The effects of dietary protein and energy on ingestion and growth were determined for nutrient ranges that correspond to primary foods in freshwaters: algae, aquatic macrophytes and organic detritus. Sixteen diets containing four levels of metabolizable energy (ME) (3.1, 6.7, 10.5, 14.1 kJ/g) and four levels of protein (3.0, 13.2, 23.2, 33.4 mg/kJ ME) were each fed ad libitum to four replicate groups of juvenile Tilapia aurea for 42 d. Protein, energy, and protein—energy interaction affected both ingestion and growth (two—way ANOVA, all P < 0.01). Increased ingestion largely compensated for lower energy levels within each protein level. Growth was proportional to diet protein content, and ingestion did not compensate for protein limitation. A second—order polynominal for growth as a function of diet protein content and energy assimilation rate fitted by linear regression accounts for 91% of variation in growth and provides a model for comparison of the relative importance of protein and energy as nutritional constraints for animals feeding on invertebrate prey, algae, aquatic macrophytes, and organic detritus. Protein appears to be the primary constraint to food value of macrophytes, and detritus, and we predict from our results that consumers of these materials will increase growth most by feeding selectively on the most protein—rich material available, as has been observed. In contrast, growth of animals feeding on algae will be increased most by increased ingestion. Omnivory is interpreted as a compromise strategy in which protein from scarce animal prey is complemented by energy from abundant primary foods.
Fouling debris composed of fish feces, excess fish food, algae, and other particulate organic matter can create environmental problems for aquaculture facilities that rear fish in Ocean net pens. Accumulations of organic debris can clog the nets and restrict water circulation which in turn can stress fish. Experiments in which red sea cucumbers Parustichopus Californicus were allowed to feed inside floating net pens at a salmon rearing facility in Southeast Alaska showed that sea cucumbers consumed fouling debris and cleared a significant amount of surface area on the nets (P < 0.0001). Sea cucumbers assimilated amino acids and other organic matter from fouling debris two to three times more efficiently than from their natural sediment diet. Muscle development of sea cucumbers consuming fouling debris inside the net pens was also significantly greater than that of sea cucumbers feeding in their natural environment (P < 0.0003). This work suggests that polyculture operations in which commercially important detritivores, like the red sea cucumber, are grown in net pens along with salmon could possibly convert the net from self-fouling to self-cleaning and could turn fouling debris into a marketable product (sea cucumber biomass).
Quantitative analyses of stomach
contents indicate Prochilodus platensis depends on detritus rather than microorganisms for its nutrition. Chemical data suggest that this detritus is of a different origin than previously supposed.
The ash-free dry mass (AFDM) of detritus, invertebrates, and algae in the diet of juvenile white sucker (Catostomus commersoni) was determined by quantitative microscopy. Fish were collected from a northern Michigan pond from January through October 1986 and their seasonal diet was compared with benthc invertebrate abundance. The quantity of detritus in sucker foreguts was inversely related to benthic microcrustacean densities. In July, microcrustacean densities were high and they comprised 95% of the AFDM in foregut contents. By October, microcrustacean densities had declined to 13% of their maximum density and detritus comprised over 90% of the sucker's diet AFDM. In laboratory aquaria, sucker that were fed detritus mixed with four different densities of Artemia ingested significantly more detritus from diets that provided lower Artemia densities. In the presence of high Artemia densities, sucker completely rejected detritus and ingested only Artemia, The fact that juvenile sucker can separate detritus from invertebrates that they swallow demonstrates that detritus is not ingested incidentally. Both laboratory and field data support the hypothesis that detritus is ingested intentionally when preferred invertebrate prey are scarce.
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