Pascal Bach

Simultaneous observations of tuna movements and their prey by sonic tracking and acoustic surveys

This paper reviews results of some experiments conducted in French Polynesia on tuna behaviour. A method based on the simultaneous use of two techniques, acoustic tracking and acoustic surveys, was used. Experiments were conducted within the framework of the ECOTAP program, a joint program between two national research institutes (IFREMER and ORSTOM), and a territorial institute (EVAAM).Acoustic tags equipped with pressure sensors were used in order to record horizontal and vertical movements of one yellowfin tuna (Thunnus albacares) and two bigeye tuna (T. obesus). Trackings lasted between 13 to 24 h. In the same time, echogram data were recorded between the surface and a depth of 500 m on board the tracking vessel. As the maximum range of the acoustic tags is small (a few hundred meters), vessel and tagged fish horizontal movements are therefore treated as equivalent. Echogram data from the sounder and data on the swimming depth of the fish given by the acoustic tag are then considered as having been obtained at the same time at the same place.Comparison between the swimming depth of the tagged fish and the echogram data from the sounder clearly shows the important role of scattering layers, assimilated as food, on vertical and horizontal tuna movements, during daytime as well as during night-time.The method used during these experiments allows to observe a new explanatory factor of tuna behaviour: the biotic environment. At small temporal and spatial scales, structure of the biotic environment and its dynamic appear to be a key factor to understanding the vertical and horizontal tuna movements. The simultaneous technique presented here must now be improved by using behavioural activities sensors. By this way, it would be possible to elucidate different tuna foraging phases in relationship with the dynamic of scattering layers.

Acoustic telemetry versus monitored longline fishing for studying the vertical distribution of pelagic fish: bigeye tuna (Thunnus obesus) in French Polynesia

Abstract This study compares detailed, nearly continuous, observations on bigeye tuna, Thunnus obesus equipped with electronic tags, with discrete observations on a larger number of individuals from fishing experiments in order to validate the use of instrumented longlines to study the vertical distribution of fish. We show that the depth distributions obtained from the two different observation techniques regarding different environmental variables (temperature, dissolved oxygen (DO), prey distribution) are similar. Bigeye tuna do not seem to be attracted by baits in the vertical dimension (no modification of their vertical distribution by the fishing gear), which allows the use of instrumented longlines to study the vertical behaviour of pelagic species. This technique, when used with appropriate deployment strategy, could therefore represent an alternative to electronic tags (acoustic or archival tags) when there is a need to determine the vertical distribution of fish species by size or sex, in different environments for the study of fishery interactions.

Co-evolution of movement behaviours by tropical pelagic predatory fishes in response to prey environment : a simulation model

Abstract Predatory fishes, such as tunas, billfishes, and sharks, coexist in pelagic regions of the tropical oceans. In situ experiments have revealed horizontal and vertical movement patterns for different pelagic species, but the influence of the biotic environment on movement behaviour has not been studied. In this paper, we propose a simple model in which the movement behaviour of these fishes is driven entirely by the biotic environment, without implementing physiological constraints. We explore this concept via computer simulations based on the Latent Energy Environments model [Menczer, F., Belew, R.K., 1996a. From complex environments to complex behaviors. Adapt. Behav. 4(3/4), 317–63]. In our model, multiple behaviours for artificial fishes evolve in a three-dimensional environment where spatial and temporal distributions of prey are patterned after hydroacoustic data taken during ultrasonic telemetry experiments on tunas in the open ocean in French Polynesia. Interactions among individuals are modeled through their shared prey resources. Movement patterns of the adapted individuals are analyzed to: (i) compare artificial individuals with real fishes (three species of tuna, three species of billfishes, and one species of shark) observed by ultrasonic telemetry; and (ii) examine how the artificial fishes exploit their environment. Most of the individuals evolved vertical patterns virtually identical to those exhibited by fishes in the wild. The agreement between our simple model and the ethological data validates the use of computational models for studies of the characteristics of multiple species inhabiting a common ecosystem.

Individual differences in horizontal movements of yellowfin tuna (Thunnus albacares) in nearshore areas in French Polynesia, determined using ultrasonic telemetry

This article describes tracking experiments conducted on eleven yellowfin tuna using ultrasonic transmitters in French Polynesia between 1985 and 1997. Nine fish were caught near Fish Aggregating Devices (FADs) while the other two were tracked in coastal areas without FADs. The fish showed different patterns of horizontal movements: tight associations with FADs lasting several days, foraging movements confirmed by simultaneous acoustic observations of prey-sized fauna, movements parallel to the shore, and traveling between FADs. This intra- and inter-individual variety of behaviour might depend on the local environment (prey), and on individual biological differences. The influence of FADs, coastlines, and prey on tuna movements is discussed. The lack of information about the surrounding environment, the internal state of the fish and the recent history of the fish usually prevent scientists from adequately interpreting the observed movements. Ideas for future research to studying tuna behaviour near FADs are discussed.

Acoustics for ecosystem research: lessons and perspectives from a scientific programme focusing on tuna-environment relationships

Fisheries management now extends from the stock to the ecosystem. The foundation for fisheries management on an ecosystem basis must lie in appropriate modelling of the ecosystems. A prerequisite for such models requires data on the two interactive components of the ecosystem: the biotope (physical environment), and the community of living species. In this context, acoustics become essential, as this tool can provide qualitative and quantitative data on various communities of species, and furthermore allows the seldom-attainable study of their interactions. In fact, acoustics allow the monitoring of entire communities, from plankton to large predators, as well as certain aspects of the physical environment, such as substratum characteristics. Acoustics have been used during the last two decades mainly to provide fishery-independent estimates of stocks. The intention of this paper is to promote the use of acoustics for studying marine ecosystems and to encourage the emergence of new generations of ecosystem models. As an example of integrative research based on acoustic data, we will present the approach and the results of a scientific programme (ECOTAP) carried out to study the tuna pelagic ecosystem in French Polynesia. We then discuss the use of acoustics as a tool for ecosystem-based studies and management.

Modeling tuna behaviour near floating objects: from individuals to aggregations

A fuzzy logic model of tuna behaviour near Fish Aggregating Devices (FADs) was developed to reproduce individual differences in horizontal movements observed from ultrasonic telemetry experiments. In this model, the behaviour of an individual is based on its surrounding environment (FADs and prey) and on its internal state (stomach fullness), which depends on its recent past actions. Internal sensors are used to determine the motivation of the fish, combined with external sensors, this determines its movements. Sensory information and motivation are modeled using fuzzy sets. A FAD attracts an individual when it is located within the FAD’s range of influence. The time spent near a FAD depends on the feeding motivation of the fish and on its surrounding environment. If the fish is not hungry, it stays near the FAD. Otherwise, the fish has to forage in order to eat, and might therefore leave the FAD if no prey is available in its vicinity. By varying the environmental conditions near FADs, the model reproduces the different horizontal movement patterns observed for tunas. The model is then extended to allow multiple individuals to co-exist, each individual modeled through the above behavioural model, without any direct or indirect interactions between them. This way, we study the effects of individual behaviour on tuna aggregation near FADs. We find that the model predicts the temporal dynamics of aggregation around FADs exhibited by tunas. By examining the effects of several FAD network models on the aggregation, we also estimate optimal spatial arrangements of FADs.

Modeling tuna behaviour near floating objects: from individuals to aggregationsModélisation du comportement des thons autour des objets flottants : des individus aux agrégations.

A fuzzy logic model of tuna behaviour near Fish Aggregating Devices (FADs) was developed to reproduce individual differences in horizontal movements observed from ultrasonic telemetry experiments. In this model, the behaviour of an individual is based on its surrounding environment (FADs and prey) and on its internal state (stomach fullness), which depends on its recent past actions. Internal sensors are used to determine the motivation of the fish, combined with external sensors, this determines its movements. Sensory information and motivation are modeled using fuzzy sets. A FAD attracts an individual when it is located within the FAD’s range of influence. The time spent near a FAD depends on the feeding motivation of the fish and on its surrounding environment. If the fish is not hungry, it stays near the FAD. Otherwise, the fish has to forage in order to eat, and might therefore leave the FAD if no prey is available in its vicinity. By varying the environmental conditions near FADs, the model reproduces the different horizontal movement patterns observed for tunas. The model is then extended to allow multiple individuals to co-exist, each individual modeled through the above behavioural model, without any direct or indirect interactions between them. This way, we study the effects of individual behaviour on tuna aggregation near FADs. We find that the model predicts the temporal dynamics of aggregation around FADs exhibited by tunas. By examining the effects of several FAD network models on the aggregation, we also estimate optimal spatial arrangements of FADs.

Improving our Understanding of Tropical Tuna Movements from Small to Large Scales

One tactical objective of behavioural studies in fisheries sciences is to understand the 3-D movements of fish, with the ambitious strategic objective of being able to explain some large-scale movements and distributions from knowledge of small-scale behaviour. The present paper reviews different possibilities to improve our understanding of tropical tuna movements, from small (days) to large scales (weeks and months). We propose some ideas for better observations of fine-scale movements of fish (sonic tags) and of the surrounding environment of the tagged fish. After determining behaviour rules, appropriate modelling should be developed in order to extrapolate results to larger-scale movements. This process can be achieved only if we can simultaneously observe the relevant factors of the environment at appropriate scales and the large-scale movements of fish (using pop-up archival tags), in order to force the models to reproduce the observed movements. This paper shows the importance of (i) small-scale studies, (ii) appropriate observations of large-scale movements and environment, and (iii) models of behaviour in order to extract relevant processes necessary to predict tuna dynamics.

Vulnerability of the Oceanic Whitetip Shark to Pelagic Longline Fisheries

A combination of fisheries dependent and independent data was used to assess the vulnerability of the oceanic whitetip shark to pelagic longline fisheries. The Brazilian tuna longline fleet, operating in the equatorial and southwestern Atlantic, is used as a case study. Fisheries dependent data include information from logbooks (from 1999 to 2011) and on-board observers (2004 to 2010), totaling 65,277 pelagic longline sets. Fisheries independent data were obtained from 8 oceanic whitetip sharks tagged with pop-up satellite archival tags in the area where longline fleet operated. Deployment periods varied from 60 to 178 days between 2010 and 2012. Tagging and pop-up sites were relatively close to each other, although individuals tended to travel long distances before returning to the tagging area. Some degree of site fidelity was observed. High utilization hotspots of tagged sharks fell inside the area under strongest fishing pressure. Despite the small sample size, a positive correlation between tag recorded information and catch data was detected. All sharks exhibited a strong preference for the warm and shallow waters of the mixed layer, spending on average more than 70% of the time above the thermocline and 95% above 120 m. Results indicate that the removal of shallow hooks on longline gear might be an efficient mitigation measure to reduce the bycatch of this pelagic shark species. The work also highlights the potential of tagging experiments to provide essential information for the development of spatio-temporal management measures.

Toothed whale and shark depredation indicators: A case study from the Reunion Island and Seychelles pelagic longline fisheries

Depredation in marine ecosystems is defined as the damage or removal of fish or bait from fishing gear by predators. Depredation raises concerns about the conservation of species involved, fisheries yield and profitability, and reference points based on stock assessment of depredated species. Therefore, the development of accurate indicators to assess the impact of depredation is needed. Both the Reunion Island and the Seychelles archipelago pelagic longline fisheries targeting swordfish (Xiphias gladius) and tuna (Thunnus spp.) are affected by depredation from toothed whales and pelagic sharks. In this study, we used fishery data collected between 2004 and 2015 to propose depredation indicators and to assess depredation levels in both fisheries. For both fisheries, the interaction rate (depredation occurrence) was significantly higher for shark compared to toothed whale depredation. However, when depredation occurred, toothed whale depredation impact was significantly higher than shark depredation impact, with higher depredation per unit effort (number of fish depredated per 1000 hooks) and damage rate (proportion of fish depredated per depredated set). The gross depredation rate in the Seychelles was 18.3%. A slight increase of the gross depredation rate was observed for the Reunion Island longline fleet from 2011 (4.1% in 2007–2010 and 4.4% in 2011–2015). Economic losses due to depredation were estimated by using these indicators and published official statistics. A loss of 0.09 EUR/hook due to depredation was estimated for the Reunion Island longline fleet, and 0.86 EUR/hook for the Seychelles. These results suggest a southward decreasing toothed whale and shark depredation gradient in the southwest Indian Ocean. Seychelles depredation levels are among the highest observed in the world revealing this area as a “hotspot” of interaction between pelagic longline fisheries and toothed whales. This study also highlights the need for a set of depredation indicators to allow for a global comparison of depredation rates among various fishing grounds worldwide.