Mercury in fish from two Nicaraguan lakes: a recommendation for increased monitoring of fish for international commerce.

We measured total mercury concentrations in water and fish of Lake Managua and Lake Apoyo. Water mercury concentrations were 10-fold higher in Lake Managua than in Lake Apoyo, although differences in mercury concentration in the most common native fish were not significant. One-fourth of the commercially fished tilapia in Lake Managua exceeded maximum recommended mercury levels for consumption among pregnant women and other at-risk groups, although bioavailability to fishes was lower than in previously studied sites in Brazil and Western Maryland. The lower bioavailiability may present important information for management options to reduce mercury exposure to fishes and humans. We recommend closer mercury monitoring among freshwater fish destined for international commerce.

Trophic eggs and parental foraging for young by the catfish Bagrus meridionalis of Lake Malawi, Africa.

Parental bagrid catfish in Lake Malawi, Africa, appear to feed their young in a manner analogous to birds and social insects. The female produces eggs which are released and consumed by the catfish young. Indirect evidence suggests that the male leaves the nest to forage and returns with benthic invertebrates in its mouth that are fed to the brood. Such behavior is hypothesized to enhance the growth rate of the young and to reduce their susceptibility to predation.

Cichlid-catfish mutualistic defense of young in Lake Malawi, Africa.

Bagrid catfish and cichlid parents engage in a mutualistic defense of their young from predators. Over 50% of the catfish broods observed contained cichlid young, primarily of three species: Crytocara pleurostigmoides, C. pictus and Rhamphocromis sp. Three catfish broods, monitored for over 50 days, had a survivorship rate 6 times greater during periods when cichlids were present than when cichlids were absent. During two and a half h of observations of catfish broods without cichlids, I observed 23 strikes by predators on catfish young. However, when cichlids were present no foraging attacks were observed upon the catfish, but 32 occurred against cichlids in two and a half hours of observations. When the catfish parents were experimently removed the cichlid young were consumed first then the catfish young. When cichlids were present the catfish young survived over 80% longer than those in broods without cichlids. It is concluded that the parental catfish increase the survival of their own young be allowing cichlids into the brood. Because of an asymmetry of costs between the two species, cichlid mothers are more likely to abandon these interspecific broods than are the catfish.

Food switching by two specialized algae-scraping cichlid fishes in Lake Malawi, Africa.

Two cichlid species morphological specialized for scraping algae were observed in Lake Malawi also to feed extensively upon zooplankton, phytoplankton, detritus, fish fry and fish eggs. These field observations confirm laboratory studies that cichlids are facultative in their feeding repertoire. Such an ability contributes to the success of the family Cichlidae. We hypothesize that the morphological specializations become critically important when food resources are low and effective cropping is required.