Contents lists available at ScienceDirect Journal of Experimental Marine Biology and Ecology journal homepage: www.elsevier.com/locate/jembe Revisiting cross-shelf transport of Dungeness crab (Metacarcinus magister) megalopae by the internal tide using 16 years of daily abundance data Leif K. Rasmuson ⁎ ,1 , Alan L. Shanks Oregon Institute of Marine Biology, University of Oregon, Charleston, OR 97420, USA ARTICLE INFO Keywords: Internal tide Larval dispersal Dungeness crab Wavelet analysis Time series analysis ABSTRACT Many marine organisms have a biphasic life beginning with a planktonic larva that undergoes development on the continental shelf. These larvae ultimately need to return to the nearshore environment to settle. One pro- posed mechanism of transport to the nearshore is the internal tide. The Dungeness crab, Metacarcinus (Cancer) magister, is one species that is hypothesized to be transported by the internal tide. In addition to daily cycles in catch of megalopae being correlated with the internal tide, the annual catch of megalopae is correlated with the Pacific Decadal Oscillation. However, intial studies occurred during a positive phase Pacific Decadal Oscillation. In this study, we analyze an additional 12 years of data (16 years total) to determine if the internal tide continues to be correlated with the transport of Dungeness crab megalopae. We used wavelet analyses and cross-Fourier analysis to relate the catch of megalopae to the daily maximum tidal range, wind speed, and sea surface tem- perature. We found that over 16 years, the correlation with the daily maximum tidal range continues to be the strongest correlate with the daily catch of megalopae. These results continue to suggest megalopae are trans- ported across the continental shelf by the internal tide. Our use of wavelets allowed us to examine the annual trends in periodicity of returning megalopae. We found that the periodicities at tidal frequencies did not persist throughout the settlement season. We hypothesize this indicates cross-shelf transport is primarily regulated by the internal tide but additional, unknown factors, interact with the internal tide affecting the number of re- turning larvae. We conclude by discussing why high-frequency sampling is an important component of ecolo- gical time series. 1. Introduction Many marine organisms begin their life as a planktonic larva that is released in the nearshore disperses to the offshore and ultimately re- turns to the nearshore to settle (Cowen and Sponaugle, 2009; Rieger, 1994). Understanding the mechanism(s) employed by larvae to migrate across the continental shelf to their near shore settlement habitats continues to be a key question in marine ecology. Previous work sug- gests the mechanism is likely a synergistic interaction between larval behavior and hydrodynamics (Metaxas 2001). Swimming behavior of larvae, either vertically or horizontally, is the primary behavior ex- hibited by larvae (Metaxas 2001). Numerous hydrodynamic processes have been proposed to transport larvae (Shanks, 1995; Queiroga and Blanton, 2005). One such mechanism is transport by the internal tide (either by bores or internal waves) (Criales et al., 2007; Pineda, 1991; Shanks, 1983; Valencia-Gasti and Ladah, 2016). In marine ecology, the word “tide” is often used to refer to the barotropic tide, which causes the high and low tides observed at shore (Bertness et al., 2001). However, the barotropic tide can also interact with ocean stratification and bathymetry to generate baroclinic internal waves and bores at tidal frequencies, collectively referred to as the internal tide (Jackson et al., 2012). Since the barotropic tide generates the internal tide, both can exhibit a two-week periodicity in magnitude (Cairns, 1968; Stanton and Ostrovsky, 1998; Suanda and Barth, 2015). However, complex interactions between the barotropic tide and both ocean stratification and topographic features cause the barotropic and baroclinic tides to often be out of phase with one another (Hibiya, 1990; Johnson et al., 2001; Vlasenko et al., 2005). Over the continental shelf, barotropic tidal currents generate the internal tide through lee wave generation at a sharp change in depth, e.g. the shelf break (Hibiya, 1990; Jackson et al., 2012). As the barotropic tide ebbs off the con- tinental shelf, the pycnocline is depressed, generating a lee wave over the continental slope. Lee waves occur at tidal frequencies and are one manifestation of the internal tide. When the barotropic tide reverses https://doi.org/10.1016/j.jembe.2020.151334 Received 1 November 2019; Received in revised form 31 January 2020; Accepted 3 February 2020 ⁎ Corresponding author. E-mail address: leif.k.rasmuson@state.or.us (L.K. Rasmuson). 1 Present address: 2040 SE Marine Science Drive, Marine Resources Program, Oregon Department of Fish and Wildlife, Newport, OR 97365, USA. Journal of Experimental Marine Biology and Ecology 527 (2020) 151334 0022-0981/ Published by Elsevier B.V. T