Most highly fecund marine fish show a steep decline in natural death rate from egg to first maturity, after which the natural mortality rate remains constant, or may even increase with age for old animals. Relatively few investigations have quantified early life-history mortality vectors for short-lived invertebrate stocks, but this overall picture is also true here for species with planktonic life stages such as penaeid shrimps, and for squids. If M decreases rapidly with age, one logical approach to demographic analysis is by subdividing the lifespan into intervals which increase in duration in proportion to the age up to the start of each interval. This time subdivision strategy is referred to as `gnomonic’. Earlier work (Caddy, 1990) showed that if a reciprocal mortality function applies with age, the product of the instantaneous annual rate of natural mortality and interval duration should be roughly constant for gnomonic intervals. This working hypothesis is shown to produce similar results to the reciprocal function for Mt, but allows a simpler approach to generating realistic life history Mt vectors in the absence of direct estimates of M for stock assessment. Values of a constant probability of death, G = Mt Δt, were used to generate vectors of M-at-age for a gnomonic series of intervals from hatching up to the mean parental age. The value of G is found by iteration that results in 2 survivors from the mean population fecundity by 1 year of age, under the assumption of steady-state population replacement for an unexploited stock. The natural mortality rate in the final, longest interval was assumed to correspond to the ‘constant adult M’ value used in stock assessment. Two extremes of reproductive strategy were chosen by comparison with data from for annual species of cephalopods or penaeid shrimps: cephalopods such as Sepia sp. and Rossia sp., with few, large yolky eggs (and/or parental care), occupy one extreme, and are contrasted with high fecundity penaeid shrimps and at least some Illex squids.

The first category has a low fecundity (130–150 eggs, and a K-selected reproductive strategy). Values of ‘adult M’ of the order of 1.0–1.3 are predicted for the last 60–80% of the annual life span. The high fecundity category (200 000 eggs or more) are opportunistic spawners such as many penaeids and some oceanic squids, and follow an r-selected reproductive strategy. An instantaneous value for pre-spawning M of the order of 2.8–3.4 is predicted for the same period mentioned above. Neither range of values falls outside those in the literature, for which a brief summary is presented.

An important research question relates to the order of magnitude of post-hatching mortality under population stability: it is suggested that irrespective of the specific model used for changes in M with age, this falls rapidly from an initial rate of some 50–75% per day for short-lived, high fecundity species in the 2 days following hatching, unless adult M values are much higher than above, and of the order of 25–40% for the low fecundity organisms over the same initial interval.

A deterministic, forecasting, equilibrium study has been conducted of the multispecies and multi-métier fisheries of the Celtic Sea, ICES divisions VIIf and g. Calculations have been made of the optimal fleet sizes and mesh sizes required to maximize objectives of joint relative sustainable yields, gross revenue and net revenue. The species modelled are the resident populations of cod (Gadus morhua), whiting (Merlangius merlangus), sole (Solea solea), plaice (Pleuronectes platessa) and Norway lobster (Nephrops norvegicus), with additional consideration given to catches of hake (Merluccius merluccius), monkfish (Lophius sp.) and megrim (Lepidorhombus whiffiagonis). The fleet métiers modelled are the beam trawls, Nephrops otter trawls and non-Nephrops otter trawls. If mesh sizes can be calculated independently foreach métier, then the yields and the gross revenue are maximized by increased effort and mesh sizes, especially in the non-Nephrops fleet. If a common mesh size is required for all fleets, then the yields and the gross revenue are maximized when (i) this mesh size is of 114 mm, (ii) the fishing effort in beam and Nephrops fleets is significantly increased, (iii) the non-Nephrops fleet is almost eliminated. Maximization of net revenue requires a mesh size similar to the current mesh of 80 mm, but with significant reductions of fishing effort in all fleets. Average profitability is increased by 50 millions EUROs per year for the Celtic Sea fishery. The model provides a basic structure for examining the management with a more realistic dynamic and stochastic description of the Celtic Sea fisheries.

The present study presents a method of assessing and fitting a stock-recruitment relationship. The method is based on a combination of an equilibrium production model and a yield per recruit model. It does not use recruitment data which are, in general, highly dispersed, but production data. Thus, the relationship obtained is not of the parametric form of classical stock-recruitment relationship such as those of the Beverton and Holt (1957) and Ricker (1954), however, in some cases, the resulting stock-recruitment curve resembles models currently used in fisheries science. It is interpreted as an expectation of the recruitment conditionally to stock spawning biomass and stock demographic structure. However, for some realistic values of the parameters, the curve thus obtained describes a loop which suggests that two different recruitment levels may be expected from the same stock spawning biomass level. This theoretical property is illustrated with simulations. As an illustrative example, the method is applied to North-West Atlantic gadoid stocks. The significance of the method, its originality, the validity of the underlying assumptions and their conceptual consequences are discussed.

The underwater sound of R/V "Johan Hjort" as a function of aspect angle has been mapped by cruising along parallel transects at various distances from the recording unit. At frequencies over 125 Hz there was a minimum in front of the vessel, and lobes of higher intensity on both sides. About 20% of the herring schools recorded during a survey in the North Sea seemed to react to the survey vessel. The distance at which the schools reacted varied from about 25 m and up to about 1000 m in front of the vessel. Within a sector of 20° on each side of the path of the vessel, the fraction of schools reacting increased to about 50%. Most of the schools that reacted seem to be herded in front of the vessel. This reaction pattern may indicate either that the schools reacted to the higher sound intensity to the side of the vessel or that the pattern occurred because of a "pursuit effect". Despite the apparent herding, more than 40% of the schools in front of the vessel were not recorded by the echo sounder. It is therefore argued that horizontal avoidance close to the vessel may have caused an underestimation of the biomass of herring of about 20%.

Fish Farming enriches the water column in dissolved organic and inorganic material. These materials excreed by the fish, elutriate from uneaten food and derive from other polluting substances. Ratios of the organic/inorganic forms of substances (for example N and P) are thus modified. Such perturbations in turn affect phytoplankton development. From in vitro analyses and tests of water taken from a turbot rearing tank, we show the changes in seawater chemical composition produced as well as the way growth was modified in three phytoplankton species: the diatom, Chaetoceros gracile, and the dinoflagellates, Gymnodinium cf. nagasakiense and Alexandrium minutum. Elutriates from the granules used for feed (0.1 to 100 mg.l−1) were found to be stimulating, particularly for Gymnodinium. Alexandrium was strongly inhibited both by turbot-tank water diluted 10-fold with coastal seawater and by diluted faeces elutriates. Urea added at low concentrations (1 to 2.5 µmol.l−1) inhibited growth in Alexandrium and Chaetoceros, but at 1 µmol.l−1 it stimulated growth in Gymnodinium. The antibiotic, Oxytetracycline was complex in its effects, with growth inhibition being observed in Chaetoceros and Gymnodinium, but stimulation in Alexandrium (at 0.2 mg.l−1). In general, we found fish farming to allow phytoplankton growth, except for Alexandrium in a few cases. Additions to the water from fish farming may regulate growth in some species. As such additions act differently on growth in different species, fish farming is likely to markedly modify species composition in its vicinity.

An overview is given of our present knowledge on taxonomy, phylogeny and biogeography of catfishes or Siluroidei, one of the worlds economically important groups of fresh and brackish water fishes. In an introductory chapter, the taxonomic position of catfishes in the ichthyological classification is discussed; their external morphology is briefly described and synapomorphies showing their monophyly are listed. In the following chapter, cattish systematics are presented. Thirty three families with 416 genera and 2 584 species are presently recognized, but these numbers are continuously changing; it is therefore obvious that the systematics of many taxa are still rather poorly known. For each family, data (if any) supporting their monophyly are given together with a short description of their external morphology. The valid genera are listed, with for each genus, the number of presently recognized species with a reference (if any) to a recent revisionary study. Contributions to the phylogenetic study of catfishes are scarce, and dealing only with a few families and with a limited number of taxa within the families. As a resultsonly few data on catfish phylogeny are presently known. Therefore, also our knowledge on historicalbiogeography of catfishes is seriously limited, until more date become available on interfamilial and intergeneric relationships.

Les stratégies de pêche, développées par les capitaines des thoniers senneurs vénézuéliens dans la mer des Caraïbes, reposent sur la recherche d'indices visuels indiquant la présence de thons à la surface de l'océan. Une fois que la détection est réalisée, le senneur peut choisir la tactique qui consiste à demander l'aide d'un thonier canneur (pêche à l'appât vivant) afin d'augmenter le taux de réussite d'un coup de senne. Pour prendre en compte la variabilité de différents facteurs, comme la saison, les indices de détection associés aux bancs, l'aide du thonier canneur, les espèces de thons et le poids moyen des poissons présents dans le banc, les probabilités de réalisation de certains événements sont estimées à l'aide de la modélisation Logit, issue de modèles Log-linéaires. Les probabilités de capturer des bancs de gros individus sont maximales lorsque l'albacore est l'espèce dominante et que la pêche a lieu sur banc libre ou sur requin-baleine; cet effet est d'autant plus marqué en saison sèche.En revanche, si le listao est l'espèce dominante, la probabilité de capturer des gros individus est faible, en particulier lorsque la pêche est réalisée sur des baleines. Les résultats de cette modélisation sont intégrés dans un arbre de décision. Si on retient le critère de l'espérance mathématique des gains potentiels, l'aide du thonier canneur ne semble pas être la meilleure des tactiques possibles. Pourtant son impact est positif sur le taux de réussite d'un coup de senne, puisque le pourcentage de calées positives passe de 58 % sans aide à 85 % avec aide. L'efficacité de cette aide dépend du type d'association rencontré. Elle augmente lorsqu'il s'agit de bancs libres ou en association avec des baleines, mais diminue lorsque le banc est associé à un requin-baleine.Une simulation par la méthode de Monte Carlo montre que, malgré l'absence de cette aide, quelques gains importants peuvent être réalisés lorsque l'objet flottant associé est une baleine. En conséquence, le choix tactique qui consiste à demander l'aide d'un thonier canneur et qui est plus fréquent chez les petits et moyens scnncurs que chez les grandes unités, pourrait traduire une attitude de prudence face au risque, sans écarter certaines contraintes physiques de ces senneurs comme leur vitesse. Si on la compare avec d'autres zones de pêche thonière, la mer des Caraïbes se caractérise par une forte proportion de coups de senne faites sur requins-baleines (37 % des coups) et sur baleines (24 %). En contrepartie, le nombre de coups de senne faits sur des épaves flottantes est trèss faible. Ceci pourrait être lié à la nature des courants marins qui entraîneraient ces épaves en dehors de la zone de pêche des senneurs vénézuéliens. La similitude des comportements adoptés face au risque, dans des milicux très différents comme la pêche et l'agriculture, est discutée brièvement.

When discards data are included in fish stock assessments, it is generally assumed that all discarded animals die. However, there are fisheries in which a significant fraction of discards are able to survive. This case can be treated with a simple extension of the usual analytical model of exploited populations, enabling Virtual Population Analysis (VPA) to be used for parameter estimation. The paper examines the effects on estimates obtained by VPA when discards are allowed to survive, and validates a procedure for deriving partial fishing mortalities in multiple-fleet fisheries where discarding and survival rates are fleet specific.

European hake Merluccius merluccius is the main commercial demersal species in the Gulf of Lions (Northwestern mediterranean). This fish is exploited by French and Spanish trawler fleets and by small-scale fisheries using gillnets and longlines. Data on landings and the demography of catches by type of gear are now available for the period 1988–1991. The assessment of the hake stock was carried out using methods of virtual population analysis (VPA), or cohort analysis, based respectively on mean pseudocohorts and cohorts by age. The results of these VPA were then analysed and compared from a methodological point of view. The general results have confirmed previous studies with regard to the level of exploitation of the species. The two methods gave quite similar results for fishing mortality rates and for mean biomass. The maximal fishing mortality rate was observed at age 4, but it was already high at age 1, due to the exploitation of juveniles by trawlers. The mean biomass was about 7 000 tons. The high values for turnover (about 80% or more) and for the maximal biomass/mean biomass ratio (23% or more) indicated a high level of exploitation but did not seem high enough to be dangerous for the stock. Yield-per-recruit analysisshowed also that the present level of exploitation is far above the optimal sustainable level.

A protocol for the incubation of Centropristis striata ovarian follicles in vitro from females that spawn repeatedly under laboratory conditions was developed. Isolated follicles underwent maturation after 15–24 h of incubation at 25 °C in 75% Leibovitz L-15 medium, containing relatively low doses of human chorionic gonadotropin (hCG) or homologous pituitary extract (50% response with approximately 2 IU/ml and 0.02 pituitary equivalents/ml, respectively). During maturation in vitro, follicles underwent a remarkable volume increase (about 340%) that was correlated with proteolysis of primarily the largest yolk proteinin the oocyte. The maturation response of follicles to hCG was size-dependent within a range from 400 to 600 m in diameter, with a maximum response displayed by 530–600 m follicles. The response of these follicles to hCG was not inhibited within a range of incubation temperatures from 15 to 28 °C, but the length of time required for the follicles to achieve 50% maturation was negatively correlated with increasing temperatures (r2 = 0.97; p ≤ 0.01). However, the largest follicles (530–600 μm) did not undergo maturation in response to high doses (1 μg/ml) of the major maturation-inducing steroids (MISs) known in teleosts, 17.20-dihydroxy-4-pregnen-3-one (17.20β) and 17.20β.21-trihydroxy-4-pregnen-3-one (20βS), even after a previous incubation period of follicles with 10 IU/ml hCG for 1.3 or 6 h. These observations indicate that C. striata follicles are very sensitive to increasing doses of gonadotropin, and that their responsiveness to this hormone is highly size-dependent within a very narrow range. The preliminary results also suggest that 17.20β and 20βS are not the MIS in this species.

Dans cet article, nous étudions les associations entre variables qualitatives. Nous présentons une approche basée sur l'utilisation du modèle loglinéaire. Appliqués aux associations entre captures par unité d'effort (cpue) d'aloses (Alosa alosa), coefficient de marée (cm) et débit fluvial (df), différents modèles mettent en évidence que les captures par unité d'effort augmentent proportionnellement à la valeur du coefficient de marée et qu'elles ont des évolutions conjointes moins structurées avec le débit fluvial.

Siluriformes, as well as Characiformes and Cypriniformes, are a diverse and widespread group of Ostariophysan fishes, but Siluriformes have a probable ancestral benthic feeding habit. They have a unique suite of morphological, physiological, ecological and behavioural traits that equip them to succeed in freshwaters but only to a limited extent in the sea. They are typically, non-aggressive stalking predators that hunt at night or in turbid water using primarily nonvisual sense organs, although there are many exceptions. The modification of the Weberian apparatus for sound production has probably resulted in some loss of buoyancy control. Catfishes are represented in all the dillerent breeding guild categories and exhibit diverse and sometimes bizarre breeding methods. Catfishes tend towards the altricial end of the altricial-precocial life-history continuum. Only two families (Ariidae and Plotosidae) have successfully colonised the sea; physiological constraints and strong competition from Elasmobranchii and Actinopterygii fishes are probable reasons, and it is notable that the two families that have succeeded have precocial lire histories that are more suited to highly competitive environments.

The African catfish, Clarias gariepinus, is a highly appreciated species for aquaculture, because of its favourable food conversion, its resistance to discases, its relatively low requirements for water quality, the possibility for high stocking density and the excellent meat quality. For those reasons even in the Netherlands, there is a modest, but Europe's largest and still expanding, African catfish farming activity. Although this species grows and matures in captivity, there is no spontaneous reproduction. We coulddemonstrate that the failure to reproduce resides in the brain-pituiary-gonad axis. Hormones required for oogenesis and spermatogenesis are being produced in sufficient quantities. However, final oocyte maturation, ovulation, spermiation and spawning behaviour do not occur, due to the lack of a gonadotropin surge. In nature, the prespawning gonadotropin surge is induced by environmental factors such as the waterlevel in the spawning area. Under farming conditions the environmental cues are hard to identifyand/or to mimic. In combination with unavoidable stress this causes a blockade of the release ofgonadotropin releasing hormone (GnRH). Consequently, gonadotropin surge release fails to occur, which is enforced by an effective hypothalamic dopaminergic inhibition. The gonadrotropin surge induces the conversion of 17 αOH-progesterone into 17 αhydroxy-20 β-dihydroprogesterone, the final maturation inducing substance. Based on these data, several protocols for artificial propagation could be developed. They include either a treatment with a GnRH analogue in combination with a dopamine receptor antagonist, a treatment with homologous gonadotropin or HCG, or a treatment with 17 αOH-progesterone. Since a number of years we have used the African catfish as a model for fundamental research onfish reproductive endocrinology. Till now one gonadotropic hormone (GTH) could be demonstrated. Itsamino acid composition and sequence was analysed and appeared to be homologous with known forms ofthe maturational GTH (GTH-II). Specific radioimmuno assays for the complete hormone and its α- and β-subunit respectively, have been developed. cDNAS encoding the subunits have been cloned. They are applied now for Northern blotting and in situ hybridization. GnRHs were fully characterised (a specific catfish-GnRH and chicken-GnRH-II). Specific antibodiesagainst these peptides were raised and the cDNAS encoding the hormone precursor molecules werecloned and used for respectively immunocytochemical localisation and radioimmunoassays, and in situ hybridisation. The importance of the two GnRH forms for gonadotropin release was studied. Chicken-GnRH-II appears to be 10 to 100 times more potent than catfish GnRH, probably due to its higher receptor affinity. Catfish GnRH, however, is present in the brain and pituitary about 100 times more than chicken GnRH-II. Steroid hormone synthesis by ovaries, testis and seminal vescicles was analysed. The sex sieroids that play a role in the negative feedback control of gonadotopin release were identified (11-keto-testosterone and testosterone) and their interaction with hypothalamic dopamine metabolism was demonstrated as one of the possible mechanisms of action. Several steroid conjugates from the seminal vesicles were shown to have pheromonal activities, involved in reproductive behaviour. They induce under certain physiological conditions attraction between conspecifics and synchronization of ovulation.

Un système expert simple, reposant exclusivement sur des statistiques de pêche, a été élaboré afin d'estimer des migrations de poissons entre zones. Les flux de migration par trimestre sont calculés à partir de la comparaison de deux estimations distinctes des effectifs par zone et par âge, la première tenant compte du phénomène des migrations, la seconde non. La première estimation est obtenue en répartissant par zone les effectifs par âge issus d'une analyse des cohortes, la seconde est calculée période par période, à partir des effectifs présents dans chaque zone à l'âge précédent. Le modèle est appliqué au listao de l'Atlantique tropical Est. Les flux de migration estimés sont alors analysés, par groupe d'âge puis pour l'ensemble de la population. Bien qu'un certain nombre de limitations inhérentes au modèle existent, les résultats obtenus apparaissent valides par rapport aux schémas connus des migrations du listao.

Neural networks provide a "black box" model for explaining and predicting trout abundance with 8 environmental variables. This work investigates the specific effect of each variable, by inputting fictitious configurations of explanatory variables and by checking the responses of the model. The comparison between this response of the model to environmental variables on one hand, and results from field observations on the other hand, shows similarities and indicates neural network modelling can be trusted. The elevation appears to be the major explanatory factor. The influence of shelters, bottom velocity and Froude number also play an important role. When considered separately, depth does not have a notable influence on the density of trout. Such confirmations of field observations suggest that these models can be used to obtain a clear identification and hierarchization of the factors influencing the abundance of trout and the mode of action of the factors. This approach can be extended to other applications in quantitative ecology in which non-linear relationships are usually observed.

Sperm was collected from testes and from three or four parts of the deferent ducts of 4 to 6 winter-spawning male rainbow trout, about every two weeks over a three month period (January toMarch). Milt volume, spermatocrit, sperm motility, 11-ketotestosterone (11-KT) and 17α-hydroxy,20β dihydroprogesterone (17,20P) levels and ionic composition were determined. Steroids and ions were also measured in blood plasma. The K+ concentrations and steroid levels in the seminal fluid from each part of the ducts did not vary significantly over the period of study. In two out of the three parts of the ducts, Na+ (parts 1 and III) Ca2+ (parts I and III) and Mg2+ (parts I and II) levels were significantly higher at the end of March than two months earlier. Motility of testicular sperm peaked (42%) in mid-February while that of sperm from ducts usually remained stable (80 to 100%) throughout the three months. Seminal fluids and blood plasma were confirmed to be isotonic (300 mmol/kg). Levels of K+, Na+, Ca2+, 11-KT and 17.20P in the plasma and seminal fluid differed significantly (respectively 0.3 vs 34 mmol, 134 vs 116 mmol, 3.4 vs 2.0 mmol, 87 vs 17 to 39 ng/ml, 11 vs 3 to 13 ng/ml), with the Na+ and steroid levels being positively correlated in both liquids. Osmolality, Cl-, Na+, K+, Mg2+ and Ca2+ concentrationsremained constant along the genital tract. In contrast, spermatocrit and 11-KT and 17.20P levels decreased significantly and progressively from testis to the posterior or middle part of the ducts (respectively 97 to 49%, 39 to 17 ng/ml, 13 to 3 ng/ml) while sperm motility (percentage of motile cells and duration of motility) increased (74 to 96% and 41 to 51 s). No significant correlation was found between sperm motility and the studied physicochemical parameters of the seminal fluid.