Changes in mean body size in an expanding population of a threatened species

Abstract

With some taxa, a reduction in the mean size of individuals may reflect over-harvesting and/or trophy hunting. However, we show that in sea turtles, a reduction in the mean size of breeding individuals may be part of the good news story of an expanding population. We describe a 70-fold increase in annual nest numbers on the island of Sal (Cape Verde, North Atlantic) between 2008 and 2020 (from 506 to 35 507 nests), making this now one of the largest loggerhead ( Caretta caretta ) nesting aggregations in the world. We use 20 128 measurements of the size of nesting turtles to show that their mean annual size has decreased by about 2.4 cm, from 83.2 to 80.8 cm. This decrease in the mean size of nesting turtles was not caused by the removal of larger turtles, for example by selective harvesting. Rather we develop a theoretical model to show than this decrease in mean size can be explained by an influx of first-time nesters, combined with a decrease in the size of those first-time nesters over time. A reduction in mean size of nesting turtles has been reported across the Atlantic, Pacific and Indian Oceans, and may be a common feature of population recoveries in sea turtles.

Long-term changes in adult size of green turtles at Aldabra Atoll and implications for clutch size, sexual dimorphism and growth rates

“…There was a clear decline in the mean size of nesting turtles at Aldabra from 1996 to 2016 that co-occurred with an increase in nesting numbers (Pritchard et al 2022), as has been reported in some other populations around the world, for example green turtles at Ascension Island (Weber et al 2014) and loggerhead turtles, Caretta caretta, in South Africa and the Cape Verde islands, respectively (Le Gouvello et al 2020;Hays et al 2022). At Ascension Island where overall nesting activity and numbers of egg clutches laid annually had increased by almost sixfold since 1977, the mean annual CCL of green turtle nesters declined from about 116 cm in 1970 to about 112 cm in 2010, i.e.…”

“…a decrease of about 1 cm per decade (Weber et al 2014), compared to the decline in size of about 0.6 cm per decade that we recorded on Aldabra, where nesting activity has increased by more than threefold between 1980 and 2016 (Pritchard et al 2022). Mean size of nesters may be influenced by an increase in the number of first-time nesters (neophytes or recruits) that are presumably smaller than established nesters (remigrants) (Hays et al 2022). Likewise, in Hawaii (USA), Piacenza et al (2016) found evidence of directional changes in size over time, suggesting shifts in age structure that could be due to recruitment.…”

“…For sea turtles, there has been protracted interest in how body size varies across populations as well as between males and females (Hirth 1980;Godley et al 2002). Recently, there has been great interest in how the mean size of breeding adults may change over time (Bjorndal et al 2013a(Bjorndal et al , 2017Weber et al 2014;Le Gouvello et al 2020;Phillips et al 2021;Hays et al 2022). Observed inter-seasonal decreases in average body size within populations have previously been attributed to fisheries related mortality of larger turtles (Hatase et al 2002;Shanker et al 2003), protection of nesting females resulting in changes in relative proportions of remigrant and neophyte (recruit) nesters (Pérez-Castañeda et al 2007;Weber et al 2014;Hays et al 2022), competition for food resources (Weber et al 2014;Sönmez 2019), and ecological change at a regional scale (Bjorndal et al 2017;Le Gouvello et al 2020;Phillips et al 2021).…”

“…Recently, there has been great interest in how the mean size of breeding adults may change over time (Bjorndal et al 2013a(Bjorndal et al , 2017Weber et al 2014;Le Gouvello et al 2020;Phillips et al 2021;Hays et al 2022). Observed inter-seasonal decreases in average body size within populations have previously been attributed to fisheries related mortality of larger turtles (Hatase et al 2002;Shanker et al 2003), protection of nesting females resulting in changes in relative proportions of remigrant and neophyte (recruit) nesters (Pérez-Castañeda et al 2007;Weber et al 2014;Hays et al 2022), competition for food resources (Weber et al 2014;Sönmez 2019), and ecological change at a regional scale (Bjorndal et al 2017;Le Gouvello et al 2020;Phillips et al 2021). Concern has been expressed that a declining size of nesting turtles may compromise the reproductive output of sea turtle populations (Le Gouvello et al 2020), since clutch size increases with body size (reviewed by Hirth 1997).…”

Trends in Reproductive Indicators of Green and Hawksbill Sea Turtles over a 30-Year Monitoring Period in the Southern Gulf of Mexico and Their Conservation Implications

“…The population’s growth depends on the recruitment of neophytes. In expanding populations, the proportion of neophytes is expected to increase over time [ 41 ] and this is often the case unless, for some unpredicted event (e.g., years with extreme temperatures, drought, and heavy rainfall affecting the incubation environment), the stock expected to recruit is completely or partially lost. It is assumed that the size of the females in a recovering population will be smaller than when populations were stable and had a balanced structure of all its components.…”