From abundance to disappearance. Identifying causes for Hippocampus guttulatus population decrease in the Ria

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Effects of anthropogenic noise as a source of acoustic stress in wild populations of Hippocampus guttulatus in the Ria Formosa, south Portugal

Publication . Palma, Jorge; Magalhães, Marisa Gonçalves; Correia, Miguel; Andrade, José Pedro

This study evaluated the effects of underwater noise as a source of acoustic stress in the wild populations of Hippocampus guttulatus in the Ria Formosa, south Portugal. Two different scenarios of underwater noise were tested: transient motor boat sound (63.4-127.6 dB) and constant sound produced by the motor of the boat directly above the animals (up to 137.1 dB). Observations were obtained in the wild between 4 and 10 m depth throughout a 3 min period, using a video camera and a hydrophone set, and compared with a control sample. A significant increase (P < 0.05) in the respiratory rate was observed in 87% of the observed fish. Opercular movements per minute (OMPM) increased from 35.7 +/- 10 (control sample) to 41.2 +/- 15.5 after the first minute, to 45.5 +/- 13.3 after the second (both under transient sound) and to 49.7 +/- 12.5 after the third (under constant sound exposure). Differences in means between the control fish and fish observed during the second (P < 0.01) and third minute of observation (P < 0.0001) were significant. Concordantly, a significant increase (P < 0.05) in the OMPM of fish observed in the first minute and the third minute was noted. In addition to the OMPM increase, 37.5% of the animals abandoned the observation location presumably in an attempt to avoid the negative sound stimuli. The noise caused by boat traffic generated an immediate physiological response, expressed as an increase in OMPM, and a behavioural response resulting in site abandonment, which together can be considered as a negative impact on the seahorse populations. This work contributes to an increasing number of studies that have shown that boat traffic can induce ecological and environmental consequences to aquatic species. Future research should evaluate how navigation exclusion areas could have a positive impact on the seahorse populations inhabiting shallow coastal areas.

2019-05Journal article

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Retinal differentiation in syngnathids: comparison in the developmental rate and acquisition of retinal structures in altricial and precocial fish species

Publication . Alvarez-Hernan, Guadalupe; Andrade, José Pedro; Escarabajal-Blazquez, Laura; Blasco, Manuel; Solana-Fajardo, Jorge; Martin-Partido, Gervasio; Francisco-Morcillo, Javier

The altricial-precocial spectrum describes the degree of morphological maturation of offspring at the moment of hatching. In fishes, precocial species develop all their structures at early stages of embryogenesis and larvae hatch at an advanced stage of development, while altricial species hatch at a less developed stage. The timing of retinal development also varies significantly between precocial and altricial fish species. Thus, retinal development is completed before hatching in precocial species. In contrast, a relatively simple retina is observed in altricial newborns and the acquisition of the adult retinal features extends until late in larval life. Therefore, retinal maturation at hatching could be considered as a morphological character to describe the developmental mode of fish newborns. Syngnathids fishes hatch with well-developed sensory systems, jaws, and feeding structures and, therefore, they are considered as precocial fish species. Using as a model the retina of two species of syngnathids (Syngnathus typhle and Hippocampus guttulatus), we describe the retinal maturity during different embryological stages and compare the results with previous studies in the retina of other altricial and precocial fish species. This will be done through a review of the pertinent literature, as well as by drawing on our own experience gathered through recent studies on fish retinogenesis. These differences in the maturity of the visual system have implications for the vision-based survival skills during the early life stages after hatching and for the overall ecology and fitness of the species.

2019-09Journal article

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Trends in seahorse abundance in the Ria Formosa, South Portugal: recent scenario and future prospects

Publication . Correia, Miguel José Teodoro; Andrade, José Pedro; Koldewey, Heather

In the last decade, seahorse population census in the Ria Formosa identified stable populations of Hippocampus guttulatus and H. hippocampus at the highest densities described for these species worldwide. However, recent studies in the Ria reported a significant decrease in seahorse numbers of 94% for H. guttulatus and 73% for H. hippocampus. Therefore, this study aimed to identify aspects responsible for the variation in seahorse abundance, first by looking at the current trend of seahorse population by comparing it with previous surveys. Secondly, new techniques were tested to improve the survey efficiency of seahorse population and new tools were developed to promote seahorse population recovery, with potential application in seahorse population management programs. In this study, it was observed that both seahorse species abundance increased since the previous survey (2008/2009) although at lower numbers than in the 2001/2002 surveys and a significant correlation between the percentage coverage and H. guttulatus density was found. The new underwater visual census tested, proved to be more effective and accurate in a low seahorse density scenario. In this study, seasonal variations in seahorse density were observed in all sites and were probably related to human activities, natural events, and holdfast availability. In addition, a new photo-identification method was tested and results indicate it as a suitable tool for capture-recapture studies and less invasive that other commonly used methods. To cope with habitat degradation, artificial holdfast units (AHU) were designed and first tested in laboratory to assess seahorse preference. The preferred AHU was then deployed in 4 locations in the Ria Formosa, that differed in depth, water current and seahorse abundance in the vicinities of the selected site. Results showed seahorse settlement at high densities and validated the usefulness of the AHU as a habitat recovery tool.

2014Doctoral thesis

Open access

Spiral beacon calibration and experiments for underwater localization

Publication . Viegas, Rúben; Zabel, Friedrich; Gomes, João; Silva, António

Underwater localization and navigation are still challenging tasks due to the underwater acoustic channel characteristics. Spiral sources are underwater transducers that create structured acoustic fields from which the angle to the source can be readily obtained. The angle estimation is obtained from the phase difference between transmitted circular and spiral fields, but for reliable operation the transducers must be properly calibrated. This paper presents a spiral source calibration procedure with the integration of a stepper motor to measure phase and amplitude features of the transmitted circular and spiral fields, at multiple bearing angles. The calibration was performed for two developed prototypes, which in turn determined the most appropriate operating frequency range. For one of the prototypes, its linearity was confirmed at all the tested frequency ranges through homogeneity and additivity tests. In addition to calibration, acoustic localization experiments were carried out with the transmission of circular and spiral fields, with a comparative analysis against footage captured from the top of the test pool. The phase difference of the mobile hydrophone was subtracted to the phase difference of the reference hydrophone to compute the angle between the spiral beacon and the mobile hydrophone. The localization results revealed noteworthy angular errors, hypothesized to be associated with the Doppler effect induced by the movement of the mobile hydrophone. These calibration and localization experiments suggest that spiral sources could be an important enabling technology for safe and reliable localization of underwater vehicles.

2024-04-15Conference object

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