Foraging and metabolic consequences of semi-anadromy for an endangered estuarine fish.

Diadromy affords fish access to productive ecosystems, increasing growth and ultimately fitness, but it is unclear whether these advantages persist for species migrating within highly altered habitat. Here, we compared the foraging success of wild Delta Smelt-an endangered, zooplanktivorous, annual, semi-anadromous fish that is endemic to the highly altered San Francisco Estuary (SFE)-collected from freshwater (<0.55 psu) and brackish habitat (≥0.55 psu). Stomach fullness, averaged across three generations of wild Delta Smelt sampled from juvenile through adult life stages (n = 1,318), was 1.5-fold higher in brackish than in freshwater habitat. However, salinity and season interacted, with higher fullness (1.7-fold) in freshwater than in brackish habitat in summer, but far higher fullness in brackish than freshwater habitat during fall/winter and winter/spring (1.8 and 2.0-fold, respectively). To examine potential causes of this interaction we compared mesozooplankton abundance, collected concurrently with the Delta Smelt, in freshwater and brackish habitat during summer and fall/winter, and the metabolic rate of sub-adult Delta Smelt acclimated to salinities of 0.4, 2.0, and 12.0 psu in a laboratory experiment. A seasonal peak in mesozooplankton density coincided with the summer peak in Delta Smelt foraging success in freshwater, and a pronounced decline in freshwater mesozooplankton abundance in the fall coincided with declining stomach fullness, which persisted for the remainder of the year (fall, winter and spring). In brackish habitat, greater foraging 'efficiency' (prey items in stomachs/mesozooplankton abundance) led to more prey items per fish and generally higher stomach fullness (i.e., a higher proportion of mesozooplankton detected in concurrent trawls were eaten by fish in brackish habitat). Delta Smelt exhibited no difference in metabolic rate across the three salinities, indicating that metabolic responses to salinity are unlikely to have caused the stomach fullness results. Adult migration and freshwater spawning therefore places young fish in a position to exploit higher densities of prey in freshwater in the late spring/summer, and subsequent movement downstream provides older fish more accessible prey in brackish habitat. Thus, despite endemism to a highly-altered estuary, semi-anadromy provided substantial foraging benefits to Delta Smelt, consistent with other temperate migratory fish

Foraging and metabolic consequences of semi-anadromy for an endangered estuarine fish

<div><p>Diadromy affords fish access to productive ecosystems, increasing growth and ultimately fitness, but it is unclear whether these advantages persist for species migrating within highly altered habitat. Here, we compared the foraging success of wild Delta Smelt—an endangered, zooplanktivorous, annual, semi-anadromous fish that is endemic to the highly altered San Francisco Estuary (SFE)—collected from freshwater (<0.55 psu) and brackish habitat (≥0.55 psu). Stomach fullness, averaged across three generations of wild Delta Smelt sampled from juvenile through adult life stages (n = 1,318), was 1.5-fold higher in brackish than in freshwater habitat. However, salinity and season interacted, with higher fullness (1.7-fold) in freshwater than in brackish habitat in summer, but far higher fullness in brackish than freshwater habitat during fall/winter and winter/spring (1.8 and 2.0-fold, respectively). To examine potential causes of this interaction we compared mesozooplankton abundance, collected concurrently with the Delta Smelt, in freshwater and brackish habitat during summer and fall/winter, and the metabolic rate of sub-adult Delta Smelt acclimated to salinities of 0.4, 2.0, and 12.0 psu in a laboratory experiment. A seasonal peak in mesozooplankton density coincided with the summer peak in Delta Smelt foraging success in freshwater, and a pronounced decline in freshwater mesozooplankton abundance in the fall coincided with declining stomach fullness, which persisted for the remainder of the year (fall, winter and spring). In brackish habitat, greater foraging ‘efficiency’ (prey items in stomachs/mesozooplankton abundance) led to more prey items per fish and generally higher stomach fullness (i.e., a higher proportion of mesozooplankton detected in concurrent trawls were eaten by fish in brackish habitat). Delta Smelt exhibited no difference in metabolic rate across the three salinities, indicating that metabolic responses to salinity are unlikely to have caused the stomach fullness results. Adult migration and freshwater spawning therefore places young fish in a position to exploit higher densities of prey in freshwater in the late spring/summer, and subsequent movement downstream provides older fish more accessible prey in brackish habitat. Thus, despite endemism to a highly-altered estuary, semi-anadromy provided substantial foraging benefits to Delta Smelt, consistent with other temperate migratory fish.</p></div

Mean stomach fullness for wild Delta Smelt by mesozooplankton density.

<p>Grey bars represent freshwater (<0.55 psu) and black bars represent brackish habitat (≥0.55 psu). Bins of low, medium, and high mesozooplankton abundance were defined by the rank of mesozooplankton abundance of each tow, 15 tows per bin in freshwater, and 15–16 tows per bin in brackish habitat. The number of fish that fell in each bin, in order of increasing mesozooplankton abundance, is 58, 76 and 83 in freshwater and 54, 48 and 50 in brackish. Note that we only tested for differences in means within freshwater or brackish habitat and that the x-axis is not continuous or to scale. Differing letters indicate statistically significant differences (there was no significant difference between the three brackish habitat means). Error bars are SEs.</p

Station IDs, the number of monthly CDFW mesozooplankton samples, mean salinity (psu) and mean temperature (°C) for each site (SD in parentheses) sampled Mar-Nov from 1972–2015.

<p>The first 8 sites are considered brackish (≥0.55 psu) in our analyses and final 4 are freshwater (<0.55 psu).</p

Mean stomach fullness (%) of Delta Smelt by salinity bin.

<p>The grey bar represents freshwater (<0.55 psu) and the black bars represent brackish habitat (≥0.55 psu). Sample sizes for each salinity bin from lowest to highest salinity = 682, 193, 186, 112, 105, and 40. Differing letters denote statistically significant differences. Error bars are SEs.</p

Mesozooplankton abundance and Delta Smelt foraging.

<p>Grey is freshwater (mean salinity <0.55 psu), black is brackish habitat (≥0.55 psu), and error bars are ±SE. (A) mesozooplankton abundance by survey (collected concurrently with Delta Smelt), (B) mean weight (mg) of prey items in Delta Smelt stomachs by survey, (C) mean number of prey items by survey in Delta Smelt stomachs, and (D) the number of prey items in Delta Smelt stomachs per mesozooplankton m^-3 by survey. STN is the Summer Townet survey (summer), FMWT is the Fall Midwater Trawl survey (fall/winter), and SKT is the Spring Kodiak Trawl survey (winter/spring).</p

Mean stomach fullness (%) for Delta Smelt by survey and salinity.

<p>STN = Summer Townet, FMWT = Fall Midwater Trawl, SKT = Spring Kodiak Trawl. Grey represents stomach fullness of Delta Smelt sampled from freshwater (<0.55 psu) and black represents stomach fullness of Delta Smelt sampled from brackish habitat (≥0.55 psu). Mean differences between freshwater and brackish habitat were significant for each survey. Error bars are ±SE, n = 1,318.</p