A summer minimum flow requirement was instituted in 1982 for the Conowingo Hydroelectric Dam, Maryland, where no such requirement existed previously. The response of fish to this minimum flow was assessed by monitoring food consumption and condition factor of three abundant, downstream species (white perch Morone americana, yellow perch Percaflavescens, and channel catfish Ictalurus punctatus), and by comparing the values from before and after the minimum flow was established. We also compared growth rates of white perch before and after the minimum flow was established by back-calculating length from otoliths. We found that fish condition (weight at length) was significantly greater for all species in years when the minimum flow was maintained. Similarly, growth rate (length at age) was significantly higher after the minimum flow requirement was instituted. Changes in food consumption appear to explain the differences in growth, as fewer empty stomachs and greater food consumption occurred after the minimum flow was instituted. Changes in bent hie invertebrate abundance appear to explain the increase in consumption. These results suggest that fish may be subject to indirect effects of flow alteration that are propagated through the food chain.

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A multiple corer for sampling the benthos of profundal sediments is described. This sampler is relatively simple to construct and is light (approximately 17.5 lb (7.9 kg)) and easy to use. Four core samples are taken simultaneously and trials reported elsewhere indicate that in soft sediments the samples taken are generally superior to those collected with most, if not all, of the more conventional samplers tested.

Standard SIPRE coring was compared with a new Subice Suction Corer and cores taken by diver for the quantitative assessment of epontic (subice) algae on first-year congelation sea ice at Resolute, N.W.T., Canada (≈75°N). The diver cores were probably most accurate but were slow and costly. SIPRE coring was as good as other techniques in late winter and early spring but gave progressively poorer (under) estimates as the season progressed, with up to 90% of the ice algae being lost from SIPRE cores by June. The Subice Suction Corer was fast, easy to operate, cheap, and gave results comparable with samples obtained by diving. Sources of error are discussed.

The exclusion efficiency of cylindrical wedge-wire screens was investigated at the Chalk Point Steam Electric Station in Aquasco, Maryland, by measuring entrainment of larval bay anchovies Anchoa mitchilli and naked gobies Gobiosoma bosci through screens with slot sizes of 1, 2, and 3 mm and through an unscreened intake. The degree of exclusion by the screens increased with fish size. Fish less than 5 mm long were not excluded by any of the screens. In contrast, more than 80% of larger ichthyoplankton were excluded by all screens. Virtually no ichthyoplankton larger than 10 mm were entrained through the 1-mm screen even when fish of this size were abundant and were entrained through the unscreened intake. The 2-mm and 3-mm-slot screens were not as effective at excluding ichthyoplankton as the 1-mm screen, but the effect of slot size on exclusion efficiency was small relative to the effect of fish size. These results suggest that entrainment through water intake structures can be successfully reduced by wedge-wire screens if the larval fish at risk exceed 5 mm in length.