine · Horse mackerel · Mackerel · Long-term · North Sea · Climate Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 284: [269][270][271][272][273][274][275][276][277][278] 2004 doliolids are connected to unusual incursions of oceanic water caused by changes in the North Atlantic Oscillation (Edwards et al. 1999).Higher than average sea temperatures are correlated with low recruitment of cod at the latitudinal limits of its range (Planque & Frédou 1999, O'Brien et al. 2000. Reid et al. (2001) related abrupt changes in the abundance and composition of the plankton and fish community ca. 1988 to recent increases in the North Sea horse mackerel fishery. Pronounced increases in tropical fish in the Bay of Biscay area have been noted (Quero et al. 1998) by scientists in France, while Swaby & Potts (1999) made the first British record of the sailfin dory Zenopsis conchifer, noting that the species is advancing northwards along the continental shelf west of the British Isles at a rate of 60 km per decade. Other studies show similar patterns. Information on first records of southerly fish species caught in Cornish waters has been collated and published (Stebbing et al. 2002), and nearly 20 completely new species have been recorded (by 2001). The fish species noted include bigeyed tunny Thunnus obesus, sailfin dory Zenopsis conchifer, short-nosed seahorse Hippocampus hippocampus and barracuda Sphyraena sphyraena. In the Irish Sea, the occurrence of the warm-water species, anchovy Engraulis encrasicolus, has increased between 1990 and 1998 according to trawl data from research surveys (Armstrong et al. 1999). British commercial fishermen have also noted change. The spider-crab Maia squinado fishery, for example, is advancing steadily further northwards (Anonymous 2003).In 1996, a paper was published describing the longterm variation in the abundance of southern species in the southern North Sea (Corten & van...

A series of baited camera and trap experiments in the eastern Mediterranean Sea between 1500 and 4264 m depth attracted a variety of opportunistic scavengers, with species composition changing with increasing depth. At the shallower stations (1500 to 1800 m), decapod crustaceans and fishes, dominated by elasmobranchs such as Hexanchus griseus, were attracted to and actively consumed the bait. Some of these species were observed at depths exceeding their previously reported ranges. This was believed to be a result of the absence of deep-water scavengers from the adjacent Atlantic due to dispersal barriers and elevated temperatures at depth. The diversity of bait-attending fauna declined with increasing depth. Elasmobranchs were not observed below 2500 m, and below 4000 m only the caridean shrimp Acanthephyra eximia and the macrourid Chalinura mediterranea were present; at this latter depth, bait consumption was negligible. This shift in species composition was reflected in changes in first arrival times. Increasing first arrival times of H. griseus suggested a decline in relative abundance from 1500 to 2500 m, whilst those of C. mediterranea indicated an increase in relative abundance from 1800 to 4264 m.

The fate of cetacean carcasses in the deep sea was investigated using autonomous deep-sea lander vehicles incorporating time-lapse camera systems, ¢sh and amphipod traps. Three lander deployments placed cetacean carcasses at depths of 4000^4800 m in the northeast Atlantic for periods of 36 h, 152 h and 276 h before being recovered. The photographic sequences revealed that carcasses were rapidly consumed by ¢sh and invertebrate scavengers with removal rates ranging from 0.05^0.4 kg h 71. In the longest experiment the carcass was skeletonized within ¢ve days. In each deployment, approximately an hour after emplacement, the grenadier Coryphaenoides (Nematonurus) armatus and large numbers of lysianassid amphipods had arrived at the food-fall. The initially high numbers of grenadiers declined once the majority of the bait had been consumed and a variety of other ¢sh and invertebrates were then observed, some taking up residence at the site. None of the ¢sh species appeared to consume the carcass directly, but preyed upon amphipods instead. Funnel traps recovered with the carcass indicated a succession in the species composition of amphipods, with the specialist necrophages such as Paralicella spp. being replaced by more generalist feeders of the Orchomene species complex.

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The oceanic abyss (depths greater than 3000 m), one of the largest environments on the planet, is characterized by absence of solar light, high pressures and remoteness from surface food supply necessitating special molecular, physiological, behavioural and ecological adaptations of organisms that live there. Sampling by trawl, baited hooks and cameras we show that the Chondrichthyes (sharks, rays and chimaeras) are absent from, or very rare in this region. Analysis of a global data set shows a trend of rapid disappearance of chondrichthyan species with depth when compared with bony fishes. Sharks, apparently well adapted to life at high pressures are conspicuous on slopes down to 2000 m including scavenging at food falls such as dead whales. We propose that they are excluded from the abyss by high-energy demand, including an oil-rich liver for buoyancy, which cannot be sustained in extreme oligotrophic conditions. Sharks are apparently confined to ca 30% of the total ocean and distribution of many species is fragmented around sea mounts, ocean ridges and ocean margins. All populations are therefore within reach of human fisheries, and there is no hidden reserve of chondrichthyan biomass or biodiversity in the deep sea. Sharks may be more vulnerable to over-exploitation than previously thought.