Non-contact robotic manipulation of floating objects: exploiting emergent limit cycles.

The study of non-contact manipulation in water, and the ability to robotically control floating objects has gained recent attention due to wide-ranging potential applications, including the analysis of plastic pollution in the oceans and the optimization of procedures in food processing plants. However, modeling floating object movements can be complex, as their trajectories are influenced by various factors such as the object's shape, size, mass, and the magnitude, frequency, and patterns of water waves. This study proposes an experimental investigation into the emergence ofrobotically controlled limit cycles in the movement of floating objects within a closed environment. The objects' movements are driven by robot fins, and the experiment plan set up involves the use of up to four fins and variable motor parameters. By combining energy quantification of the system with an open-loop pattern generation, it is possible to demonstrate all main water-object interactions within the enclosed environment. A study using dynamic time warping around floating patterns gives insights on possible further studies.

A Multi-AUV Maritime Target Search Method for Moving and Invisible Objects Based on Multi-Agent Deep Reinforcement Learning.

Target search for moving and invisible objects has always been considered a challenge, as the floating objects drift with the flows. This study focuses on target search by multiple autonomous underwater vehicles (AUV) and investigates a multi-agent target search method (MATSMI) for moving and invisible objects. In the MATSMI algorithm, based on the multi-agent deep deterministic policy gradient (MADDPG) method, we add spatial and temporal information to the reinforcement learning state and set up specialized rewards in conjunction with a maritime target search scenario. Additionally, we construct a simulation environment to simulate a multi-AUV search for the floating object. The simulation results show that the MATSMI method has about 20% higher search success rate and about 70 steps shorter search time than the traditional search method. In addition, the MATSMI method converges faster than the MADDPG method. This paper provides a novel and effective method for solving the maritime target search problem.

Robust cooperative trajectory tracking control for an unactuated floating object with multiple vessels system.

This paper proposes a robust cooperative trajectory tracking control scheme for an unactuated floating object with multiple vessels under environmental disturbances. The object and multiple vessels are connected by using towlines. The proposed control scheme consists of three parts: a virtual controller for the object, a control allocation algorithm and a distributed robust time-varying formation controller for vessels. The virtual controller is first designed to obtain the control forces of the object to track the reference trajectory. To compute the optimal tension of each towline, the control allocation algorithm is introduced. Then, the time-varying relative positions from the object to vessels are gained by using a nonlinear towline model and the towline attachment geometry. Furthermore, the distributed robust time-varying formation controller is devised for vessels based on dynamic surface control technique, an adaptive law and graph theory. It is proved that the tracking errors of the object and vessels are bounded. Simulations substantiate that the proposed method can achieve good cooperative control performance and robustness, and the unactuated object can track the reference trajectory with high accuracy.

Distribución espacial y variables explicativas de capturas de Thunnus albacares (Perciformes: Scombridae) y especies no objetivo por la flota internacional de cerco en el Pacífico de Costa Rica / Spatial distribution and correlates for catches of Thunnus albacares (Perciformes: Scombridae) and non-target species by the international purse-seine fleet in the Pacific of Costa Rica

Introducción: En los ambientes marinos del Pacífico de Costa Rica, la actividad pesquera por la flota cerquera internacional es importante debido a su amplio alcance y fuerte impacto. Objetivo: Evaluar la distribución espacial de la captura de atún aleta amarilla y especies no objetivo por esa flota, y posibles correlaciones con variables ambientales y pesqueras. Métodos: Aplicamos estadísticas geoespaciales y modelos de regresión múltiple en datos de la Comisión Interamericana del Atún Tropical, del 2002 al 2011. Utilizamos una grilla del tamaño de la Zona Económica Exclusiva (ZEE) con 1 124 celdas de 22 x 22 km (unidad de análisis), para un total de 11 240 celdas-año. Resultados: El atún mediano (presumiblemente inmaduro en su mayoría), delfines, mantas y rayas fueron capturados mayormente dentro del Domo Térmico (noreste de la ZEE). Las más altas capturas de atún pequeño (inmaduro en su totalidad), tiburones, picudos, dorado, peto y diversas especies pequeñas, ocurrieron en el extremo sur. El atún grande (maduro) fue capturado especialmente en el centro de la ZEE, hacia el este del Área Marina de Manejo Montes Submarinos y el Parque Nacional Marino Isla del Coco. Los atunes, dorado, delfines, mantas y rayas mostraron gran fidelidad de sitio por asociación con variables geográficas y ambientales, a la vez que el atún pequeño estuvo asociado a lances sobre objetos flotantes y el atún grande a lances sobre delfines. La heterogeneidad de las especies no objetivo probablemente escondió la mayoría de las correlaciones. Conclusiones: La pesca está afectando los niveles tróficos más altos y es necesario un diseño de zonificación ajustado para proteger mejor el domo térmico y considerar la susceptibilidad al uso de redes de cerco.

Introduction: In the Pacific marine ecosystems of Costa Rica, fishing activity by the international purse-seine fleet is important for its wide scope and large impact. Objective: To evaluate the spatial distribution of the catch of yellowfin tuna and non-target species by this fleet, and possible correlates with environmental and fishing variables. Methods: We applied geospatial statistics and multiple regression models to Inter-American Tropical Tuna Commission data, covering from 2002 to 2011. We used a grid the size of the Exclusive Economic Zone (EEZ) with 1 124 cells of 22 x 22 km (unit of analysis), for a total of 11 240 year-cells. Results: The medium tuna (presumably immature for the most part), dolphins, mantas and rays dominated catches within the Thermal Dome (Northeast of the EEZ). Small tuna (all immature), sharks, billfishes, dorado, wahoo, and various small species were caught mostly in the extreme South. Large tuna (mature) was caught specially in the zone center, towards the east of the Submarine Mounts Marine Management Area and the Isla del Coco National Marine Park. Tuna, dorado, dolphins, mantas and rays showed high site fidelity due to their association with geographic and environmental variables, at the time small tuna was associated with sets on floating objects, and big tuna was associated with sets on dolphins. The heterogeneity of non-target species probably hid most correlations. Conclusions: Fishing is affecting the highest trophic levels and an adjusted zoning design is needed to better protect the thermal dome and to take into account susceptibility to the use of purse seines.

Effects of habitat modifications on the movement behavior of animals: the case study of Fish Aggregating Devices (FADs) and tropical tunas.

BACKGROUND: Aggregation sites represent important sources of environmental heterogeneity and can modify the movement behavior of animals. When these sites are artificially established through anthropogenic actions, the consequent alterations to animal movements may impact their ecology with potential implications for their fitness. Floating objects represent important sources of habitat heterogeneity for tropical tunas, beneath which these species naturally aggregate in large numbers. Man-made floating objects, called Fish Aggregating Devices (FAD), are used by fishers on a massive scale to facilitate fishing operations. In addition to the direct impacts that fishing with FADs has on tuna populations, assessing the effects of increasing the numbers of FADs on the ecology of tuna is key for generating sound management and conservation measures. METHODS: This study investigates the effects of increasing numbers of FADs (aggregation sites) on the movements of tunas, through the comparison of electronic tagging data recorded from 146 individuals tunas (yellowfin tuna, Thunnus albacares, and skipjack tuna, Katsuwonus pelamis) tagged in three instrumented anchored FAD arrays (Mauritius, Oahu-Hawaii and Maldives), that differed according to their distances among neighboring FADs. The effect of increasing inter-FAD distances is studied considering a set of indices (residence times at FADs and absence (travel) times between two visits at FADs) and their trends. RESULTS: When inter-FAD distances decrease, tuna visit more FADs (higher connectivity between FADs), spend less time travelling between FADs and more time associated with them. The trends observed for the absence (travel) times appear to be compatible with a random-search component in the movement behaviour of tunas. Conversely, FAD residence times showed opposite trends, which could be a result of social behavior and/or prey availability. CONCLUSION: Our results provide the first evidence of changes in tuna associative behavior for increasing FAD densities. More generally, they highlight the need for comparing animal movements in heterogeneous habitats in order to improve understanding of the impacts of anthropogenic habitat modifications on the ecology of wild animals.

Floating novel object recognition in adult zebrafish: a pilot study.

The novel object recognition (NOR) tasks can be used to quantify memory function in zebrafish similarly to rodents. The development of zebrafish learning and memory tests provides a means for testing the effects of pharmacological manipulations of memory. Several authors reported on the successful application of different objects in NOR tests placed either at the bottom of test tanks or submerged into the tank water of zebrafish. This pilot study was designed to test the suitability of floating objects in NOR tests using adult zebrafish. Floating objects such as crumpled aluminum balls and pink plastic hollow pearls were found to be suitable for NOR tests when small groups of zebrafish are used as experimental animals. Adult zebrafish of both sexes were capable of distinguishing between the different colors and surface consistencies of certain floating objects. A significantly higher number of mouth-object contacts were recorded when either floating aluminum balls or floating plastic pearls were used as novel object during NOR tests.

Trapped modes around freely floating bodies in a two-layer fluid channel.

Unlike the trapping of time-harmonic water waves by fixed obstacles, the oscillation of freely floating structures gives rise to a complex nonlinear spectral problem. Still, through a convenient elimination scheme the system simplifies to a linear spectral problem for a self-adjoint operator in a Hilbert space. Under symmetry assumptions on the geometry of the fluid domain, we present conditions guaranteeing the existence of trapped modes in a two-layer fluid channel. Numerous examples of floating bodies supporting trapped modes are given.