Osmotic properties of eggs of the herring Clupea harengus

Yolk osmolality of developing eggs of the herring Clupea harengus L. is strongly hypoosmotic to seawater: about 440 mOsm during the first week of development decreasing to 360 mOsm before hatching. The perivitelline fluid (PVF) of the eggs is isoosmotic to the ambient medium. The PVF equilibrates within 10 min to changes in the ambient seawater. The content of Na⁺, K⁺, Cl^- and free amino acids amount to 26, 52, 48, and 54 n mol egg^-1, respectively, on Days 1 to 3 after fertilization, increasing to 63, 69, 80, and 79 n mol egg^-1, respectively, prior to hatching (Days 18–20). The apparently conflicting findings of a decreasing yolk osmolality and a simultaneous increase in the amount of egg solutes are at present unexplained.     

Factors influencing predation of Hyperoche medusarum (Hyperiida: Amphipoda) on larvae of the Pacific herring Clupea harengus pallasi

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.This study was sponsored by the International Bureau of the Gesellschaft für Kernenergiever-wertung in Schiffbau und Schiffahrt in connection with the German Canadian agreement on scientific and technical cooperation.     

High natural mortality of Baltic herring (Clupea harengus) eggs caused by algal exudates?

Results from field studies in 1978 and 1982 together with results from a laboratory study carried out in 1984 indicate that the unusually high natural mortality of Baltic herring (Clupea harengus L.) eggs observed in situ is presumably caused by toxic exudates released by filamentous brown algae during a limited period which happens to coincide with the peak of spawning. Oxygen deficiency is no longer believed to be the major cause of the observed unnaturally high mortality. It cannot, however, be entirely ruled out.     

Biological effects of oil on early development of the Baltic herring Clupea harengus membras

The effects of petroleum hydrocarbons from two crude oils and one fuel oil (No. 1) were studied on the ontogenic development of the Baltic herring Clupea harengus membras L. Herring eggs exposed to water-soluble fractions of the oils at the time of fertilization showed no decrease in numbers of fertilized eggs compared to eggs exposed 6 or 72 h after fertilization. During embryongenesis, treatment with 3.1 to 8.9 ppm or 3.3 to 11.9 ppm total oil hydrocarbons from light fuel oil and the two crude oils respectively, gave rise to alterations in embryonic activity, decreased heart rate, and premature or delayed hatching. Although many larvae hatched from eggs exposed to contaminated water (3.1 to 11.9 ppm total oil hydrocarbons), the majority of the (70 to 100%) were malformed or dead 1 day after hatching. Exposure of eggs to 5.4–5.8 ppm total oil hydrocarbons resulted in significantly (P<0.001) decreased lengths of the larvae. Increased temperature (from 9° to 14°C) aggravated the effects of the oils. The results are discussed in relation to the potential effects of oil spills and chronic oil pollution on fish eggs and larvae in the Baltic Sea.     

Light intensity and the feeding behaviour of herring,Clupea harengus

An infra-red sensitive video-recording technique was used to study the effects of darkness and light intensities from 0.0001 to 270 photopic lx on the feeding behaviour of herring (Clupea harengus L.). When offered natural zooplankton, consisting of a mixture ofCalanus finmarchicus, Euchaeta norvegica, Oithona similis, Balanus sp. nauplii, and crustacean nauplii as prey, the fish fed by biting (snapping) at light intensities above a threshold of 0.001 lx and were size-selective, taking the larger organisms first. When fed on pure cultures of CaliforniaArtemia sp. nauplii (San Francisco Bay brand), the threshold light intensity was 0.01 lx. Swimming speed increased with increasing light intensity when the fish were actively feeding by biting. When the fish were filter-feeding on high densities ofArtemia sp. nauplii in the light, they continued to school and swimming speed was not related to light intensity.     

Lipid class and fatty acid composition of brain lipids from Atlantic herring (Clupea harengus) at different stages of development

Little is known about the changes in composition of brain lipids and fatty acids at different stages of development in fish. Wild Atlantic herring (Clupea harengus L.) were collected from Loch Linnhe and the Firth of Clyde, Scotland, from August 1990 to March 1991. Lipid class and fatty acid compositions of brain lipids were studied at four different stages of development: larvae at the end of the yolk sac stage, two juvenile stages and sexually mature adults. The total lipid content in brains increased during development, and larval brains contained higher proportions of neutral lipids and lower proportions of polar lipids than the brains of juvenile or adult herring. Increased proportions of polar lipids in juvenile and adult herring brains were mainly due to increased percentages of phosphatidylcholine (PC), phosphatidylethanolamine (PE), cerebrosides and sulphatides. The increase in the proportions of the glycolipid classes suggested increasing levels of myelination with development. In total lipids, saturated fatty acids generally decreased and monounsaturated fatty acids and dimethyl acetals (derived from PE-plasmalogen) increased from larvae to adults. However, the proportions of polyunsaturated fatty acids in individual phosphoglycerides were generally highest in juvenile stages, due mainly to increased 22:6n-3, and were lowest in adult fish. Relatively high percentages of 24:1 isomers were found in all the phosphoglycerides, but primarily PC, and these increased during development from larvae to adult. Fatty acids were distributed between individual phosphoglycerides with a characteristic pattern that did not change with development, although the relative amounts of individual fatty acids were altered. The variations and roles of the different lipid components of herring brain are discussed with respect to lipid compositions and functions in brains of other fishes and vertebrates.     

Cercopagis pengoi and Mysis spp. alter their feeding rate and prey selection under predation risk of herring (Clupea harengus membras)

The anti-predator behaviour of Baltic crustacean planktivores was studied in feeding experiments under predation pressure of herring. The experiments were conducted with pelagic mysids: Mysis mixta and Mysis relicta, and with Cercopagis pengoi, a non-indigenous cladoceran, which invaded the Baltic Sea in 1992. Zooplankton was offered as prey. Two kinds of experiments were performed in the absence and presence of chemical predator cues: (1) two-prey experiments with prey, which have poor or good escape responses and all three planktivores and (2) natural prey experiments with mysids in natural zooplankton assemblages. The results showed that all three species reacted to the chemical cue of herring by decreasing their feeding rate and altering prey selection. C. pengoi selected easily captured prey (rotifers) in two-prey experiments under predation risk while selection for any prey was evident in mysids in natural prey experiments only in the absence of predator cues. This indicates that planktivores have different anti-predator strategies, which are modified by their own prey capture abilities. C. pengoi was a very efficient predator on small prey with size-specific prey consumption rate 5 to 18 times the rate of mysids. Results show that the studied planktivores are capable of adjusting their feeding behaviour to decrease their conspicuousness in order to increase survival under predation risk. Further, results support the view that C. pengoi has adapted well to the Baltic ecosystem, sharing food niche with pelagic mysids and most probably having a strong influence on the whole pelagic food web.     

Escape behaviour of solitary herring (Clupea harengus ) and comparisons with schooling individuals

The escape behaviour of solitary herring (Clupea harengus L.) startled by a sound stimulus was observed by means of high-speed video-filming. The results were compared with data from a previous study on the escape behaviour of schooling herring. Escape responses were divided into “away responses” and “towards responses” according to the orientation of the C-bend of the body relative to the stimulus. The proportion of away responses was smaller for solitary than for schooling herring. In solitary herring, the subsequent escape trajectories of fish making initial away responses showed a bimodal pattern of distribution, with modes at 130 and 180° from the stimulus. Trajectories following towards responses, however, were mainly within the semicircle directed at the stimulus, and their pattern of distribution differed from that of away responses. This result contrasts with observations on schooling herring, whose trajectories following both initial away and towards responses are directed away from the stimulus. In addition, we measured the response latency, defined as the interval of time between stimulus presentation and the first detectable movement of the fish. Solitary herring showed a higher proportion of short-latency responses (latency <50 ms) than schooling herring. Different behaviours appear to be exhibited by herring depending on whether they are solitary or within a school. We hypothesize that schooling may raise the threshold for initiation of fast escape responses, giving longer latencies and slower responses which are more appropriate in their directionality and reduce the possibility of collisions with neighbours. In addition, we suggest that schooling behaviour enhances the directionality and co-ordination of the escape response of the whole school, possibly increasing the probability of surviving a predator attack. Received: 2 October 1996 / Accepted: 17 October 1996     

RNA:DNA ratios and growth of herring (Clupea harengus) larvae reared in mesocosms

Autumn-spawned North Sea herring larvae (Clupea harengus L.) were released in two outdoor mesocosms of 2500 m³ (A) and 4000 m³ (B). The mesocosms were monitored for temperature, salinity, oxygen, chlorophyll a, zooplankton and herring larvae abundance. The density of suitable prey for first feeding larvae (mainly copepod nauplii) was initially low in Mesocosm A (<0.11^-1) compared to in Mesocosm B (>11^-1). Half-way through the experiment the situation was reversed, with higher densities of prey in Mesocosm A (>31^-1) as compared to Mesocosm B (～11^-1). The average temperature declined steadily in both mesocosms from 18°C at release to 11–12°C by the end of the experiment 60 d later. The RNA:DNA values of individual herring larvae were related to protein growth rates and temperature adjusted according to Buckley (1984). A corresponding DNA growth index (Gdi) was given as: Gdi=0.68 TEMP+3.05 RNA:DNA-9.92. The RNA:DNA based growth indices were significantly correlated with other somatic growth estimates. The average estimated protein growth rate in the two mesocosms followed the same temporal pattern as the somatic growth rate, but with a lag of 2 d or more. Residual analysis of the regression of ln RNA versus ln DNA also showed the same temporal pattern as the RNA:DNA ratios, but the shift in condition as estimated by this method occurred more in synchrony with the other somatic growth measures. Larvae in Mesocosm A had RNA:DNA values similar to the starvation control kept in the laboratory the first days after release, confirming that larvae in Mesocosm A initially were in poor nutritional condition. On the other hand, the majority of the herring from Mesocosm B were characterised as starving or in poor nutritional condition towards the end of the experiment. The assessment of growth and nutritional condition were in accordance with independent survival estimates which suggested that the majority of the total mortality occurred during the first 15 d in Mesocosm A and there-after in Mesocosm B.     

Temperature influences muscle differentiation and the relative timing of organogenesis in herring (Clupea harengus) larvae

Eggs from spring spawning stocks of herring (Clupea harengus L.) were fertilized and reared at either 5, 8 or 12°C in 1991 and 1992. The differentiation of myotomal muscle fibres was investigated in relation to the development of other organs and tissues using light and electron microscopy. The gut, notochord, eyes and haemocoel appeared at the same relative point in development between fertilization and hatching at all temperatures. In contrast, the formation of the spinal cord, pronephros, pectoral fin buds and muscle fibres was relatively retarded at 5°C compared with 8 and 12°C. Myogenesis in the presumptive inner muscle mass occurred after 12 to 16 d at 5°C, 7 to 10 d at 8°C and 3.5 to 6 d at 12°C. Myoblasts aligned in orderly rows running from myosept to myosept prior to fusion to form myotubes. Actin and myosin filaments were synthesised throughout the cytoplasm in associated with presumptive Z-lines at the periphery of myotubes and immature muscle fibres. Differentiation of the superficial and inner muscle fibres types of larvae occurred at around the same time. Following this initial period of myogenesis, the number of myotomal muscle fibres remained constant until after hatching, so that increases in muscle bulk in the late embryo were entirely due to fibre hypertrophy. At hatching, the number of superficial muscle fibres present in myotomes just posterior to the yolk-sac was significantly less at 5°C (108±12) than at either 8°C (132±10) or 12°C (140±10) (mean±SD, 12 fish/temperature). In contrast, there were around 280 inner muscle fibres/myotome, comprising 90% of the trunk cross-sectional area, at all three temperatures. Myofibrillargenesis occurred relatively slowly at low temperatures, so that the volume density of myofibrils in the inner muscle fibres of larvae at hatching was significantly less at 5°C (39.2±9.0) than at either 8°C (49.6±8.8) or 12°C (50.2±9.8) (mean ±SD, 20 fibres/temperature from total of 5 fish). Undifferentiated myoblasts remained at hatching to form a population of presumptive myosatellite cells. The number of presumptive myosatellite cells per mm² cross-sectional area of muscle fibre was more than two times higher at 8°C (1493±335) than at either 5°C (478±102) or 12°C (924±233) (mean±SD, 5 fish/temperature). The results suggest that temperature can influence the commitment of myoblasts to differentiation at a critical stage in embryogenesis, thereby providing a potential mechanism for influencing future growth characteristics. Correspondence to: I.A. Johnston at Gatty Marine Laboratory     

Effects of sulfuric water pollutants on fertilization,embryonic development and larvae of the herring,Clupea harengus

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.     

Effects of a transitory,low food supply in the early life of larval herring (Clupea harengus) on mortality,growth and digestive capacity

In order to assess possible effects of a transitory, low food supply on later development, three groups of Clyde herring larvae (Clupea harengus L.) were exposed in 1989 to different feeding regimes immediately after yolk resorption. Group 1 received a high daily ration of 80 copepods larvae^–1 for 31 d, Group 2 a low daily ration of 15 copepods larva^–1 for 10 d followed by a high ration (80 copepods larva^–1) for 21 d and Group 3 a low ration of 15 to 20 copepods larva^–1 for 31 d. After 31 d of feeding, digestive capacity, expressed as the sum of trypsin and trypsinogen, was markedly reduced in Group 2 compared to Group 1, while Group 3 had an even lower digestive capacity. After the switch from low to high ration Group 2 exhibited compensatory growth and caught up with Group 1 both in standard length and content of soluble protein. Group 3 had the lowest growth rates. Mortality was equal in Groups 1 and 2, while Group 3 showed an excess mortality of 40% of the start population. Although Group 2 larvae had caught up with Group 1 in growth at the end of the study, content of trypsin and trypsinogen in Group 2 was only half of that found in Group 1. Thus, comparing effects of a short period of food limitation on future growth, mortality and content of digestive enzymes, the study indicates content of trypsin and trypsinogen to be the most sensitive variable for detection of food limitation in the early stages of exogenous feeding.     

Variations in the content of trypsin and trysinogen in larval herring (Clupea harengus) digesting copepod nauplii

Trypsin and its proform trypsinogen were quantified by radioimmunoassay in herring (Clupea harengus L.) larvae subjected to different prey densities. During the first weeks of larval life, the enzyme content fluctuated in a threephased pattern. Yolk resorption (Phase 1) was characterized by an increase in enzyme. During the first few days after yolk resorption (Phase 2), there was a sharp decline in enzyme. Older larvae (Phase 3) exhibited a second period of intensive enzyme synthesis. Amounts of trypsin in intestines of feeding larvae were analysed. At first feeding, a basal level of gut enzyme of approximately 30ng was recorded, and the amount of additional enzyme secreted from the pancreatic tissue into the intestine appeared to be dependent upon the numbers of prey items ingested. The enzyme-substrate ratio in the intestine was approximately 1 to 4. Prey availability affected amount of trypsinogen. Larvae experiencing a high prey density had an approximately two-fold higher specific enzyme content in Phase 2 compared to larvae exposed to a low prey density. A proposed nutritional strategy for first feeding herring larvae is discussed.     

Effect of light intensity on activity and food-searching of larval herring,Clupea harengus: a laboratory study

Larvae of Clupea harengus were reared from spawning herring caught in March 1982 and 1983 in the Firth of Clyde, Scotland. An infra0red observation technique was used to record the behaviour of larval herring both in shallow dishes using a top view and in a tank 2 m deep using a side view. The amount of time larvae spent swimming, which was minimum in complete darkness, increased with increasing light intensity and as the larvae grew. Maximum swimming speeds of feeding larvae were recorded at light intensities between 10 and 100 lux. The presence of food organisms (Artemia sp., Brazilian strain) at light intensities below the feeding threshold (0.1 lux) caused an increase in the proportion of time spent active, but light intensities above the threshold had different effects, depending on developmental stage: larvae of 12 mm increased swimming speed, but 21 mm larvae decreased speed. In the 2 m deep tank in darkness, larvae displayed inactive periods wherein they sank head first, interspersed with periods of upward swimming. As light intensity increased, vertical swimming was replaced by horizontal swimming. These results are discussed with reference to food searching and vertical migration of larval herring in the sea.     

Growth and mortality in young larval herring (Clupea harengus); effects of repetitive changes in food availability

Following yolk resorption, laboratory-reared larval Baltic herring (Clupea harengus L.) were exposed to two sequences of food restriction for 5 d and re-alimentation for 10 d. Comparisons regarding larval growth (standard length and content of water-soluble protein), mortality and content of the sum of trypsin and trypsinogen were made with larvae at a continuous high ration. Larvae exposed to varying prey abundance grew less in length than the control, and during the second high-ration period (Day 22 to 32) growth in length ceased. From the first low-ration period onwards, the content of water-soluble protein in these larvae was lower than that of the control larvae, and the survival rate of the low-high ration group was 59% compared to 77% in the larvae at a continuous high ration. In contrast, the effects of varying food availability were minor on larval content of trypsin and trypsinogen. Results are compared with previous findings in larval Clyde herring, and the effects of larval stock and timing and duration of food restriction on larval growth performance are discussed.     

Food and feeding habits of the herring Clupea harengus and the sprat C. sprattus in inshore waters of the west coast of Scotland

The food and feeding habits of the herring Clupea harengus L. and the sprat C. sprattus (L.) as 0-group and older fish in inshore waters of the west coast of Scotland, were studied from April, 1970 to March, 1972. The two species occurred together. The diets of 0-group herring and sprats are almost identical, both qualitatively and quantitatively. Their daily feeding rhythms also coincide. It is probable that competition between these two species in their first year of life could occur if the food supply is limiting. Changes in diet between 0-group and older fish are more pronounced in herring than in sprats. The diet itself is mainly crustacean in both clupeids, with copepods contributing the major share.     

Assimilation of the wax esters of marine zooplankton by herring (Clupea harengus) and rainbow trout (Salmo gairdnerii)

Herring (Clupea harengus L.) and rainbow trout (Salmo gairdnerii) fed frozen-thawed zooplankton for 7 days assimilated more than 90% of the dry matter in the zooplankton consumed. Less than 5% of the total lipid ingested, largely wax esters, was excreted in the faeces. Faecal lipids from both the herring and the trout, especially the latter, were rich in wax esters and free fatty alcohols. Faecal fatty alcohols were enriched in 22:1 and deficient in 16:0 and 16:1 alcohols compared to zooplankton fatty alcohols. The growth of rainbow trout fed frozen-thawed zooplankton over 3 months was about one-half of that of trout fed commercial pellets. The fish fed on zooplankton appeared normal and were more pigmented than the fish fed on commercial pellets.     

Effects of ocean acidification on the early developmental stages of the horned turban,Turbo cornutus

To estimate the impact of CO₂-driven ocean acidification on the early life stages of gastropods, the effects of increased partial pressure of seawater carbon dioxide (pCO₂) (800–2,000 μatm) on the early developmental stages and larval shell length of the commercially important gastropod, the horned turban snail, Turbo cornutus were investigated. Increase in experimental seawater pCO₂ had an increasingly negative impact on the early developmental rate; the proportion of embryos or larvae displaying retarded development increased at higher pCO₂. The proportion of embryos that developed to the 4-cell stage at 2 h after fertilization decreased linearly with increasing pCO₂. At ~1,000 μatm pCO₂, retarded development was observed in ~50 % of larvae. No embryos developed to the 4-cell stage at 2,000 μatm pCO₂ within 2 h of fertilization. A similar trend continued until 24–26 h after fertilization; the proportion of larvae attaining veliger stage by 24–26 h also decreased with increasing pCO₂. The shell length of T. cornutus veligers decreased gradually as seawater pCO₂ increased, but markedly decreased in seawater under nearly unsaturated and unsaturated conditions (≤1.04) of the aragonite saturation state (Ω _aragonite). The results indicate that increased pCO₂ seawater has a progressive and acute effect on embryonic and larval T. cornutus, and imply that the extended early developmental period and/or the downsized larval shell produced by ocean acidification will have a negative impact on survival, settlement and recruitment well into the future.