Marine reserve effects on population density and size structure of commonly and rarely exploited limpets in South Africa

Abstract

Marine reserves are commonly used to conserve living resources, but their effectiveness where policing is difficult is unclear. We compared monthly population density and size structure data collected over 20 months for two rarely and two commonly exploited intertidal limpets inside and outside reserves in South Africa. Densities were greater inside reserves, but significantly so in very few months. Unexpectedly, no significant reserve effect was detected for the territorial Scutellastra longicosta, shown by surveys to be favoured by harvesters. Reserve effects for the rarely exploited S. granularis probably result from indirect effects of higher barnacle cover in reserves where trampling is reduced, while interviews indicated that Cellana capensis is not targeted but large individuals are sometimes misidentified and hence are harvested outside the reserve. There were few reserve effects on mean or mean maximum size. The results indicate a gradient of exploitation among species and sites. One non-reserve site was more heavily exploited than the other, while one reserve experienced more poaching than the other. The effectiveness of marine reserves thus differed between reserves and among species. Exploited limpets were generally larger and at higher densities in reserves, but these effects were rarely statistically significant, indicating that reserve effects are weak compared to natural variability, probably reflecting ineffective policing.

Assessing the effects of no-take zones in a marine protected area spanning two ecoregions and rock substrate types

“…By contrast, Scutellastra argenvillei, which inhabits semi-exposed to exposed areas (Branch and Steffani, 2004), was recorded as being harvested at only one site in any numbers. The absence of S. longicosta, S. cochlear and S. granularis from any catch records supports the view that they are rarely harvested in the region we investigated, although S. longicosta is harvested on the more easterly shores of South Africa (Hockey et al, 1988;Nakin et al, 2012;Nakin and McQuaid, 2014;Nakin and McQuaid, 2016).…”

“…Similar findings were reported for S. granularis at Dwesa Nature Reserve, with larger sizes outside than inside the reserve (Nakin and McQuaid, 2014). The occurrence of larger individuals in harvested areas may reflect reduced competition due to the removal of harvested competitors, although that has not yet been demonstrated empirically.…”

“…The densities of S. longicosta that we recorded were magnitudes less than those recorded in Dwesa Nature Reserve (Nakin and McQuaid, 2014). This might reflect that the species is reaching the western limit of its distribution range in the TMNP, resulting in low densities (Hidas et al, 2010).…”

“…Densities and sizes (shell lengths) of three limpet species commonly harvested in the region, Cymbula granatina, C. oculus and Scutellastra argenvillei (Eekhout et al, 1992;Bustamante et al, 1995;Branch and Odendaal, 2003), and three species of limpet rarely harvested there, S. cochlear, S. granularis and S. longicosta, were compared between harvested and no-take areas. Classification of species as 'commonly harvested' or 'rarely harvested' was based on our own observations, as reported in the results, and those of Lasiak (1991b) and Nakin and McQuaid (2014). These authors did report Scutellastra longicosta as being harvested on the southeast coast of South Africa, but there was no evidence of this in our study area.…”

“…It is possible that that its abundance is influenced more by the arrival of the alien mussel M. galloprovincialis (Griffiths et al, 1992;Branch et al, 2008;Branch et al, 2010) than any other factor, as this mussel greatly boosts this limpet's recruitment (Van Erkom Schurink and Griffiths, 1990). In the Agulhas Ecoregion, S. granularis density is also often restricted by barnacles such as Tetraclita serrata and Chthamalus dentatus (Branch, 1976;Nakin and McQuaid, 2014), which are substantially less abundant in the Southern Benguela than the Agulhas (Boland, 1997). There were significant and complex interactions among ecoregion, rock types and protection level for S. granularis and S. cochlear as a result of these variable outcomes, which was not unexpected in the absence of any likely direct effect of protection.…”

Assessing the effects of no-take zones in a marine protected area spanning two ecoregions and rock substrate types

“…By contrast, Scutellastra argenvillei, which inhabits semi-exposed to exposed areas (Branch and Steffani, 2004), was recorded as being harvested at only one site in any numbers. The absence of S. longicosta, S. cochlear and S. granularis from any catch records supports the view that they are rarely harvested in the region we investigated, although S. longicosta is harvested on the more easterly shores of South Africa (Hockey et al, 1988;Nakin et al, 2012;Nakin and McQuaid, 2014;Nakin and McQuaid, 2016).…”

“…Similar findings were reported for S. granularis at Dwesa Nature Reserve, with larger sizes outside than inside the reserve (Nakin and McQuaid, 2014). The occurrence of larger individuals in harvested areas may reflect reduced competition due to the removal of harvested competitors, although that has not yet been demonstrated empirically.…”

“…The densities of S. longicosta that we recorded were magnitudes less than those recorded in Dwesa Nature Reserve (Nakin and McQuaid, 2014). This might reflect that the species is reaching the western limit of its distribution range in the TMNP, resulting in low densities (Hidas et al, 2010).…”

“…Densities and sizes (shell lengths) of three limpet species commonly harvested in the region, Cymbula granatina, C. oculus and Scutellastra argenvillei (Eekhout et al, 1992;Bustamante et al, 1995;Branch and Odendaal, 2003), and three species of limpet rarely harvested there, S. cochlear, S. granularis and S. longicosta, were compared between harvested and no-take areas. Classification of species as 'commonly harvested' or 'rarely harvested' was based on our own observations, as reported in the results, and those of Lasiak (1991b) and Nakin and McQuaid (2014). These authors did report Scutellastra longicosta as being harvested on the southeast coast of South Africa, but there was no evidence of this in our study area.…”

“…It is possible that that its abundance is influenced more by the arrival of the alien mussel M. galloprovincialis (Griffiths et al, 1992;Branch et al, 2008;Branch et al, 2010) than any other factor, as this mussel greatly boosts this limpet's recruitment (Van Erkom Schurink and Griffiths, 1990). In the Agulhas Ecoregion, S. granularis density is also often restricted by barnacles such as Tetraclita serrata and Chthamalus dentatus (Branch, 1976;Nakin and McQuaid, 2014), which are substantially less abundant in the Southern Benguela than the Agulhas (Boland, 1997). There were significant and complex interactions among ecoregion, rock types and protection level for S. granularis and S. cochlear as a result of these variable outcomes, which was not unexpected in the absence of any likely direct effect of protection.…”

Assessing the effects of no-take zones in a marine protected area spanning two ecoregions and rock substrate types

“…By contrast, Scutellastra argenvillei, which inhabits semi-exposed to exposed areas (Branch and Steffani, 2004), was recorded as being harvested at only one site in any numbers. The absence of S. longicosta, S. cochlear and S. granularis from any catch records supports the view that they are rarely harvested in the region we investigated, although S. longicosta is harvested on the more easterly shores of South Africa (Hockey et al, 1988;Nakin et al, 2012;Nakin and McQuaid, 2014;Nakin and McQuaid, 2016).…”

“…Similar findings were reported for S. granularis at Dwesa Nature Reserve, with larger sizes outside than inside the reserve (Nakin and McQuaid, 2014). The occurrence of larger individuals in harvested areas may reflect reduced competition due to the removal of harvested competitors, although that has not yet been demonstrated empirically.…”

“…The densities of S. longicosta that we recorded were magnitudes less than those recorded in Dwesa Nature Reserve (Nakin and McQuaid, 2014). This might reflect that the species is reaching the western limit of its distribution range in the TMNP, resulting in low densities (Hidas et al, 2010).…”

“…Densities and sizes (shell lengths) of three limpet species commonly harvested in the region, Cymbula granatina, C. oculus and Scutellastra argenvillei (Eekhout et al, 1992;Bustamante et al, 1995;Branch and Odendaal, 2003), and three species of limpet rarely harvested there, S. cochlear, S. granularis and S. longicosta, were compared between harvested and no-take areas. Classification of species as 'commonly harvested' or 'rarely harvested' was based on our own observations, as reported in the results, and those of Lasiak (1991b) and Nakin and McQuaid (2014). These authors did report Scutellastra longicosta as being harvested on the southeast coast of South Africa, but there was no evidence of this in our study area.…”

“…It is possible that that its abundance is influenced more by the arrival of the alien mussel M. galloprovincialis (Griffiths et al, 1992;Branch et al, 2008;Branch et al, 2010) than any other factor, as this mussel greatly boosts this limpet's recruitment (Van Erkom Schurink and Griffiths, 1990). In the Agulhas Ecoregion, S. granularis density is also often restricted by barnacles such as Tetraclita serrata and Chthamalus dentatus (Branch, 1976;Nakin and McQuaid, 2014), which are substantially less abundant in the Southern Benguela than the Agulhas (Boland, 1997). There were significant and complex interactions among ecoregion, rock types and protection level for S. granularis and S. cochlear as a result of these variable outcomes, which was not unexpected in the absence of any likely direct effect of protection.…”

Assessing the effects of no-take zones in a marine protected area spanning two ecoregions and rock substrate types

“…By contrast, Scutellastra argenvillei, which inhabits semi-exposed to exposed areas (Branch and Steffani, 2004), was recorded as being harvested at only one site in any numbers. The absence of S. longicosta, S. cochlear and S. granularis from any catch records supports the view that they are rarely harvested in the region we investigated, although S. longicosta is harvested on the more easterly shores of South Africa (Hockey et al, 1988;Nakin et al, 2012;Nakin and McQuaid, 2014;Nakin and McQuaid, 2016).…”

“…Similar findings were reported for S. granularis at Dwesa Nature Reserve, with larger sizes outside than inside the reserve (Nakin and McQuaid, 2014). The occurrence of larger individuals in harvested areas may reflect reduced competition due to the removal of harvested competitors, although that has not yet been demonstrated empirically.…”

“…The densities of S. longicosta that we recorded were magnitudes less than those recorded in Dwesa Nature Reserve (Nakin and McQuaid, 2014). This might reflect that the species is reaching the western limit of its distribution range in the TMNP, resulting in low densities (Hidas et al, 2010).…”

“…Densities and sizes (shell lengths) of three limpet species commonly harvested in the region, Cymbula granatina, C. oculus and Scutellastra argenvillei (Eekhout et al, 1992;Bustamante et al, 1995;Branch and Odendaal, 2003), and three species of limpet rarely harvested there, S. cochlear, S. granularis and S. longicosta, were compared between harvested and no-take areas. Classification of species as 'commonly harvested' or 'rarely harvested' was based on our own observations, as reported in the results, and those of Lasiak (1991b) and Nakin and McQuaid (2014). These authors did report Scutellastra longicosta as being harvested on the southeast coast of South Africa, but there was no evidence of this in our study area.…”

“…It is possible that that its abundance is influenced more by the arrival of the alien mussel M. galloprovincialis (Griffiths et al, 1992;Branch et al, 2008;Branch et al, 2010) than any other factor, as this mussel greatly boosts this limpet's recruitment (Van Erkom Schurink and Griffiths, 1990). In the Agulhas Ecoregion, S. granularis density is also often restricted by barnacles such as Tetraclita serrata and Chthamalus dentatus (Branch, 1976;Nakin and McQuaid, 2014), which are substantially less abundant in the Southern Benguela than the Agulhas (Boland, 1997). There were significant and complex interactions among ecoregion, rock types and protection level for S. granularis and S. cochlear as a result of these variable outcomes, which was not unexpected in the absence of any likely direct effect of protection.…”