Kenneth R. McKaye

Controlling Vectors and Hosts of Parasitic Diseases Using Fishes A case history of schistosomiasis in Lake Malaŵi

Current estimates of the global prevalence of parasitic diseases range from 600 million cases caused by protozoan and metazoan parasites (Kolberg 1994) to more than 1 billion total cases (Goodman 1994). Strategies for reducing the transmission of these diseases include: immunization programs; reduction of parasite loads in human populations via chemotherapy; modification of cultural practices through public health education, clean water, and sanitation projects; habitat modification (e.g., drainage of stagnant water bodies); and biological and chemical vector and host control (Warren and Mahmoud 1990, WHO 1990). Due Much of the published research on the use of

African Tilapia in Lake NicaraguaEcosystem in transition

L ake Nicaragua contains more than 40 species of fish, including 16 recognized species (Thorson 1976) of native cichlids and additional undescribed cichlids.1 The lake is also inhabited by several large marine predators, such as the bull shark (Charcharhinus leucas), sawfish (Pristis perotteti), tarpon (Tarpon atlanticus), grunt (Pomadasys spp.), and snook (Centropomus parallelus; Thorson 1976). Eleven of the 16 cichlid species are confined to San Juan Province, which contains the lake (Miller 1966). The most heavily exploited fishery resources have traditionally been the native cichlids, estimated currently to constitute 58% of Lake Nicaraguas fish biomass. Lake Nicaragua (Figure 1), the largest (8264 km2) tropical lake outside of Africa (Incer 1976), is lo-

Characteristics that influence male reproductive success on a lek of Lethrinops c.f. parvidens (Teleostei : Cichlidae)

Abstract Sexual selection has long been proposed as a mechanism leading to the diverse cichlid (Teleostei: Cichlidae) fauna of Lake Malawi, Africa. Many of the shallow-water, sand-dwelling, bower-building cichlid species are particularly well suited for studies of sexual selection because they participate in leks. Since females in lekking systems appear to acquire only genetic material from their mates, it has been suggested that leks are ideal systems to study female mate choice. The objectives of the investigation were to examine Lethrinops c.f. parvidens male bower characteristics (i.e., bower size and location) as well as other male characteristics (i.e., length, gular color, and duration on the lek) for their influence on male mating success as measured by the number of visits, circles, and eggs laid by females. These measures are nested in that a visit by a female may or may not lead to circling, and circling by a female may or may not lead to egg-laying. We found increased bower height and higher numbers of conspecific neighbors (analogous to shallow-water, near-shore bower positions) to be positively, significantly associated with the number of visits by females. The only significant correlate with the number of circles was visits, and similarly circles was the only significant correlate with the number of eggs laid. The R2 value for the egg-laying regression was quite low (19.8%) compared with visits (54.3%) and circling (78.9%), suggesting that females may be using additional cues, that we failed to measure, when in close proximity to males or simply that a small proportion of the females were ready to spawn. Both indirect selection and direct selection pressure due to egg predation may have influenced female choice on the lek.

DEFENSE OF A PREDATOR'S YOUNG BY A HERBIVOROUS FISH: AN UNUSUAL STRATEGY

One species of cichlid fish Cichlasoma nicaroguense was observed defending the young of the largest cichlid in the lake, the predator C. dovii. The behavior of these fish was analogous to that of cichlid parents defending young to which they were genetically related. With this added help, C. dovii parents were not required to attack predators upon their own young as often as conspecific parents without helpers. Survivorship of C. dovii young was increased by the added protection afforded by these nonrelated individuals. A model is given to explain this unusual behavior in terms of individual natural selection. The arguments presented account for this behavior in terms of delayed return benefit (Trivers 1971) in that the individual (or his close kin) who defends the predators young will later be repaid. This repayment is derived from the predator feeding on the main competitors of the altruist.

Schistosomiasis in Lake Malawi: Relationship of Fish and Intermediate Host Density to Prevalence of Human Infection

Prior to 1985, the open waters of Lake Malawi were free from schistosome transmission. Over the past decades, however, the prevalence of urinary schistosomiasis has increased dramatically in the southern part of the lake. We found the prevalence of human schistosomiasis in school-aged children to be negatively correlated with the density of molluscivorous fishes. Specifically, the increased infection rate in southern Lake Malawi between 1978 and 1991 is coincident with the reduction in numbers of snail-eating fishes. During 2003, we determined the relative abundance of molluscivorous fishes and snail density at 18 sites throughout the lake and schistosome infection in school-aged children living in selected lakeshore communities of Lake Malawi. At the 18 sites sampled in 2003, we found that snail abundance decreased with an increase in abundance of snail-eating fishes. Furthermore, the 2003 samples showed that the abundance of snail-eating fishes increased and there was a reduction in schistosomiasis in school-aged children in Chembe Village. We believe that we will not observe a return to the 1978 infection rates until these fishes continue to increase and inhabit shallower waters.

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

SummaryBagrid 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.

Schistosomiasis transmission in Lake Malawi

Schistosomiasis (bilharziasis) transmission due to Schistosoma haematobium has for many years been known to occur along protected shorelines in Lake Malawi, but the recent finding that transmission also can also occur along open shorelines with sandy sediment has had a detrimental effect on tourism to Lake Malawi. The present paper shows that transmission, as evidenced from presence of intermediate hosts, along open shorelines is occurring in the southern part of the Lake. Relatively simple precautions can be taken to ensure minimal risk of attracting schistosome infection, and the schistosomiasis risk should not deter people from visiting and diving in Lake Malawi.

Initial Six-year Expansion of an Introduced Piscivorous Fish in a Tropical Central American Lake

Two populations of the piscivorous fish Gobiomorus dormitor (the ‘guabina’) were examined in two separate crater lakes in Nicaragua, Central America. At Lake Apoyo, G. dormitor were introduced in May 1991 by local fishermen and have invaded successfully; at Lake Xiloá, the population is naturally occurring. To provide baseline life history data for G. dormitor in both lakes and to test hypotheses related to population growth and invasion success, this study aimed to (1) document and define the population growth and depth distribution of the fish through video transects in Lake Apoyo; (2) compare diet composition between lakes; and (3) compare life history parameters between lakes, including relative growth rates and reproduction. Videotaped transect studies, designed originally to examine cichlid breeding, began in both lakes in 1990 and have documented the subsequent increase of G. dormitor. Yearly means generally fit the exponential growth model. G. dormitor foraged mainly in shallow waters in Lake Apoyo and preyed mostly upon the atherinid Melaniris cf. sardina throughout seasons. In Lake Xiloá, cichlid fry constituted the majority of the diet overall and cannibalism was more common, although this trend varied with the cichlid breeding season. Relative growth rates were significantly different, based on otolith mass regressions on standard length. Lake Apoyo G. dormitor generally behaved as a successful invading species, displaying exponential growth, year-round reproduction, and higher foraging success than the natural population of Lake Xiloá. Further research is planned to explore the G. dormitor invasion when the population becomes integrated into the community.

Experimental Evidence of Female Choice in Lake Malawi Cichlids

Abstract To date, most research on sexual selection has focused on the discrimination between attributes of individuals of the opposite sex. We have demonstrated female discrimination of a male behavioral character, bower size, by experimentally manipulating bower height in a lek of cichlid fishes from Lake Malawi, Africa. Eggs increased significantly (p<0.01) with large (15 cm) and extra large (30 cm) artificial bowers. Males with extra large bowers received a 50% greater increase in eggs laid than those with just large bowers.

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

SummaryParental 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.