The present thesis aims to deepen the ethological and ecological knowledge associated with the scalloped hammerhead shark, Sphyrna lewini. Current information regarding populations of this species in the Red Sea is limited due to the scarcity of studies conducted in the region. Even more limited is the knowledge about its behavior, ecology, and daytime aggregations in the central Red Sea reefs. Although new investigative methodologies have been introduced, quantifying aggregations or social interactions in these marine organisms remains a significant challenge. The ethology and ecology of this shark and its daytime aggregations in the central Red Sea reefs were studied through the analysis of several key characteristics, including the average number of individuals per sampled area, their spatial distribution in the water column, and the different behavioral patterns exhibited both in the presence and absence of divers. Additionally, the presence/absence of these organisms and their aggregations was estimated in relation to dive sites, depth, and time of day. Statistical analysis of video footage and observational data collected during expeditions conducted between March and May from 2010 to 2012 and from 2014 to 2019 in the central-western Red Sea reefs, as well as in May 2022 in the central-eastern Red Sea reefs, enabled the description of different aggregation phases of S. lewini specimens. More specifically, comparing the number of individuals observed at different times and depths during single and multiple dives with the total number of individuals observed for each dive allowed the identification of three main types of aggregations: aggregations consisting of 1–4 individuals, 5–24 individuals, and 25 or more individuals. These three types of aggregations appear to differ not only in terms of frequency of observation but also in their spatial distribution within the water column and in the ethological characteristics exhibited. A combined observational-descriptive approach, integrated with qualitative and semi-quantitative analysis of data derived from video footage, experimental observations, and parameters collected during scientific expeditions conducted as part of the Hammerhead Schooling Research project by the Danishark Elasmobranch Research Association since 2004, made it possible to investigate in greater depth the large aggregations of S. lewini in the central Red Sea reefs. The results obtained are of significant scientific interest, given the critical knowledge gap regarding aggregations of this species. Sphyrna lewini has been listed as Critically Endangered (CR) on the IUCN Red List since November 8, 2018, in both the regional and global assessments.
La presente tesi vuole approfondire le conoscenze etologiche ed ecologiche associate allo squalo martello smerlato, Sphyrna lewini. Attualmente le conoscenze riguardanti le popolazioni di questo squalo nel Mar Rosso sono scarse dato il numero limitato di studi affrontati. Ancor più limitate le informazioni relative alla sua etologia ed ecologia ed alle aggregazioni diurne nei reef del Mar Rosso centrale. Sebbene ad oggi sia stato possibile integrare nuove metodiche di indagine, quantificare aggregazioni o interazioni sociali in questi organismi marini rappresenta ancora una vera e propria sfida. L’etologia e l’ecologia di questo squalo e delle sue aggregazioni diurne nei reef del Mar Rosso centrale sono state studiate attraverso l’analisi di alcune caratteristiche importanti, come il numero medio di individui per area campionata, la distribuzione spaziale degli stessi in colonna d’acqua e i differenti pattern comportamentali esibiti sia in presenza che in assenza di subacquei. È stata poi stimata la presenza/assenza di questi organismi e delle loro aggregazioni in relazione ai siti d’immersione, alla profondità e al momento della giornata. L’utilizzo a fini statistici dei filmati e dei relativi dati osservativi acquisiti nell’arco delle spedizioni condotte tra i reef del Mar Rosso centro-occidentale tra marzo e maggio degli anni dal 2010 al 2012 e dal 2014 al 2019 e tra i reef del Mar Rosso centro-orientale nel maggio del 2022, hanno permesso di descrivere differenti fasi aggregative degli esemplari di S. lewini. Più nello specifico, il confronto del numero di individui osservati in momenti e a profondità differenti, nell’arco della medesima e di molteplici immersioni, con il numero totale di individui osservati per ciascuna delle immersioni condotte, ha permesso di identificare tre principali tipi di aggregazione: aggregazioni con numero di esemplari di S. lewini compreso tra 1 e 4, tra 5 e 24, e maggiore o uguale a 25. Oltre che per frequenza di osservazione, le tre tipologie di aggregazione sembrerebbero distinguersi, sia per distribuzione spaziale nella colonna d’acqua, sia per caratteristiche etologiche esibite. L’utilizzo di un approccio osservativo-descrittivo, integrato all’analisi qualitativa e semi-quantitativa delle informazioni derivanti da filmati, dati e parametri acquisiti sperimentalmente, nell’arco delle spedizioni scientifiche condotte nell’ambito del progetto di ricerca Hammerhead Schooling Research, dall’Associazione Danishark Elasmobranch Research dal 2004 in poi, ha permesso di indagare più a fondo le grandi aggregazioni di S. lewini nei reef del Mar Rosso centrale. I risultati ottenuti sono di particolare interesse scientifico visto l’importante deficit conoscitivo riguardante le aggregazioni della specie, catalogata dall’8 Novembre 2018 nella IUCN Red List sotto la dicitura di Critically Endangered (CR), sia nella valutazione regionale che in quella globale.
Aggregazioni di squali martello, Sphyrna lewini (Griffith & Smith, 1834), nei reef del Mar Rosso centrale
GINELLI, ANDREA
2023/2024
Abstract
The present thesis aims to deepen the ethological and ecological knowledge associated with the scalloped hammerhead shark, Sphyrna lewini. Current information regarding populations of this species in the Red Sea is limited due to the scarcity of studies conducted in the region. Even more limited is the knowledge about its behavior, ecology, and daytime aggregations in the central Red Sea reefs. Although new investigative methodologies have been introduced, quantifying aggregations or social interactions in these marine organisms remains a significant challenge. The ethology and ecology of this shark and its daytime aggregations in the central Red Sea reefs were studied through the analysis of several key characteristics, including the average number of individuals per sampled area, their spatial distribution in the water column, and the different behavioral patterns exhibited both in the presence and absence of divers. Additionally, the presence/absence of these organisms and their aggregations was estimated in relation to dive sites, depth, and time of day. Statistical analysis of video footage and observational data collected during expeditions conducted between March and May from 2010 to 2012 and from 2014 to 2019 in the central-western Red Sea reefs, as well as in May 2022 in the central-eastern Red Sea reefs, enabled the description of different aggregation phases of S. lewini specimens. More specifically, comparing the number of individuals observed at different times and depths during single and multiple dives with the total number of individuals observed for each dive allowed the identification of three main types of aggregations: aggregations consisting of 1–4 individuals, 5–24 individuals, and 25 or more individuals. These three types of aggregations appear to differ not only in terms of frequency of observation but also in their spatial distribution within the water column and in the ethological characteristics exhibited. A combined observational-descriptive approach, integrated with qualitative and semi-quantitative analysis of data derived from video footage, experimental observations, and parameters collected during scientific expeditions conducted as part of the Hammerhead Schooling Research project by the Danishark Elasmobranch Research Association since 2004, made it possible to investigate in greater depth the large aggregations of S. lewini in the central Red Sea reefs. The results obtained are of significant scientific interest, given the critical knowledge gap regarding aggregations of this species. Sphyrna lewini has been listed as Critically Endangered (CR) on the IUCN Red List since November 8, 2018, in both the regional and global assessments.File | Dimensione | Formato | |
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https://hdl.handle.net/20.500.12075/21073