Science Service System

Summary of Proposal OCE2355

TitleWhales and waves: zooplankton accumulation, fish and humpback whale foraging response, and the SAR observation of internal waves at Stellwagen Bank (Massachusetts, U.S.A.)
Investigator da Silva, Jose - University of Porto, Geosciences, Environment and Spatial Planning
Team Members
Dr. Helfrich, Karl - Woods Hole Oceanographic Institution, Physical Oceanography
SummaryDespite the ubiquitous nature of NLIWs in the coastal oceans and their potential ecological impact on vertical distribution and trophic interactions of juvenile and adult fish and sharks, knowledge on the response of nekton to NLIWs is either very scant or null, yet this knowledge may be a key for understanding nektonic prey and predator distribution. Our previous findings funded by NOAA SeaGrant at WHOI suggest new research, which will be some of the first to address the influence of NLIWs on fish and shark ecology. Do whale aggregation and foraging at Stellwagen Bank (SB) southern flank depend on sand lance abundance, and do whales associate in NLIWs only when sand lance is present? Do sand lance respond to NLIWs by changing their vertical distribution and crossing isopycnals? What are the changes in dynamic pressure and circulation associated with the NLIWs that may elicit a response in sand lance? Do sand lance and dogfish (a common name for a kind of small shark that lives in the study region) represent contrasting modes of nekton response to the NLIWs? Is the vertical distribution of dogfish structured by the NLIWs in phase with the oscillations of the NLIW, and does the internal tide “compresses” the vertical distribution of the dogfish? To answer these questions we have created a multi-disciplinary research program using a variety of techniques at multiple scales of resolution including (1) observations of NLIWs with TerraSAR-X, acoustics, currents and temperature, (2) mathematical modeling, and (3) direct in situ observations of fish, shark and humpback whale behavior, distribution, and abundance in the presence and absence of NLIWs. Objectives: 1. Determine in situ patterns of fish, shark and whale response to the NLIWs. Measure NLIW activity; Measure sand lance and dogfish distribution at the SW corner of the bank before, during, and after NLIWs; Measure whale behavior, distribution, and abundance in the Southern flank (recorded by expert observers) before, during, and after NLIWs. 2. Compare the observations of NLIWs to a fully non-linear mathematical model of large-amplitude internal solitary waves. Determine wave properties such as phase speed and use the model to predict conditions within the water column (e.g., changing pressure and bottom current velocity) that we do not measure directly. 3. Relate characteristics of NLIWs as measured in situ with the behavioral response of the fish, sharks and whales. Response will be inferred from timing in distributional change, timing of the IWs, and expected sites of zooplankton accumulation (e.g. near the surface at leading IW depressions). Use theoretical solutions in 2. as initial conditions in a 2D nonhydrostatic numerical model and to explore motion of fish and sharks that are either passively advected or swim in response to the wave passage. 4. Analyze SAR images to investigate (1) whether in addition to NLIWs generated at Race Point channel, other NLIWs generated elsewhere hit the area and interact with Race Point waves, locally causing very high amplitude displacements and convergence currents, and (2) whether interannual variability in the occurrence of Race Point ISWs is a clue to the variability in whale distribution and sand lance abundance. The in situ work will be completed from the R/V Auk (a NOAA ship) anchored at ca. 55 m near a location where internal waves generated from Race Point hit Stellwagen Bank. Work will be completed in each one of 6 days on year 1 (2014), and 5 days of year 2 (2015), in June-August. Sampling days are chosen such that expected Race Point waves occur at a different time of the day, to avoid a potential bias. Shipboard measurements includes CTD profiles, single and dual beam 200 and 120 kHz acoustic backscatter (Biosonics DTX), and live underwater video (SplashCam Deep Blue) with underwater flashlights fitted with light diffusers. We will support the cruises providing real time SAR images.

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