2 research outputs found
Modification of the biological intercept model to account for ontogenetic effects in laboratory-reared delta smelt (Hypomesus transpacificus)*
We investigated age, growth, and ontogenetic effects on
the proportionality of otolith size to fish size in laboratory-reared delta smelt (Hypomesus transpacificus)
from the San Francisco Bay estuary. Delta smelt larvae were reared from hatching in laboratory mesocosms for 100 days. Otolith increments from known-age fish were enumerated to
validate that growth increments were deposited daily and to validate the age of fish at first ring formation. Delta smelt were found to lay down daily ring increments; however, the first increment did not form until six days after hatching. The relationship between otolith size and fish size was not biased by age or growth-rate effects but did exhibit an interruption in linear growth owing to an ontogenetic shift at the postflexon stage. To back-calculate the size-at-age of individual fish, we modified the biological intercept (BI) model to account for ontogenetic changes in the otolith-size−fish-size relationship and compared the results to the time-varying
growth model, as well as the modified Fry model. We found the modified BI model estimated more accurately the
size-at-age from hatching to 100 days after hatching. Before back-calculating size-at-age with existing models,
we recommend a critical evaluation of the effects that age, growth, and ontogeny can have on the otolith-size−fish-size relation
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Pre-Screen Loss and Fish Facility Efficiency for Delta Smelt at the South Delta's State Water Project, California
Water exports have been implicated in the decline of fish populations in the upper San Francisco Estuary, California. We evaluated the relation between delta smelt salvage at the John E. Skinner Delta Fish Protective Facility (SFF) and underlying entrainment losses at the State Water Project (SWP, south Delta). We used cultured delta smelt in mark–recapture experiments in February and March 2009 (adults) and June 2009 (juveniles) to estimate: (1) the percent of fish recaptured at the SFF of the total released at the entrance of the SFF (fish facility efficiency), (2) the percent of fish recaptured at the SFF of the total released in Clifton Court Forebay (CCF), a reservoir for SWP exports, and (3) the fish losses in CCF and before the SFF (pre-screen loss). Mean fish facility efficiency was lower in successive releases: February (53.2%), March (44.0%) and June (24.0%). The mean percent recapture of fish released at the CCF entrance was also lower over time: February (3.01%); March (0.41%) and June (0.03%). Correspondingly higher mean pre-screen losses occurred over time: February (94.3%); March (99.1%) and June (99.9%). We concluded that: (1) entrainment losses of delta smelt could be higher at times, compared to other species previously studied at the SWP; (2) pre-screen loss was the largest source of mortality for delta smelt; (3) increased release distance from the SFF and residence time in CCF—and decreased exports—resulted in a lower percentage of recaptured fish at the SFF; and (4) salvage of delta smelt at the SWP does not seem to be a consistent index of entrainment