Freshwater turtle populations as bioindicators following an oil spill: Delayed demographic changes reveal long-term impacts

Chronic, long-term impacts of oil spill disasters on wildlife often exceed short-term, highly visible mass mortalities and widespread oiling of individuals. Species with long lifespans, late maturation, and low recruitment rates are particularly vulnerable to long-term population-level impacts but can be useful as indicator species for ecosystem recovery. In 2010, one of the largest freshwater oil spills in the U.S. occurred in the Kalamazoo River, MI, when 3.2 million L of spilled oil impacted 56 km of river and associated wildlife. During cleanup and restoration efforts in 2010-2011, thousands of northern map turtles (Graptemys geographica) were captured, cleaned, and released. During 2019-2020 northern map turtles were captured to evaluate changes in the population size, demography, and size classes nine to ten years later. Population demography shifts occurred in the first few years after the spill in a species that otherwise exhibits minimal demographic fluctuation under "normal" conditions. In comparing demographic parameters at the time of the oil spill to values similar to 10 years post-spill, a nearly 30% reduction in population size was detected, distribution of body size shifted to smaller males and females, and there was a shift in the population sex ratio between 2011 and 2019. There were also signals of failed recruitment in cohorts that would have hatched during the years immediately before and after the oil spill. These data suggest that beyond the direct mortality caused by the spill, declines in the estimated population size and shifts in the size distribution of northern map turtles are likely indicative of negative demographic impacts incurred following the 2010 oil spill and resulting cleanup.