Horseshoe crab spawning in the Delaware Bay: a potential response to sea surface

The American horseshoe crab Limulus polyphemus (HSC) spawns in their greatest densities in the Delaware Bay; however, over-harvests have reduced their population size by 90% in the late 20th century and climate change is degrading their spawning habitat. Because their eggs are an important food source for threatened shorebird species and adjust the timing they spawn due to ocean temperatures efforts were made to understand sea surface temperatures (SSTs) throughout the Bayshore area, patterns of egg cluster abundances and surface densities, identify a SST threshold for peak spawning activity, and use physical characteristics related to specific sites identified as increasing egg abundances. The highest egg abundances – specifically clusters – were in the southern region and decreased northward, while SSTs showed the opposite pattern. Surface egg densities were also the most abundant in the Southern region but are believed to be transported widely within and between regions. The CART analysis identified that the five-day moving average for SST of 17° C when abundance shifted from pre-peak to post-peak and currently occurs between May 29 and June 5. This temperature threshold was determined to have advanced by several days for all regions and beaches with the Northern region experiencing the most significant advancement, suggesting that a potential does exist for a mismatch to develop if HSCs spawn earlier to match this advancement. Beach nourishment activities will be important for the continued existence of suitable habitat for HSC spawning in the Bayshore area and should focus on increasing the physical parameters that increase cluster abundance such as having sand depth of at least 40 cm and beach width of at least 22 m. As climate change continues to alter the physical habitat in the Bayshore area, continued monitoring of when HSCs are spawning as a response to SST will be important to understand if this mismatch is developing, potentially jeopardizing the recovering red knot population. The MUR dataset is a useful and time and cost-efficient way to monitor SSTs in the Bayshore area for both beaches and regions, allowing it to be implemented in surveying HSC spawning to identify more specific temperature patterns.M.S.Includes bibliographical reference