ABSTRACT. Laboratory experiments were conducted on juvenile summer flounder Paralichthys dentatus (41 to 80 mm total length) to determine low temperature tolerance (2 to 3 "C) a t 10, 20, and 30 %salinity, and to measure feeding rate, assimilation efficiency, growth rate and growth efficiency at 2, 6, 10. 14, 18 "C and 10, 20 and 30 '7' salinity. There was 100 % sunival at temperatures above 3 'C, suggesting that juvenile summer flounder are able to survive most winter water temperatures encountered in north/central Mid-Atlantic Bight (MAB) estuaries. Mortality was 42 % after 16 d at 2 to 3 'C, and was highest in fish < 50 mm TL (1 g). Mean specific growth rates were not significantly different between 2 and 10 "C (mean = 0.14 % d-'), and these rates were not significantly different from zero. Mean growth rate increased to 2.4 "0 d-' at 14 ' C and 3.8 O/ O d-' at 18 OC. Ad libitum feeding rate showed a similar relationship to temperature. Mean assimilation efficiency (60.1 %) was not affected by temperature. Mean growth efficiency ( K , ) was significantly lower at 6 "C (-23.1 "h) than a t 14 and 18 "C (18.4 and 22.1 %, respectively), and was highly variable. Salinity had no significant effect on any parameters measured, suggesting that factors other than salinity are controlling spatial distributions. Mortality resulting from acute exposure to low temperature probably occurs during one 2 to 4 wk period each winter Recruitment success from north/central MAB estuaries may be lower in years with late winter cold periods (i.e. March vs December) due to increased numbers of fish being exposed to lethal low temperatures. Additional mortahty probably results from low growth rates caused by sub-optimal temperatures (i.e. < 10 "C) throughout the spring. The annual contribution of new recruits from northern estuanes appears to b e dependent on winter temperature regime, particularly on the magnitude and timing of temperature minima.
Spatial and temporal differences In habitat characteristics of coastal nursery grounds can have a large ~m p a c t on growth rate survival, and subsequent recruitment of species w~t h e s t u a r~n edependent early life history stages Stone flounder Karelus bicoloratus IS a temperate North Pacific flatfish species characterized by large recruitment variability and an estuanne-dependent juvenile stage Post-larvae settle from m~d -J a n u a r y to early Apnl In inshore and estuanne nursery grounds, and luveniles subsequently rnove farther inshore using selective t~d a l stream transport Laboratory btud~es were conducted to determine the s e n s i t~v~t y of juven~le stone flounder growth rates to changes In temperature and feeding rates at conditions common durlng settlement (8°C) and post-settlement (12cC) The relat~onship between RNA DNA ratio and growth rate {+as developed in the laboratory, and was used to measure in sltu growth rates of juven~les from 5 dltferent habltats durinq settlement (March) and after (April) Juvenile stone tlounder in the laboratory giew reldt~vely quickly feeding ad l~b~t u m at 8°C(4 1 body xveiqht d l ) and 12°C (7 O", bw d 'I, but growth rate was very sensitive to changes in feeding rate at both temperatures Growth rdte of starved juven~les was not significantly different between 8 and 12°C and averaged -1 3 % bw d ' RN,\ DNA ratios were good pred~ctors of growth rate (R--0 881, with temperature as a covariate In situ growth rates at all 5 stat~ons ( 1 estuary 1 seagrass bed, and 3 deeper inshore settlement areas) were higher In Apnl than in March In sltu growth rates were hlghest during both months at the low-salinity estuanne s t a t~o n w h e~e fish had h~g h e s t gut fullnesses Growth rates wele also h~g h at a deeper inshore station near a sewage treatment outfall characterized by higher prey abundances than those typically found in open water aleas of Sendai Bay, Japan Growth rates were consistently lowest at the vegetated (Zostera spp bed) habitat, although retention In this h a b~t a t between March and April was hlgh Spatial differences In s e d~m e n t grain slze and prey abundances may be the primary factors responsible for the large vanability In habltat-specific growth rates Temperature-corrected compansons of growth l i m~t a t~o n suggest that d~s c r e t e habitats maintaln their relative values as nursery grounds over tune although the magnitude of growth llm~tation was much g~e a t e r durlng settlement (March) than post-settlement (Aprll) KEY WORDS Stone flounder Kareius bicoloratus Growth Habitat RNA:DNA . Recruitment
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