We developed a method of using external morphological characteristics to determine the sex of yellow perch Perca flavescens. The characteristics used were (1) the shape of the urogenital opening, (2) the shape of the anterior edge of the urogenital opening when thumb pressure was applied to the abdomen just anterior to the anus, and (3) the presence or absence of swelling around the anus. In females, the urogenital opening was crescent or half-moon shaped, its anterior edge remained linear when pressure was applied to the abdomen, and a ring-shaped area of swelling was present around the anus. In males, the urogenital pore was circular, its anterior end distended to a V or U shape (pointed anteriorly) when the abdomen was pressed, and the area around the anus was not generally swollen. The accuracy of using this method was 97.4% for fish 190-280 mm total length, 93.1% for fish 111-150 mm, and 82.7% for fish 80-110 mm.
This study used six 0.04 ha plastic-lined ponds to compare the e¡ects of a fertilization regime using a one-time initial application of an organic fertilizer (alfalfa pellets) with the current regime of weekly applications of organic fertilizers on the abundance and distribution of aquatic invertebrates and walleye, Sander vitreus, ¢ngerling production. Walleye, 3^4 days post hatch, were stocked on 1 May 2002 and harvested on 6^7 June 2002. Throughout the growing season, a ratio of 7:1total nitrate-nitrogen to total phosphorus was maintained in all ponds regardless of the treatment. Once fry were stocked, ponds in Treatment #1 were fertilized weekly with organic fertilizer (alfalfa pellets; 112 kg ha À 1 week À 1 ) for a total of 795 kg ha À 1 pond À 1 . Ponds in Treatment #2 only received an initial application of alfalfa pellets (112 kg ha À 1 ). Ponds inTreatment #1had sig-ni¢cantly higher ammonia and nitrate levels as well as higher chironomid larvae but not zooplankton compared with the other treatment. At harvest, walleye in the Treatment #1 ponds were signi¢cantly longer and heavier; however, the survival and relative weight were not signi¢cantly di¡erent. These results suggest that weekly applications of organics are important for the benthic food base and growth of ¢ngerling walleye reared in plastic-lined ponds.
Twelve 0.26-m 3 , shallow, plastic pools at an Iowa fish culture facility were used as mesocosms to determine the mode (airborne versus water supply) and rate of colonization by benthic chironomid larvae without the predation pressure of fingerling walleyes Sander vitreus. Chironomid larvae have particular importance in the culture of walleye fingerlings. Concurrent with walleye hatchery operations in nearby plastic-lined production ponds, pools were filled and then fertilized using the same protocols as the production ponds. A 2 3 2 factorial experimental design was used that included screened (1,000-lm mesh over the top of the pools) and nonscreened pools and filtered (75-lm-mesh sock) and nonfiltered water in the pools. Chironomid larvae were sampled twice during the 5-week culture season with Hester-Dendy samplers. At the end of the walleye culture season, there were no significant differences in chironomid density between filtered (108 larvae/m 2 ) and nonfiltered (657 larvae/m 2 ) pools, but nonscreened pools (592 larvae/m 2 ) had larger densities of chironomid larvae than screened pools (174 larvae/m 2 ). These results suggest that airborne movement and egg deposition by chironomid adults, rather than transfer of organisms from the water supply reservoir to the culture facility, constitute the primary method of chironomid colonization in plastic-lined ponds.
Introduction Materials and Methods Results Discussion Acknowledgements 2 Literature Cited 3 CHAPTER 3. EFFECTS OF FORMULATED FEED ON THE PRODUCTION OF FINGERLING WALLEYE IN PLASTIC-LINED CULTURE PONDS Abstract 6 Introduction 7 Materials and Methods Results Discussion Acknowledgements Literature Cited CHAPTER 4.
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