David R. Thompson

Migratory shearwaters integrate oceanic resources across the Pacific Ocean in an endless summer

Electronic tracking tags have revolutionized our understanding of broad-scale movements and habitat use of highly mobile marine animals, but a large gap in our knowledge still remains for a wide range of small species. Here, we report the extraordinary transequatorial postbreeding migrations of a small seabird, the sooty shearwater, obtained with miniature archival tags that log data for estimating position, dive depth, and ambient temperature. Tracks (262 ± 23 days) reveal that shearwaters fly across the entire Pacific Ocean in a figure-eight pattern while traveling 64,037 ± 9,779 km roundtrip, the longest animal migration ever recorded electronically. Each shearwater made a prolonged stopover in one of three discrete regions off Japan, Alaska, or California before returning to New Zealand through a relatively narrow corridor in the central Pacific Ocean. Transit rates as high as 910 ± 186 km·day−1 were recorded, and shearwaters accessed prey resources in both the Northern and Southern Hemisphere’s most productive waters from the surface to 68.2 m depth. Our results indicate that sooty shearwaters integrate oceanic resources throughout the Pacific Basin on a yearly scale. Sooty shearwater populations today are declining, and because they operate on a global scale, they may serve as an important indicator of climate change and ocean health.

Changes in fisheries discard rates and seabird communities.

It is clear that discards from commercial fisheries are a key food resource for many seabird species around the world. But predicting the response of seabird communities to changes in discard rates is problematic and requires historical data to elucidate the confounding effects of other, more ‘natural’ ecological processes. In the North Sea, declining stocks, changes in technical measures, changes in population structure and the establishment of a recovery programme for cod (Gadus morhua) will alter the amount of fish discarded. This region also supports internationally important populations of seabirds, some of which feed extensively, but facultatively, on discards, in particular on undersized haddock (Melanogrammus aeglefinus) and whiting (Merlangius merlangus). Here we use long-term data sets from the northern North Sea to show that there is a direct link between discard availability and discard use by a generalist predator and scavenger—the great skua (Stercorarius skua). Reduced rates of discarding, particularly when coupled with reduced availability of small shoaling pelagic fish such as sandeel (Ammodytes marinus), result in an increase in predation by great skuas on other birds. This switching of prey by a facultative scavenger presents a potentially serious threat to some seabird communities.

The chemical form of mercury stored in South Atlantic seabirds

Concentrations of total mercury and organic (methyl) mercury were measured in the liver tissue of adults of 12 seabird species collected at colonies on Gough Island, South Atlantic Ocean. Total mercury levels showed both great intra- and interspecies variation, ranging from a mean of 1343.0 microg g(-1) dry weight in wandering albatrosses to a mean of 0.8 microg g(-1) dry weight in broad-billed prions. Organic mercury levels were less variable both between, but especially within, species. Organic mercury levels, expressed as a percentage of total mercury levels, ranged from a mean of 2.6% in wandering albatrosses up to a mean of 92.6% in littee shearwaters. Within each species, individuals with the highest total mercury levels tended to have the lowest percentage organic mercury, this trend being statistically significant in several species. Two species exhibited a significant positive correlation between organic mercury levels, in absolute terms, and total mercury levels. When all 12 species were considered, a highly significant negative correlation between mean percentage organic mercury and mean total mercury was found (rs = -0.888, P < 0.001). These results provide evidence to suggest that some seabirds may be capable of demethylating organic mercury in a species-dependent and that eliminatory pathways for the excretion of dietary mercury may influence the mode of response of a particular species.

Mercury accumulation in great shuas Catharacta skua of known age and sex, and its effects upon breeding and survival

(1) Great skuas Catharacta skua Brunnich have among the highest tissue mercury concentrations of British seabirds, and many of the birds breeding on Foula, Shetland in 1988 and 1989 had concentrations well in excess of those known to affect reproduction adversely in some terrestrial bird species. Comparison of mercury concentrations in feathers of great skuas collected before 1900 and in 1988-89 indicates a significant increase during this century, which is assumed to be due to anthropogenic activities (Thompson 1989). (2) Mercury in muscle tissues of great skuas was entirely methyl mercury, whereas approximately half the mercury in liver and kidney tissue was inorganic. Feather mercury concentrations correlated with total mercury concentrations in soft tissues of birds culled during incubation. (3) Mercury concentrations in feathers of adult great skuas on Foula, Shetland were found to be higher than those in feathers of chicks, but were independent of adult age and sex. (4) There was no evidence of accumulation of inorganic mercury in soft tissues with age. This suggests that dietary variation and specialization are more important than age as determinants of mercury concentrations in this species. (5) There was no relationship between diet during the breeding season and feather mercury concentration, either comparing mercury concentrations in different years covering a period of major change in diet at the colony, or comparing different birds with markedly different diets in the same year. Great skuas disperse widely during the winter, and differences in exposure to mercury during this period may be more important than differences in diet during the breeding season. (6) Despite evidence of increased mercury pollution (Thompson 1989), there was no evidence of a relationship between the mercury concentrations of individuals and their breeding performance or survival.

Metal concentrations in seabirds of the New Zealand region

Concentrations of the heavy metals cadmium, copper, lead, zinc, mercury and, in some individuals, methyl mercury were determined in a range of tissues of 64 tropical, subtropical, subantarctic and antarctic seabird taxa mostly from the New Zealand region. Although apparently natural, levels of cadmium and mercury in some species greatly exceed those known to have toxic effects in some terrestrial birds. Copper and zinc levels exhibited less inter-species variation than the non-essential metals cadmium and mercury. Cadmium concentrations were highest in kidney tissues but uniformly low in feathers. Total mercury concentrations showed most inter-species variation. Mean methyl mercury levels in liver tissues of several large procellariiforms represented less than 5% of the corresponding mean total mercury level. Lead concentrations were generally low or below the limits of detection, but elevated levels were measured in some coastal or scavenging species. In a significant number of species, mean concentrations of liver cadmium and mercury and kidney cadmium were greater in adults than in young birds. The reverse was true for copper. Mean zinc levels in liver did not differ between adults and young. High levels of cadmium in some species seem likely to be due to diet, whereas high levels of mercury probably reflect more closely the moult intervals which constrain the ability of birds to eliminate methyl mercury.

Seabirds as biomonitors of mercury inputs to epipelagic and mesopelagic marine food chains

Mercury concentrations are presented for time-series of feather samples from seabirds feeding near the apex of epipelagic and mesopelagic marine food chains in the south-east and north-east North Atlantic. These quantify historical increases in surface and deep water mercury concentrations due to mercury pollution. Our results highlight the importance of mercury methylation in mesopelagic water as the factor responsible for the greatest increases in seabird mercury concentrations, which occur in seabirds feeding primarily on mesopelagic prey. Seabirds which feed on prey occupying the epipelagic zone exhibited more modest increases in mercury concentration than mesopelagic feeders, but the rates of increase in the epipelagic-feeding birds are in agreement with recent model predictions regarding anthropogenic inputs of mercury to the environment. We investigated the importance of mesopelagic prey in the diet of seabirds, as opposed to trophic status, in determining mercury concentrations. Analysis of stable isotope ratios of nitrogen revealed that trophic status was not the principal factor in determining mercury concentration and that the presence of mesopelagic prey in the diet was likely to contribute more to the patterns of mercury burdens.

Stable-isotope ratios of carbon and nitrogen in feathers indicate seasonal dietary shifts in Northern Fulmars

a stepwise enrichment of •SN at each trophic level (Minagawa and Wada 1984, Schoeninger and DeNiro 1984, Hobson and Welch 1992, Hobson 1993). Carbonisotope ratios give weak trophic information, but indicate relative contributions from differing sources (e.g. marine vs. terrestrial; biota from these two systems having distinct b•3C signatures based on differences in b•C of carbon incorporated at photosynthesis) to the diet of particular species (Fry et al. 1983, Dunton and Schell 1987, Hobson 1987,1990, Mizutani et aL 1990, Hobson and Sealy 1991). When combined, stable-isotope ratios of nitrogen and carbon can be used to discriminate between species on the basis of dietary input to a greater extent than that afforded by one ratio alone (Hobson 1991, 1993). However, most such studies to date have utilized internal tissues

Do Heavy Metal Concentrations Pose a Threat to Marine Turtles from the Mediterranean Sea

Abstract Concentrations of heavy metals (Hg, Cd and Pb) were determined in internal organs and nest contents of green turtles Chelonia mydas and loggerhead turtles Caretta caretta from northern Cyprus, eastern Mediterranean Sea. Concentrations of mercury in liver tissue were higher in loggerhead turtles (median 2.41 μg g−1 dry weight) than in green turtles (0.55 μg g−1 dry weight). Preliminary data suggest cadmium concentrations to be highest in kidney tissue of loggerhead turtles (median 30.50 μg g−1 dry weight) but in liver tissue of green turtles (median 5.89 μg g−1 dry weight). Concentrations of lead in internal tissues were often below analytical detection limits in both species, but when measurable, tended to be higher in loggerhead turtles. Concentrations of mercury and cadmium in nest contents from both species were low, often below analytical detection limits, while those of lead were relatively high in loggerhead turtle hatchlings (up to 10.56 μg g−1 dry weight). When measurable, concentrations of all three metals tended to be higher in loggerhead turtle nest contents than in green turtle nest contents. Results presented here are consistent with inter-specific differences in diet and trophic status. Heavy metal burdens in loggerhead turtles and green turtles from the Mediterranean are similar or lower than corresponding concentrations in turtles from Japan and Hawaii, but some lead concentrations in Mediterranean loggerhead hatchlings are at levels known to cause subclinical toxic effects in other vertebrates.

Feathers as a means of monitoring mercury in seabirds: Insights from stable isotope analysis.

Mercury concentrations, together with nitrogen and carbon stable isotope signatures, were determined in body feather samples from northern fulmars Fulmarus glacialis and great skuas Catharacta skua, and in different flight feathers from great skuas. There were no significant relationships between trophic status, as defined using isotope analysis, and mercury concentration in the same feather type, in either species. Mercury concentrations in body feather samples were markedly different between fulmars and skuas, reflecting differences in diet, but there was no corresponding difference in trophic status as measured through nitrogen stable isotope signatures. We conclude that mercury concentrations and stable isotope values in feathers are uncoupled, mercury concentrations apparently reflecting the body pool of accumulated mercury at the time of feather growth whilst stable isotope values reflect the diet at the time of feather growth. There were significant positive correlations between the different flight feathers of great skuas for all three parameters measured. These were strongest between primary 10 and secondary 8, suggesting that these two feathers are replaced at the same time in the moult sequence in great skuas. Stable isotope analysis of different feathers may provide a means of investigating moult patterns in birds.

Bioamplification of mercury in great skua Catharacta skua chicks: The influence of trophic status as determined by stable isotope signatures of blood and feathers

Abstract Biomagnification of mercury was investigated via combined mercury and stable isotope analysis of the blood and feathers of great skua chicks from two colonies in the north-east Atlantic. There were significant positive correlations between δ15N signatures and mercury concentrations in the blood from chicks at both colonies suggesting that dietary specialization influences intra-specific variability in mercury burdens. The relationships were slightly weaker in feathers and therefore blood is probably a better monitoring unit. Adult blood was also assessed and in terms of biomonitoring may provide an index of mercury intake over the whole breeding season.