Distribution, Abundance, and Settlement of Slope-spawning Flatfish during Early Life Stages in the Eastern Bering Sea Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/x920g0226

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  • Changes in environmental conditions in marine ecosystems could directly or indirectly influence distribution, abundance, settlement, and size at settlement of flatfish. Understanding species-specific and age-specific responses to environmental variability is important for managing commercially important flatfish stocks. Slope-spawning flatfish whose offspring rely on extensive drift from the slope (spawning) to the shelf (settlement) and which require specific habitat for settlement could be especially vulnerable to environmental variability. Arrowtooth flounder (ATF; Atheresthes stomias), Greenland halibut (GH; Reinhardtius hippoglossoides), and Pacific halibut (PH; Hippoglossus stenolepis) are commercially and ecologically important slope-spawning flatfish species in the eastern Bering Sea (EBS), which has experienced fluctuating warm and cold periods since 2000. Although the three species share many attributes, their population trajectories have fluctuated differently. This difference could result from contrasting responses to environmental variability during early life history. To understand how physical variability of the Bering Sea can differentially affect flatfish ecology from pre-settlement to post-settlement phases, I used a combination of field data, biophysical modeling, and statistical modeling to characterize early life stage attributes (chapter 2), settlement success (chapter 3), and size, abundance, and distribution at settlement (age-0) and age-1 (chapter 4). Based on historical ichthyoplankton survey data for GH and PH, I found that there were species-specific differences in the spatial distribution (vertically and horizontally) and juvenile nursery areas between the two species during early life stages in the EBS. Specifically, I found that PH larvae abruptly move to shallower water as they grow, and cross onto the shelf earlier than GH. This ontogenetic movement has the benefit of allowing PH larvae to take advantage of on-shelf transport to reach their settlement locations. However, an early transition from the slope to the shelf may not equally benefit GH, whose settlement locations are further from the spawning ground. Using a bio-physical modeling approach parameterized on the field data summarized in chapter 2, I found that species-specific variability of early life attributes causes interannual and species-specific variability of GH and PH settlement success in the EBS. GH settlement increased with increasing along-shelf (northwestward) flow whereas PH settlement decreased. GH that spawned in November and December were highly successful at settling while PH settlement was most successful when they spawned in January and February. Furthermore, GH settlement is affected by temperature dependence of pelagic larval duration, but not PH, indicating a strong resilience of PH to temperature induced variations in development and dispersal duration. Using otolith microstructure analysis, I found that variations in size at settlement for ATF are significantly correlated with latitude of sampling location. For GH, their size at settlement is associated with bottom water temperature and sea ice extent. Especially, sea ice coverage has a strong negative correlation with on-shelf winds, which drive along-shelf Ekman transport to southeast impacting dispersal pathways and duration. Size at settlement for ATF increased with increasing latitude of sampling location, which could be impacted by currents. For GH, size at settlement decreased with decreasing bottom water temperature and increasing sea ice extent. Also, my results showed that settlement habitat increases for GH in cold years whereas that of ATF increases in warm years. The bottom temperature of age-0 habitat for both ATF and GH affected on their age-1 abundance; GH age-1 abundance increased with decreasing bottom temperature of age-0 habitat, but no clear directionality was found for ATF. The findings from this study have implications for understanding settlement success and recruitment of slope-spawning flatfish in the EBS. In most cold years when along-shelf flow is generally strong, the level of larval supply of GH to their settlement areas is higher than in warm years. Size at settlement for GH decreased in cold years. The larger amount of suitable habitat for settlement and post-settlement stages could result in lower competition and less predation in comparison to warm years. In support of this hypothesis, I found greater age-1 abundance in cold years, indicating size at settlement in GH may not be critical compared to suitability of habitat features and larval supply to settlement grounds. On the other hand, in cold years with strong along-shelf transport to northwest, PH (or ATF), which settle in the southern part of the EBS, have lower numbers of successful settlers. Size at settlement for ATF increased in cold years, and I assumed that size at settlement for PH may have similar patterns. The amount of suitable habitat after settlement would be smaller, resulting in lower recruitment due to increased competition for limited resources. By studying how physical factors and their variability influence these three flatfish during early life stages, this study provides valuable insight into the response of flatfish stocks to past and future climate changes in the eastern Bering Sea – a system that is especially vulnerable to warming.
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