This work presents results from analytical as well as ecotoxicologic investigations of sediment and water samples from the North and Baltic Seas. A bioassay-directed procedure was used to investigate cause-effect relationships between observed effects in acute laboratory bioassays (luminescent bacteria assay with Vibrio fischeri and embryo test with Danio rerio) and analyte concentrations in extracted samples. Brominated phenols and indoles-including 4-bromophenol; 2,4-dibromophenol; 4- and 6-bromoindole; 3,4-, 4,6-, and 3,6-dibromoindole; and tribrominated compounds-were identified in partly remarkable concentrations (up to 40,000 ng g(-1) total organic carbon TOC for 4-bromophenol) in North Sea sediment samples and water samples (913 ng L(-1) 3,6-dibromoindole) from the German Bight. The toxicity of some of the identified brominated substances was low, with median effect concentration levels (EC(50)) ranging from 0.08 to 21.7 mg/L for V. fischeri and 4.3 to 46.3 mg/L for D. rerio. Comparison of the concentrations of analytes with ECs showed a toxicity contribution of brominated phenols and indoles to overall toxicity of the fraction. In the case of one water sample from the German Bight, brominated phenols and indoles accounted for the observed toxicity. Brominated phenols and indoles, which are assumed to be of biogenic origin, have rarely been discussed so far in the context of ecotoxicologic effects in marine ecosystems.
The European eel Anguilla anguilla has shown decreased recruitment in recent decades. Despite increasing efforts to establish species recovery measures, it is unclear if the decline was caused by reduced numbers of reproductive-stage silver eels reaching the spawning area, low early larval survival, or increased larval mortality during migration to recruitment areas. To determine if larval abundances in the spawning area significantly changed over the past three decades, a plankton trawl sampling survey for anguillid leptocephali was conducted in March and April 2011 in the spawning area of the European eel that was designed to directly compare to collections made in the same way in 1983 and 1985. The catch rates of most anguilliform leptocephali were lower in 2011, possibly because of the slightly smaller plankton trawl used, but the relative abundances of European eel and American eel, Anguilla rostrata, leptocephali were much lower in 2011 than in 1983 and 1985 when compared to catches of other common leptocephali. The leptocephali assemblage was the same in 2011 as in previous years, but small larvae of mesopelagic snipe eels, Nemichthys scolopaceus, which spawn sympatrically with anguillid eels, were less abundant. Temperature fronts in the spawning area were also poorly defined compared to previous years. Although the causes for low anguillid larval abundances in 2011 are unclear, the fact that there are presently fewer European and American eel larvae in the spawning area than during previous time periods indicates that decreased larval abundance and lower eventual recruitment begin within the spawning area.
Leptocephali of several elopomorph families can reach sizes of 100-300 mm or larger, but it is questionable if these large eel or notacanth larvae are effectively sampled by small-mesh sampling gear in the open ocean because of net avoidance or insufficient fishing effort. A sampling survey in the Sargasso Sea using both an Isaacs-Kidd Midwater Trawl (IKMT) with fine mesh and a large pelagic trawl with large mesh sizes found that fewer species and individual large leptocephali >100 mm were collected by 25 IKMT tows compared to 4 trawl deployments. Net avoidance of the IKMT by the larger leptocephali appeared to occur at least during the day, and the large trawl did not catch any small leptocephali because of the large mesh size. A combination of net avoidance of the IKMT and greater sampling effort of the much larger mouth-opening trawl seems to have resulted in more large leptocephali being collected by the trawl. This indicates that IKMT surveys undersample large leptocephali species and that large trawls do not sample the whole assemblage of leptocephali. To fully understand the biodiversity and abundance of leptocephali in the world's oceans, large fine-mesh sampling gear like the IKMT and very large trawls with smaller mesh will likely be needed. This may be important because leptocephali are probably more abundant in the open ocean than is realized, and their role in the ocean surface layer communities and carbon cycle is not understood.
The availability of site-specific data on the EROD baseline level, its random variation and its annual cycle is a necessary prerequisite for monitoring. If monitoring is to be carried out only for a limited time period of the year, a season with low background variability in EROD activity (autumn) should be chosen to avoid the need for a compensation of the temperature-triggered shift in sexual cycles and the resulting changes in EROD activity.
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