Pupfishes (genus Cyprinodon) have existed for long periods in small desert springs and streams that often lack competitors, major predators, and large environmental changes. How their populations are regulated in these systems is poorly known. We experimentally examined the role of emigration in regulating populations of pupfish by (1) comparing the dynamics of populations held in four pools open to emigration with those of populations held in four pools closed to emigration over a 22—mo period and (2) comparing the rates of emigration from open pools having different densities of pupfish in relation to resources. When pupfish populations were prevented from emigrating, they exhibited symptoms of overpopulation similar to those well documented for fenced populations of small mammals. Their numbers increased more than populations in open pools, mortality was high, and body condition and recruitment were low. Again, as in small—mammal populations, these responses were absent in pupfish populations in open pools, where 84% of the total numbers of fish produced emigrated. The timing and magnitude of pupfish emigration was similar in all four open pools and was related to seasonal temperature changes. More males emigrated than females, and emigrants had significantly lower condition factors than residents. When densities of pupfish in relation to resources were doubled in two open pools, the percentages of pupfish that emigrated (41.8 and 42.2%) were almost equal to the 50% drop in resources. In contrast, only 15.5 and 16.0% emigrated from two control pools, where pupfish densities in relation to resources were left unchanged. Our results, demonstrating that emigration is a potentially significant mechanism for regulating populations of desert pupfish, are consistent with data from a variety of other animals (small mammals, hydra, roe deer), suggesting that population regulation via a behavioral spacing mechanism may be common among mobile animals.

Lake Bonney is a permanently ice-covcrcd lake of 3.2 km2 in upper Taylor Valley, Victoria Land, Antarctica.The mean depth, without ice cover, is 18.7 m. Thk lake probably occupies a glacially ovcrdccpcncd valley. The lake is meromictic. The maxirhal temperature of 7C coincides with the chcmoclinc at about 15 m; the tempcraturc decreases to a range of +2 to -4C in the bottom waters and to less than 1C bcncath the ice. Calculations of supcrficial water inflow and of water loss by sublimation of ice indicate that the lake is slowly shrinking.Old shorelines above the prcscnt lake level attest to shrinkage. Part of the difference between water gain and water loss is made up by inflow of warm-spring water at depth. Such springs are probably the major source of heat for the lake. Total solids in the monimolimnion range up to 407.3 g/liter, Sodium and magnesium chloride account for 96,s of the dissolved salts. An analysis of ionic ratios suggests that the lake waters may consist of trapped scawatcr highly modified by subscqucnt concentration by evaporative processes, by addition of ions from surrounding soils, and by addition of warm-spring water. The ionic concentrations arc altcrcd in part by the precipitation of CaSOe, CaC03, and Na2SOh.

Previous workers inferred from stomach analyses that threadfin shad (Dorosomapetenense) ate plankton by both filter and particulate feeding. These inferences were confirmed in this study by laboratory experiments in which both types of feeding were observed. Threadfin shad consumed relatively small food particles ( < 0.39 mm) by filtration, while larger prey (7.5 mm) were eaten individually. The shad were able to filter feed on small foods (brine shrimp nauplii and phytoplankton) at all light intensities from 0 to 9 x 10' fL. These data indicate that under natural conditions shad can filter feed at any time of the 24 h period if food conditions are sufficient to trigger feeding. Filter feeding is probably induced by chemoreception rather than vision. The particulate feeding rate decreased as light intensity decreased, reaching a minimum between 9 x lo-' and 9 x fL. From these data it is inferred that particulate feeding is a visual process in this species, requiring intensities equivalent to bright moonlight or greater. Filter and particulate feeding abilities allow threadfin shad to consume most of the different types of plankton and to change their diet with seasonal changes in the composition of the plankton.

SynopsisBrown trout (Salmo trutta) were more efficient than Arizona trout (Salmo apache) in eating brine shrimp at starlight fL) light levels. Arizona trout required light levels 2 moonlight (10-VL) to feed. In bright light (50 fL), brown trout utilized cover to a much greater extent in both field and laboratory. Our study indicates that factors other than competition for food or habitat are probably causing the displacement of Arizona trout by brown trout when browns are stocked into the native habitat of Arizona trout.

A landlocked lake of sodium-mixed-anion type in lower Taylor Valley has a salinity ranging from 1/35 to 1/5 that of sea water. The lake seems to be chemically stratified into three distinct layers. Several possible sources are postulated for the dissolved salts. The chemical zonation may have been initiated by past climatic variation; however, a thermal or magmatic origin for some of the waters is also indicated. No single origin for the lake waters or the stratification seems likely.

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