Stable-isotope ratios of carbon ( 13 C/ 12 C) and nitrogen ( 15 N/ 14 N) are widely used in the analysis of animal diets. However, using these stable-isotope ratios to infer dietary changes depends on precise knowledge of turnover rates of carbon and nitrogen. In the present study, carbon and nitrogen turnover rates were determined for recently settled juvenile winter flounder Pseudopleuronectes americanus in the laboratory using naturally occurring stable isotopes as dietary tracers. Flounder were reared at 13°C on a diet of rotifers Brachionus plicatilis of known isotopic composition from the time that the larvae began feeding until they reached metamorphosis and began to settle to the benthic habitat. At settlement, the fish were assigned to 1 of 2 temperature treatments (13 and 18°C). A subset of fish at each temperature was maintained on rotifers to serve as controls. The remaining fish were switched to a diet of brine shrimp Artemia sp. (known to be isotopically distinct from rotifers) and then sampled systematically over a 16 d period. Temperature had a significant effect on both carbon and nitrogen turnover rates. At 13°C, the half-life of carbon was 4.1 d (± 0.6), and of nitrogen, 3.9 d (± 0.7). At 18°C, the half-life of carbon was 2.2 d (± 0.3), and of nitrogen, 3.1 d (± 0.3). The change in isotopic composition closely followed predictions based entirely on the production of new tissue. KEY WORDS: Stable isotopes · Diet shifts · Pseudopleuronectes americanus · Winter-flounder habitat · TemperatureResale or republication not permitted without written consent of the publisher

We collected weekly, quantitative ichthyoplankton samples over 6 years (1989-1994, 1309 samples) to identify temporal scales of variability in the abundance and occurrence of larval fish assemblages near Little Egg Inlet in southern New Jersey, U.S.A. We collected species that spawn in the estuary (30%), both the estuary and continental shelf (35%), continental shelf (25%), and the Sargasso Sea (10%). The following analyses suggest an annually repeated seasonal progression of species assemblages: (i) the rank abundance of the 20 dominant species did not change significantly from year to year, (ii) variation in the density of the dominant species was primarily explained by intraannual rather than interannual variation, and (iii) multivariate analysis of the assemblage matrix identified five seasonal assemblages that occurred during all six years. We found that the timing and duration of each of these seasonal groups were correlated with two characteristics of the annual temperature cycle, magnitude (higher or lower temperature) and trajectory (increasing vs decreasing temperature). We suggest that the repeated occurrence of larval fish assemblages in temperate estuaries along the U.S. coast may, in part, be driven by local environmental processes.

In laboratory experiments, we determined the relative amount of predation before, during and after settlement with winter flounder Pleuronectes americanus as prey and sevenspine bay shrimp Crangon septemspinosa as predators. Wild-caught winter flounder at various stages of morphological development and settlement lfrom presettled, pre-eye-migration (511 mm standard length. SL) to eyes fully migrated, settled individuals (10.0 to 34.3 mm SL)] were exposed to predation by adult shrimp (47 to 74 mm total length). Mortality in treatments involving small settled flounder (78%) was twice that of presettled individuals (30 to 45 %). However, subsequent mortality decreased with flounder size, and was 0 % for flounder 220 mm SL. Predator density affected rate of predation on settled juveniles (8.7 to 19 mm SL); mortality increased from 0 to 80% as predator density increased from 0 to 10.6 m-2 At densities 210.6 m-Z, mortality reached an upper asymptote of 80 to 90%. These experiments suggest that predation by shrimp may be an important determining factor of habitat selection, size and mortality during settlement of winter flounder and other benthic fishes.

We examined the rate of isotopic change of nitrogen (N) and carbon (C) and isotopic fractionation at two temperatures during several ontogenetic diet transitions in summer flounder (Paralichthys dentatus). We examined prefeeding larvae as they metabolize the maternal source of nutrition and evaluated three diet transitions in the early life stages of this species: (i) yolk dependency to first feeding on zooplankton, (ii) one zooplankton (rotifers) to another (Artemia), and (iii) zooplanktivory to piscivory. We used experimental results and simulations to contrast time- versus growth-based models for estimating parameters of isotopic change and fractionation. All rates of isotopic change were lowest for the transition between zooplanktivory and piscivory, most rapid for the transition from yolk dependency to first feeding, and generally lower at cooler temperatures. Estimates of fractionation were greater for N (2.8‰–3.8‰) than for C (0.23‰–0.91‰) but appeared to be unaffected by water temperature or fish life stage. Simulation results suggested that parameter estimates of isotopic change and fractionation may be compromised when growth rates are low unless sampling design is modified to address slow growth rates. We conclude that sampling strategy and the model used can influence the accuracy and precision of estimates of isotopic change and fractionation.

Giant kelp Macrocystis pyrifera is a foundational species that forms a 3-dimensional habitat and supports numerous high-value fisheries species. Constant grazing of kelp holdfasts by overabundant sea urchins causes catastrophic ecological and economic impacts on rocky reefs worldwide. Overgrazing creates urchin barrens that persist for decades in the absence of ecological forcing that would shift the ecosystem back to a kelp-dominated state. Annual surveys of kelp forest and urchin barren sites in the Southern California Bight were performed from 2011 to 2020 to assess changes in kelp forest communities as a result of restoration efforts through sea urchin culling. However, that time period also encompassed a sea urchin mass mortality event. Following drastic reductions of sea urchin densities, rocky reefs returned to a kelp-dominated state within approximately 6 mo and remained stable through the remainder of the study. Benthic cover, fish, and kelp and macroinvertebrate communities inside former urchin barrens became more similar to that of kelp forest reference sites and continued to do so for the next 5 yr. Giant kelp density increased significantly compared to existing kelp forests, while benthic indicators of urchin dominance (i.e. crustose coralline algae and bare rock cover) decreased. Kelp restoration through sea urchin culling essentially mimics sea urchin mass mortality events. If culling can produce similar declines in urchin density, it may be a viable management tool to rapidly restore persistent urchin barrens at moderate spatial scales, while a mass mortality event can drive recovery of kelp forest communities at more extensive spatial scales.