71 research outputs found
Changes in fish stocks in an Estonian estuary: overfishing by cormorants?
No full textAbstract
Vetemaa, M., Eschbaum, R., Albert, A., Saks, L., Verliin, A., Jürgens, K., Kesler, M., Hubel, K., Hannesson, R., and Saat, T. 2010. Changes in fish stocks in an Estonian estuary: overfishing by cormorants? – ICES Journal of Marine Science, 67: 1972–1979. In Estonia, the cormorant Phalacrocorax carbo sinensis is a newcomer, and its numbers have increased rapidly since 1985. In the shallow protected (no fishery) Käina Bay in Väinameri (West Estonia), the colony was established in 1995. Gillnet sampling indicated that roach was the most abundant spawning fish species in 1995. Ten years later, when the study was repeated, the catch per unit effort was already more than 100 times lower than in 1995. The number of spawning perch decreased tenfold from 1995 to 2005. During the same period, commercial fishing effort in the entire Väinameri area decreased several times. The change in fish abundance in the Käina Bay and in the coastal fish-monitoring areas in the archipelago sea nearby, together with an analysis of food of cormorants, indicates that the decline in fish abundance might be related to the increased numbers of cormorants. The conclusion is drawn that the establishment of a cormorant colony could have seriously damaged or even prevented normal functioning of historically important spawning grounds and affected fish recruitment to adjacent areas. Therefore, expanding bird colonies might play a role similar to an expanding fishing fleet, by overexploiting the resource.</jats:p
Läänemere idaosa litoraali kalakoosluste varieeruvus ja selle põhjused
No full textVäitekirja elektrooniline versioon ei sisalda publikatsiooneLitoraalialad on vee-elustikule oluliseks sigimis-, kasvu- ja toitumisalaks. Muutlike abiootiliste ja biootiliste keskkonnategurite koosmõjul varieerub litoraalialade kalakoosluste liigiline koosseis väga suurel määral nii ajas kui ka ruumis. Käesoleva töö eesmärkideks oli välja selgitada: 1) Läänemere litoraali asustavate kalaliikide ööpäevaringseid ja aastasiseseid käitumismustreid; 2) litoraali kalastiku sesoonse toitumisökoloogia eripärad meritindi näitel; 3) anadroomsete kalade (lõhe ja meriforell) noorjärkude alternatiivseid rändestrateegiaid jõgede ja Läänemere litoraalialade vahel. Selgus, et: 1) Läänemere litoraali asustavate kalaliikide ööpäevaringne käitumine on seni kirjeldatust oluliselt mitmetahulisem. Seega, mingi konkreetse litoraali piirkonna ööpäevas muutuva liigilise koosseisu täielikuks kirjeldamiseks on vajalik proovipüükide tegemine nii hommikul, keskpäeval, õhtul kui ka öösel; 2) Läänemere idaosa litoraaliala asustava meritindi toitumiskäitumine erineb avamere elupaikades kirjeldatust; 3) Läänemere riimveelised litoraalialad võivad olla seni arvatust olulisemaks kasvualaks alternatiivseid rändestrateegiaid kasutavatele lõhilaste noorjärkudele; 4) läbi riimvee vooluveekogu vahetavad noorkalad võivad osaliselt olla ka anadroomsete lõhilaste populatsioonides esinevate „eksijate“ varem kirjeldamata tekkemehhanismiks; 5) Läänemere litoraali asustavatel kalaliikidel võib esineda nii piirkonnaspetsiifilisi ööpäevaringseid toitumisrändeid (meritint) kui ka elukäigumustreid (lõhe, meriforell).The shallow littoral is an important reproduction, nursery, and foraging area to coastal marine biota. Due to variability in abiotic and biotic environmental parameters, near-shore fish communities of the Baltic Sea are highly variable. Thus, the occurrence and abundance of fish species may be spatially and temporally highly variable. The objectives of the present thesis were to: 1) assess diel variations in the composition of the fish assemblage in the surf-zone of the non-tidal Baltic Sea; 2) estimate the predatory role of European smelt in a littoral habitat by describing seasonal variation of prey composition; 3) investigate whether littoral zone of the Baltic Sea also function as a habitat for early out-migrating Atlantic salmon and anadromous brown trout (ABT) fry and parr. The main results and conclusions are the following: 1) fine-scale variations in littoral fish assemblages are more complex and may take place within shorter time frame than previously known; 2) European smelt inhabiting littoral areas of the Baltic Sea occupy different ecological niche than smelt in offshore areas; 3) alternative migrations of juvenile salmonids between freshwater and brackish environments indicate that the shallow littoral zone of the Baltic Sea may play a significant role as a permanent or provisional nursery area to these species; 4) the phenomenon of stream shifting through the marine environment may constitute at least one possible mechanism behind the straying behaviour documented during the spawning runs of Atlantic salmon and ABT in the Baltic Sea basin; 5) some coastal fish species inhabiting the Baltic Sea might show sub-basin specific behaviour in terms of their regular diel movements and alternative life history pattern
Muuga lahel ja Käsmu lahel talvituvate aulide (Clangula hyemalis) toitumisökoloogia: kaaspüügil põhineva andmestiku analüüs
No full text
Angerja (Anguilla anguilla) looduslike ja asustatud populatsioonide arvukuse hinnang Eesti veekogudes
No full textA Thesis for applying for the degree of Doctor of Philosophy in Applied Biology.Väitekiri filosoofiadoktori kraadi taotlemiseks rakendusbioloogia erialal.ABSTRACT. Eels, due to their mysterious life cycle and appearance have long fascinated scientists and fishers alike. In Estonia, similarly to the rest of Europe, eel populations have faced dramatic changes over the last decades, giving researchers a tough task to find out the factors behind these changes.
My thesis focused on different aspects regarding the fisheries and welfare of eel in Estonian coastal and freshwater habitats. We studied a long-time series of commercial landings, scientific surveys, restocking efforts, and environmental variables, to shed light on the dynamics of these mysterious creatures.
One observation from our investigation is the decline in both commercial and recreational landings of eels along the Estonian coast since the 1970s where data was available. This trend is consistent with a broader pattern seen all across Europe. However, with the fall of the Soviet Union at the beginning of the 1990s there was a sudden growth in eel landings on the Estonian coast. This was most probably due to changes in fishing practices, improvement in registering the catches, and growth in fishing effort rather than the recovery of local eel stocks. Since 2008 the landings of small fyke nets in particular dropped sharply with the main reason aside from the declining eel population being reduced effort as well as the diminishing profitability of using such gear. As opposed to small fyke nets, the number of large fyke net licenses remained unchanged. As eels are not specially targeted by large fykes the changes in the eel landings also reflect the decline of eel biomass in the coastal areas.
Restocking is a part of eel management strategies in many European countries, including Estonia. Initially aimed at preserving traditional eel fisheries, most notably in Lake Võrtsjärv, restocking programs have evolved in response to declining stocks and regulatory frameworks. Our research highlights the importance of optimizing restocking efforts, considering factors such as stocking density, water conditions, and prey availability. For example, eels released to waterbodies as glass eels, tend to fare better in lakes with lower stocking densities, mixed water column, and abundant benthic invertebrates. Inland water bodies vary in their suitability for eel restocking, with well-mixed, nutrient-rich lakes offering the most optimal conditions for growth. Environmental factors, such as the prevalence of cyanobacteria, can negatively impact restocked eel populations, highlighting the overall importance of ecosystem health concerning eel management.
While there have been numerous studies on eel migration, the topic is complicated and often raises a lot of questions while providing not so many answers. Restocked eels show the ability to navigate from freshwater habitats to the sea, however, their journey can be far from straightforward. Some individuals have displayed a surprising behavior of returning to their rivers of origin, spending extended periods in freshwater before resuming their migration towards the ocean. While this behavior exposes them to additional risks from predators and fisheries, it once again confirms the complexity of eel migration strategies.
By investigating the Estonian eel populations, our study affirms the need for holistic management approaches that consider the interconnectedness of ecosystems and human activities both locally and internationally. When we understand the processes that influence the dynamics of eel populations, we can work towards the recovery of these mystic and unique creatures.Publication of this thesis is supported by the Estonian University of Life Sciences.LÜHIKOKKUVÕTE. Tänu oma ainulaadsele ja saladuslikule elutsüklile on angerjad juba pikka aega teadlaste, kalurite, looduskaitsjate ning laiema üldsuse arutelusid täitnud. Nagu mujal Euroopas on ka Eesti angerjapopulatsioonid viimaste aastakümnete jooksul drastiliselt muutunud ning nende muutuste põhjuste väljaselgitamine on teadlaste jaoks olnud keeruline ülesanne
Antud doktoritöö keskendub erinevate Eesti angerjapopulatsioone mõjutavate faktorite analüüsimisele nii mage- kui merevee elupaikades. Uuringu raames töötati läbi pikaajalisi kutselise püügi-, taasasustamise- ning keskkonnaandmeid ja viidi läbi erinevaid teadusprojekte, et angerjate müstilist elukäiku Eesti vetes paremini mõista.
Me nägime, et sarnaselt ülejäänud Euroopale on nii kutseliste kui harrastajate saagid Eesti rannikumeres olnud pikas languses. Kuigi Nõukogude Liidu kokkuvarisemise järel 1990ndate alguses saagid lühiajaliselt kasvasid, ei olnud selle põhjuseks suurenenud angerjate hulk Eesti rannikul vaid pigem kalapüügimeetodite ja -vahendite paranemine, varasemast täpsem saagi registreerimine ning püügivõimsuse tõus. Alates 2008. aastast hakkasid eelkõige angerjaspetsiifiliste püügivahendite, nagu näiteks rivimõrrad, saagid langema, mille peamiseks põhjuseks vähenevate angerjavarude kõrval oli Eesti Angerjamajanduskavast tulenev püügivõimsuse vähendamine ning ka sellise püügi madal kasumlikkus. Samas võib ka angerjavarude vähenemist märgata analüüsides suuremate mõrdade saagikust, mille püügilubade arv on püsinud oluliselt muutumatuna ning mis ei ole spetsiaalselt angerjapüügiks mõeldud.
Nagu mitmel pool Euroopas on ka Eestis üheks oluliseks angerja majandamise vahendiks taasasustamine. Algselt kohaliku püügivaru täiendamise eesmärki täitnud tegevusel on tänu hõbeangerjate väljarändele potentsiaalselt oluline osa ka üle-euroopalise populatsiooni taastootmisel. Meie uuringu tulemused näitavad, et taasasustamise juures on angerjate kasvu ja heaolu silmas pidades üliolulisel kohal asustustihedus, keskkonnatingimused veekogus ning toidu kättesaadavus. Angerjatele sobivad madala asustustihedusega, hästi segunenud ning rohkelt bentilisi suurselgrootuid omavad järved, nagu näiteks Võrtsjärv. Samas peab silmas pidama, et on ka negatiivseid faktoreid, mis sellistes järvedes võivad angerjaid mõjutada. Üheks selliseks on näiteks sinivetikate vohamine ning kõrge suvine veetemperatuur taasasustamise aastal.
Angerjate liikumismustrid on üheks keerulisemaks osaks nende elutsüklis. Siiani on näiteks ebaselge, mis tegelikult põhjustab angerjate hõbestumise ning miks võib angerjate rändekäitumine väga erinev olla. Taasasustatud angerjad on võimelised mageveest merre rändama, kuid see ei pruugi olla niivõrd selge nagu sageli arvatakse. Merre rännanud hõbeangerjad võivad oma rände katkestada ning tagasi magevette pöörduda, enne, kui pärast pikemat perioodi uuesti merre laskuvad. See teadmine on oluline, sest need naasnud angerjad võivad seeläbi sattuda näiteks uuesti püügisurve alla või kiskjate ohvriks.
Meie uuring näitas, et Eesti angerjapopulatsioonide uurimisel on oluline mõista tervikut, analüüsida erinevaid keskkonna- ja inimmõjusid ning arendada koostööd erinevate huvigruppide vahel nii lokaalselt kui piiriüleselt. Ainult nii on võimalik angerjavarude taastumisele kaasa aidata.Publication of this thesis is supported by the Estonian University of Life Sciences
Natural re-establishment of Atlantic salmon reproduction and the fish community in the previously heavily polluted River Purtse, Baltic Sea
No full textDeliverable 3.4.1 Maps of the status of spawning grounds
No full textKalade koelmu- ja turgutusalade (edaspidi: taastootmisalade) hea seisund ja toimine on
aluseks kalaasurkondade ja -varude jätkusuutlikkusele. Teadmised taastootmisalade
ökoloogilisest seisundist on seega kriitilised ka kalavarude jätkusuutlikul ekspluateerimisel ja
majandamisel. Kuigi teadlased on ühel meelel taastootmisalade olulisuses, puudub senini
täpne arusaam millised keskkonnaparameetrid on kõige määravamad hästi toimivate
taastootmisalade toimimisel (Kraufvelin et al. 2018). Seda kuidas taastootmisalade arv ja
kvaliteet mõjutab kalade populatsioonidünaamikat on harva kirjeldatud ning veelgi harvem
on seda informatsiooni kasutatud teadusliku soovitusena kalavarude majandamises. Kuna
kaladele olulised taastootmisalad on vähemal või rohkemal määral tugeva inimmõju all, siis
aitaks nende alade toimimise uurimine mõista, miks osad alad on täiendi mõttes
produktiivsemad kui teised.
Eesti riimveelist rannikumerd asustavad aastaringselt erinevad mageveekalad nagu näiteks
haug (Esox lucius), säinas (Leuciscus idus), luts (Lota lota), särg (Rutilus rutilus) ja teib
(Leuciscus leuciscus), kes suuremal või vähemal määral koevad mageevees. Nimetatud
(pool)siirdekalad rändavad kudemiseks magevette (vooluveekogud, järved) või mageveelise
mõjutusega veekogudesse (lahed, rannajärved ja -lõukad). Selliseid veekogusid leidub
Saaremaal arvukalt, kuid tihti on probleemiks nende kehv ühendus merega, st veerežiim, mis
ei võimalda kaladel (vähemalt mitte igal aastal) sinna siirduda või seal edukalt kudeda. Samuti
on paljud vooluveekogud õgvendatud või muud moodi inimese poolt tugevalt mõjutatud, mis
on vähendanud nende sobilikkust taastootmisalana. Ka pealtnäha hea kvaliteediga
taastootmisalade puhul on ilma täpsemate uuringute läbiviimiseta raske öelda, kas ikka on
tegemist taastootmisalaga, mis annab olulist iga-aastast panust täiendisse
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.)
No full textA review of the literature on acoustic herding and attraction of fish A 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 percids The 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.Rapporten består av tre olika rapporter.</p
A review of the literature on acoustic herding and attraction of fish [Elektronisk resurs] : Visual ecology of fish - a review with special reference to percids : Reproduction biology of the viviparousblenny (Zoarces viviparus L.)
No full textA 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.</p
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