Unexpected stability in faunal population abundances following an estuary-wide collapse of oysters

Abstract

Live oyster reefs are considered a critical recruitment habitat for estuarine faunal populations as localized in situ or mesocosm studies have demonstrated many faunal species prefer live oyster habitat. It has therefore been assumed that the loss of live oyster habitat would precipitate faunal population declines, but this has been largely untested at large (estuary) scales. Here, we assessed how estuary-wide faunal populations were affected by a 95% loss of live oyster habitat following the 2012 oyster collapse of Apalachicola Bay, FL, which previously supported one of the largest oyster fisheries in the United States. We standardized long-term fisheries-independent monitoring seine and trawl data to create relative indices of resident, associated, and transient faunal species' overall abundance and recruit abundance (restrictive to sizes between 15% and 35% of $L_{\infty }$). We expected that both relative abundance indices would decrease following the oyster collapse, particularly among species that reside on or recruit to oyster reefs. However, analyses via a series of one-sided Bayesian t tests did not indicate that faunal recruitment or overall abundance significantly declined in 2012 post-collapse. As the response of the faunal population could be lagged relative to the 2012 collapse, we also conducted change point analyses to search for lagged declines. Of the 24 relative abundance time series, only two had significant change points post-collapse, and only black sea bass overall relative abundance declined with an associated change point at the end of the time series. The surprising paucity of faunal decline following oyster loss may be due to the use of alternative habitat types, exceptionally lagged faunal responses, or, perhaps most compelling, a disconnect between preferred and required habitats. Our failure to detect faunal consequences following an oyster population collapse suggests that assumptions of habitat loss (or restoration) effects on estuarine fauna at ecosystem scales are not straightforward and the extrapolation of sub-estuary-scale studies may result in poor predictions of future outcomes.