Plaice Larvae Research Articles

Monitoring the Vertical Movements and Light Responses of Herring and Plaice Larvae

The movements of herring and plaice larvae of different age, from hatching to about 8 weeks old, were monitored at different levels in a vertical tube by means of matched pairs of thermistors controlled and ‘balanced’ by bridge circuits. Movement of larvae past the thermistors upset the balance and the resulting change in the circuit could be followed on a pen-recorder. Having established, by visual comparisons, the interpretation of the pen record, experiments could be performed over extended periods or at low light intensities when visual observations would have been tedious or impossible. The numbers of larvae present were shown by the number of spikes on the pen record per unit time, while a measure of their size or activity was given by the height of the spikes.The light intensity could be altered by interposing neutral-density filters between an artificial light source and the surface of the tube. Alternatively, natural daylight could be beamed in by a mirror above the tube when placed by a window. Using artificial light, changes of very large intensity had little effect on the larvae until a critical level was reached (equivalent to the intensity at late dusk or early dawn) when a vertical migration took place. A similar vertical migration took place when the natural light at dusk was beamed in; many larvae then moved to the surface and remained there, apparently very active throughout the night untill the light increased at dawn. Vertical migration could be simulated in the laboratory at any time by imposing an artificial light-cycle of appropriate amplitude on the larvae.

Studies on the skin of plaice (Pleuronectes platessa L.)

Histological and ultrastructural examinations were carried out on the integuments of larval plaice from hatching to 60 days. The skin of the newly hatched larva consisted of a delicate epidermis overlying a fluid filled dermal space. As the fish matured this became thicker, the fibrous dermis developed and the mitochondrion rich ‘chloride cells’ degenerated. Around the 42nd day the eosinophil granule cell appeared in the basal layers of the epidermis and by the sixtieth day the epidermis could be considered fully developed.

Brightness Discrimination in Larvae of Plaice and Sole

ABSTRACT The brightness discrimination of larvae of plaice and sole was tested by subjecting them to a choice of moving phototactically to one of two horizontally opposed light sources. At higher light levels, above 100– 10−1 m-candles, they moved to the brighter of the two lights; below 10−1– 10−2 m-candles they moved to the dimmer. Brightness discrimination, expressed as the increment (ΔI) discriminated as a percentage of the dimmer light (I) (the Weber fraction), was of the order of 60– 200 %.

Some effects of food density on the growth and behaviour of plaice larvae

The effect of food density on the growth, survival, and swimming activity of plaice larvae (Pleuronectes platessa L.) was investigated under controlled laboratory conditions. Suboptimal food concentrations decrease the growth of the height of the body musculature relative to length, and increase the amount of time spent searching for food. Older larvae are able to withstand much longer periods without food than young larvae. On the basis of these experiments, it is suggested that food limitation in a larval plaice population is likely to result in a concave mortality curve.

The influence of some byproducts from vinylchloride production on fertilization, development and larval survival on plaice, cod and herring eggs

The effect of byproducts from Norwegian vinylchloride production on fertilization, egg development and larval survival was tested. Eggs and larvae of plaice, cod and herring apparently seemed unaffected by concentrations up to 10 ppm, but concentrations of 100 and 50 ppm delay and stop development of plaice eggs in an early stage. Cod larvae seemed to survive high concentrations of the toxic substances for far longer periods than did I and II year groups of saithe.

Selective breeding of marine fish: I. Automatized feeding of pelagic fish larvae under controlled environmental conditions

1. (1) A rearing apparatus for pelagic fish larvae, with automatic Artemia feeding, is described. 2. (2) The metamorphosis of the plaice larvae occurs in a shorter time than in other described equipment. The growth of the larvae is satisfactory during the first 80 days. 3. (3) Plaice larvae reared in high temperatures metamorphose in a shorter time than larvae reared at lower temperatures. The growth of the larvae reared at high temperatures is also significantly better compared to larvae reared at lower temperatures.

Hydrographic Observations in the Eastern Irish Sea With Particular Reference to the Distribution of Nutrient Salts

INTRODUCTIONStudies on the physical hydrography of the Irish Sea, based mainly on temperature and salinity observations, have been made by Bowden (1955) and Lee (1960). The current systems in various parts of the Irish Sea have been investigated by Bowden & Sharaf El Din (1966), Harvey (1968), Ramster & Hill (1969) and Hill & Ramster (1971). Various smaller-scale physical observations made by other workers have been listed by the above authors.There are not as many references in the literature to the distribution of nutrient salts in the Irish Sea. Jones & Haq (1963) measured phosphate in Liverpool Bay as part of an investigation into the distribution of the alga Phaeocystis. Slinn & Offlow (1968) have made regular routine measurements of phosphate, nitrate and silicate over a number of years off Port Erin, Isle of Man. Ewins & Spencer (1967) measured phosphate, organic phosphorus, nitrate and silicate in the Menai Straits, and Liss (1969) surveyed the distribution of silicate in the western Irish Sea.During recent years the Fisheries Laboratory at Lowestoft has investigated the distribution and ecology of plankton and larval plaice in the Irish Sea, and a supporting programme of both physical and chemical hydrographic observations has been made in the area. Ramster & Hill (1969) and Hill & Ramster (1971) have summarized the results of the Lowestoft current measurements. The present paper describes an investigation into the distribution of temperature, salinity, phosphate, nitrate and silicate. The occurrence of Phaeocystis is also reported as an extension of the earlier work by Jones & Haq (1963).

Visual Thresholds and Spectral Sensitivity of Flatfish Larvae

ABSTRACT Plaice and sole larvae have a pure-cone retina and no retinomotor responses. Observations on the extinction of phototactic and light-dependent feeding behaviour, using both white and coloured light, enabled visual thresholds and spectral sensitivity to be determined. Visual threshold decreased (that is, sensitivity increased) with age. The values for extinction of negative phototaxis ranged from 10−4−10−6 m.c. Sole larvae could feed in the dark from the early post-hatching stage, plaice larvae only at metamorphosis. Thus light intensity thresholds for feeding were only determinable in younger plaice larvae. The values ranged from 10°−10−2 m.c. The spectral sensitivity curves were plateau-like and photopic in nature. Thus the larval cones have spectral properties similar to those of adult teleosts. The differences in threshold obtained by the two techniques are probably a measure of the extent of recruitment of high-threshold cones required for image formation in feeding. No evidence of a dermal light sense was found.

The Feeding of Plaice and Sand-Eel Larvae in the Southern North Sea

Larvae of the sand-eel,Ammodytes marinus, appear in large numbers in the Southern Bight during March, some 2–4 weeks after the peak hatching period of plaice eggs. The sand-eel larvae outnumber those of the plaice about tenfold, but the factor may be 60 or more locally.The principal food components in sand-eel guts in 1961 were copepod nauplii and appendicularians, while plaice were feeding almost exclusively on the latter. Larvae of both species show identical feeding patterns, with activity ceasing at night, and both aggregate during the day at depths of about 2–6 fm. Thus the sand-eel larvae are not separated from the plaice larvae by any pattern of behaviour, and are partially exploiting the same food reserve.Consideration of prey size suggests that any direct competition from the larval sand-eels would first affect the smallest plaice larvae, which are in any case the most vulnerable to shortage of food. But any reduction in the crop of small appendicularians will later be reflected in the number of larger ones, and in this way older plaice larvae might be affected. While the food supply appears to have been adequate up to the time of the investigation, the reserve apparently represented only 3 days' rations. The significance of this observation is unfortunately obscured by our ignorance of appendicularian biology; but, should the food reserve become so diminished, competition between plaice and sand-eel larvae might be expected. The extent of its impact on survival is only likely to emerge from more comprehensive surveys.

A predator-prey size relationship for plaice larvae feeding on Oikopleura: Shelbourne, J. E., 1962. J. Mar. Biol. Ass., U.K., 42 (2): 243–252

A predator-prey size relationship for plaice larvae feeding on Oikopleura: Shelbourne, J. E., 1962. J. Mar. Biol. Ass., U.K., 42 (2): 243–252

The Swimming Speeds of Plaice Larvae

ABSTRACT The swimming speeds attained by hatchery-reared plaice larvae have been determined by measurements made (a) in still water, and (b) against a current flowing through a long glass tube. In still water contact stimuli were used to initiate swimming. No optomotor response was found, and it was not possible to use the ‘fish-wheel’ of Bainbridge (1958). Larvae in the experimental tube stemmed the current for brief periods only, their position then being near the periphery of the upper quadrant. The speeds were found to show a linear relationship to larval length multiplied by the height of the post-anal musculature. Observations in still water show that these values correspond to those of steady swimming: they are not maximum speeds. Speeds ranged from about 1·5 cm./sec. when newly hatched to 9 cm./sec. in late larvae. Darting velocities of up to 15 cm./sec. were recorded with half-grown (stage III) larvae. In such larvae cruising speeds were about three fish-lengths (3L) per second, and the maxima about 10L/sec. The design of gear for collecting fish larvae at sea is discussed. Only a high-speed tow-net, towed at about ten times the velocity attainable by the largest larvae and giving almost no warning of its approach, can be considered to collect a quantitatively balanced sample.

A predator-prey size relationship for Plaice larvae feeding on Oikopleura

SUMMARYPelagic plaice larvae in the southern North Sea normally feed exclusively on the appendicularian Oikopleura dioica, and the size of prey eaten can be estimated from the Oikopleura faecal pellets found in the larval gutPredator-prey size measurements show that smaller plaice larvae are restricted to smaller prey, and that, as larval growth proceeds, so their feeding versatility increases, to include all sizes of Oikopleura at a late pelagic stage of development. Competition for food will therefore be keenest in the early larval stages, when prey-size restriction prevails. This accords with Hjort's hypothesis regarding brood strengths.As growth proceeds, so plaice larvae begin to discriminate for prey size. At the close of the pelagic phase, the degree of size selection is roughly equal to that of a 6o-mesh to i in. plankton net.

The feeding and condition of plaice larvae in good and bad plankton patches

In January and March, during the 1955 plaice spawning season, plankton samples were collected with a Heligoland larva net at stations on a grid around a floating radio buoy in the southern North Sea. There was a decided scarcity of suitable food for plaice larvae in the January patch, and this famine was reflected in the deteriorating physical condition of those larvae caught at the transition stage of development, when yolk reserves were becoming exhausted and an adequate external food supply essential. By March, the spring plankton outburst was in full swing. The condition of transitional larvae improved in this good food patch. Feeding started about the mid-yolk phase, mainly on plants. By the time most of the yolk had been resorbed, the appendicularians Oikopleura and Fritillaria had become the principal food items, and remained so throughout pelagic larval life.

Rearing of Plaice

IN NATURE of July 12 (p. 251), there is an interesting note on the rearing of larval plaice at Plymouth, by Mr. J. T. Cunningham, in which it is mentioned that they have been reared to the age of thirty-seven days; but it is not stated how long the incubation went on. It may be interesting to say that at the Fishery Board's Marine Hatchery, at Dunbar, I succeeded in preserving many millions of larval plaice from twenty-four to thirty-three days, counting from the time of fertilisation; and some were reared in jars for longer. On one occasion I kept them in a thriving condition to the forty-seventh day after impregnation of the eggs, at which age they were carried away by an accidental overflow. The eggs were fertilised on April 3, hatched on April 19, and laræ reared until May 20, when the accident occurred. A description in full will be given in the Fishery Board's report.