The present paper suggests a method for estimating the fishing power of vessels and for analysing fleet dynamics. The approach is based on quantification of stocks catchability (q), derived from fishing mortality coefficients (F) as calculated by virtual population analysis. Catchabilities for each harvested stock are thus estimated relatively to the fishing effort (fn) of each vessel, according to the equation: q = (F/fn). A linear model is then fitted to these catchabilities. The model allows the identification and quantification of trends in average mortality rates per fishing hour for each stock. Under some assumptions, trends are interpreted as variations in the overall fishing power of each fleet. The approach is applied to three industrial and semi-industrial fleets of Brittany (Lorient, Concarneau and Douarnenez) and to the main gadoid stocks they exploit off the west coast of Scotland (ICES area VIa), and in the Celtic Sea (ICES area VIIf,g,h) for Concarneau. Results show large variations in fishing power. Particularly, a marked increase trend in the fishing power exerted on saithe (Pollachius virens) is highlighted for the three fleets, over the period 1983-1989. These variations can be explained by the redirection of fishing strategies, which may occur on a large scale. Thus, we show how the collapse of saithe stock at first led the three fleets to intensify the harvesting of saithe, and from 1989 on, to adopt different strategies. The possible causes of the observed dynamics are discussed, as well as their consequences for fisheries management. In particular, the relevance of direct control of fishing effort as a regulatory tool is questioned.

The in vivo distribution and fate of 14C-cholesterol injected in prepuberal female prawns, Penaeus japonicus Bate, intact (E+) or deprived of their eyestalks (E−), were investigated. Animals were injected at the beginning of the experiment. Three days later, half of them (24 specimens) supported bilateral eyestalks ablation within a 48 hours period. Four days after the ablation, the ovaries of eyestalkless animals were relatively more developed than those of intact animals as seen by the increase in ovarian weight. The total content 14C-cholesterol and/or its metabolic products (total dpm) in whole animals was not significantly different between the two lots of animals and remained nearly constant throughout the experiment.

Distribution of radioactivity (dpm/mg wet weight of tissue) in carcass, muscle, gut, hepatopancreas,eyes and eyestalks and ovaries from E+ and E− after 192 hours of 14C-cholesterol injection indicate that ovaries presented the highest concentration of label (6 to 40 times higher than other organs) while these differences were not statistically significant between E+ and E−. The study of incorporation of 14C-cholesterol in those organs from E+ and E− independent of their volume, expressed as percentage organ radioactivity/whole animal radioactivity, at different time points, indicate that major accumulation was achieved by the carcass and muscle although radioactivity in the muscle progressively increased in inverse correlation with the decrease observed in the carcass. Hepatopanereas, eyes and eyestalks, gut and ovaries showed small incorporation that was kept constant with time in eyes and guts and significantly decreased in ovaries and hepatopancreas. At 192 hours after 14C-cholesterol injection there was an apparent increase in 14C label retention in ovaries from E− compared to E+ that was not statistically significant.

Results indicate that the ovary is presumably the major site of cholesterol metabolism followed bythe hepatopancreas. Muscle seemingly is the major site for storage of large amounts of cholesteroland/or its metabolites while eyes and gut have negligeable retention. Furthermore, in our experimental conditions, the requirement for cholesterol in different tissues does not vary significantly with eyestalk ablation suggesting that the phenomenon could be independent of ovarian maturation and moulting.

Atlantic salmon juveniles were subjected to a precocious (day 104 after first feeding) photoperiodic (P) and thermic (T) control. After rearing the fry at 16 °C, constant (16 hours of light and 8 hours of darkness, 16 °C) (C) and shortened simulated natural S photoperiod and temperature regimes were used in a factorial experimental design until transfer to seawater. The growth obtained with constant (C) regimes was better than with S regimes. In all cases, we observed very early (day 141, mid-June) a clear segregation of the population into two different subpopulations. Among the conditions, all the differences between the upper and lower modes were growth-dependent. We observed different developmental trends of the gill (Na+-K+)-ATPase activity: the S regime gave a very progressive increase of the enzymatic activity, while we had a rapid decrease after a first phase of increase with a constant photoperiod. The absolute levels of the gill (Na+-K+)-ATPase activity were slightly lower than those observed in one year-old smolts. However, they are quite similar to those of upper-mode fish that have a good salinity tolerance. After transfer to seawater, fish subjected to shortened simulated temperature and photoperiod had the lowest mortality and osmotic disturbance (after 48 hours and 3 weeks). Although the freshwater phase generally lasts more than 15–17 months for this species in a natural environment, we concluded that it is possible to obtain a good salinity tolerance after 7–8 months, with shortened simulated natural photoperiod and temperature regimes, in spite of the absence of a real smoltification; these regimes, however, must be applied during the "dynamic" presmolting phase when the growth is intense.

The Rideau Canal in Canada was constructed in the 1800s to enable strategic military andcommercial transport in eastern Ontario between Lake Ontario and the Ottawa River. Today,the Rideau Canal is managed by Parks Canada and remains an engineered ecosystem,particularly in the urban Ottawa reach (7 km long) where it is largely channelized and ispartially drained during the winter to protect canal infrastructure (i.e., locks) andprovide recreational opportunities (i.e., skating). Interestingly, the Ottawa reach of theRideau Canal supports one of the few wild urban muskellunge (Esoxmasquinongy) fisheries in North America supported by natural reproduction.However, little is known about the spatial ecology, residency patterns or survival of thisapex freshwater predator in urban environments or canal systems. Radio-telemetry was usedto study adult (N = 10; total size range of 64 to 122 cm) muskellunge inthe Ottawa reach of the Rideau Canal over a multiple year period. The seasonal movementsand home range of muskellunge were greatest during spring corresponding with the periodwhen water levels in the canal were raised and muskellunge were presumably seeking outsuitable spawning habitats following ice-off. During the summer fish ranged throughout theentire 7 km urban reach of the canal and by early fall muskellunge moved to the deeperparts of the reach and became largely restricted to an embayment known as Dow’s Lake priorto the lowering of the canal where they stayed for the entire winter. The same pattern ofseasonal movements persisted across several years. None of the tagged muskellunge werestranded from the fall canal drainage. One fish emigrated from the urban reach of thecanal during the study period, moving upstream through a lock to an exurban reach. Inaddition, a single tagged fish died from a winterkill event. Environmental influences suchas seasonality and water depth (associated with canal operations) are believed to be theprimary mechanisms contributing to habitat selection and movement patterns of muskellungein this reach of the Rideau Canal. Data on the spatial ecology of muskellunge in the urbanreaches of the Rideau Canal will inform management of this unique population of fish toensure that the historic and ecological values are balanced to preserve this iconicCanadian natural heritage site and its biota.

The ultrastructure of spermatozoon and sperm production were observed in wolffish (Anarhichas lupus), a species with internal fertilization, from a broodstock. The mature spermatozoon has an elongated head, eccentric flagellum, and comparatively well developed middle piece. Based on the observations of sperm cells with the optical microscope, the mean lengths of spermatozoon head and flagellum were 3.3 and 21.2 μm respectively. A high variability of sperm cell size was observed due to the presence of spermatids and degenerated spermatozoa in the ejaculate. In most males, sperm production was registered throughout the whole year with a peak in December and January. Sperm production decreased to the end of the breeding season. The difference in volumes of ejaculate and sperm concentrations between males kept at two light cycles (18D: 6L and 6D: 18L) was not significant. Features of wolffish sperm are compared with data reported in the literature on other internally and externally fertilizing fish.

One method to assess fish stocks using a suite of indicators is the traffic light approach. In this approach, the time series of the different indicators are mapped on a common colour scale to highlight alerts that occur when indicators cross reference limit values. Until now, however, the procedure has lacked a statistical framework. Here, we propose the cumulative sum (CUSUM) monitoring scheme as a suitable statistical framework. CUSUM is a statistical process control method that detects deviations from a reference mean, according to defined performance criteria. With the CUSUM monitoring scheme, alarm signals can be triggered when indicators cross defined in-control limits that correspond to defined probabilities of false alarm and non-alarm (i.e., precision and power of the CUSUM monitoring scheme). A table of CUSUM out-of-control deviations is constructed to serve as a diagnostics table. In this table, the deviations in the different indicators are quantitative and given in similar units of variance, which facilitates their integrated assessment. The CUSUM out-of-control table also shows how deviations accumulate over time and thus provides a view of the stock history. The procedure was applied to the North Sea cod stock to illustrate how a fishery-independent integrated assessment can be achieved using a suite of indicators derived from research survey data. The indicators used were related to the spatial distribution, abundance, length structure, length at maturity and apparent mortality. The stock was found to be outside its reference limits from 2001 and has shown a continued degradation in status since this time.

La diversité spécifique est généralement considérée comme un facteur-clé de la résilience des écosystèmes en réponse à la pression anthropique et notamment à la pression de pêche. Dans cet esprit, on analyse ici l’impact de la pêche sur la diversité spécifique, à partir du cas des ressources démersales côtières de Guinée. Celles-ci ont la particularité d’avoir subi récemment un très rapide et important accroissement de la pression de pêche. En une quinzaine d’années, on est ainsi passé d’une situation dans laquelle les ressources démersales étaient considérées comme étant « à l’état vierge », à une surexploitation globale. Cette évolution mérite d’être considérée comme un « modèle accéléré » de l’histoire d’une exploitation. L’analyse se fonde sur les données de 18 campagnes scientifiques de chalutage démersal effectuées dans la zone côtière guinéenne entre 1985 et 1992. Le niveau de diversité spécifique, par campagne ou par trait de chalut, est quantifié par des indices synthétiques : la richesse spécifique, la diversité de Shannon et l’équitabilité. Les indices par trait de chalut sont analysés par les méthodes de modélisation en analyse de variance, afin de dégager une évolution temporelle des indicateurs. Les diagrammes rang/fréquence de chaque campagne sont également établis. On montre que les indices de diversité et d’équitabilité restent sensiblement constants, alors que l’abondance et la richesse spécifique diminuent significativement au cours de la période. Les espèces les moins abondantes et particulièrement des espèces de la communauté à Sparidés, tendent ainsi à disparaître des prélèvements. Enfin, l’année 1989 présente des indices de diversité faibles, liés à des conditions environnementales favorables pour certaines espèces entraînant un recrutement exceptionnel de celles-ci. La pertinence des indices utilisés est discutée.

We investigated spatio-temporal patterns of fish assemblages in four small coastal rivers in South-East Ivory Coast. The samples were collected between July 2003 and March 2005 at 8 sampling sites (2 per river: 1 upstream and 1 downstream). A total of 59 fish species belonging to 39 genera, 23 families and 11 orders were captured. Perciforms (30% of the families and 33% of the species), followed by Siluriforms (22% and 22%), Osteoglossiforms (13% and 17%), Characiforms (9% and 10%) and Pleuronectiforms (9% and 3%) were the most abundant orders. Among the families sampled, Cichlidae (20% of the species), Mormyridae (13%), Clariidae (10%), Cyprinidae (10%) and Characidae (8%) were largely represented. Among the 59 fish species captured, we identified fifteen marine/brackish species and two introduced species. To analyse patterns of fish assemblages, we used a non-linear clustering technique, the self-organizing map (SOM). Using SOM, samples were classified into 4 clusters, mainly related to the spatial location of the sampling sites. Except for the distance from the source of the river, environmental variables (width, depth, current velocity) did not show a clear distribution gradient on the SOM map. This pattern was explained by the abundance of estuarine/marine species (i.e. 25% of the species) characterizing the lower course of the rivers studied. Such a distribution of estuarine/marine species colonizing the lower and middle course of the rivers was explained by the absence of dams and contrasts with the patterns observed in more fragmented rivers of Ivory Coast. Therefore, we suggest that these small streams should be preserved in order to permit estuarine/marine species to migrate between streams and the Aby lagoon, thus allowing their large distribution and their reproduction in favourable habitats. We suggest particular attention be given to the Soumié River basin which represents the only preserved stream ecosystem within the Bia River basin.

While fishing resources available to fishermen are becoming increasingly scarce, management actions that regulate these resources are becoming more restrictive, to such an extent that some fisheries remain closed for years. The anchovy fishery in the Bay of Biscay, which was closed for 5 years, from 2005 to 2009, is one of the most important sources of revenue for the Basque pelagic purse-seiner fleet. Throughout the course of this closure the fleet was forced to find alternative revenue streams. In the present study, the fishing alternatives were analysed before and during the anchovy fishery closure, defining an adaptability indicator to measure the ability of the fleet to cope with fisheries closures. Four differently-behaved segments were found within the purse-seiner fleet. Only one of these segments managed to maintain the same level of turnover during the anchovy closure, while the others could not reach a minimum level of adaptability. Compensation of the fishery closure through financial aid was studied, analysing the impact that public aid had on the profit of each segment and on the adaptability indicator.

Les travaux sur l'élevage des poissons Siluroidei originaires d'Amérique du Sud sont rares et se situent, pour la plupart, au niveau expérimental; cependant le potentiel de ces espèces dans le développement de l'aquaculture dans cette région est considérable. L'essentiel de l'information disponible concerne la reproduction induite. Des méthodes de reproduction contrôlée ont été établies pour plusieurs espèces, comme Pseudoplatystoma fasciatum, P. coruscans, Leiarius marmoratus, Sorubim lima. Pimelodus blochii, Calophysus macropterus, Rhamdia sapo (Pimelodidae) et Eremophylus mutisii (Trichomycteridae). Pour les Siluroidei dont le corps est couvert de plaques osseuses, Hoplosternum littorale (Callichthyidae) et Hypostomus sp (Loricariidae) peuvent aussi être mentionnés, de même que Arius herzbergii (Ariidae) pour les espèces d'origine marine. Chez les Pimelodidae, les techniques d'incubation sont similaires à celles habituellement employées avec succès pour les autres espèces de tendance rhéophile, du fait de la nature pélagique des œufs (flottants ou semi-flottants), d'une fécondité élevée et d'une absence de soins parentaux. Ainsi, les méthodes de reproduction employées sont voisines de celles qui ont été définies pour Colossoman macroponum (Serrasalmidae, Characoidei). espèce originaire du bassin amazonien et désormais élevée à l'échelle commerciale. L'élevage des larves et des juvéniles est reconnu comme la phase la plus critique de la gestion de l'élevage de ces poissons-chats. Ceci résulte principalement de la qualité des aliments et un mode de distribution, et d'un important cannibalisme. Les données sur la production et la commercialisation des alevins restent limitées vraisemblablement parce que la production n'est pas consolidée; celle-ci tend cependant à se mettre en place. en particulier pour les Pimelodidae, pour lesquels il existe une forte demande sur le marché des poissons d'ornement. Pour réduire le cannibalisme et ses conséquences chez des espèces d'importance économique, la production d'hybrides entre espèces de Pimelodidae piscivores et omnivores a été tentée avec un certain succès pour les croisements suivants : P. fasciatum, L. marmoratus, P. blochii, Practocephalus hemiliopterus, P. blochi, P. fasciatum. Les résultats montrent une bonne compatibilité génétique.La production piscicole des Siluroidei d'Amérique du Sud reste très faible. Les statistiques de pêche se rapportant à ces poissons sont incomplètes, voire inexistantes dans la région. L'élevage des espèces sud-américaines de poissons-chats pourra se dèvelopper lorsque leurs cycles biologiques en captivité seront mieux maîtrisés. l'élevage larvaire notamment, et lorsque les paramètres économiques conditionnant la rentabilité des exploitations auront été mieux évalués.

A year-long study in the Bay of Banyuls-sur-Mer (France) assessed the inputsfrom the local intermittent Baillaury River and the stimulationof nearshore phytoplankton blooms.During flash-floods, dissolved inorganic nitrogen, phosphateand dissolved organic carbon concentrations in the river were neither correlated to the river discharge nor to the season.Silicate was correlated to the river discharge.The particulate organic matter load was diluted in a mineral matrix (3–4%)but had a high nutritional quality at the beginning of the flash-flood. The offshore response to the single major flow event (October 2005) was monitored dailyduring two weeks at 350 m (8 m water depth) and 1.4 km from the river mouth (long-term monitoring station, 27 m water depth).The flash-flood signal was partly hidden at the shallowest site by swell resuspension of bottom sediments. At the deepest station, three phases were identified: (1) at the beginning of flow, river dissolved inputs dilute conservatively leading to increased dissolved inorganic nitrogen and silicate concentrations and decreased seawater salinity, (2) during the main turbid pulse, phosphateis released and (3) after the water column cleared,diatom photosynthesis occurred at 3 m below the water surface, leading seven days after the flow peak discharge to a high of 2 µgL−1 of chlorophyll a, which vanished three days later.The nutrients and salinity recovered their pre-flow values at that time.This dynamics of nutrients and chlorophyll during a flash-flood event is consistent with long-term monitoring data.

An adaptation of the von Bertalanffy growth model is formulated to describe the phenotypic plasticity of fish somatic growth in relation to trophic conditions. The model is developed for the North sea Downs herring (Clupea harengus). It suggests that annual growth variability during 1974-1990 was mainly due to the combined effects of herring abundance and wind-induced turbulence (coincident with the spring stratification of the water column). Springtime turbulences cause reduced and delayed planktonic blooms preceding the annual foraging period of Downs herring. The negative relation observed between herring abundance and growth is hypothesized to be due to intra-specific competition for trophic resources. Incorporated into the calculation of yield per recruit, the established growth model provides slightly more optimistic diagnoses while dropping the classic assumption of constant weight at age.

The “Erika” oil spill has killed more seabirds than any before in Europe: nearly 70 000 guillemots (Uria aalge) were found dead or alive on beaches, and many more are thought to have been killed. This unexpectedly high number highlighted how poor our knowledge was on spatial and temporal patterns in seabird distribution in the Bay of Biscay. The purpose of our research project, “ERIKA-Avion”, was to fill this gap, providing the first distribution maps and abundance estimates of seabirds wintering in the entire shelf of the Bay of Biscay. In particular, we analysed fine-grained distribution maps for the wintering auks, comparing their areas of highest density with the oil drift area, and proposing for the first time (although preliminarily) minimal abundance estimates for these birds in this area.

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%.

The gulf of Cariaco, Venezuela, is a rather isolated piece of coastal ocean where spatio-temporal variability has been measured. In 1986, three repeated surveys have been carried out in this gulf. The important differences in biomass estimates are attributed to sampling error only. The biological nature of this variability is studied. It is due to a high concentration of the biomass in a few schools and correlatively to the hit or the miss of these large schools, during the sampling. Structural information for predicting them is sought. Also, the number of schools per elementary sampling distance unit (ESDU) and the biomass in each school were measured by echointegration. It is found that there is no clear relation between these two variables. Thus their spatial structures are studied separately, using geostatistics. The number of schools per ESDU is structured in space. Its histogram is not very dissymmetrical. The error variance of its mean estimate is reasonable. The biomass per school is not structured in space. Its histogram is highly skewed. The error variance of its mean estimate is high. The major error variance term on the abundance is due to the imprecision on the estimate of the mean biomass per school. The major quantitative problem is the indeterminancy at the sampling stage of the maximum biomass per school and correlatively the major problem is the characterization of the skewness of the biomass per school histogram. The possibility of using the number of schools as an index of biomass is discussed.

The aquaculture production of all-female triploid (sterile) populations is recognized as being of potential advantage for many species of fish where sexual maturation is not desired. All-female triploids have been produced by inducing triploidy in eggs that had been fertilized with monosex female sperm. However, monosex stocks to produce this type of sperm are currently available only for a limited number of economically important species. To circumvent this problem, an alternative method would be the direct fertilization of tripoid embryos. In Pacific salmon, pressure shocks applied shortly after fertilization have consistently resulted in high yields of tripoids, and feminization by direct estrogen treatment has also been shown to be highly effective if applied shortly after hatching. In this study, coho salmon eggs were made triploid by a pressure shock of 633 kg/cm2 for 4 min, applied 20 min after fertilization and incubation at 10 °C. Some of the resulting embryos were then feminized with estradiol-17β in single 2-hour immersion treatments at 400 μg/liter, administered 1 and 8 days after hatching. Six months later, when the fish were juveniles, analysis of the DNA content of erythrocytes by flow cytometry revealed a 100% induction of triploidy, while histological examination of the gonads showed 82% females, which were completely devoid of oocyte development and thus genetically sterile. These manipulations reduced survival during early stages of development but survival became stable later. Together, these results suggest that the production of sterile fish by direct feminization of induced triploids could be of particular advantage for those species in which all-female stocks are yet not available or difficult to obtain.

Une analyse phylogéographique de quinze échantillons de Dicentrarchus labrax dont cinq sont originaires de Méditerranée occidentale, sept de Méditerranée orientale et trois échantillons d'élevage en provenance de trois piscicultures françaises a été effectuée à partir du polymorphisme de six locus microsatellites. Parmi les échantillons orientaux, trois ne se regroupent pas en fonction de leur origine géographique, mais plutôt avec le groupe occidental. Ces échantillons présentent également une diversité allélique légèrement réduite, indiquant qu'ils proviennent d'un nombre limité de géniteurs d'origine occidentale. Parmi les stocks d'élevage, un seul montre une réduction significative de la variabilité génétique, ce qui indique que ces cheptels sont ouverts et font largement appel à des géniteurs sauvages. L'utilisation d'alevins d'origine occidentale pour ensemencer les premières fermes d'élevage du bassin oriental remonte au plus tard au début des années 80. Ceci soulève la question des mécanismes biologiques expliquant le maintien de poissons d'origine occidentale dans un contexte oriental pendant au moins deux ou trois générations.

Daily striae on the shell of the scallop, Argopecten purpuratus, were used to investigate its growth in a protected population within La Rinconada Bay, near Antofagasta (2nd Region, Northern Chile), and to determine how environmental forcings control shell growth. This sclerochronological approach was useful to describe daily shell growth a posteriori. Mean shell growth rates were calculated daily between February and October 1999 from shells of six specimens harvested on 15 October 1999. A multiple regression analysis performed on shell growth and environmental data shows that a temperature increase stimulates shell growth whereas particulate organic carbon concentration might be stressful above 2.5 mg L−1. The lunar cycle might also have an effect on growth, most potent around New Moons, by synchronizing an endogenous oscillator. The age at commercial size (90 mm) was only 10–11 months in La Rinconada Marine Reserve and the von Bertalanffy growth parameters were K = 2.24 y−1 and H ∞ = 117 mm. Differences in shell growth performances between this study and previous ones dealing with A. purpuratus growth may be explained by the different methods used for shell growth modelling, higher growth rate on the seabed compared to suspended cultures, and particular environmental forcings (e.g. El Niño Southern Oscillation-ENSO events). Growth performance seems better during La Niña than during El Niño events, probably because of the large temperature increase (above the highest temperatures recorded in “normal” conditions) and the decline in coastal upwelling (which affects food availability) during El Niño. La Rinconada Marine Reserve appears as a very favourable site for A. purpuratus growth, probably because of the active upwelling zone off Antofagasta. These new insights on A. purpuratus shell growth confirm the importance of La Rinconada Marine Reserve as an area supporting repopulation of endangered wild stocks and scallop aquaculture in Chile.

Experiments conducted at the Qualark Creek acoustic site on the Fraser River have shown unexpected systematic errors in the split-beam angle measurement. A tethered dead salmon with a target strength of approximately –30 dB was used as a target at 3.7 m range. A signal-to-noise ratio of ~12 dB was observed. This target strength and signal-to-noise ratio are typical in some rivers where migrating adult salmon are observed. The target's location was measured both from the frame used to position the target in the beam and from the acoustic data. Comparison of these two sets of measurements demonstrated a bias that increased with the target's distance off the beam axis. The acoustically measured target locations were closer to the beam axis than the actual locations. Each acoustic estimate represented the mean location obtained after 3 000 pings. Results from a simulation model that includes the effects of random noise on the split-beam angle measurement reproduce the systematic underestimate of these angles. This bias increases rapidly with increasing off-axis target location and decreasing signal-to-noise ratio. The bias has been observed for circular and elliptical beams and the model accurately predicts the magnitude and direction of the systematic part of the angle measurement error in both cases. Results for a circular transducer are used here to illustrate the situation. The presence of this bias can affect measurements of fish density and migration speed. At the low signal-to-noise ratios often obtained in riverine environments, those who use a split-beam system for observation or estimation of fish should be aware of this phenomenon.