# Warming and Change in Ocean Productivity Alter Phenology of an Expanding Loggerhead Population in Cabo Verde

## TL;DR

Warmer temperatures are causing loggerhead turtles in Cabo Verde to nest earlier and more frequently, but reduced ocean productivity is lowering their reproductive success.

## Contribution

This study provides population-specific evidence of how climate warming and ocean productivity changes affect loggerhead turtle reproductive phenology and output.

## Key findings

- Warmer sea surface temperatures advanced the nesting season's start, peak, and end.
- Higher temperatures reduced ocean productivity, leading to fewer and smaller clutches.
- Larger females nested more frequently in warmer years, but overall reproductive output declined over time.

## Abstract

The diverse responses of sea turtles to climate warming highlight the need for population-specific studies. Here, we used long-term monitoring data from one of the largest loggerhead turtle nesting populations in the world to investigate the reproductive phenology and output in relation to climate warming. We found that warmer years were associated with earlier phenology and longer nesting seasons. At the same time, increased temperatures reduced ocean productivity, which prolonged foraging periods and consequently decreased clutch frequency and size. This decline in reproductive output may ultimately compromise population resilience and slow recovery in the face of ongoing climate warming.

Climate warming can alter reproductive timing of species, yet the capacity for phenological adjustment in long-lived species, particularly marine ones, remains elusive. Using 17 years of monitoring data from one of the largest loggerhead turtle (Caretta caretta) populations, we investigated the environmental drivers of reproductive phenology and output. We found that warmer sea surface temperatures (SST) in both the feeding ground and the nesting ground advanced the start, peak, and end of the nesting season. We provide evidence for waves of arrival at the nesting ground, suggesting more turtles produce fewer clutches than previously thought. Inter-nesting intervals were shorter during episodes of higher SST, particularly in larger females, likely underpinned by metabolic scaling variation in reproductive pacing. Conversely, remigration intervals lengthened over time in all size classes, reflecting the detected continuous decrease in productivity in the feeding ground. As a result of reduced ocean productivity, both clutch size and clutch frequency also declined over the study period. Moreover, the declining trend in body size further reduces reproductive output, as smaller females produce smaller clutch sizes. Overall, we show that sea turtle population dynamics correlate with environmental parameters. The sustained decline in reproductive output underscores the need to mitigate the impacts of climate warming on the foraging area to safeguard this population, which, given its size, holds global significance.