On the presence of basking shark (Cetorhinus maximus) in the Mediterranean Sea

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Abstract

A B S T R A C T. -The presence of basking shark C e t o rhinus maximus Gunnerus, 1765 throughout the Mediterranean Sea was investigated using data stored in the "Large Elasmobranch Monitoring" programme database (Italy), with new records coming from the French area and also including the first data from the eastern sector of the Mediterranean basin. More than 500 records of information on basking sharks were included in the database, with older records coming from the 19 th century, but also including the most recent literature. Data revealed the presence of specimens of quite different sizes that ranged from very young (about 2 m TL) to large adults (8-9 m TL) encompassing large part of the body length range known for this shark at global scale. Most of the occurrences of the species in Mediterranean are reported off the coastal areas of the western central sectors of the Mediterranean basin, where it is often caught incidentally by trammel nets in spring. T h e numerous records registered in the last two decades may be related rather to an increasing scientific and public interest on the species than to an actual abundance trend of the Mediterranean population. The knowledge on the distribution and population structure of basking shark in the Mediterranean basin plays an important role in the implementation of conservation measures of such protected species.

Key takeaways

AI generated

The study compiles over 500 records of basking shark presence in the Mediterranean Sea.
Basking sharks range from 2 m to 9 m total length (TL), showing significant size variation.
Incidental catches in trammel nets primarily occur in spring, indicating seasonal patterns in sightings.
The Mediterranean population is classified as endangered, necessitating conservation measures for this protected species.
Data collection relies on historical records, scientific interest, and collaboration across Mediterranean countries.

FAQ's

AI generated

What seasonal trends are evident in basking shark sightings in the Mediterranean?add

The data indicates that basking shark sightings peak during spring, particularly in March, reflecting seasonal clustering around feeding hotspots.

How does the distribution of basking sharks vary across the Mediterranean regions?add

Basking sharks are primarily concentrated in the Western and Central Mediterranean sectors, particularly near the Liguro-Provençal area due to favorable upwelling conditions.

What factors influence the frequency of basking shark observations?add

Observations are influenced by meteorological conditions and research interest, with records showing strong correlation with water temperature and chlorophyll concentration.

What evidence supports repeating seasonal return behaviors in basking sharks?add

Some individual basking sharks exhibit philopatric tendencies, returning annually to the same coastal feeding locations, as seen in tagged individuals tracked across seasons.

What gaps exist in the current data on basking shark populations in the Mediterranean?add

Quantitative estimates of basking shark populations remain elusive due to the scarcity of targeted data collection efforts and monitoring in Eastern Mediterranean waters.

Figures

Figure 2. - Frequency of incidental catches of C. maximus by year in the Mediterranean. [Fréquence annuelle des captures acciden - telles de C. maximus en Méditerranée.] Figure 1. - Geographical allocation of observations and captures of Cetorhinus maximus in the Mediterranean Sea. B = Balearic region; T = Tyrrherian region; A = Adriatic region; E = Eastern Mediterranean; a = Imprecise geographical position; ? = Lacking of data. [Localisation des observations et des captures de Cetorhi nus maximus en Méditerranée. ]

Figure 3. - Frequency of incidental catches of C. maximus by month in the Mediterranean. [Fréquence mensuelle des rani accidentelles de C. maximus en Méditerranée. ]

Figure 4. - Geographical allocation of findings of C. maximus per season in Mediterranean Sea. [Localisations saisonniéres des observations de C. maximus en Méditerranée. ]

Figure 5. - Length frequency distribution of Cetorhinus maximus. [Distribution des tailles de C. maximus. ]

Figure 7. - Length frequency distribution of C etorhinus maximus per oT [Distribution des tailles de C. maximus en fonction de a saison. Figure 6. - Length frequency distribution of C etorhinus maximus per Sex. [Distribution des tailles de C. maximus en fonction du SeXe,

Figure 8. - Incidental catches of Cetorhinus maximus split by fishing gear (only for the specimens for which TL was specified). Others: anchovy larsen trawl (n = 1); lamparo net (n = 2); seine net (n =2); rifle (n = 1); not specified caught (n =27). [Captures accidentelles de C. maximus en fonction de |’ engin éche (seulement pour les spécimens pour lesquels la longueur totale était connue).

Figure 9. - Length frequency distribution of C. maximus per region. B =Balearic region; T = Tyrrhenian region; A : Adriatic region; E = Eastern Mediterranean. [Distribution des tailles de C. maximus suivant les régions.]

Figure 10. - Mediterranean average concentration of chlorophyll- like pigment (mg/m) for season (Satellite imagery: © OCEAN Project, 2000). [Concentration saisonniére moyenne des pigments de chlorophylle (mg/m) en Méditerranée.]

Research claims

535 records of basking sharks were collected from 1795 to 2002; primarily confined to Western and Central Mediterranean.View claim detailsarrow_forward

Incidental catches peaked in spring and winter, with March having 25% of observations; linked to increased scientific interest since 1990.View claim detailsarrow_forward

Length frequency distribution shows 50% young individuals in autumn; adult specimens peak at ~7 m TL in winter.View claim detailsarrow_forward

Quantitative population estimates for basking sharks in the Mediterranean are not possible due to insufficient data.View claim detailsarrow_forward

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