Riders on the Storm: Selective Tidal Movements Facilitate the Spawning Migration of Threatened Delta Smelt in the San Francisco Estuary

W. A. Bennett,J.R. Burau

doi:10.1007/s12237-014-9877-3

Abstract

Migration strategies in estuarine fishes typically include behavioral adaptations for reducing energetic costs and mortality during travel to optimize reproductive success. The influence of tidal currents and water turbidity on individual movement behavior were investigated during the spawning migration of the threatened delta smelt, Hypomesus transpacificus, in the northern San Francisco Estuary, California, USA. Water current velocities and turbidity levels were measured concurrently with delta smelt occurrence at sites in the lower Sacramento River and San Joaquin River as turbidity increased due to first-flush winter rainstorms in January and December 2010. The presence/absence of fish at the shoal-channel interface and near the shoreline was quantified hourly over complete tidal cycles. Delta smelt were caught consistently at the shoal-channel interface during flood tides and near the shoreline during ebb tides in the turbid Sacramento River, but were rare in the clearer San Joaquin River. The apparent selective tidal movements by delta smelt would facilitate either maintaining position or moving upriver on flood tides, and minimizing advection down-estuary on ebb tides. These movements also may reflect responses to lateral gradients in water turbidity created by temporal lags in tidal velocities between the near-shore and mid-channel habitats. This migration strategy can minimize the energy spent swimming against strong river and tidal currents, as well as predation risks by remaining in turbid water. Selection pressure on individuals to remain in turbid water may underlie population-level observations suggesting that turbidity is a key habitat feature and cue initiating the delta smelt spawning migration.

Highlights

Migration is a widespread life history strategy that optimizes the use of spatial and temporal variability in habitat quality to increase reproductive success and fitness of individuals (Dingle 1996)
We investigate the effect of tidal currents and water quality variables on the individual movement behaviors used during the spawning migration by the threatened delta smelt, H. transpacificus, endemic to the northern San Francisco Estuary (SFE), California, USA. (Fig. 1)
During the 2009–2010 winter of the Pilot Study, river flows were relatively low, peaking at only about 1,557 m3 s−1 (Fig. 2(a)), but were apparently sufficient to mobilize higher levels of turbidity (~350 nephelometric turbidity units (NTU) at Freeport, Fig. 2(b)) than those observed in the following winter season

Summary

Introduction

Migration is a widespread life history strategy that optimizes the use of spatial and temporal variability in habitat quality to increase reproductive success and fitness of individuals (Dingle 1996). Characterizing this fascinating and complex phenomenon involves perspectives at various levels of biological organization, such that migration is most readily defined by the behavior of individuals, but only fully understood in terms of population outcomes or consequences (Roff 1992; Dingle 1996). Selection pressure to conserve energy for reproduction is more likely to be stronger on smaller or long-distance migrants and when spawning occurs soon after migrating (Kinnison et al 2001; Crossin et al 2004; Jonsson and Jonsson 2006)

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