Abstract
While there is substantial information about the upstream migration of commercially and recreationally important fishes, relatively little is known about the upstream migration of small-bodied species, particularly through estuaries. In the San Francisco Estuary, there is a major need to understand the behavior of delta smelt Hypomesus transpacificus, a small pelagic fish listed under the state and federal endangered species acts. The spawning migration period may be critical as upstream movements can result in entrainment in water diversions. In general, delta smelt live in the low-salinity zone of the estuary and migrate upstream for spawning. During the fall pre-migration period, delta smelt remain primarily within the low-salinity zone in the western Sacramento–San Joaquin Delta and Suisun Bay. There were no significant upstream shifts of fish into fresher water during late fall, suggesting that delta smelt do not show pre-migration staging behavior. Following winter “first flush” flow events that appear to trigger migration, upstream movement rates are relatively rapid, averaging 3.6 km/d, a finding consistent with results from particle-tracking simulations, laboratory studies, and other fishes. Like some other native fishes, delta smelt apparently “hold” in upstream areas following migration; most do not spawn immediately. Overall, delta smelt fit the pattern of a diadromous species that is a seasonal reproductive migrant. Emerging data suggest that there is variability in the migration behavior of delta smelt, a pattern contrary to the reigning viewpoint that all smelt migrate in winter.
Highlights
Animal migrations have long intrigued humans, movements by food species such as waterfowl, ungulates, and game fishes
Analyses of the Fall Midwater Trawl Survey (FMWT) showed that the distribution of delta smelt varied by year, but the pre-migration distribution over the past two decades has consistently been in west Delta and Suisun Bay, the region immediately downstream of Chipps Island (Figure 2)
The pre-migration distribution occurs in the low-salinity zone of the estuary as illustrated by the strong association between fish distribution and X2 during fall (Figure 3)
Summary
Animal migrations have long intrigued humans, movements by food species such as waterfowl, ungulates, and game fishes. In estuaries and their tributaries, the seasonal passage of anadromous fishes represents the most dramatic migration by aquatic species. Given the impressive numbers of salmonids that migrate through estuaries and rivers of the northern hemisphere, it is relatively easy to understand why these movements have regional cultural significance (Roche and McHuchison 1998). Migration represents a critical part of the life history for a variety of organisms. Seasonal or ontogenetic migrations have been documented for a broad diversity of taxonomic groups, including fish, mammals, reptiles, birds, and insects (Baker 1978). Many organisms undergo smaller-scale diel migrations, in aquatic habitats. Many organisms undergo smaller-scale diel migrations, in aquatic habitats. Northcote (1978) has proposed that there are three basic functional categories of migrations: (1) reproductive (spawning) migration, (2) migration toward food, and (3) refuge migration
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