Distributions of larval rockfishes Sebastes spp. across nearshore fronts in a coastal upwelling region

Abstract

We sampled larval fishes along cross-shelf transects off Granite Canyon, California, USA, during the upwelling seasons of 1993 and 1994 to determine whether coastal upwelling fronts affect the cross-shelf distribution of larval rockfishes Sebastes spp. during the earliest period of planktonic life. Rockfish larvae occurred in relatively high densities near surface fronts and were distributed in patches oriented along sloping pycnoclines contiguous with surface fronts. Qualitative comparisons between observed distributions of larval rockfishes in relation to hydrographic structure and predictions from models of plankton distributions at convergent fronts support the hypothesis that convergent circulation contributes to observed distributions. Our results indicate that (1) coastal upwelling fronts influence larval rockfishes at an earlier life history stage than has previously been documented, and (2) the influence of upwelling fronts on distributions of larval rockfishes is similar to the influence of hydrographic fronts on distributions of larval fish reported for a variety of oceanographic settings. In light of the plausible effects of upwelling fronts on larval transport and ecology, our findings suggest that upwelling fronts merit further investigation for their potential role in translating variability in upwelling dynamics into fluctuations in recruitment to coastal rockfish populations along the west coast of North America. KEY WORDS: Larval fish distribution · Coastal upwelling · Upwelling fronts · Recruitment mechanisms · Rockfishes · SebastesResale or republication not permitted without written consent of the publisher

Shoreward swimming boosts modeled nearshore larval supply and pelagic connectivity in a coastal upwelling region

“…Fronts are associated with phytoplankton blooms and aggregate phytoplankton and larvae, and these phenomena are well-documented in central California (Bjorkstedt et al, 2002;Graham et al, 1992;Ryan et al, 2010Ryan et al, , 2014Traganza and Conrad, 1981;Woodson et al, 2009). Fronts are common and persistent in the CCS (Castelao et al, 2006;Shanks et al, 2003a), and are spatially correlated with higher recruitment of both intertidal and subtidal species .…”

“…Highly variable in space and time, ocean currents have unpredictable effects on transport (Mitarai et al, 2008), especially over shorter, non-seasonal timescales. For example, larvae can be entrained in seaward-growing filaments (Bjorkstedt et al, 2002;Strub et al, 1991) or otherwise transported offshore, but they also can be entrained in transiently stationary eddies or fronts that may result in little net transport (Fig. 1), at least temporarily.…”

“…Larvae swim or migrate vertically to help regulate their retention or dispersal, either continuously or through diel or ontogenetic vertical migrations (Bradbury and Snelgrove, 2001;Epifanio and Cohen, 2016;Epifanio and Garvine, 2001;Morgan, in press;Queiroga and Blanton, 2005). Larvae can swim vertically to find favorable horizontal currents (Paris and Cowen, 2004), or swim against vertical velocities to maintain their depth, often resulting in aggregations at fronts (Bjorkstedt et al, 2002;Franks, 1992;Graham et al, 1992;Ryan et al, 2014;Shanks et al, 2000;Shanks et al, 2003a) or the coast (Genin et al, 2005;Shanks and Brink, 2005). Modeling studies show relatively small changes in depth Fig.…”

Recruitment of coastal fishes and oceanographic variability in central California

“…Many authors have found higher densities of larval rockfishes associated with upwelling fronts in central California (Larson et al, 1994;Sakuma and Ralston, 1995;Bjorkstedt et al, 2002), indicating that upwelling conditions play an integral role in the transport and delivery of larval fishes. For instance, years of above average onshore transport of surface waters in the California current eco system resulted in successful widow rockfish (Sebastes entomelas) recruitment classes (Norton, 1987), while strong upwelling during the winter months resulted in low overall juvenile rockfish abun dance as examined in seabird diets (Ainley et al, 1993).…”

“…Equatorward wind stress displaces coastal surface waters offshore via Ekman transport, which are replaced by colder, nutrient rich, subsurface waters (Huyer, 1983). Because of their relatively limited locomotory abilities, planktonic larvae may become entrained in the surface Ekman layer and accumulate in offshore coastal fronts (Bjorkstedt et al, 2002). These coastal fronts interact with alongshore current systems, and provide a means for larval dispersal to connect distant populations.…”