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Notes: Abstract In winter 1965/66 larvae of Downsherring were reared in aquaria at the Marine Station of the Biologische Anstalt Helgoland. They were fed with wild plankton caught on Helgoland Roads. About 10% of the actively feeding larvae were lost due to two endo-parasites and two ecto-parasites. The endoparasites are: a nematode (Contracoecum sp.) which is taken in by the larvae with the food and enters the host's body cavity from the gut, and a cestode (Scolex pleuronectis) which is found in the posterior part of the gut. The ecto-parasites are: a copepodite stage of a lernaeocerid, which attaches itself preferably close to the anus area of the herring larvae, and the copepode Caligus rapax. The nematode and copepodite were observed when the larvae had reached a total length of 9 to 13 mm. The cestode was found for the first time in larvae of 15 to 18 mm, and Caligus rapax in larvae of 20 to 25 mm total length.

Notes: Abstract Using herring (Clupea harengus) eggs and larvae from “Baltic spring spawners”, the biological effects of sulfuric pollutants (largely FeSO4 and H2SO4) which are scheduled to be released daily into the North Sea in large quantities, have been tested. Dilutions (1:8000, 1:16000, 1:24000 and 1:32000) of the sulfuric pollutants were used as “test media”. Throughout all experiments water temperature was maintained at 8.0°±0.2°C, salinity near 16.5‰; eggs were artificially fertilized 1 or 6 to 7 h after catching the parent individuals and, attached to glass plates, incubated in 1 l aerated containers. In all test media brownish precipitates, which resulted from diluting the sulfuric pollutants with 50% sea water, tend to adhere to the egg surfaces, thereby possibly interfering with gaseous and other exchanges between egg and surrounding medium. Under conditions of maximum test medium effectiveness, percentages of successful fertilization and of egg survival are considerably reduced; diameter of fertilized eggs remains smaller; embryonic growth rate is retarded while the heart frequency tends to increase (indicating physiological stress); duration of incubation is shortened: percentage of successful hatching decreases and structural abnormalities of freshly hatched larvae increases. Dilutions down to 1:24000 and 1:32000 have rather limited effects. Exposure of 1 to 3 day old, healthy herring larvae to the 4 test media leads to failure of performing prey catching manoeuvres in 1:32000 and 1:24000, to impaired locomotory performances in 1:16000, and practically to paralysis, shrinkage, permanently bent bodies and death within a few days in 1:8000. Although the present study needs deepening through further and more detailed experiments, it can be said that the pollutants under consideration represent a danger to herring eggs and larvae at least up to a dilution of 1:32000.

Notes: Abstract The biological effects of dinitrophenol (DNP) were tested on Clupea harengus eggs of Baltic spring and autumn spawners. Experimental water temperature was maintained at 10.8°C±0.2C° (experiment 1) and 11.8°C±0.2C° (experiments 2 and 3); salinity ranged between 14 and 16‰. Eggs were artificially fertilized 1 to 4 h after catching the parent individuals; they were attached to glass plates and incubated in 1 l aerated containers. Thirty, 48 and 55 h after fertilization, eggs were transferred to various test concentrations of DNP (dilutions of 2,4- and 2,5-DNP used are listed in Table 1). 2,4-Dinitrophenol is known as a true uncoupling reagent, which inhibits the phosphorylation of ADP. The effects of DNP on embryo activity, heart rate, rates of body malformations and mortality are studied. Embryo activity rose above control values between the 6th and 7th days of the incubation period in low DNP concentrations (0.01 to 0.05 mM/l), and heart activity deviated after 5 to 7 days from that of the controls. Mortality rate depended on inhibitor concentration and time of poisoning (Table 4). Various malformations of embryos were observed (anophthalmy, symmetrical and unilateral microphthalmy, curled bodies, de-differentiations, Fig. 7).

Notes: Abstract Herring larvae were obtained via artificial spawning (Baltic spring spawners, Downs herring). Eggs were immediately transported to the Marine Station (“Meeresstation”) of the Biologische Anstalt Helgoland, transferred into 140] tanks, and incubated at about 10°C. Sea water was circulated through an internal filter. Artificial illumination (neon tubes) was kept at about 1000 Lux (water surface) during 12 h per day; it was than decreased gradually to complete darkness within 30 min. Dawn was also simulated in order to avoid abrupt changes in light intensity. Food consisted of wild plankton (mainly crustacean nauplii) caught every day on Helgoland Roads, and of Artemia salina nauplii. The larvae were fed 1 to 3 times a day; they took the food always within the first half hour after it was offered. Over periods of 5 min each, the time spent for various activities (different modes of swimming, feeding) were recorded. The behavioural patterns of comparable larvae were filmed. The initial phase of prey catching consists of s-shaped body bending; usually the main bend of the body (upper arrows in Figs. 2 and 3) bears a typical directional relationship to the swimming path of the prey focussed (lower arrows). Such body bending is not always succeeded by subsequent steps of prey catching. In the normal prey catching process, aiming is followed by sudden stretching of the body and swallowing of the prey within 0.2 to 0.3 sec. Yolk sac larvae can use their pectoral fins, larvae of more then 15 mm total length also their tail- and dorsal-fins, for stabilization and correction of prey catching movements. In yolk sac larvae, complete prey catching lasts about 1 to 3 sec. Percentage successful prey catching manoeuvres increases with age and experience (Table 2). Initial success percentage was about 1% in Baltic Sea larvae (Kiel) and about 10% in Downs larvae; it rose within 30 to 35 days in Kiel larvae to nearly 60%, in Downs larvae to over 70%. The possible reasons for these differences are discussed; they may be related to body size and composition of planktonic food. Visual perception of food depends on optic capacities of larvae, size and distance of prey, visibility, and “duration of presentation” (time span during which the image of the prey is projected onto the retina). This, in turn, appears to be subject to frequency and amplitude of undulating movements of the head during swimming. The percentage of body positioning for prey catching attains maximum values at prey distances of 2 to 8 mm in yolk sac larvae (Downs), and of 3 to 40 mm in larvae of 15 to 20 mm body length; it decreases steadily with increasing prey distance. Larvae up to 15 mm total length take mainly copepod nauplii, larger larvae preferably copepodites. Distance of prey perception is wider in the horizontal than in the vertical plane; in fact, larvae do not perceive prey underneath the horizontal plane.

Notes: Abstract Predation of different-sized Hyperoche medusarum (Hyperiida: Amphipoda) on larvae of the Pacific herring Clupea harengus pallasi was studied in the laboratory. The attacking rate of H. medusarum was a function of herring larvae size as well as size of the predator, and varied from 0.15 to 0.95 larvae attacked h-1 per hyperiid. In the range of 7.55 to 16.05 mm total larval length, vulnerability to predation was highest for 13.3 and 13.7 mm larvae. Large hyperiids swam faster and covered a wider area during searching and were more effective predators than small ones. Predation seemed to be influenced by light, and its intensity was dependent on the duration of previous food deprivation of the hyperiid.

Notes: Abstract In rearing experiments with herring eggs (temperature=14.0°±0.1°C; salinity=15‰), oxygen consumption under normal conditions and after addition of 2,4-DNP (concentration=0.1 mM/l; pH=8.1) was measured over the period of embryonic development by means of the Wabburg-technique. Additionally, the concentration of low molecular sugars, polysaccharides, free amino-acids, and adenosintriphosphate (ATP) was determined. The oxygen consumption increases during embryonic development; this increase is not linear. Periods of high intensity of oxygen consumption are followed by others with only slight increase. Immediately before hatching, the respiration curve distincly declines (Fig. 1). Under the influence of 2,4-DNP (dinitrophenol), the embryos increase their respiration intensity after a short period of incubation. The maximum rise in percentage over the normal values reaches up to 400% at the beginning of gastrulation, falls to 50% even before the locking of the blastopore, and decreases slightly to about 30% until hatching. The immense decline in the percentage increase in respiration following the addition of 2,4-DNP at the end of the first day of development is caused by the rapid increase in normal respiration. After poisoning with DNP at different stages of development, the uncoupled respiration curves are normally almost equal. This holds both for the temporal position of the respiration maxima (about 12 to 24 h after poisoning), and for the absolute amounts of the increased respiration over the normal values (5 to 7 μl/h/100 embryos). Excluded from these regularly repeated findings are two stages of development: (1) the stage of epiboly after exceeding the yolk equator until shortly before locking of the blastopore (26 to 32 h after fertilization at 14°C); (2) the period at the end of the 4th day of development when the eyes become pigmented (100 to 120 h after fertilization). These two stages are characterized by the fact that, at the moment of poisoning, the normal respiration shows retarded activity. On the other hand, these two stages are well able to undergo periods of development in which long-living embryonic deformations can occur after uncoupling of respiration with 2,4-DNP. The content in low molecular sugars and polysaccharides decreases slightly in the course of embryonic development and, following the addition of 2,4-DNP, decreases considerably during the first 24 h. After 48 h, accelerated decomposition of carbohydrates continues. Under the influence of 2,4-DNP, the embryos metabolize more carbohydrates in 1 day than during the whole normal development period. The changeover of the metabolism to increased decomposition of carbohydrates can be explained as a dislocation of the energetic sources from the respiration chain to glycolytic phosphorylation. In accordance with these facts, the concentration of free amino-acids, almost equal during normal embryonic development, remains unchanged under the influence of 2,4-DNP.

Notes: Abstract Herring larvae were obtained through artificial spawning of Downsherring caught in the southern North Sea near Ijmuiden (Netherlands) on November 18/19, 1965. The eggs were transported to the Marine Station of the Biologische Anstalt Helgoland and transferred into 140 l tanks (Blaxter and Hempel, 1961) using a closed water circulation (inside filter; Fig. 1). All tanks were placed in a dark room at a constant temperature of 10±0.1 °C. Artificial light was kept at a level of about 1.500 lux at the water surface of the tanks for 12 h per day. Dawn was simulated over a period of 30 min in order to avoid abrupt changes in light conditions. The first observations on schooling behaviour were made about 60 days after hatching: at this time the larvae had reached a total length of about 25 to 30 mm. Grouping began with “contact behaviour” of two single individuals: these swam together in the same direction for a matter of seconds only. Groupings of two individuals occurred simultaneously at different places in the same aquarium. Sometimes the larvae formed greater aggregations involving up to 20 individuals. The distances between the larvae were not constant and varied considerably. Shortly after the formation of a school, the initial groups could be distinguished (Fig. 3). Starved larvae tended more towards schooling than well-fed individuals. If food were offered, schooling behaviour ended abruptly.

Notes: Abstract The garpike Belone belone enters the Wadden Sea in April, to spawn in May and June. The large eggs (3mm) bear numerous, long hair-like filaments. Embryonic and larval development were investigated during rearing experiments in 1970 and 1971. The development of the embryo is described, with special reference to the circulatory system and “Kupffer's vesicles”. The embryos display ventilation with pectoral fins and gill opercula when still in the egg. They hatch after 2 or 3 weeks at 20° and 16°C, respectively. Newly hatched larvae accept a wide variety of prey, including dry, aquarium fish food. Growth, feeding behaviour, swimming performance, and survival of the juveniles were investigated in the laboratory and in a small outdoor pond. Young garpikes (1 to 3 cm standard length) survive at temperatures ranging from 13° to 25°C, and salinities from 7 to 50%. They may reach 12 to 15 cm in their first summer. Their cruising speed is estimated to be 1 to 2 body lengths/sec. Garpikes disappear from The Wadden Sea in October, and probably migrate offshore. Observations on the behaviour of adults (40 to 70 cm total length) in a large indoor tank, indicate that they avoid high light intensities in winter. Adults display panic reactions when the water temperature drops below 6° to 7°C; this indicates that garpikes probably migrate in winter to greater depth (lower light intensity), to avoid water temperatures below 6°C and rough weather conditions in the upper water layers.