Long-term stability in the population levels of the eyefluke Tylodelphys podicipina (Digenea: Diplostomatidae) in perch

Annual changes in the population size of the metacercariae of Tylodelphys podicipina in the eyes of perch in a small eutrophic lake were studied over ten years by following changes in prevalence, abundance and overdispersion of parasites throughout the life of each year class of fish. The population increased rapidly in the first two years after its introduction to the lake, but for the next six years fluctuated within very narrowly constrained limits before declining as a result of a catastrophic decline in the perch population. No evidence of parasite-induced host mortality was found: the decline in parasite abundance throughout the life of each year class was due to density-independent parasite mortality within the fish as a result of natural variation in the life span of the metacercariae. Levels of infection in the first year of a year class were determined principally by transmission processes, as transmission could only occur over a very brief period when release of cercariae from parent generation snails coincided with the presence of perch fry. Transmission appeared to be density-independent, and infection levels in perch to be determined by the dimensions of the transmission' window' and subsequently modified by parasite mortality. No evidence of host responses or any other regulatory factor of fish origin was found, and the infrapopulations in perch appear to be non-equilibrial and unstable. Although the possibility of regulatory processes acting on infrapopulations of the parasite in other hosts remains, the importance of transmission windows in determining infrapopulation levels in fish is emphasized.     

Long term studies on the population biology of two specie of eyefluke, Diplostomum gasterostei and Tylodelphys clavata (Digenea: Diplostomatidae), concurrently infecting the eyes of perch, Perca fluviatilis

Changes in the size of the infrapopulations of metacercariae of Diplostomum gasterostei and Tylodelphys clavata were studied in a small lake for a period of seven years in perch Perca fluviatilis . Tylodelphys clavata appeared in the lake at the commencement of the study and its subsequent population increase and changes were typical of those of a species recently established in a new habitat. Population size increased slowly at first, then rapidly, then declined again as population density approached equilibrium level. By contrast, the population size of D. gasterostei declined throughout the study. Two processes were responsible for the decline: a decrease in the proportion of fish carrying heavy infections which was continuous throughout the seven years and a dramatic decline in recruitment rate in young fish during the last two years. In earlier years recruitment rate had changed annually and changes in overall infrapopulation size had largely reflected the recruitment rate two years previously. It was concluded that these two processes were independent of each other and related to changes in the population density of other species of eyeflukes. The decline in the proportion of heavily infected fish was significantly negatively correlated with the increase in population density of T. clavata and the decline in recruitment rate in young fish was coincident with the appearance and infrapopulation increase of T. podicipina metacercariae in the eyes of young perch. It is suggested that these two species interact negatively with D. gasterostei and are currently partitioning the perch habitat between themselves.     

The epidemiology of the metacercariae of Diplostomum baeri and D. spathaceum in perch (Perca fluviatilis) from the warm water effluent of a nuclear power station

Thermal effects on the dynamics of infection with metacercariae of Diplostomum baeri and D. spathaceum were monitored between May and September, in a year-class of perch Perca fluviatilis, in an artificial lake receiving warm water discharges from a nuclear power station and in an unheated reference site, for a two year period. In the heated area the prevalence of infection of the retinal form, D. baeri, was always 100%, whereas in the unheated site there was an increase from 93% in May to 100% in June 1986. The relative density of D. baeri was found to increase gradually during the first summer in both areas, although the accumulation rate of metacercariae was significantly increased in the heated area. In August of the first year the relative density of D. baeri peaked in the heated area, whereas it continued to increase in the unheated control. However, in September of the following year, the relative densities were at the same level in both thermal regimes. A concomitant decline in the degree of overdispersion of metacercariae within the host population was observed in the heated area as the population density of metacercariae decreased, whereas the index of dispersion remained at the same level throughout the study in the unheated area. However, it was not possible to sample perch in the unheated area between October 1986 and May 1987 and changes in the parasite population could have occurred during this period. The prevalence and relative density of D. spathaceum, the lens form, was on the contrary low, especially in the unheated site where it was recorded only occasionally. The infection of D. baeri exhibited a convex pattern in both thermal regimens, although the peak infection was noticed earlier in the heated area. Regulation of the parasite infrapopulation may have been achieved by the combined effects of a decreased transmission rate of cercariae with increasing age of the host, the mortality of metacercariae as a natural termination of the life span and to selective predation of heavily infected hosts. However, regardless of an increased accumulation rate of metacercariae in the heated area, the relative densities of D. baeri became equal in both thermal regimes at the end of the study. The mortality rate of metacercariae in the heated area was therefore presumably increased as compared with the unheated reference site.     

Seasonal occurrence of Dollfustrema vaneyi (Digenea: Bucephalidae) metacercariae in the bullhead catfish Pseudobagrus fulvidraco in a reservoir in China

The seasonal population dynamics of metacercariae of the bucephalid Dollfustrema vaneyi (Tseng, 1930) Echmann, 1934 in the bullhead catfish Pseudobagrus fulvidraco (Richardson) were investigated in Jiangkou reservoir, Jiangxi Province, east China, during the period from April 1990 to August 1991. In total, 523 fish were obtained, and the overall prevalence of the metacercariae was 89.87% and mean abundance 136.25 +/- 308.09 (mean +/- SD). A pattern of seasonal changes in prevalence and mean abundance was observed, with higher levels of metacercariae infection in late spring and summer. An analysis of the distribution of D. vaneyi in different organs of P. fulvidraco suggested that the eyes might be a suitable location for the parasite. Furthermore, the possible role of metacercariae in bullhead catfish was discussed in relation to the life cycle of this parasite.     

Mediterranean Diplodus annularis (Teleostei: Sparidae) and its brain parasite: unforeseen outcome

Patterns of parasite load and aggregation of the bird trematode Cardiocephaloides longicollis in its main intermediate host in the Mediterranean, the annular sea bream, Diplodus annularis, were studied in a large sample collected off Valencia (Spain) and are discussed within the context of the parasite induced host mortality hypothesis. The metacercariae were located within large composite cysts of host origin in the ventricles of the optic lobes of the cerebrum. A weak immunological response was detected in older fish, which was significantly associated with the total parasite load. Although the mean abundance of C. longicollis showed a tendency to increase with host size, the infection levels were generally homogeneous with a noticeable plateauing in the intermediate size classes. The distribution of the metacercariae was aggregated and agreed with the negative binomial distribution. There was a marked decline in parasite aggregation in the largest size-class, suggesting parasite-induced host mortality in the oldest fish possibly due to predation by large non-host fish predators. On the other hand, levelling off of abundance and decrease in heterogeneity of parasite distribution within the intermediate age cohort could indicate that these sizes are being rapidly and/or constantly removed from the host population due to by-catch fishing. The overall high infection levels and the continuous recruitment across age cohorts provides evidence that an enhanced parasite transmission is taking place in the Gulf of Valencia due to increased spatial overlap of the hosts involved in the life cycle. We suggest a human-induced facilitation of the digenean life cycle due to the fact that gulls in the area feed extensively on discards, thus indicating the possibility of an unforeseen effect of fishing practices in a marine littoral system.     

Distribution of metacercariae of Centrocestus formosanus (Trematoda: Heterophyidae) on the gills of Aplocheilus panchax

The metacercariae of Centrocestus formosanus (Nishigori 1924) occur on the gills of freshwater fishes in association with blood vessels. The microecology of the metacercariae on the gills of the cyprinid fish Aplocheilus panchax (Ham. & Buch.) was investigated. The cysts showed a preference for anterior gills and for the dorsal section of each gill. Variations in the density of infection, season and size of the fish do not affect the spatial distribution of metacercariae. It is presumed that ventilation water currents spread the cercariae onto the anterior gills and then the cercariae actively select the preferred habitat.     

Population biology of Camallanus oxycephalus Ward and Magath, 1916 (Nematoda: Camallanidae) in white bass in western Lake Erie.

The distribution and abundance of the nematode Camallanus oxycephalus infecting white bass, Morone chrysops, in western Lake Erie was studied for over 2 years. Infection was generally more frequent and of higher intensity in large fish. The frequency distributions of nematode abundance in all segments of the fish population followed the negative binomial distribution. The data show seasonal cycles in population structure, site selection, intensity of infection, maturation, and reproduction. Infection occurs during July and August with a resulting peak in population density; during late summer and autumn, mortality, probably density-dependent, reduces the population by 30 to 60%; surviving worms are eliminated at 1 year of age. Growth and development of female worms is arrested from November to April, then proceeds at a rapid rate until the worms release their larvae and die. This growth pattern is probably related to temperature but may also involve host hormone cycles. The dispersal period of the nematode coincides with the annual maximum density of the intermediate host, a cyclopoid copepod,and is interpreted as an adaptation which increases the probability of successful transmission. Because the number of larvae produced by each female worm is a function of body volume, natural selection has favored rapid spring growth and attainment of large body size relative to the male worm. Both seasonal timing in the life cycle and the number of larvae produced are important factors in determining the abundance of this and perhaps other parasites. Evidence is presented suggesting that fluctuations of environmental parameters may disrupt the timing of transmission and alter the distribution and abundance of the parasite. It is hypothesized that the magnitude of such changes in parasite abundance may be related to the complexity of the host-parasite system.     

The occurrence and natural alleviation of stunting in a population of roach, Rutilus rutilus (L.)

Two sudden and pronounced changes in the growth rate of roach, Rutilus rutilus (L.), in Slapton Ley, Devon, have been observed between 1972 and 1976. At first the growth rate declined to the point at which the population became stunted, then subsequently it improved again. These changes were correlated with population density; the numbers of roach were increasing during the late 1960's/early 1970's, and this increase was substantially reinforced by very strong year classes in 1972, and, to a lesser extent, in 1973. The increase in population density resulting from the strong 1972 year class was apparently adequate to accelerate the onset of the stunting observed. The improvement in growth rate in 1976 followed immediately after extensive mortalities amongst the roach during 1975, caused by an epidemic of the parasite Ligula intestinal is (L.). Some improvement in the growth of O group roach had been evident during 1975, but this was not observed in the population. Ford-Walford plots showed a decline in the value of L after the onset of stunting in 1972.
Differences in the reproductive biology of the fish were found following the alleviation of stunting; the gonads of both sexes achieved a greater relative weight, and correspondingly the fecundity of the females was higher, although this latter effect became progressively less marked with increasing size. It was not apparent amongst the largest individuals. There was also some evidence of a decline in the proportion of fish maturing at minimum size. The changes in the reproductive biology were associated with the improvement in conditions for individual fish following the decline in population density.     

Seasonal occurrence of Opechona pyriforme metacercariae (Digenea: Lepocreadiidae) in Eirene tenuis medusae (Hydrozoa: Leptothecata) from a hypersaline lagoon in western Gulf of Mexico

Seasonal occurrence of Opechona pyriforme metacercariae in the Eirene tenuis population from Laguna Madre, Mexico was analyzed in zooplankton samples collected in October 1997 and January, May, and July 1998. Eirene tenuis medusae were present in May, July, and October, although parasites were found only in October and July samples. Host population density was positively correlated with both surface water temperature and salinity. Total sample prevalence was 3.74%, mean abundance of the parasite was 0.06, and intensity of infection ranged between 1 and 59 metacercariae/host. Prevalence differed significantly among seasons, while intensity of infection did not. Parasite distribution was found to be highly aggregated. Although O. pyriforme infected hydromedusae of all sizes, prevalence was higher in sizes above 2.11 mm umbrella diameter. Additionally, a tendency towards increasing intensity of infection values with increasing umbrella diameter of medusae was observed. This is the first record of E. tenuis as a second intermediate host of O. pyriforme, as well as the first record of this helminth in the Gulf of Mexico.     

Seasonal variation in the diet of the king penguin (Aptenodytes patagonicus) at sub-Antarctic Marion Island

King penguins are important consumers of marine resources, throughout the year, at the Prince Edward Islands. Meal size varied from 8.5–12.6% of adult mass, depending on the method of determination. In spite of the biases in the analysis favouring the overestimation of squid, fish and, in particular, myctophid fish accounted for the largest proportion of the stomach samples, 87% by wet mass, 75% by numbers and 69% by reconstituted mass. The relative abundance of fish in the diet dropped markedly in winter followed by a subsequent rise to nearly 100% in summer. This rise coincided with an increase in the chick growth rate and the king penguin population at the island and suggests the rise in relative abundance offish reflects a real increase in the availability of fish around the islands.
Juvenile and adult Krefftichthys anderssoni/Protomyctophum tenisoni and adult Electrona carlsbergi were the most common fish consumed. There was an increase in the modal size of K. anderssoni/P. tenisoni throughout the year which we interpret as growth of a single fish population. Juvenile Kondakoviu longimana was the important squid species taken by king penguins. Crustaceans were only rarely recorded in the diet and may have come from digestion of fish and squid stomachs.
This is the first study of the diet of a Southern Ocean pelagic predator that has identified myctophid fish as a major component of its diet. All three important fish species taken by king penguins at Marion Island have a wide distribution throughout the Southern Ocean and consequently may prove to be important dietary components of other Southern Ocean pelagic predators.     

Histopathological and ultrastructural observations of metacercarial infections of Diplostomum phoxini (Digenea) in the brain of minnows Phoxinus phoxinus

The spatial distribution and histopathological changes induced by metacercariae of the digenean trematode Diplostomum phoxini (Faust, 1918) in the brains of European minnows Phoxinus phoxinus (L.) from the River Endrick, Scotland, were studied by light and electron microscopy. Post-mortem examination of a sample of 34 minnows revealed that 50% (n = 17) of the population was infected with 13.7 +/- 2.6 (mean +/- SE; range 1 to 38) metacercariae per infected host. Serial histological sections of the infected minnow brains revealed that the metacercariae were unevenly distributed throughout the brain, with aggregations occurring in the cerebellum, the medulla oblongata and the optic lobes. In fish with highest intensities of infection, over 40% of the cerebellar area and about 30% of the medulla oblongata area were occupied by larvae. Metacercariae disrupt the integrity of brain tissue, with individuals being found in small pockets surrounded by cellular debris. Metacercariae were rarely encountered on the surface of the brain. Electron microscopic examination of infection sites revealed that the granular layer surrounding metacercariae was necrotic, exhibited nuclear degradation and was marked by vacuolation of the cytoplasm. Rodlet cells, the only inflammatory cell types recorded in this study, were found only in parasitized brains and in close proximity to the teguments of metacercariae. It is hypothesised that secretions released from the teguments of metacercariae are a counter response to protect the metacercariae from the fish brain's cellular defence mechanisms.     

色林错裸鲤感染匙形双穴吸虫的种群动态和寄生偏好

文章通过种群动态研究匙形双穴吸虫感染在时空上的变化,并研究该虫感染是否存在对宿主性别、左右眼及眼球不同部位的寄生偏好性,摸清匙形双穴吸虫在色林错裸鲤中的感染情况,分析种群消长原因,探究其生活史策略。跨年按不同季节采捕色林错裸鲤,记录全长、体重和性别,采集并统计匙形双穴吸虫囊蚴数量,计算不同时空下的感染率和平均丰度,通过独立样本非参数检验,判断不同性别宿主、左右眼及眼球不同部位的感染数量是否存在显著差异性,以检验匙形双穴吸虫是否存在寄生偏好。共剖检色林错裸鲤165尾[全长28.7—49.5 cm,平均全长(37.9±4.0) cm,体重196.9—827.2 g,平均体重(473.3±127.9) g,包括雌性82尾,雄性83尾],共检出匙形双穴吸虫515只,最大寄生量为32只/尾。在时间上,匙形双穴吸虫感染率和平均丰度在2020年夏季最高,秋季和春季呈下降趋势, 2021年夏季感染率下降,平均丰度却呈上升趋势, 2021年秋季与2020年秋季感染率与平均丰度较为接近。在空间上,根据色林错裸鲤全长范围,以5 cm为分组间隔,将其分为5个全长组,感染率和平均丰度在25 cm≤TL<3...     

The population biology of the acanthocephalan Pomphorhynchus laevis (Müller) in the River Avon

Regular samples of Gammarus pulex and dace Leuciscus leuciscus and occasional grayling Thymallus thymallus and chub L.cephalus were examined from the River Avon, Hampshire, for the presence of the acanthocephalan Pomphorhynchus leavis . The parasite only occurred in medium sized Gammarus due to lower probability of contact with small gammarids and stunted growth and selective mortality amongst older infected ones. No cycles in incidence or development of the parasite in G.pulex were observed. The parasite infected gammarids and grew in all months, and cystacanths were available throughout the year. Despite seasonal feeding activity and dietary preferences, fish fed on Gammarus and acquired infections in all months. Dispersion of P.laevis within the fish population was related to host feeding behaviour. No evidence of seasonal cycles in incidence or intensity of infection in fish was found, and observed monthly changes in the parasite population were related to changes in size structure of the host sample. In dace and grayling P.laevis grew little and matured only in summer, but in chub it grew and produced acanthors all year. The parasite population in fish appeared to be in a state of dynamic equilibrium and gain and loss of parasites took place throughout the year with the level of infection at any moment being determined primarily by the feeding behaviour of the host. This relationship between host diet, water temperature and parasite population size is discussed, and P.laevis in the R. Avon compared with other localities and other parasites.     

The occurrence of metacercariae of Bucephaloides gracilescens (Digenea: Gasterostomata) in an intermediate host, the four-bearded rockling, Enchelyopus cimbrius (Gadidae)

Infections with metacercariae of Bucephaloides gracilescens in the four-bearded rockling Enchelyopus cimbrius were studied for a year in a Norwegian fjord. The prevalence was at all times high, but abundance showed a marked decline in spring to a low level during summer. A rise in abundance, variance-to-mean ratio and frequency of smaller metacercariae in early autumn (August-September) shows infection to have occurred mainly in autumn. There were no differences in level of infection between host sexes, but a highly significant positive correlation of parasite burden with host length. No influence on the condition factor of the host was revealed, the parasite cysts in the central nervous system are believed to have low pathogenicity. A detailed account of the distribution of metacercarial cysts is given. No support for infection being via the lateral line canals is revealed, cercarial penetration is believed to be through the general surface of the head and through the unpaired fins of the trunk and tail.     

Linking genetic diversity and temporal fluctuations in population abundance of the introduced feral cat (Felis silvestris catus) on the Kerguelen archipelago

Linking temporal variations of genetic diversity, including allelic richness and heterozygosity, and spatio-temporal fluctuations in population abundance has emerged as an important tool for understanding demographic and evolutionary processes in natural populations. This so-called genetic monitoring was conducted across 12 consecutive years (1996-2007) at three sites for the feral cat, introduced onto the Kerguelen archipelago fifty years ago. Temporal changes in allelic richness and heterozygosity at 18 microsatellite DNA loci were compared with temporal changes in the adult population abundance index, obtained by typical demographic monitoring. No association was found at the island spatial scale, but we observed an association between genetic diversity and adult population indices from year to year within each study site. More particularly, the magnitude of successive increases or decreases in the adult population abundance index appeared to be the major factor linking the trajectories of genetic diversity and adult population abundance indices. Natal dispersal and/or local recruitment, both facilitated by high juvenile survival when the adult population size is small, is proposed as the major demographic processes contributing to such an observed pattern. Finally, we suggested avoiding the use of the harmonic mean as an estimator of long-term population size to study the relationships between demographic fluctuations and heterozygosity in populations characterized by strong multiannual density fluctuations.     

Natural parasitism of Bradybaena similaris (Férussac, 1821) by Postharmostum gallinum Witenberg, 1923

A total of 4,298 snails Bradybaena similaris were collected from the rural area of Rio de Janeiro County and from the Counties of Paracambi and Itaguai, State of Rio de Janeiro. Two thousand and five of these snails had metacercariae in their pericardial cavity. The collecting was done between October 1983 and September 1986. The annual average prevalences were: 42.89% (1st year), 43.42% (2nd year) and 55.86% (3rd year). Experimental infection in chicks allowed the identification of these metacercariae as Postharmostomum gallinum. The small variation in size observed among metacercariae from the same host indicates that recruitment does not occur on a continuous basis. Statistical analysis showed no correlation between the snail size and the number of metacercariae in the pericardical cavity, and that the snail mortality observed in the laboratory was due to the parasitic infection.     

The effects of predation by the mummichog, Fundulus heteroclitus (L.), on the abundance and distribution of the salt marsh snail, Melampus bidentatus (Say)

Enclosure and exclosure experiments were conducted in Canary Creek marsh to examine how predation by a killifish, Fundulus heteroclitus (L.), affects the abundance and size distribution of the salt marsh snail, Melampus bidentatus (Say). Enclosures (7.6 × 19.7 m) were stocked with Fundulus heteroclitus at densities of one-half normal, normal, twice normal, and four times normal density. Fish exclusion pens were also built. In both years of study, the mean density of snails increased significantly in pens where fish were excluded or their density reduced. During the same period in each year, the density of snails in pens containing higher than normal fish density fell by ≈50%.

Fish density also affected the size distribution of snails within pens. In both years, mean shell length of snails in the pen with the highest density of fish was significantly greater, and mean shell length of snails within fish exclusion pens was significantly lower than in all other treatments. Gape size limitation of F. heteroclitus causes selective predation on small snails and apparently is responsible for the difference in mean shell length among treatments.

Density and size distribution measurements of the natural Melampus bidentatus population in Canary Creek marsh were also taken. Larger snails were found to occur in the low marsh zone, whereas smaller snails occurred in the high marsh zone. Since the high marsh area is flooded less often than the low marsh, and the grass types found in the high marsh zone are known to afford protection from fish predation, the distribution of snails in the marsh is consistent with the idea that fish predation is an important factor influencing the distribution of snails in Canary Creek marsh.     

The Role of Carrion Supply in the Abundance of Deep-Water Fish off California

Few time series of deep-sea systems exist from which the factors affecting abyssal fish populations can be evaluated. Previous analysis showed an increase in grenadier abundance, in the eastern North Pacific, which lagged epibenthic megafaunal abundance, mostly echinoderms, by 9–20 months. Subsequent diet studies suggested that carrion is the grenadier''s most important food. Our goal was to evaluate if changes in carrion supply might drive the temporal changes in grenadier abundance. We analyzed a unique 17 year time series of abyssal grenadier abundance and size, collected at Station M (4100 m, 220 km offshore of Pt. Conception, California), and reaffirmed the increase in abundance and also showed an increase in mean size resulting in a ∼6 fold change in grenadier biomass. We compared this data with abundance estimates for surface living nekton (pacific hake and jack mackerel) eaten by the grenadiers as carrion. A significant positive correlation between Pacific hake (but not jack mackerel) and grenadiers was found. Hake seasonally migrate to the waters offshore of California to spawn. They are the most abundant nekton species in the region and the target of the largest commercial fishery off the west coast. The correlation to grenadier abundance was strongest when using hake abundance metrics from the area within 100 nmi of Station M. No significant correlation between grenadier abundance and hake biomass for the entire California current region was found. Given the results and grenadier longevity, migration is likely responsible for the results and the location of hake spawning probably is more important than the size of the spawning stock in understanding the dynamics of abyssal grenadier populations. Our results suggest that some abyssal fishes'' population dynamics are controlled by the flux of large particles of carrion. Climate and fishing pressures affecting epipelagic fish stocks could readily modulate deep-sea fish dynamics.     

Long-Term Changes in the Diet of Gymnogobius isaza from Lake Biwa,Japan: Effects of Body Size and Environmental Prey Availability

Body size and environmental prey availability are both key factors determining feeding habits of gape-limited fish predators. However, our understanding of their interactive or relative effects is still limited. In this study, we performed quantitative dietary analysis of different body sizes of goby (Gymnogobius isaza) specimens collected from Lake Biwa between 1962 and 2004. First, we report that the diet was composed mainly of zooplankton (cladocerans and copepods) before the 1980s, and thereafter, shifted to zoobenthos (gammarids). This foraging shift coincided with, and thus can be linked to, known historical events in the lake at that time: decrease in zooplankton abundance with the alleviation of eutrophication, increase in fish body size resulting from fish population collapse, and increase in gammarid abundance due to reduced fish predation pressure. Supporting this view, our data analyses revealed how the long-term changes in the diet composition would be co-mediated by changes in fish body size and environmental prey availability. Specifically, while zoobenthos abundance strongly affected the fish diet composition, larger (smaller) fish preferred zoobenthos (zooplankton). Furthermore, the body size effects were stronger than those of prey availability. These results provide the best long-term evidence that fish feeding habits vary over decades with its body size and prey community due to anthropogenic disturbances.     

LONG-TERM TRENDS IN ABUNDANCE AND DISTRIBUTION OF MANATEES (TRICHECHUS MANATUS) IN THE NORTHERN BANANA RIVER, BREVARD COUNTY, FLORIDA

Four aerial survey projects were conducted between 1977 and 1986 to determine the abundance, density and distribution of West Indian manatees (Trichechus manatus), in the northern Banana River, Kennedy Space Center, Florida. Manatee density and distribution within selected portions of the 78.5 km² study area were determined. Peak numbers of manatees occurred in spring of each year. The maximum counts increased from 56 in 1978 to 297 in 1986. Manatee abundance was lowest in the winter of each year. Mean density per flight increased from 0.52 manatees/km2 in 1977–78 to 2.73/km² in 1984–86. This increase may reflect increases in the east coast population or shifts in the population distribution. Distributional changes were observed in the study area through time, with a lower percentage of manatees occurring in industrial areas and a correspondingly higher percentage of manatees in nonindustrial areas by 1985.