High annual survival suggested by size structure of Kemp’s ridley sea turtles captured by coastal research trawling in the Northwest Atlantic Ocean since 1990

Abstract

Abundance of Kemp’s ridley sea turtle Lepidochelys kempii Garman, 1880 in the Northwest Atlantic Ocean (NWA) is far less than in the Gulf of Mexico (GOM), but encounters of this species in the NWA have increased in recent decades. Consequently, improved opportunity exists to evaluate population structure, which is of particular interest given suggestions of renewed decline in annual survival rates in the GOM. Here we use size structure for Kemp’s ridley sea turtles captured (n = 617) by research trawling in the South Atlantic Bight (SAB) to assess survival since 1990 following age assignment using recently published size-at-age keys. With limited exception, the ratio of older (≥age 10) to younger (ages 1 to 9) sea turtles captured randomly was overwhelmingly stable and best aligned with high (≥0.9) annual neritic survival in theoretical models with fixed annual hatchling (H) recruitment. When annual H recruitment reflected exponential increase in H production since 1985, following low and generally stable production between 1966 and 1984, reduced H emigration proportion from the GOM to NWA and further increase in annual neritic survival were required to achieve the size/age structure reported for in-water data. Stepwise regression using monthly values for 5 climate indices simulated (adj. r2 = 0.77) proportionate annual H decline, which better explained observed survey size/age structure than climate-based prediction (adj. r2 = 1.00) of published particle emigration rates. Temporal decline in proportionate H recruitment to the NWA should increase retention of juveniles in the GOM to further enhance multi-generational population rebuilding.