SynopsisGizzard shad, Dorosoma cepedianum, in Acton Lake, Ohio, ingested foods of varying nutritional quality during the 19131-1983 growing seasons. Adult (ages 2-4) fish fed on a mixed diet (ORG>30%; C:N<7:1) of zooplankton and organic detritus in early summer, and on detrital materials (ORG<16% ; C:N>ll:l) during the remainder of the growing season. Age 0 (<35 mm standard length) fish ingested only detrital materials. The nutritional quality (ORG = 10 -20%; C:N<11:1) of these foods displayed little seasonal variation, but was higher than that of organic detritus taken by adult fish in late summer and autumn. Growth and condition of gizzard shad were poor when the diet consisted of detrital materials; however, age 4 fish (1983) grew rapidly and condition improved when zooplankton were consumed. These results suggest that ingestion of poor-quality detritus can reduce the growth and condition of gizzard shad in Acton Lake, whereas the seasonal inclusion of high-quality zooplankton in the diet can result in rapid growth and improved condition.

Selective feeding on and digestion of detritus in surface sediments (organic matter < 10%, nitrogen < 0.4%) by age‐0 and adult (ages 3 and 4) gizzard shad Dorosoma cepedianum in Acton Lake, Ohio (water temperature = 15–18°C) were compared with those of age‐0 fish fed two particulate diets (low‐quality diet: organic matter = 10%, nitrogen = 0.3%; high‐quality diet: organic matter = 89%, nitrogen = 6.7%) at 18°C in the laboratory. Fish feeding on natural detritus and the low‐quality diet exhibited mean selection efficiencies, ratios of nutrient weight percentages in the gizzard to those in the sediment or food, of 1.8–2.9 for total organic matter, 2.4–4.4 for total carbon, and 3.2–13.2 for total nitrogen. Fish fed the high‐quality diet exhibited no selective feeding. Mean digestive efficiencies, 100 [ 1 – (feces value ÷ gizzard value)], for total organic matter, total carbon, and total nitrogen in the diets had ranges of 50–66, 51–64, and 62–84%, respectively. Age‐0 fish feeding on natural detritus and the two laboratory diets exhibited similar digestive efficiencies. In addition, age‐0 and adult gizzard shad were equally efficient at digesting the components of natural detritus. Gizzard shad apparently can ingest selectively the more nutritious portions of low‐quality particulate diets in the field and laboratory and digest a substantial proportion of the nutrients present.

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Orconectes rusticus currently is undergoing an explosive range expansion in the midwestern U.S.A., but information on the potentially important effects of water temperature on the species' biology is lacking. The thermal ecology of O. rusticus in southwestern Ohio, U.S.A., was examined by determining 1) the effects of four water temperatures (16, 20, 25, and 29°C) on survival and growth of juveniles, 2) the responses of juveniles and adults to a thermal gradient (7-27° C), and 3) the thermal tolerances (critical thermal maximum, CTMax, and critical thermal minimum, CTMin) of free-living, field-acclimatized juveniles and adults on a biweekly basis throughout the summer. Month-long growth experiments predicted maximum growth rates of juveniles at water temperatures between 26 and 28°C, but greatest survival between 20 and 22° C. Laboratory-acclimated (22° C) adults and field-acclimatized (2.5° C) juveniles both had an acute preferred temperature of 22° C. CTMaxs and CTMins of juveniles were 0.5-2.6° C higher than those of adults throughout the summer, suggesting that juveniles were exposed to water temperatures 1.5-6.8° C warmer than those of adults. Juvenile and adult O. rusticus prefer habitats where water temperatures favor maximum survival, but they usually are not found together in the same habitat; adults apparently displace the juveniles into warmer habitats. Warmer temperatures can decrease survival of juveniles but improve their growth rates, leading to enhanced fecundity and competitive ability. The past and future success of O. rusticus in expanding its range may depend, in part, on the species ability to adjust to new thermal environments occupied by other species of crayfish.