Disentangling exploitation of the intertidal grazer Phorcus sauciatus (Gastropoda: Trochidae) in an oceanic archipelago: Implications for conservation

Abstract

Harvesting of intertidal grazers such as topshells is known to affect negatively the exploited populations by altering population structure and decreasing abundance. Phorcus sauciatus has a wide geographic distribution in the North‐eastern Atlantic Ocean and is subject to increasing levels of harvesting pressure due to the expansion of human population on coastal areas. The effect of proximity to human settlements and coastal accessibility on the size structure and abundance of P. sauciatus populations was examined in Madeira archipelago. Mean size, proportion of reproductive individuals, and abundance of this species were generally smaller in areas closer to human settlements and in more accessible coastal areas. Marine protected areas returned the highest mean sizes evidencing their effectiveness in preserving the size structure of this species. The results highlight the necessity to regulate the harvest of P. sauciatus in Madeira archipelago, as well as the implementation of management measures aiming at the sustainable exploitation and conservation of this species, exploited in this region since the early 15th century.

Weight–length relationships of four intertidal mollusc species from the northeastern Atlantic Ocean and their potential for conservation

“…In this region a negatively allometric growth was obtained, this may result from the high level of harvesting pressure leading to lower population densities and to alterations in population dynamics and size structure (Riera et al 2016;Sousa et al 2018). This is corroborated by the study of Sousa et al (2019) that found a similar pattern of growth, smaller mean size populations and lower proportion of reproductive individuals in this region.…”

“…Harvesting of keystone species such as topshells negatively affects the exploited populations by altering population structure and decreasing abundance (Sousa et al 2019) and thus the knowledge of species and ecosystems is important to promote the implementation of management measures. Marine Protected Areas (MPAs), defined as no-take zones, are a popular measure for the conservation of harvested marine populations, since they promote the recovery of populations by removing harvesting pressure (Halpern and Warner 2002).…”

Unravelling the effects of exploitation on the size–structure of the intertidal topshell Phorcus sauciatus in harvested and non-harvested Atlantic regions

“…The present study showed that P. sauciatus from Madeira archipelago exhibited the smallest total length within the four studied locations, this may result from a high level of harvesting pressure, known to negatively affect the exploited populations by altering their population size-structure (Riera et al, 2016;Tuya et al, 2006). Recently, it was suggested that harvesting has a great impact on the population dynamics of P. sauciatus from Madeira archipelago with more emphasis on harvested populations that show a smaller mean size and lower proportion of reproductive individuals (Sousa et al, 2019b).…”

“…The differences found between size of harvested and non-harvested populations in mainland Portugal and the Canaries were smaller than those observed in Madeira archipelago. This is related to the lack of regulation in Madeira, with uncontrolled total catch of this species (Sousa et al, 2019b). Also, the lack of widespread traditional harvest of topshells in mainland Portugal results in less marked differences between harvested and non-harvested areas.…”

Island effect in the shell phenotypic plasticity of an intertidal gastropod

“…Despite being highly exploited since the fifteenth century for human consumption in Madeira, the regional morphological variation of this species is unknown. Most of the studies concerning Phorcus sauciatus from Madeira (NE Atlantic Ocean) have focused on the biological parameters and exploitation rate (Sousa et al, 2019a) and in the effects of harvesting pressure on the size-structure and abundance of populations (Sousa et al, 2019b).…”