In support of the IOTC ecosystem report card: Advances in monitoring the impacts on and the state of the “foodweb and trophic relationships” ecosystem component

In support of the development of the ICCAT Ecosystem Report Card, this paper addresses the “foodweb/trophic relationships” ecosystem component. Specifically, it contributes towards developing the following elements: (1) we describe what this component means in the context of ICCAT species and fisheries and the importance of monitoring it; (2) we describe the role of ecological indicators and ecosystem models in monitoring this ecosystem component; (3) we present a list of candidate ecological indicators that could be estimated to monitor this component; (4) we discuss the main challenges in monitoring this ecosystem component and indicator development; and finally (5), we draft a work plan to guide our future work. We invite the ICCAT community and others to contribute towards the development of ecological indicators and ecosystem models to monitor this ecosystem component. If interested, contact the corresponding authors to find out how you can contribute to this initiative

A Preliminary Habitat Suitability Model for Devil Rays in the Western Indian Ocean

The European tropical tuna purse seine fishery incidentally captures three highly migratory and endangered species of devil rays, spinetail devil ray Mobula mobular, sicklefin devil ray M. tarapacana, and bentfin devil ray M. thurstoni in the Indian Ocean. Due to their global decreasing populations, understanding the factors of their spatial and temporal distributions and the associated environmental conditions are fundamental for their management and conservation. Yet, the spatial and temporal distribution of devil rays in the Indian Ocean is poorly understood. Here we developed a habitat suitability model for devil rays in the Western Indian Ocean depicting the seasonal and interannual changes in their spatial distributions and underlying environmental conditions. We used bycatch data collected between the period 2010-2020 by the EU tropical tuna purse seine observer programs to determine which environmental variables influence the occurrence of devil rays using generalized additive models. A separate modelling was done for the spinetail devil ray, and for the three species of devil rays combined, since many individuals are only recorded at the genus level. The environmental variables associated with the presence of devil rays were chlorophyll, sea surface height and sea surface temperature fronts. When modelling the habitat suitability for spinetail devil ray, the most influential environmental variables were net primary production of phytoplankton and sea surface temperature fronts. Both the interannual and seasonal variability in habitat suitability of devil rays were explained by these environmental variables. We also showed that devil rays are associated to permanent hotspots in the Mascarene Plateau and Central Indian Ridge, and to seasonal hotspots in the Western Arabian Sea and Equatorial regions where there is a high occurrence of devil rays during winter monsoon. We found that setting on large tuna schools decreases the chances of devil ray bycatch. Both models predicted a higher probability of incidental catch of devil rays in fishing sets on free swimming schools of tunas than in sets on fish aggregating devices. The identified hotspots and associated environmental characteristics provide information about the habitat use and ecology of the devil rays in the Western Indian Ocean. Furthermore, the habitat suitability models, and biological hotspots identified in this study could also to be used to inform the development of future spatial management measures, including time-area closures, to minimize the interaction of pelagic fisheries with these vulnerable species

A preliminary habitat suitability model for oceanic whitetip shark in the western Indian Ocean

Understanding the temporal, spatial and environmental factors influencing species distributions is essential to minimize the interactions of vulnerable species with fisheries and can be used to identify areas of high bycatch rates and their environmental conditions. Classified as critically endangered by the International Union for the Conservation of Nature, the oceanic whitetip shark (Carcharhinus longimanus) is the second main shark species incidentally caught by the tropical tuna purse seine fishery in the western Indian Ocean. In this study, we used the European Union purse seine fishery observer data (2010-2020) and generalized additive models to develop a habitat suitability model for juvenile oceanic whitetip shark in the western Indian Ocean. Sea surface temperature was the main environmental driver suggesting a higher probability of occurrence of this shark with decreasing temperatures. The type of fishing operation also was an important predictor explaining its occurrence, suggesting a higher probability of incidentally catching this species when using fish aggregating devices as set type. Moreover, predictive maps of habitat suitability suggested the area offshore of Kenya and Somalia are an important hotspot with higher probabilities of incidentally catching this species during the summer monsoon (June to September) when upwelling takes place. The habitat suitability models developed here could be used to inform the design and testing of potential time-area closures in the Kenya-Somalia basin with the objective of minimizing the bycatch of this critically endangered species with the least possible impact on fishing operations and fishery yields of target tuna

In support of the ICCAT ecosystem report card: advances in monitoring the impacts on and the state of the “foodweb and trophic relationships” ecosystem component.

In support of the development of the ICCAT Ecosystem Report Card, this paper addresses the “foodweb/trophic relationships” ecosystem component. Specifically, it contributes towards developing the following elements: (1) we describe what this component means in the context of ICCAT species and fisheries and the importance of monitoring it; (2) we describe the role of ecological indicators and ecosystem models in monitoring this ecosystem component; (3) we present a list of candidate ecological indicators that could be estimated to monitor this component; (4) we discuss the main challenges in monitoring this ecosystem component and indicator development; and finally (5), we draft a work plan to guide our future work. We invite the ICCAT community and others to contribute towards the development of ecological indicators and ecosystem models to monitor this ecosystem component. If interested, contact the corresponding authors to find out how you can contribute to this initiative.Versión del editor

The rostral micro‐tooth morphology of blue marlin, Makaira nigricans

Billfish rostra potentially have several functions; however, their role in feeding is unequivocal in some species. Recent work linked morphological variation in rostral micro‐teeth to differences in feeding behavior in two billfish species, the striped marlin (Kajikia audax) and the sailfish (Istiophorus platypterus). Here, we present the rostral micro‐tooth morphology for a third billfish species, the blue marlin (Makaira nigricans), for which the use of the rostrum in feeding behavior is still undocumented from systematic observations in the wild. We measured the micro‐teeth on rostrum tips of blue marlin, striped marlin, and sailfish using a micro–computed tomography approach and compared the tooth morphology among the three species. This was done after an analysis of video‐recorded hunting behavior of striped marlin and sailfish revealed that both species strike prey predominantly with the first third of the rostrum, which provided the justification to focus our analysis on the rostrum tips. In blue marlin, intact micro‐teeth were longer compared to striped marlin but not to sailfish. Blue marlin had a higher fraction of broken teeth than both striped marlin and sailfish, and broken teeth were distributed more evenly on the rostrum. Micro‐tooth regrowth was equally low in both marlin species but higher in sailfish. Based on the differences and similarities in the micro‐tooth morphology between the billfish species, we discuss potential feeding‐related rostrum use in blue marlin. We put forward the hypothesis that blue marlin might use their rostra in high‐speed dashes as observed in striped marlin, rather than in the high‐precision rostral strikes described for sailfish, possibly focusing on larger prey organisms.Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/501100001659Peer Reviewe

Patterns and mechanisms of seabird-environment interactions in southern Africa : population and individual studies

The interactions between seabirds and their environment, notably their prey, include complex spatial patterns and mechanisms that span over different scales of processes (e.g. physiology, behaviour, population). To understand large-scale patterns in seabird populations it is necessary to develop insight in the respective fields of study of physiology, behaviour and population ecology, and to reconcile these levels. Working along this line of research, I propose in this thesis to appreciate seabird-environment relationships from different perspectives: at the level of the population and of the individual. The first part of the thesis investigates whether regional trends of South African seabird populations (African penguin, Cape gannet, Cape cormorant and swift tern) follow a major shift in the distribution of their prey [Paper 1]. I demonstrate that for these seabirds the breeding populations respond at the level of the colony to the spatio-temporal variability of their prey, and I propose potential mechanisms for such responses [Paper 2]. The second part is dedicated to the analysis of patterns in prey distribution and individual seabird movements in relation to oceanographic mesoscale features such as fronts. I evidence that frontal structures are predictable areas where to find prey [Paper 3] and that seabirds like Cape gannets adjust their foraging strategies to such structures in order to optimize their foraging success [Papers 4 and 5]. This thesis underlines the importance of the spatial dimension in the relationships between seabirds and their prey, and highlights the importance of oceanographic features as catalysts of seabird-prey interactions