The paper explores the apparent contradiction between a high trawling pressure on juveniles and sustained production of hake that has occurred over the last decade in many Mediterranean fisheries. The practical consequences are followed of assuming rapid declines in natural mortality rate M in the first few years of life to a low, constant adult natural mortality, as well as the observation, for small-mesh trawl cod ends, of declining availability with age. Several approaches are proposed for fitting declining M-with-age with a reciprocal function for hake, using criteria based on mean life-time fecundity, mean age at egg production, existing estimates of adult M, and vectors based on stock productivity assumptions. All vectors of M-at-age were similar to MSVPA estimates of North Sea stocks.

The implications of the changes in mortality with age for stocks harvested by fine-mesh trawls wereexplored in yield per recruit calculations under 2 different hypotheses: 1) using current estimates of growth and adult mortality, 2) M-at-age vectors for juveniles, dropping rapidly from age 0+, and declining availability to trawling for older fish. These hypotheses were compared within yield per recruit analyses. Under the new assumptions, given current F>>M (adults), yield isopleths predict no significant increases in Y/R with stretched mesh > 40 mm, but a substantial decline in fecundity per recruit with small increases in effort by gill nets or longlines, aimed at mature fish. These results are linked to the refugium concept for older fish, and it is speculated that this may be in part responsible for the continued productivity of other sustained fisheries for juvenile resources elsewhere.

The Code Activated Transponder (CAT) ingestible fish tag capable of operating at depths of 6 000 m is described. The CAT is acoustically interrogated by a scanning sonar mounted on the AUDOS free-fall camera lander vehicle. The CAT, 12.5 mm diameter and 65 mm long works at ultrasonic frequencies (77 KHz) and can be tracked to a range of 500 m. The AUDOS lander incorporates the new TRATEX (Transponding Acoustic Tracking Experiment) system that logs direction and range of individually tagged fish together with current and compass information. Typical results are shown from a track of 3 Coryphaenoides (Nematonurus) armatus (Macrouridae) taken at the Porcupine Seabight in the North Atlantic (49° 35'N-13° 43'W), 3 500 m deep. The mean fish swimming speed was 0.087 m.s−1. Despite their low speed, fish moved independently of the bottom current, either cross-current or against the current, optimising the chance of encountering odour plumes from new food falls.

Scanning electron microscopic observation of embryos and larvae of the carp (Cyprinus carpio) in the course of development was undertaken to further understanding of the relationship between larval behaviour and their chemo- and mechanoreceptors. Free neuromasts and olfactory pits equipped with mechano- and sensory-cilia were found on carp embryos. During larval development, the number of neuromasts and the olfactory mechano- and sensory cilia steadily increases and they become more morphologically differentiated. The olfactory features, presented in early larval stages, indicate that although the olfactory organ at this early life history has not yet attained its final configuration, it can perceive odour stimulation. The neuromasts found on the forehead of the newly hatched larvae seem to assist in the early behavioural selection, approach and attachment to a substratum, before the stage of filling of the gas bladder. The increasing number of neuromasts during development reflects the larvae's capability of capturing food items in the absence of visual stimulation. The rapid maturation of free neuromasts, the lateral-line system and the olfactory organ, is evidence of their post-metamorphosis position in the search for, location and successful capture of food.

Sampling operations are not routinely conducted in France to monitor the landings of bluefin tuna caught by purse seiners in the Mediterranean. The length, weight and age frequencies of bluefin caught by this fleet are predicted based on sale records, length-at-age and weight-length relations, trends in fishing and natural mortality rates, and other ancillary data. The resulting length frequencies are more accurate than those predicted by Liorzou and Bigot (1992) using a simpler algorithm, and are spread more evenly across adjacent size classes. Catch-per-unit-effort (CPUE) was expressed as the numbers of age 2 bluefin caught per successful seine fishing trip in the Gulf of Lions fall fishery. CPUE indices were computed by several methods, and compared to that of the spanish baitboat fishery in the Bay of Biscay. The index trends of the french and spanish fleets exhibit a general increase during 1970–95, but the results do not demonstrate whichof these is the best indicator of juvenile abundance. Complementary surveys are required to determine the temporal and spatial distributions of surface schools and fishing effort, and identify the most reliable index of abundance.

In this study, the remuneration system applied in the French Guyana shrimp fishery is analyzed in relation to the market changes that this fishery has experienced since 1985. The contract used for the crew members is not the classical share contract usually observed in fisheries. It is a piece rate wage with quality-bonuses. In the classical contract theory literature the piece rate contract is thought to avoid effort shirking but to induce quality shirking. The bonuses are introduced here in order to limit the quality shirking which may occur during grading and storing operations on board. The skippers' contract also differs from the share contract. It is based on remuneration parameters controlled by the companies. By changing these parameters between 1987 and 1991, the companies led the skippers to modify their fishing strategies and especially to concentrate their efforts towards small sized shrimp. The companies thus became able to respond to the increasing demand for small shrimp categories which characterises the French market. The evolution of the spatial distribution of the fleet effort over the same period is consistent with this hypothesis: the fleet has moved towards fishing zones where small shrimp are known to be in abundance.

A biotelemetry experiment was designed to determine whether the selection of a residence area by adult barbel Barbus barbus (Cyprinidae) was dependent on the availability of physical habitat features, on the presence of fish shoals and/or on the presence of specific partners. The experiment took place in 3.0 km stretch of the River Ourthe (Belgium), where the latter features had been characterised by habitat and electric fishing surveys (1 190 fish ≥ 15  cm, at 265 locations). Six barbel (34.8–49.5 cm) belonging to a single shoal of 34 fish were tagged with 40 MHz radio transmitters, transplanted in pairs at different distances (−1 km, −30 m, +1 km) from the capture site and tracked from 3 September to 7 October 1990.

Fish showed variable mobility patterns and home range sizes (200–2 400 m). They selected 17 residence areas of which the cumulative frequency of occupation ranged from 0.1 to 79.5 day × fish. The comparison between the residence areas selected, habitat suitability and the location of fish shoals in the study area indicate that the presence of a shoal of at least 10 resident fish within a suitable habitat (normalised suitability index ≥ 1.0) is necessary but not sufficient to guarantee its long term selection and occupation. Homing movements of transplanted fish were directed towards the activity area rather than to the residence area itself. The fish that did not home, selected residence areas in a part of the river with a higher availability of potential feeding areas. These elements are discussed within the context of habitat utilisation and foraging strategies by a gregarious species. It is suggested that resident fish of the same species may ease the exploitation of a non familiar environment by a naive fish, and that this may positively trade off the benefit provided by home site fidelity, depending on the availability of food resources.

Results are presented from laboratory-based experiments in which we investigated short-term responses of Mytilus edulis L. to experimental changes in the amount and composition of suspended seston. Working with large quantities of cultured algae, we have studied feeding behaviour over ranges of food availability and quality that extend well beyond earlier limits. Findings confirm the ability of mussels to selectively reject inorganic particles as pseudofaeces prior to ingestion, thereby enriching the organic content of ingested matter by 30% more than the organic content of natural filtered seston. Our findings also establish acclimation both of selective and absorptive processes, indicating that the extent to which growth of M. edulis can be stimulated in the short-term will depend upon prior nutritional history. Present maximal growth of 14.8% dry soft tissue d−1 in a standard M. edulis of 1 g dry soft tissue was much higher than has previously been documented for any adult mussel. Maximal growth was achieved when natural seston that had been enriched to more than about 60% organic content with a mixture of algal monocultures was available at concentrations above about 11.5 mg total particulates 1−1. Neither ingestion rate nor net energy balance were improved with further increases in food availability, associated with regulatory reductions in the rate of water filtration (clearance rate) that maintained organic ingestion rate independent of increases in both the amount and organic content of available seston. These findings suggest that digestive processes had become saturated when organic ingestion reached about 6.5 mg organics g−1 dry soft tissue h−1 in a standard M. edulis of 1 g dry soft tissue, representing as much as 20.6% of all soft tissue organic mass mussel−1 d−1. Such saturation of organic ingestion is consistent with previous conclusions based on comparative allometries showing that limitations to growth in M. edulis and other bivalves are associated with the rate of food processing.

L'étude présente une application de la méthode géostatistique transitive à une dimension (1D) pour estimer la précision des évaluations de biomasse effectuées au Sénégal par écho-intégration. Le problème majeur rencontré dans cette étude a été la prise en compte de la variabilité temporelle dans l'estimation de la variance d'échantillonnage. Lors des campagnes acoustiques, l'écho-intégration se fait en continu le long de la route du navire. Lorsque le plan d'échantillonnage est constitué de transects parallèles, cette caractéristique permet de considérer chaque transect comme une unité d'échantillonnage. En faisant la somme des valeurs mesurées le long des transects, l'estimation à 2D est simplement ramenée à une estimation à 1D. Si de plus les transects sont équidistants, la méthode géostatistique transitive à 1D offre une solution correcte et simple au calcul de la variance d'estimation. La méthode a été appliquée à la série des campagnes acoustiques sénégalaises qui ont toutes été effectuées selon le même réseau de transects équidistants et parallèles d'orientation Est-Ouest. Les densités sont sommées le long des transects. Les variations latitudinales de la biomasse par transect sont présentées sous forme de profils à 1D. La biomasse, la répartition spatiale et l'autocorrélation sur ces profils présentent des différences importantes d'une campagne à l'autre. Les profils ont été regroupés en 4 catégories présentant des caractéristiques différentes. L'autocorrélation moyenne (structure) par catégorie a été estimée et modélisée. Une valeur moyenne de l'erreur relative d'estimation est fournie pour chaque catégorie. L'erreur relative d'estimation varie en fonction des différences de structures entre catégories et aussi en fonction de différentes hypothèses pour la modélisation de la structure aux petites distances. Deux cas ont été considérés : continuité de la distribution (absence d'un effet pépite) ou discontinuité (présence d'un effet pépite). L'erreur relative maximale estimée est d'environ 31 %. Le problème majeur rencontré à la modélisation de la structure 1D des biomasses par transect a été de choisir de prendre en compte ou non un effet de pépite. L'interprétation de cette pépite serait la variabilité temporelle à l'échelle de la radiale.

Surgery procedures were used to implant telemetry transmitters into the body cavity of adult (574–1 033 g) tilapias Oreochromis aureus in aquaculture tanks (4 m2, 1.5 m3, 26.5 ± 0.5 °C, ≥ 5.0 mg O2 l−1) and their effect on fish survival, growth and behaviour was evaluated. Only one out of 35 implanted fish died. With one exception, all fish consistently retained their transmitter until the end of the study (up to 30 months). Healing was faster when the incision was sutured with polyamide monofilament (5–14 days) than with other suture materials, due to tunnelling with atraumatic needles for catgut or fouling of braided silk. In all 10 fish sacrificed after 30 and 50 days, the transmitter had become encapsulated by connective tissue. No infection or damage to the viscera was observed.

The activity of four tilapias (903–1 033 g) equipped with motion sensitive transmitters was telemetered during the recovery from anaesthesia and surgical procedures. All four fish maintained a normal diurnal activity rhythm pattern throughout the study but had low levels of activity during the first 12–24 h. Based on the evolution of their resting posture after surgery, it is suggested that tilapias need 3 to 4 days to completely compensate the negative buoyancy resulting from anaesthesia and tagging.

The objective of the study was tu evaluate the capacity of rainbow trout to discriminate diets containing micro-granulated or powdered novel fluoroquinolone in comparison with a positive (containing sulfamerazine sodium) and a negative control (placebo). In a first trial, groups of trout had access to a self-feeder containing the placebo feed for 10 days, and then to one of the four feeds during another period of 10 days. Fish reduced their voluntary feed intake (VFI) by more than 50 % when they had access to the positive control, and by more than 20 % when they had access to feed containing micro-granulated premix. There was no significant decrease in VFI when the fish had access to feed containing powdered premix, but in this latter case, an increase in feed refusals was observed. In a second trial, the capacity of the trout to discriminate between two self-feeders containing different feeds was tested. During a first period of 10 days, all the feed hoppers were loaded with the placebo feed, to determine the feeder preference relative to any dietary-independent factor. In the following period of 10 days, fish had access simultaneously to the two medicated fecds. During a third period of 10 days, a validation of the results was attempted, using the placebo feed or the positive control in one of the feeders. It was shown that trout were able to discriminate between the two medicated feeds and that micro-granulated premix was better accepted than powdered premix. In addition, an increase in feed refusals was observed when the preferred feeder was loaded with feed containing powdered premix. It is concluded that this tested fluoroquinolone could decrease significantly VFI. The powdered premix was more detectable by the fish than the micro-granulated premix and led to some refusals.

Cette étude s'appuie sur l'utilisation successive de plusieurs méthodes d'analyses multivariées pour réaliser une typologie de la flottille chalutière du port de Sète, principal port de pêche français de la Méditerranée. Les unités de pêche étudiées sont représentées à l'aide d'un profil annuel de la composition spécifique de leurs débarquements. Cinq stratégies de pêche ont ainsi pu être identifiées. La mise en œuvre d'une méthode de segmentation permet une caractérisation formelle de ces différentes stratégies à l'aide d'objets symboliques. Les stratégies sont étudiées en référence à plusieurs caractéristiques générales couramment utilisées en gestion des flottilles de pêche (puissance, longueur, âge du bateau, etc.). L'analyse typologique mise en œuvre au cours de l'étude permet de caractériser nettement deux grandes logiques d'exploitation, l'une orientée vers la recherche d'un très petit nombre d'espèces (Engraulis encrasicolus, Sardina pilchardus), l'autre déterminée par une grande diversité des captures. Elle permet également d'apprécier comment les captures d'espèces peu représentées dans certaines stratégies peuvent contribuer significativement à l'exploitation d'une ressource.

Yield-per-recruit models are often used to provide management guidance for the efficient use of a fish cohort and to estimate the biological reference points, such as F0.1 or FMAX. Although model parameters are usually age-specific, these parameters are thought to be more likely related to length than age. In this study, age- and length-structured models are compared in calculating yield per recruit, F0.1, and FMAX at which the yield per recruit is maximized. Using a Monte Carlo simulation approach, I also compare differences in effects of uncertainties of the model parameters on yield-per-recruit analysis and estimation of F0.1, and FMAX using age- and length-structured models. This study shows that differences are small in estimating yield per recruit, F0.1, and FMAX using two types of models when there are no error variances in the model parameters. However, such differences increase with the uncertainties of the model parameters. The yield-per recruit, F0.1, and FMAX estimated using the age-structured model tend to have smaller values and variations compared with those estimated using the length-structured model. This study indicates that although the length-structured model can better incorporate the information observed from fisheries, age-structured yield-per-recruit model can provide estimates of yield per recruit, F0.1, and FMAX, more precisely and more conservatively, and thus is preferred from the conservation viewpoint in fisheries management.

The purse-seiners of the Java Sea use light and fish aggregating devices (FAD) to concentrate and catch small pelagic fish (Decapterus sp., Amblygaster sirm, Sardinella gibbosa). This sets a question about the interaction between fish and fishing vessels. Relevant estimates of fishing mortality and fishing effort require an understanding of this interaction. To gain information on this relationship, acoustic surveys were performed on the fishing grounds and the present paper analyses these data. Using acoustic surveys, the fish density was measured and the number of schools was counted per sea surface unit. The fishing vessels were located by radar. Acoustic data were stratified into two groups, one where fishing vessels were present and the other where they were not. This stratification was tested for by resampling the data. Day and night sampled data were analysed in each strata separately. Variograms were computed to characterise spatial structure both for the fish and the purse-seiners. Contingency tables between number of schools and fish density were computed to characterise the relation between biomass and schools for the day and the night when seiners are absent or present. At night, in the absence of purse-seiners, the number of schools increase and fish rise from the bottom. In areas where purse-seiners are present this natural phenomenon is stronger. It also changes the spatial structure both of the fish density and the school abundance. The spatial structure of the vessels is similar to that of the fish density. The fishermen seem to be able to detect by day fish concentrations which will be fished later by night.

This paper presents a multispecific approach in order to assess the main demersal resources exploited off Senegal (West-Africa). In absence of reliable data on the actual fishing effort, two series of theoretical fishing effort are estimated, using available species abundance indices from 1971 to 1991. These two series correspond to different hypotheses concerning the trends of fleets' fishing power: an optimistic one and a pessimistic one. For each species or taxa, a surplus production model is calculated with landings statistics and effort. The combining of monospecific models provides a multispecific diagnosis. The analysis is completed by the calculation of diversity indices of catches at the equilibrium level, expressed as a function of fishing effort. While considering the uncertainty around this first assessment, we show that Senegalese groundfish resource is globally full or over-exploited, according to the considered hypothesis. Under the pessimistic hypothesis, 9 among the 11 taxa of Pisces were clcarly over-exploited, with fishing effort some-times as much as twice or three times the effort of maximisation (fMSY). The situation is very critical for Arius sp., Epinephelus aeneus, Pagellus bellottii and Pomadasys sp. Using the multispecific approach, pessimistic diagnosis indicates a fishing effort nearly twice fMSY, with a Maximum Sustainable Yield at about 53 000 t for the 11 taxa together. Conversely, in 1991, octopus and cuttlefish appeared under-exploited. The diversity analysis shows that an increase in fishing effort would lead to a decrease in catch diversity. This result is interpreted as a sign of fragility in the exploited ecosystem.

La gestion halieutique des lacs exige une estimation réaliste du potentiel exploitable. Aussi, est-il pertinent d'utiliser des relations empiriques développées sur des systèmes semblables et liant des facteurs abiotiques aux rendements de poisson ? L'utilisation des données abiotiques de 65 lacs africains montre que les ajustements avec les modèles antérieurs faisant intervenir l'indice morpho-édaphique et les rendements (relation non significative), la surface totale et les captures totales (R2 = 0,76) ou l'effort de pêche et les rendements (relation non significative) sont peu précis et insuffisamment fiables pour prédire les captures potentielles. Les relations faisant intervenir les rendements et l'altitude ou la latitude des lacs ne sont pas significatives. Celles qui impliquent la profondeur moyenne, la surface ou le volume le sont mais n'expliquent qu'un faible pourcentage de la variance (respectivement 13,11 et 17 %). L'indice morpho-édaphique reste le paramètre explicatif le moins mauvais (r2 = 0,42) lorsque l'effort de pêche est supérieur à 2 pêcheurs par km2. Ces mauvais résultats semblent largement dus à la qualité médiocre des données disponibles. Il paraît donc important de remédier à cette situation en favorisant la mise en place sur de nombreux lacs africains d'observatoires des pêches impliquant des méthodes de collectes homogènes et assurant des résultats fiables.

Swimming capabilities and in situ measurement of muscle activity from adult Atlantic salmon (Salmo salar) at two seasonal temperatures were measured using radio transmitted electromyogram (EMG) signals. Forced sustained levels of activity and critical swimming speeds were determined and correlated to radio transmitted EMG signals using a modified Blazka swim speed chamber. There were no differences in swimming performance levels between tagged and untagged individuals. At 18 °C, sustained activity and critical swimming speeds were approximately 70% and 20% higher than at 12 °C, respectively. No differences in burst activity were observed at these temperatures. EMGs recorded from salmon during ascent of an artificial flume at cold temperatures revealed that overall muscle activity is greater than that observed for critical swimming speeds. This implies that white muscle may be recruited at this temperature. However, in contrast, most activity at 18 °C is below that observed during critical swimming speed. Moreover, salmon required almost twice as long to traverse the flume at 18 °C than at 12 °C. Together, our data demonstrates that salmon may recruit white muscle fibres and incur an oxygen debt at colder temperature as a strategy for ascending velocity obstructions at a quicker rate.

The uptake and release of material by oysters (Crassostrea gigas) and mussels (Mytilus edulis) in Marennes-Oléron Bay (SW France) were measured in situ using benthic ecosystem tunnels (BEST). There was a large variation in seston concentrations, owing to high sedimentation/erosion dynamics. Organic matter was diluted by resuspended sediment, but had a low C/N ratio, i.e. a high quality, owing to resuspension of microphytobenthos. In many cases a significant decrease of chlorophyll concentrations was observed in tunnels with oysters and mussels. Average clearance rates per tidal cycle ranged from 4 to 7, and from 0.9 to 2.7 1.g−1AFDW.h−1 throughout the season, for oysters and mussels respectively. Calculation of the filtration pressure (the fraction of the primary production filtered per day), indicated that bivalves in the bay were able to filter much more than the daily pelagic primary production. It seemed obvious that the bivalve stocks depended not only on phytoplankton but also on microphytobenthos. Significant regeneration of nitrogen could be demonstrated from musselbeds on the sediment, at higher rates than direct excretion of ammonium. No significant release of nutrients was observed for the oysters or mussels, when they were put on tables, as practised by the oyster farmers. lt seemed likely that biodeposits were flushed away from the tunnels in this case, and mineralization occurred elsewhere. It was concluded that the carrying capacity of the bay for herbivores is extensively exploited by the shellfish. Mineralization of bivalve biodeposits represent a large potential for nitrogen regeneration.

The aim of the present study is to test the extent to which different intensities of continuous additional light affect somatic growth and sexual maturation in Atlantic salmon postsmolts in sea cages. Postsmolts (9 200 individuals) were randomly distributed among four commercial sized sea cages and exposed to either natural light or natural light + continuous additional light of low, medium or high intensity from January to June. In May the high light intensity group had a signiticantly higher mean live body weight than the other groups, and at the end of the experiment in June, all groups differed in mean gutted weight. There was apositive logarithmic regression between level of light intensity exposure during night and mean gutted weight. A small proportion of fish matured sexually (after 1.5 years in seawater) in the natural light group, whereas no maturation was detected in the light groups. These results suggests that to affect growth and proportion or maturation in Atlantic salmon by additional light, there might be different threshold values of light intensity.

Gastrointestinal evacuation of inert X-ray dense marker by turbot, Psetta maxima, was followed by use of radiography. Groups of individually tagged turbot aged 1 + years (136.5 g), were held at 16 °C and acclimatized to a feeding period of 4 hours per day. Fish were fed on pelleted feed in excess. The experimental design allowed a test of the effects of handling stress and post-prandial feeding on gastrointestinal evacuation. Repeated handling affected feed intake, but there were no significant treatment effects on gastrointestinal evacuation rate. Fish that ingested large meals displayed higher evacuation rates of markers than fish which ingested smaller meals, but total evacuation time was longer for the fish that consumed the largestmeals. Marker particles started to be voided from the gastrointestinal tract approximately 10 hours following initiation of a meal. and evacuation of marker was completed within 52 hours under the holding conditions employed in the present study.

La répartition spatiale des nasses à poisson et à langoustes sur le plateau sud-est de la Martinique est étudiée au moyen d'observations en mer et de données bathymétriques. A l'intérieur du récif-barrière, l'essentiel de l'effort est déployé le long de la ligne de tombants qui borde la côte ; à l'extérieur du récif, la répartition individuelle des nasses est pratiquement indépendante du relief sous-marin, mais il existe une forte corrélation entre les moyennes d'effort et d'indice de relief par tranches bathymétriques de 5 m. La relation seulement indirecte entre effort de pêche et relief du fond à l'extérieur du récif semble être le résultat de conditions d'exploitation difficiles (éloignement de la côte, état de la mer) mais aussi de choix délibérés des pêcheurs. Ces résultats conduisent à discuter la notion de « surface exploitée » utilisée pour calculer des captures ou des efforts par unité de surface. L'utilisation de surfaces corrigées à l'aide des indices de relief conduit à une image plus réaliste de la répartition de la pression de pêche sur le plateau insulaire de Martinique.