A review of the literature on acoustic herding and attraction of fish : Visual ecology of fish - a review with special reference to percids : Reproduction biology of the viviparousblenny (Zoarces viviparus L.)

• A review of the literature on acoustic herding and attraction of fishA literature study of fishing methods using acoustic herding, passive acoustic steering and acoustic attraction is presented. All three techniques are used world-wide in traditional fishing, but their applications to modern fisheries are very few. Optimization in terms of selectivity and increase in catches seems promising for acoustic attraction, and many successful trials have been carried out on various fish species of different hearing abilities. The results from acoustic herding are more negative and a more thorough knowledge of fish behaviour is needed before such techniques can be improved. When examining passive acoustic steering, little evidence has been found that fish actually use acoustic cues to detect fishing gear. Theoretical calculations show that claims that fish can detect the Aeolean tones generated by the water flowing through the net can probably be discounted, but measurements of the acoustic field around the fishinggear have to be made to finally confirm this. However, it has been shown that the fishinggear leading structures currently used are far from optimal. Studies of the sensory basis of gear detection by fish are needed to improve such structures. Psychoacoustic studies have shown that fish are essentially sensitive to very low frequency sounds. Therefore, improving acoustic fishing techniques demands an efficient, low-frequency sound source. It is shown that the fishing boat itself can be modified to become a relatively efficient transducer at the desired frequencies.

• Visual ecology of fish - a review with special reference to percidsThe function and morphology of the fish visual apparatus is in many respects similar to terrestrial vertebrates. Light passes through a cornea, is concentrated by a lens and absorbed by photoreceptors in the retina, which sends nervous signals via the optical nerve to the brain for interpretation. But there are also some important dissimilarities. Instead of changing the shape of the lens, fish accommodates by moving its lens in relation to the retina. Fish has no air—cornea interface, which deprives it of nearly 80% of their optical power compared to terrestrials. Fish as a group has also a more diverse set of visual pigments than terrestrialvertebrates since they have adapted to a large number of different light environments.The light environment in water is depending on both above surface, at surface and below surface conditions. The proportion of light transmitted through the surface depends on the angle of the incident light and whether the surface is still or rough. The more dislocated the angle is from the perpendicular the more light is reflected. A rough surface reflects more light than a still. Once light has entered the water column it is absorbed and scattered by the water itself and dissolved and particulate substances. Depending on the composition of different substances in the water, light of different wavelengths is more or less diminuted. There are four major agents absorbing light in aquatic environments. 1) The water itself, which absorbs most of the light in the ultraviolet, green and redparts of the spectrum 2) Dissolved organic substances (so called Gelbstoffe) absorbing maximally in the UV, violet, blue and green part of the spectrum 3) Phytoplankton, which absorbs light depending on pigment composition. Chlorophyll, the dominating pigment group absorbs most light in the blue and red part of the spectrum. 4) Particulate in organic substances absorbing light of different wavelengths in a more even fashion. The absorbance characteristics of these substances nevertheless may vary between different types.Percids can be grouped into two major classes by their visual adaptations for foraging in different light environments. Pikeperches (Stizostedion sp.) mainly andruffe (Gymnocephalus sp.) feed crepuscularly or nocturnally, and are thus equipped with a visual apparatus designed for light sensitivity. These species has a so called tapetum lucidum, a reflective layer of the retina consisting of a substance called 7,8-dihydroxyanthopterin. The arrangement induces extra reflections of light back and forth between the rodswith additional absorptions at each reflection. Perches (Perea spp.) are instead diurnal feeders, foraging in bright light. They lack a tapetum lucidum and have instead a visual physiology enhancing visual acuity in bright light.Larval perch are equipped with UV-sensitive cones. These are believed to be supportive when they feed on zooplankton in the pélagial. When the perch larvaego through metamorphosis and switch habitat, from the pélagial to the benthiclittoral, the UV-sensitive cones are regressed.As with most other freshwater fishes, the retina of percids absorbs maximally in regions of the spectra slightly deviating from the regions where water transmits maximally. This is believed to be advantageous when detecting objects underwater. Since light reflecting off an object usually contains larger portions of light in regions spectrally deviating from the background, which nearly eclipses the regions where light transmission in the particular water is maximum. Having a retina that absorbs more light of such wavelengths will increase the perceived contrast between the object in particular and the background. For deep-sea fishes on the other hand there seems to be a clear match between the spectral characteristics of their visual pigments and the spectral region of maximum transmission in water. This is probably due to the fact that the only light that reaches down to their ambient environment is in the regions of the spectrum where lighttransmission in seawater is maximal.The importance of contrast for prey recognition has been demonstrated for many freshwater fishes. For percids it has been studied at early developmental stages. Both for perch (Perea fluviatilis) and pikeperch (Stizostedion lucioperca)larvae, contrast seems to be an essential factor for prey recognition. One important factor affecting the light environment in aquatic environment is turbidity. Increasing turbidity decreases the distance of a fish’s visual field, increases luminosity, affects contrasts, defocuses and depending on type, selectively changes the composition of wavelengths of light transmitted through the water. All these factors influence a fish’s foraging ability. Not necessarily negative. By changing the colour of the background light it may actually enhance the contrast of certain prey items. It will also affect the light attenuation with depth. High levels of turbidity will decrease the maximum distance of light penetration thus imposing spatial constraints on fish foraging.

• Reproduction biology of the viviparousblenny (Zoarces viviparus L.)Reproduction biology was studied in a Kattegat population of the viviparous blenny (Zoarces viviparus L.). The females attained maturity at the age of 1+ or2+ and the males at the age of 1+. Fertilisation of the females took place within 2-3 days. At spawning time the feeding activity of females decreased or stopped until the second half of October when it was resumed and increased up to mid January when the study was finished. The condition factor was lowest in December.Size and age of the females did not influence egg size or the duration of the development of eggs and larvae. Lengthand weight growth rate of larvae was synchronous within and between broods.The mortality of larvae during intra-ovarian development varied between females, and reproduction success was slightly lower in the younger females. High mortality in some broods led to increased growth rate of surviving larvae. This suggests that relative fecundity may influence larval growth and that growth rate is limited by the maternal supply of energy.Theoretically, viviparous blenny may maximise reproduction by producing a high number of protoplasmic oocytes, and during later gametogenesis and intraovarian offspring development reduce functional fecundity by stepwise mortality. Such a reproductive strategy would ensure a sufficient growth rate of surviving larvae, a maximal number of larvaefully developed to leave ovary, and an optimal parturition time. This hypothesis was verified by observations suggesting that compared to other species of fish the viviparous blenny produces high numbers of abnormal oocytes and larvae with serious malformations, which are dying during pregnancy.

Ämnesord

NATURVETENSKAP  -- Geovetenskap och miljövetenskap -- Miljövetenskap (hsv//swe)

NATURAL SCIENCES  -- Earth and Related Environmental Sciences -- Environmental Sciences (hsv//eng)

LANTBRUKSVETENSKAPER  -- Lantbruksvetenskap, skogsbruk och fiske -- Fisk- och akvakulturforskning (hsv//swe)

AGRICULTURAL SCIENCES  -- Agriculture, Forestry and Fisheries -- Fish and Aquacultural Science (hsv//eng)

Nyckelord

fishing

acoustic herding

fish behaviour

fish visual apparatus

reproduction biology

viviparous blenny (Zoarces viviparus L.)

fiske

akustisk vallning

fiskens beteende

fiskens syn

reproduktionsbiologi

National

Nationellt finansierad miljöövervakning

Ett rikt växt- och djurliv

A Rich Diversity of Plant and Animal Life

Levande sjöar och vattendrag

Flourishing Lakes and Streams

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