Predation of Northern Pike (Esox lucius L.) Eggs: A Possible Cause of Regionally Poor Recruitment in the Baltic Sea

Northern pike (Esox lucius) spawning habitat and egg mortality were studied in three spawning areas in 2001 along the Swedish coast of the Baltic proper: the Blekinge Archipelago, Kalmar Sound and coastal freshwater streams. Spawning peaked during the last week of April in streams, at temperatures ranging from 7.7 to 8.9 °C and during the first week of May in brackish waters, at temperatures ranging from 8.9 to 13.8 °C. Spawning occurred in shallow waters, at depths between 0.2–1.5 m, but generally most of the spawn was found in the shallowest areas. In streams, eggs were mainly attached to emersed vegetation, while in brackish sites pike eggs were well scattered among flooded emersed plants, submersed plants and filamentous algae. Mean egg density varied between 469–1829 m^–2 with the lowest density observed in Kalmar Sound. The calculated egg loss occurring from approximately one day after spawning to one day before hatching ranged from 41±7% in coastal streams to 67±6% in the Blekinge Archipelago and 100% in Kalmar Sound. The significant removal of eggs from spawning sites in Kalmar Sound and Blekinge was most likely due to predation from several fish species. In situ observations and stomach analyses suggested that many pike eggs in Kalmar Sound were lost to the three-spined stickleback (Gasterosteus aculeatus), a species that presently dominate the littoral small fish community. This study therefore suggests that egg predation by sticklebacks and other fishes may be a possible cause of the reported poor recruitment of coastal pike populations in the Kalmar Sound region.     

Population trends and reproductive success of the European shag Phalacrocorax aristotelis on the Iberian Peninsula following the Prestige oil spill

In 2003, immediately following the Prestige oil spill in Galicia, Spain, we studied the population trends and reproductive performance of European shags (Phalacrocorax aristotelis) at oiled and unoiled colonies. This bird is an important member of the nearshore marine community, breeding in the area affected by the Prestige oil spill. The European shag feeds around the breeding colonies throughout the year, making it a useful indicator of environmental change. Before the oil spill, population trends were similar between oiled and unoiled colonies. Nevertheless, colonies located within the path of the oil suffered greater declines (ca. 10%) compared with pre-spill trends and with population trends at unoiled colonies. In 2003, the breeding success was 50% lower in oiled colonies compared with unoiled colonies. The data available from pre-spill years suggest that the annual reproductive success did not differ among colonies before the impact. European shags breeding at colonies affected by oil showed a negative initial impact from the Prestige oil spill. The reduction in reproductive success at oiled colonies may be due to sub-lethal effects of oil exposure or low food availability after the oil spill.     

Communal Spawning in the Common Frog Rana temporaria– Egg Temperature and Predation Consequences

Females of the common frog, Rana temporaria, spawn synchronously at communal breeding sites, forming a communal egg mass of individual spawn clumps. A total of four spawning sites were studied. In the centre of the communal egg mass, daily maximum temperature was higher and minimum temperature lower than further out. Spawn clumps in the centre of the communal egg mass developed faster than single spawn clumps. In the centre of the egg masses, some spawn clumps suffered almost total egg mortality. This was probably the consequence of a cold spell with sub‐zero temperatures in combination with a tendency for low minimum temperatures in the centre. In a laboratory study, worm leeches, Erpobdella spp., fed readily on eggs of common frog, while horse leeches, Haemopis sanguisuga, hardly ate any eggs. At the spawning sites, most worm leeches were found in single spawn clumps, far from the communal egg mass. Females which spawned late, when most of the spawning was over, preferred to spawn in the centre of the communal egg mass. This suggests that there is a net advantage for eggs in spawn clumps laid in the centre.     

Flavobacterium psychrophilum in Baltic salmon Salmo salar brood fish and their offspring.

Baltic salmon brood fish were investigated for the presence of Flavobacterium psychrophilum in the kidney, spleen, brain and sexual products (ovarian fluid, unfertilised eggs and milt). Samples for bacteriology were taken at capture, when the fish were ascending their native river to spawn, and after a period of captivity in indoor pools, at stripping. During captivity, abnormal wiggling behaviour was recorded in some of the fish. Bacterial samples were taken to determine if F. psychrophilum had any role in the aetiology of the condition. Furthermore, the presence of F. psychrophilum on egg surfaces during incubation was investigated. F. psychrophilum was isolated from internal organs and/or sexual products in 7 out of 50 (14.0%) fish sampled at capture and 63 out of 272 (23.2%) fish sampled at stripping. The bacteria was isolated from either spleen or gonads in 2 out of 19 (10.5%) fish with abnormal wiggling behaviour but no bacteria was isolated from the brain. No F. psychrophilum was isolated from eggs at the eyed stage. Just before hatching, the bacterium was isolated from 5 out of 15 (33.3%) family groups. The present study shows that Baltic salmon brood fish are carriers of F. psychrophilum during their spawning migration. The presence of the bacteria in sexual products from both females and males indicates that transmission from the brood fish to the offspring should be considered an important route of infection.     

The deposition and mortality of the eggs of the Baltic herring Clupea harengus membras L., on different substrates in the south-west archipelago of Finland

The environmental factors affecting the deposition and mortality of the eggs of the spring-spawning Baltic herring were studied in the inner Archipelago Sea of south-western Finland. On four spawning grounds, 27 study squares (area 1 m²) were surveyed by divers. In each square, one quantitative egg sample was taken and the following data were recorded: depth, temperature, bottom quality of the square and under the eggs, egg substrate, cover (%) of plants and/or the bivalve Mytilus edulis , and the heterogeneity of the environment, expressed as the total number of all bottom materials, plants and Mytilus found in the square.
Eggs were found in the depth zone of 1–4 m. Their density ranged from 6000 to 2·3 millions of eggs per square metre. Egg number had a significant negative correlation with both depth (r=-0·545, P <0·01, d.f. = 25) and temperature of the sea water ( r =-0.479,. P <0·01,d.f. = 25), and a positive correlation with the total cover (%) in the squares ( r = 0·375, P < 0·05, d.f. = 25).
The highest numbers of eggs were found on Cladophora glomerata, which the most preferred spawning substrate of those present. Mortality of the eggs varied according to the substrate. In eggs attached to Cladophora, Potamogeton and Mytilus, the proportion of dead eggs was 0.15·5%; in those attached to red algae it was significantly higher ( Furcellaria , 5·0–63·2%; Phyllophora , 0–95·2%).     

Decline in condition of gorgonian octocorals on mesophotic reefs in the northern Gulf of Mexico: before and after the Deepwater Horizon oil spill

Hard-bottom ‘mesophotic’ reefs along the ‘40-fathom’ (73 m) shelf edge in the northern Gulf of Mexico were investigated for potential effects of the Deepwater Horizon (DWH) oil spill from the Macondo well in April 2010. Alabama Alps Reef, Roughtongue Reef, and Yellowtail Reef were near the well, situated 60–88 m below floating oil discharged during the DWH spill for several weeks and subject to dispersant applications. In contrast, Coral Trees Reef and Madison Swanson South Reef were far from the DWH spill site and below the slick for less than a week or not at all, respectively. The reefs were surveyed by ROV in 2010, 2011, and 2014 and compared to similar surveys conducted one and two decades earlier. Large gorgonian octocorals were present at all sites in moderate abundance including Swiftia exserta, Hypnogorgia pendula, Thesea spp., and Placogorgia spp. The gorgonians were assessed for health and condition in a before-after-control-impact (BACI) research design using still images captured from ROV video transects. Injury was modeled as a categorical response to proximity and time using logistic regression. Condition of gorgonians at sites near Macondo well declined significantly post-spill. Before the spill, injury was observed for 4–9 % of large gorgonians. After the spill, injury was observed in 38–50 % of large gorgonians. Odds of injury for sites near Macondo were 10.8 times higher post-spill, but unchanged at far sites. The majority of marked injured colonies in 2011 declined further in condition by 2014. Marked healthy colonies generally remained healthy. Background stresses to corals, including fishing activity, fishing debris, and coral predation, were noted during surveys, but do not appear to account for the decline in condition at study sites near Macondo well.     

Potent phototoxicity of marine bunker oil to translucent herring embryos after prolonged weathering

Pacific herring embryos (Clupea pallasi) spawned three months following the Cosco Busan bunker oil spill in San Francisco Bay showed high rates of late embryonic mortality in the intertidal zone at oiled sites. Dead embryos developed to the hatching stage (e.g. fully pigmented eyes) before suffering extensive tissue deterioration. In contrast, embryos incubated subtidally at oiled sites showed evidence of sublethal oil exposure (petroleum-induced cardiac toxicity) with very low rates of mortality. These field findings suggested an enhancement of oil toxicity through an interaction between oil and another environmental stressor in the intertidal zone, such as higher levels of sunlight-derived ultraviolet (UV) radiation. We tested this hypothesis by exposing herring embryos to both trace levels of weathered Cosco Busan bunker oil and sunlight, with and without protection from UV radiation. Cosco Busan oil and UV co-exposure were both necessary and sufficient to induce an acutely lethal necrotic syndrome in hatching stage embryos that closely mimicked the condition of dead embryos sampled from oiled sites. Tissue levels of known phototoxic polycyclic aromatic compounds were too low to explain the observed degree of phototoxicity, indicating the presence of other unidentified or unmeasured phototoxic compounds derived from bunker oil. These findings provide a parsimonious explanation for the unexpectedly high losses of intertidal herring spawn following the Cosco Busan spill. The chemical composition and associated toxicity of bunker oils should be more thoroughly evaluated to better understand and anticipate the ecological impacts of vessel-derived spills associated with an expanding global transportation network.     

Female within-nest spawning-site preference in a paternal brooding blenny and its effect on the female mate choice

Female spawning-site preference within a nest and its effect on the female mate choice in a paternal brooding blenny Rhabdoblennius ellipes , were examined in rocky intertidal pools using artificial nests. The number of eggs deposited at the nest entrance site was lower than the number deposited at the center and at deep sites. Moreover, the within-nest difference in the egg developmental stage indicated that eggs at the nest entrance site were deposited after those at the center and deep sites. These results indicated that females prefer to spawn eggs in the center and deep sites rather than at the entrance site. Owing to the higher egg mortality rate at the entrance site, females may avoid spawning at such sites. An analysis of the nests in the study area suggested that the within-nest site preference affects female mate choice that is females avoided nests where only the entrance site was available for spawning and instead spawned in nests where the center and deep sites were available.     

溢油事故中渔业资源损失的数值模拟评估模式

海洋溢油污染不仅关系到天然渔业资源、海鸟等生物、海域环境、海岸线生态的破坏,而且对渔业、捕捞业、旅游业都会造成巨大损失,甚至会直接或间接地危害人类的健康。短期来看,一方面石油、燃料油等进入海洋后,对海洋生物资源造成影响;另一方面会危害附近海区的海洋环境,侵害海洋生物以及海鸟赖以生存和栖息的环境。长期来看,持续的海洋污染会导致的生态环境失衡,海洋的生产力也随之下降。溢油事故中的渔业资源损失评估作为追究污染事故责任,尽快恢复海域资源与环境的重要一环需要不断改进创新。为了定量确定海洋溢油事故发生后渔业资源的损失程度,将传统的评估模式与现代科学技术相结合,通过海洋动力学、流体力学、海洋生物学、环境化学等多个学科交叉融合,将流场风场模型、溢油模型、海域调查监测、卫星遥感技术、毒性效应和渔业资源的损失评估方法相结合,形成一种渔业资源损失评估的数值模拟评估模式,以完善溢油事故中渔业资源损失评估体系。在溢油事故现场监测数据的基础上,运用数学计算理论选择相对应的溢油模型,结合具体溢油事故案例的潮流数据和风场数据,模拟海上溢油污染的时空分布情况。采用卫星遥感技术根据不同油品在海水中不同的亮度表现,处理得到溢油油膜信息,与模拟得到的油膜信息进行比对验证,并对模型进行修正,通过模拟得出污染海域油浓度分布与溢油污染范围信息,结合溢油污染对不同海洋生物的毒性效应,得到渔业资源的损失程度,为溢油事故的渔业资源损失评估提供一种思路,为溢油事故发生后的损失评估和事故处理起到一定的参考辅助作用。     

Structure and Functioning of the “Egg Bank” of a Fairy Shrimp in a Temporary Pool: Chirocephalus ruffoi from Pollino National Park (Southern Italy) as a Case Study

To investigate their distribution and total numbers, resting eggs of the anostracan Chirocephalus ruffoi were collected from the bed of a temporary pool in southern Italy. Samples were taken at 0.5 m intervals along six transects oriented at 30° from each other, by means of a cylindrical core sampler. The horizontal distribution of intact resting eggs was extremely patchy, with cyst number per core ranging from 191 to 1,400 (CV = 32.7%), corresponding to a mean of between 0.8 and 4.3 cysts cm⁻³. Differences observed were related to core position and transect orientation, total cyst numbers being markedly higher in the leeward area of the pool compared to the windward area. Marked variation was also evident in vertical distribution, a significant, though weak correlation was recorded between egg density and sediment depth. Cyst‐bank size (± 95% confidence limits) of the pool bed, estimated from the mean cyst number cm⁻³ obtained for the 6 transects, ranged between 1.0 × 10⁸ and 1.3 × 10⁸ cysts. Hatching in the laboratory was very erratic. Despite significant differences in hatching, the observed variation was unrelated to most of the variables considered (position within sections, cores and transects, pre‐incubation treatment) and was explained only by initial sediment conditions (moist/dry). In none of the experimental conditions tested was synchronous hatching obtained. Possible causal factors (mixing of the bottom sediments by cattle, egg age, storage conditions, differential exposure to environmental cues as well as variability in hatching response even at clutch level) are discussed. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The ‘World Prodigy’ oil spill in Narragansett Bay,Rhode Island: acute effects on macrobenthic crustacean populations

On June 26,1989, the tanker World Prodigy ran aground just outside the mouth of the West Passage of Narragansett Bay, Rhode Island, USA. About 922 metric tons of No. 2 fuel oil were released into the water and drifted over a total area of about 120 sq miles. Three days after the spill only a small fraction of the oil remained.The effects on macrobenthic crustaceans within the first five weeks after the spill were studied at five stations with a varying degree of oil exposure, including one control site never reached by oil from the spill. Significant differencies between these stations were noted for total amphipod abundance, the amphipod genus Ampelisca and ostracods (retained on a 0.3 mm mesh), but not for amphipods of the genus Corophium. At the most heavily impacted station (23 µg oil g^–1 sediment dry weight), the total amphipod abundance, dominated by Ampelisca verrilli, decreased by 86% within the first two weeks after the spill. Decreases in total amphipod abundance significantly larger than at the control site were noted also at two other stations, one of which with only trace amounts of oil detected in the sediment. The amphipod populations at these sites were dominated by juvenile specimens.These findings confirm the extreme sensitivity to oil pollution of amphipods and ostracods, noted in earlier field and experimental studies.     

银鲫雌核发育的细胞学观察

在受精卵内,精子核呈凝缩状态,不形成雄性原核,没有看到两性原核融合。在刚产出的成熟卵子中没有看到极体,直到受精后10分钟(在水温19°—21℃)才有唯一的一个极体排出。在同一尾银鲫产出的同批卵子中,绝大多数具有成熟分裂中期的卵核,并在精子入卵后继续进行发育(雌核发育),接着排出唯一的一个极体;授精7分钟的少数卵子具有三极纺锤体状的核。根据现在的观察,双凤水库的银鲫是以雌核发育方式繁殖的种群。       

Variability in biochemical components of the mussel (Mytilus galloprovincialis) cultured after Prestige oil spill

The biochemical composition (proteins, carbohydrates, glycogen, total lipids and lipid classes) of the mussel Mytilus galloprovincialis was investigated during an experimental culture using mussel seed from areas with different degree of exposure to the Prestige oil spill. The aim of the study was to identify alterations in the biochemical composition of mussel seed from natural populations commonly used in Galicia for mussel raft culture that might be linked to previous oil exposure. We have selected three mussel seed populations from Pindo, Miranda and Redes, that were characterised in a previous study according to the oil exposure three months after the spill. These populations were transplanted to a raft culture system in the Ría de Ares-Betanzos where our experimental culture followed standard commercial techniques from March 2003 to February 2004. Mussels from Pindo (characterised as the most affected area by the oil spill) showed marked differences in lipid composition with regard to other populations in the content of triacylglycerols, (P<0.001), free fatty acids (P<0.001) and phospholipids (P<0.05) at the onset of the culture. Although these differences in lipid composition might reflect their previous exposition to hydrocarbons, this pattern did not last longer most likely due to depuration of hydrocarbons stored in the tissues or by the development of certain tolerance to PAHs. These significant differences were not detected between Miranda (designed as hardly affected area) and Redes (designed as reference area) which may reflect that Miranda mussels were not affected or only hardly affected by the spill. With the exception of the onset of the culture, biochemical composition showed similar patterns in all mussel populations. Then, the fact of being cultured in a common environment seemed to be more responsible for the long-term variability in the energetic reserve than the origin of the populations or their previous biochemical status.     

Oiling rates and condition indices of shorebirds on the northern Gulf of Mexico following the Deepwater Horizon oil spill

The coastline of the Gulf of Mexico in the United States is an important wintering and stopover region for migratory shorebirds. The Deepwater Horizon oil spill (April–August 2010) impacted more than 1700 km of this coastline and could potentially affect shorebirds through long‐term exposure to toxins, degraded habitats, and altered food chains. We investigated the exposure to Deepwater Horizon oil of seven species of shorebirds that winter or stopover along the northern Gulf of Mexico. From October 2010 to May 2012, we captured and banded 691 shorebirds at six sites that experienced varying levels of oil contamination. Of birds sampled, 22 were lightly oiled, with species that forage on the coast having higher rates of oiling than those that forage in more estuarine habitats. Although only 8.6% of birds captured from October 2010 to May 2011 and 0.6% of the birds captured from August 2011 to June 2012 showed signs of oiling, an unknown, but potentially larger, number of shorebirds were likely exposed to indirect effects of the spill, such as decreased foraging time due to oiling of sites or disturbance from cleanup activities. Fuel stores and fattening rates of Dunlins (Calidris alpina) during spring migration, as measured using plasma metabolites, were not influenced by site oiling level. However, the level of disturbance at study sites was a significant predictor of both fuel stores and glycerol levels, suggesting that Dunlins stopping over during spring migration may have had difficulty reaching necessary fuel stores in spring 2011 due to disturbance from cleanup activity on oiled beaches. These effects from disturbance were only observed at sites with high cleanup activity, suggesting that the impact of oil‐spill cleanup on shorebirds may be minimized by limiting cleanup activities to specific areas and times of day.     

Spatial aspects of reproduction in buffalo sculpin,Enophrys hison

Synopsis The spatial distribution and parental care of demersal egg masses of buffalo sculpin (Enophrys bison) were studied at 11 areas in Puget Sound, Washington, during 1972–1973. Adult buffalo sculpin inhabited rock and rock-sand habitats at depths from the intertidal to minus 20 m throughout the year, but 84% of all spawn clusters were found at depths between –1 m and +1 m within localized rocky areas. Areas used for spawning were exposed to strong tidal currents and other water movements. A single male fish guarded and fanned spawn, but eggs were temporarily deserted when less than about 1 m of water covered the spawning site. The cyclical nature of male desertion, behavior of spawn predators, and spawn mortality due to physical factors while emergent suggest several possible advantages to low intertidal spawning. These advantages include spatial and temporal escape from marine predators by embryos, time for the tending parent to forage, and optimal physical conditions for embryonic development.     

POTENTIAL IMPACT OF OIL SPILLS ON CALIFORNIA SEA OTTERS: IMPLICATIONS OF THE EXXON VALDEZ SPILL IN ALASKA

Based on the survival of sea otters held at rehabilitation centers during the 1989 Exxon Valdez oil spill in Alaska, we built two models of otter mortality. One was based on the relationship between mortality and distance from spill origin, the other was based on the relationship between mortality and time from the spill origin. These models are simplistic and are meant as first steps in arriving at realistic risk estimates and in providing a conceptual framework for relating oil spills and sea otter mortality. Using the distance model, we simulated the impact of an Exxon Valdez event occurring at different locations along the California coast. A spill at the Monterey Peninsula had the greatest impact, exposing 90% of the California sea otter population to oil and killing at least 50% of the individuals. The time model was used to predict the mortality of otters exposed to oil of various ages and for various periods of time. It suggested that efforts to rehabilitate otters should be discontinued 20-30 d after a spill. The limitations of the data available from the Exxon Valdez spill emphasize the importance of being prepared to conduct appropriate research during the next oil spill in sea otter habitat.     

The effect of extreme northerly climatic conditions on the life history of the minnow, Phoxinus phoxinus (L.)

Over its exceptionally wide geographical range the minnow displays a remarkable variability in its life history. In southern England many fish mature after one year and few fish survive to their third birthday. Very few fish spawned for more than one season but a 65-mm female is estimated to spawn up to 3172 eggs over the extended spawning season. In the River Utsjoki in Finnish Lapland maturity was strongly size-dependent and delayed until the fish reached 5, 6 or even 7 years of age, with a maximum age of 13 years at a length of only 75 mm. Unlike some other small-sized species in unproductive environments, individual females continued to spawn successive clutches of eggs though over a much shorter spawning season. The maximum estimated egg production was 824 for a 65-mm fish. Temperature had a strong direct or indirect effect on growth, with significantly lower growth increments in cold summers. At a second Arctic site in eastern Lapland growth rates were higher and maturity earlier, yet clutch size was greatly reduced with a seasonal fecundity estimate of only 314. There were only small differences in size of eggs or larvae between the populations. Evidence from other species indicates that most of the observed changes over the range of the minnow are phenotypic responses to the contrasting environments. However, the different strategies displayed at the two Arctic sites could represent the results of selection for differences in pre- and post-reproductive survival.     

Reproductive morphology, mating behavior, and spawning ecology of cephalaspid sea slugs (Aglajidae and Gastropteridae)

Abstract. Several recent experimental studies have assessed the behavioral reproductive ecology of four opisthobranch species belonging to the families Aglajidae and Gastropteridae (Cephalaspidea). In order to extend the scope of this earlier work, here we document the reproductive morphology and behavior of 36 taxa belonging to the same two sea slug families. Measurements focus on insemination patterns, spawning characteristics, as well as the size and shape of male and female reproductive organs (i.e., prostate, penis, bursa copulatrix, seminal receptacle). We found that behavioral and morphological traits vary considerably between species, even between closely related taxa. Mating behavior covers the full spectrum from unilateral to simultaneously reciprocal insemination and includes putatively antagonistic mechanisms such as hypodermic injection in some cases. With respect to morphology, particularly great variation was present in traits related to the male sexual function, such as prostate size and penis type. In traits associated with the female function, noticeable variation in those traits accessible for measurement was limited to the relative size of the sperm‐digesting bursa copulatrix. These data provide the basis for future comparative analyses on the evolution of reproductive characters in a phylogenetic context. We further explore the relationship between characteristics and body size. Our data corroborate previous findings that in species with exclusively planktotrophic development, investment per embryo (using mean egg diameter per species as a proxy) is independent of mean species body size. In contrast to egg size, spawn weight and the number of eggs per spawn tightly increased with body size, both within and among species. Hence, larger individuals among planktotrophs increased reproductive investment by increasing offspring quantity rather than investing more into each single offspring. Comparably large eggs with few eggs per spawn in our only two species with facultatively intracapsular metamorphosis are also consistent with findings in other opisthobranchs.     

DNA damage in eelpout (Zoarces viviparus) from Göteborg harbour

The relationship between DNA damage and the exposure of marine organisms to environmental contaminants was examined in the G?teborg harbour area. This research is part of a wider ecotoxicological study planned to evaluate the biological impact of chemical contamination in the River G?ta estuary, following a bunker oil (10-100 tonnes) spill occurred in June 2003. Here we present data on the DNA strand breaks derived using the comet assay and the presence of apoptotic cells using the diffusion assay in nucleated erythrocytes of the eelpout (Zoarces viviparus) from the study area and at a clean reference site. Polycyclic aromatic hydrocarbon metabolites were also analyzed in the bile of exposed fish. The results showed a high level of damaged DNA, paralleled by a peak in bile PAH metabolites, in fish from the most impacted site, 3 weeks after the oil spill. A significant recovery was observed in specimens from the spill site, 5 months later, but not in fish caught in the middle part of G?teborg harbour, which is chronically subjected to heavy chemical pollution. The levels of apoptic cells did not show any marked variations, but a significant recovery was observed in fish from the oil impacted site 5 months after the spill.     

Developmental defects in pelagic fish embryos from the western Baltic

In February/March 1983 and 1984 a survey of pelagic fish eggs was conducted in the western Baltic (Kiel Bight), employing a horizontally towed plankton net (1 m Ø and 300 μm mesh). Maximum egg numbers in the upper meter of the S=21×10^?3 salinity layer were 200·100 m^?3. The most abundant eggs were cod (up to 142 eggs·100 m^?3), followed by plaice (up to 74 eggs·100 m^?3) and flounder (20 eggs·100 m^?3). A considerable percentage of embryos of all species displayed aberrant development. In 1983 18% of cod, 22% of flounder and 24% of plaice eggs caught contained defective embryos; in 1984 this number was larger, ranging from 28% in plaice over 32% in cod to 44% in flounder. Early developmental stages showed the highest malformation rates (up to 51% in the case of early flounder embryos). With progressive development, malformations decreased in numbers, being lowest prior to hatching. Highest rates of malformations were recorded in the Mecklenburg Bight in 1983. A second area with high incidence of malformation rates was located south and east of the island of Langeland. Several reasons, including environmental and anthropogenic factors, for the occurrence of malformed embryos in pelagic fish eggs are discussed. The potential of malformation rates in embryos of pelagic fish eggs as a tool for monitoring is considered.