In a field experiment where the presence or absence of piscivorous pike (Esox lucius) in ponds was manipulated, the morphology of crucian carp (Carassius carassius) diverged, such that individuals became deeper bodied in pond sections with pike. A laboratory experiment confirmed that the presence of this predator induced a change in body morphology in the carp. Estimation of prey vulnerability to predation by pike, a gape-limited predator, revealed that this increase in body depth resulted in crucian carp reaching a size that provided refuge from predation. However, this change in morphology incurs a cost through an increase in drag when the carp are swimming. Because crucian carp are limited by resources in the absence of piscivores and by the substantial cost of the defensive morph in their presence, phenotypic plasticity should be the optimal strategy for this species.

No abstract

Studies of sun fish Lepomis spp. have demonstrated that predation regulates sizespecific habitat use and competitive interactions both within and among species. However, all of these studies were conducted in clear-water systems with dense littoral vegetation, resulting in well-defined littoral and open-water habitats. In contrast, little is known about predator-prey interactions among fishes that inhabit turbid lakes and reservoirs. In these systems, turbidity reduces detection (reaction) distance for both predator and prey, and can eliminate submerged vegetation that serves as a refuge. In laboratory experiments, we quantified reaction distance of juvenile bluegills Lepomis mac roc hi r us to a predator, largemouth bass Micropterus salmoides, and determined shifts by bluegills between nearshore and offshore habitats across a turbidity gradient. Bluegill reaction distance declined as a negative power function of turbidity from less than 2 m in clear water to 23 cm at 10 nephelometric turbidity units (NTU) and 9 cm at 50 NTU. However, bluegill reaction distance was always greater than largemouth bass reaction distance. In tilted laboratory pools with a depth gradient of 0-50 cm and a largemouth bass predator, bluegills used deepwater habitat (>20 cm deep) less than 20% of the lime in clear water and more than 80% of the time at all turbidities greater than 10 NTU. The presence of an obligate open-water prey for largemouth bass, gizzard shad Dorosoma cepedianum, did not influence bluegill habitat use. The apparent reduction in open-water predation risk with a relatively minor increase in turbidity suggests that size-specific habitat use by bluegills in turbid systems may not be as simply defined as in clear-water lakes.

No abstract

. 1993. Interactive influence of turbidity and light on larval bluegill (Lepornis rnacrochirusj foraging. Can. j. Fish. Aquat. Sci. 50: 704-788.In a series of in situ enclosure experiments with larval bluegill (lepomis rnacrochirus), we demonstrate that turbidity from suspended sediments reduces bluegill consumption of crustacean zooplankton, primarily cyclopoid copepods and cogepod nauplii. However, this reduction occilrred only when light intensity in parts of enclosures fell below a threshold, estimated at <450 Ix. Following recent studies demonstrating copepod die1 vertical migration in response to predators, it appears that copepods in our experiments used IOLV-light strata as a refuge. Without this apparent refuge present, larval bluegill consumption increased with increasing turbidity, but prey were smaller on average. Thus, prey biomass consumed by larval bluegill did not differ with turbidity in high-light conditions. We postulate that the shift to smaller prey across taxa at higher turbidity, when light intensity exceeded 450 ix, derives from increased prey-background contrast. In low-light conditions, larval hluegill consumed larger, but fewer, zooplankton with increasing turbidity, resulting in lower prey hiomass consumed. Thus, we demonstrate the field conditions causing negative turbidity effects on larval fish foraging success, and thus growth and recruitment.Daws une s6riedfexp6riences in situ (dans une enceinte) avec des tarves de crapet arlequin (Lepomis macrocb~irus), nous avons dkmontr6 que la turbidit6 provoquee par les sediments en suspension reduisait la consommation de crustaces nooplanctoniques, surtout des cspepsdes cyclopTdes et des nauplii de cop6yodes. Toutefois, cette redilcteon ne s'observait que lorsque ifintensit6 lumineilse dans certaines partie de I'enceinte etait inferieure h une valeur seuil estirnee & <450 IX. Des etudes de suivi recentes ayant rnontre la migration verticale nycthemeral des cop6podes en r6ponse 2 I'action des predateurs, il sernble que dans nos experiences les copepodes aient utrlis6 les couches de faible intensit6 lurnineuse csmrne refuge. Sans la pr4sence de ce refuge apparent, la csnssmmatiow des crapets larvaires augmentait avec B'accroissernent de \a turbidite, mais les proies etaient en moyenne plus petites. Ainsi, la biornasse des proies sonsommees par les crapets larvaires n'a pas changk avec la turbidit6 dans des conditions d'intensite lurnineuse @levee. Nous avons postule que le changement vers la consommation de proies plus petites dans Ifensemble des taxons en presence d'uwe turbidit6 plus elevbe, lorsque l'intensite lurnineuse &passe 458 IX, est attribuable une augmentation du contraste entre les proies et le fond.Bans des conditions de faible lurninosit6, les crapets larvaires snt consorr~rne du zooplancton rno~ns nombreux, mais plus gros, au fur et 2 mesure qu'augmentait la turbidite, avec le resultat que la biomasse des proies cowsomm6es est plus petite. Ainsi, nous avons d6montr6 les conditions qui, sur le terrain, song 3 I'origine des effet...