Environmental factors modify post-settlement survival and growth of winter flounder Pseudopleuronectes americanus in the absence of predation

Post-settlement mortality and growth in flatfish has the potential to profoundly impact year-class strength. However, the abiotic drivers of post-settlement mortality and growth are difficult to disentangle from the effects of predation. Young-of-the-year (YOY) winter flounder Pseudopleuronectes americanus, collected from Shinnecock Bay, NY (USA), were kept in predator-exclusion cages from May through August in 2016 and 2017, with simultaneous recording of dissolved oxygen, temperature and salinity at each caging site. Daily mortality rates in cages were 7 to 5.9 times less than mortality estimated for wild fish from the same area. We modeled the effect of 9 time-varying environmental variables, as well as the effect of cage location and depth, on survival and growth of caged fish using a Cox proportional hazards model with time-dependent covariates. The site that had the best survival in 2016 was not the site that had the best growth within that year, highlighting the potential for trade-offs between survival and growth. We found evidence that environmental conditions can impose both acute and chronic stress to YOY fish in estuarine environments, as well as potential threshold effects that warrant further exploration. Future monitoring efforts and studies of habitat suitability should take into account the degree and duration of exposure to temperature, salinity and dissolved oxygen extremes, not just the average conditions. We argue that risk can more accurately be captured by measuring aspects of the frequency, duration and intensity of both acute and chronic manifestations of environmental stressors, revealing potential thresholds and sublethal effects.