Yield isopleths of Tilapia esculenta Graham 1928 in Lake Victoria and Tilapia nilotica (Linnaeus) 1757 in Lake Albert

The yield equation given by BEVERTON and HOLT (1957) has several parameters which are difficult to estimate for tropical freshwater fish species. Nevertheless, some simplifying assumptions can be made and the most relevant parameters used to enable the construction of yield isopleths. Tilapia esculenfa has the following parameters: maximum length (L ∞=33.8 c.m. growth rate (K) = 0.32, natural mortality rate (M)=0.17 and the length at maturity (1 m)=22 cm. The optimum yield is obtained by catching the fish at a length of first capture of 26 em and a fishing mortality rate of 0.5. Tilapia nilotica with L ∞=49 cm, 1 m=36 cm, K=0.50 and M= 0.30 gives optimum yield when caught at a length of first capture of 35-36 cm with a fishing mortality rate of 0.5-0.6. The stuned Tilapia nilotica of Lake Albert has L ∞=17 cm, K=2.77,1 m=12 cm and M=3.37. With such a very high natural mortality, maximum yields would be obtained hy using a length of first capture less than 9 cm and a fishing mortality rate exceeding 1.8.

On the ecology and explorations of the fisheries of an East African rift valley lake, Part 1: On the bionomics and population structure of Tilapia nilotica Linnaeus 1757 in Lake Baringo, Kenya

An ecological survey of the fisheries of Lake Baringo, Kenya was carried out between August, 1972 and August, 1973. The bionomics and population structure of T. nilotica is described. Sampling was done with multifilament gillnets of graded mesh sizes from 51 mm to 178 mm in approximately 12.5 mm increments. The Lake was divided into three sampling and ecologically different zones - the south, central and north zones. The size range of T. nilotica of both sexes caught was between 5 and 27 cm (mode 16 cm) with a mean length of 16.07 cm. For all the collections, males dominated (55.3%) and a higher proportion of males were caught in January, August and November. The smallest mature male and female was 9 and 10 cm respectively. Males grow faster and mature at larger sizes than females. 50% of all males and females mature at 17.4 and 16:4 cm respectively. The periods of intense spawning were between August and October and January to April. The Tilapia were feeding best in central and north zones and the feeding intensity was reduced in January. Two endoparasites Contracaecum sp. and Clinostomum sp. were isolated from the Tilapia. The "condition" of the fish was better in the north than in the other two zones.

Factores que afectan la distribución de salmónidos en Argentina

The introductions of salmonids in Argentina can be considered as successfull. The have become established as reproducing populations in southern and middle-western lakes and reservoirs. An extensive sampling programme on 112 lakes and reservoirs was carried out during the summers of 1984 to 1987. Environmental features that determine their present distribution were analyzed. The hypothesis, which we used as starting-point, is that it has been attempted to introduce at least once and least one species of salmonids in all lakes and reservoirs of Argentina.

Marketable fish production: seasonal ponds

(4 pp.)

The History, Present Condition, and Future of the Molluscan Fisheries of North and Central America and Europe: Volume 3, Europe

(PDF file contains 248 pages.)

The History, Present Condition, and Future of the Molluscan Fisheries of North and Central America and Europe: Volume 2, Pacific Coast and Supplemental Topics

Over 100 molluscan species are landed in Mexico. About 30% are harvested on the Pacific coast and 70% on the Atlantic coast. Clams, scallops, and squid predominate on the Pacific coast (abalone, limpets, and mussels are landed there exclusively). Conchs and oysters predominate on the Atlantic coast. In 1988, some 95,000 metric tons (t) of mollusks were landed, with a value of $33 million. Mollusks were used extensively in prehispanic Mexico as food, tools, and jewelry. Their use as food and jewelry continues. Except in the States of Baja California and Baja California Sur, where abalone, clams, and scallops provide fishermen with year-round employment, mollusk fishing is done part time. On both the Pacific and Atlantic coasts, many fishermen are nomads, harvesting mollusks wherever they find abundant stocks. Upon finding such beds, they build camps, begin harvesting, and continue until the mollusks become so scarce that it no longer pays to continue. They then look for productive beds in other areas and rebuild their camps. Fishermen harvest abalones, mussels, scallops, and clams by free-diving and using scuba and hooka. Landings of clams and cockles have been growing, and 22,000 t were landed in 1988. Fishermen harvest intertidal clams by hand at wading depths, finding them with their feet. In waters up to 5 m, they harvest them by free-diving. In deeper water, they use scuba and hooka. Many species of gastropods have commercial importance on both coasts. All species with a large detachable muscle are sold as scallops. On the Pacific coast, hatchery culture of oysters prevails. Oyster culture in Atlantic coast lagoons began in the 1950's, when beds were enhanced by spreading shells as cultch for spat. (PDF file contains 228 pages.)

The History, Present Condition, and Future of the Molluscan Fisheries of North and Central American and Europe: Volume 1, Atlantic and Gulf Coasts

This three-volume monograph represents the first major attempt in over a century to provide, on regional bases, broad surveys of the history, present condition, and future of the important shellfisheries of North and Central America and Europe. It was about 100 years ago that Ernest Ingersoll wrote extensively about several molluscan fisheries of North America (1881, 1887) and about 100 years ago that Bashford Dean wrote comprehensively about methods of oyster culture in Europe (1893). Since those were published, several reports, books, and pamphlets have been written about the biology and management of individual species or groups ofclosely related mollusk species (Galtsoff, 1964; Korringa, 1976 a, b, c; Lutz, 1980; Manzi and Castagna, 1989; Shumway, 1991). However, nothing has been written during the past century that is comparable to the approach used by Ingersoll in describing the molluscan fisheries as they existed in his day in North America or, for that matter, in Europe. (PDF file contains 224 pages.)

Coastal brownfields and adaptation to climate change: Discussions on potential hazards from contaminated groundwater displacement due to saltwater incursion

Toxic-waste associated with coastal brownfield sites can pose serious risks to human and environmental health. In light of anticipated sea-level rise (SLR) due to global climate change, coastal brownfields require heightened attention. The primary intent of this study is to pose questions and encourage discussion of this problem among policy makers. Impacts from SLR on coastal zones are examined within a brownfield policy framework and, current coastal brownfield policy discussions with respect to SLR are also examined. (PDF contains 4 pages)

Integrating methods for determining length-at-age to improve growth estimates for two large scombrids

Fish growth is commonly estimated from length-at-age data obtained from otoliths. There are several techniques for estimating length-at-age from otoliths including 1) direct observed counts of annual increments; 2) age adjustment based on a categorization of otolith margins; 3) age adjustment based on known periods of spawning and annuli formation; 4) back-calculation to all annuli, and 5) back-calculation to the last annulus only. In this study we compared growth estimates (von Bertalanffy growth functions) obtained from the above five methods for estimating length-at-age from otoliths for two large scombrids: narrow-barred Spanish mackerel (Scomberomorus commerson) and broad-barred king mackerel (Scomberomorus semifasciatus). Likelihood ratio tests revealed that the largest differences in growth occurred between the back-calculation methods and the observed and adjusted methods for both species of mackerel. The pattern, however, was more pronounced for S. commerson than for S. semifasciatus, because of the pronounced effect of gear selectivity demonstrated for S. commerson. We propose a method of substituting length-at-age data from observed or adjusted methods with back-calculated length-at-age data to provide more appropriate estimates of population growth than those obtained with the individual methods alone, particularly when faster growing young fish are disproportionately selected for. Substitution of observed or adjusted length-at-age data with back-calculated length-at-age data provided more realistic estimates of length for younger ages than observed or adjusted methods as well as more realistic estimates of mean maximum length than those derived from backcalculation methods alone.

Long-term Trends in Catch Composition from Elasmobranch Derbies in Elkhorn Slough, California

Long-term trends in the elasmobranch assemblage of Elkhorn Slough, Monterey Bay, California, were analyzed by documenting species composition and catch per unit effort (CPUE) from 55 sport fishing derbies that occurred during May, June, and July, from 1951 until 1995. The most abundant species (bat ray, Myliobatis californica; shovelnose guitarfish, Rhinobatos productus; and leopard shark, Triakis semifasciata) were also analyzed for size-weight relationships, trends in size class distribution, stage of maturity, and sex ratios. Changes in species composition over the course of the derbies included the near complete disappearance of shovelnose guitarfish by the 1970’s and a slight increase in the abundance of minor species (mainly smoothhounds, Mustelus spp., and thornback, Platyrhinoidis triseriata) starting in the mid 1960’s. The relative abundance of bat rays in the catch steadily increased over the years while the relative abundance of leopard sharks declined during the last two decades. However the average number of bat rays and leopard sharks caught per derby declined during the last two decades. Fishing effort appeared to increase over the course of the derbies. There were no dramatic shifts in the size class distribution data for bat rays, leopard sharks, or shovelnose guitarfish. The catch of bat rays and leopard sharks was consistently dominated by immature individuals, while the catch of shovelnose guitarfish was heavily dominated by adults. There was evidence of sexual segregation in either immature or mature fish in all the species. Female bat rays and shovelnose guitarfish were larger than their male counterparts and outnumbered males nearly 2:1. Female and male leopard sharks were more nearly equal in size and sex ratio. Changes in species composition are likely due to fishing pressure, shifts in the prevailing oceanographic conditions, and habitat alteration in Elkhorn Slough. The sex ratios, stage of maturity, and size class distributions provide further evidence for the theory that Elkhorn Slough functions as a nursery habitat for bat rays and leopard sha

Lethal Parasites in Oysters from Coastal Georgia with Discussion of Disease and Management Implications

Extensive mortalities of oysters, Crassostrea virginica, occurred from 1985 through 1987 in coastal waters of Georgia. Fluid thioglycolate cultures of oysters collected from 16 of 17 locations revealed infections by the apicomplexan parasite Perkinsus marinus. An ascetosporan parasite, Haplosporidium nelsoni, was also observed in histopathological examination of oysters from 4 of the locations. While the range of H. nelsoni currently is recognized as the east coast of the United States from Maine to Florida, this is the first report of the parasite in Georgia waters. This paper documents the occurrence of these two lethal parasites in oysters from coastal waters of Georgia, along with potential disease and management implications. Results of an earlier independent and previously unpublished survey are also discussed which document the presence of P. marinus in Georgia as early as 1966.

Skin lesions on common bottlenose dolphins (Tursiops truncatus) from three sites in the Northwest Atlantic, USA

Skin disease occurs frequently in many cetacean species across the globe; methods to categorize lesions have relied on photo-identification (photo-id), stranding, and bycatch data. The current study used photo-id data from four sampling months during 2009 to estimate skin lesion prevalence and type occurring on bottlenose dolphins (Tursiops truncatus) from three sites along the southeast United States coast [Sarasota Bay, FL (SSB); near Brunswick and Sapelo Island, GA (BSG); and near Charleston, SC (CHS)]. The prevalence of lesions was highest among BSG dolphins (P=0.587) and lowest in SSB (P=0.380), and the overall prevalence was significantly different among all sites (p<0.0167). Logistic regression modeling revealed a significant reduction in the odds of lesion occurrence for increasing water temperatures (OR=0.92; 95%CI:0.906-0.938) and a significantly increased odds of lesion occurrence for BSG dolphins (OR=1.39; 95%CI:1.203-1.614). Approximately one-third of the lesioned dolphins from each site presented with multiple types, and population differences in lesion type occurrence were observed (p<0.05). Lesions on stranded dolphins were sampled to determine the etiology of different lesion types, which included three visually distinct samples positive for herpesvirus. Although generally considered non-fatal, skin disease may be indicative of animal health or exposure to anthropogenic or environmental threats, and photo-id data provide an efficient and cost-effective approach to document the occurrence of skin lesions in free-ranging populations.

Culture potential of Amblypharyngodon mola with carps in polyculture in farmers' pond of northern regions of Bangladesh

To assess the culture potential of mola (Amblypharyngodon mola) along with carps in polyculture systems, an experiment consisted of three treatments each with five replications was conducted for 4 months in two villages of Parbatipur upazilla under Dinajpur district. In the first treatment (SS), catla, rohu, mrigal, grass carp, Thai punti, common carp and a higher density of silver carp (8 per 40m²) were stocked. In the second treatment (SM), stocking density of silver carp was reduced to half and mola was added at a stocking density of 12,500/ha with all other fishes used in SS. In the third treatment (MM), no silver carp was stocked and mola was added at a stocking density of 25,000/ha with all other fishes used in SS. All treatments were subjected to the same regime of feed and fertilizers. The yields of large carps were 2035 kg/ha, 1757 kg/ha and 1326 kg/ha for treatments SS, SM and MM, respectively. Catla, grass carp and carpio showed better growth and production performance in presence of mola at a higher density, while rohu, Thai punti and mrigal showed better performance when stocking density of mola was relatively low. Mola yield was almost two times higher (184 kg/ha) in absence of silver carp (MM) than (62 kg/ha) in presence of silver carp (SM). The result showed that there was a significantly (p<0.01) lower total fish production in treatment MM. But there were no significant difference in total production between treatment SS and SM.

On the fishes and fisheries of Lake Baringo

Six fish species are known to occur in Lake Baringo. Tilapia nilotica Linnaeus 1757, Barbus gregorii Boulenger 1902, Clarias mossambicus Peters 1852 and Labeo cylindricus Peters 1852 were recorded in 1930-31. In 1969, two more fish species were identified: Aplocheilichthys sp. and Barbus lineomaculatus Boulenger 1903. T. nilotica is the only fish species commercially exploited. But the catches, catch per unit effort and the mean size of fish caught in commercial gillnets have declined since 1968. B. gregorii is important in the subsistence rod-and-line fishery. L. cylindricus, C. mossambicus, B. lineomaculatus and Aplochelichthys sp. are not commercially exploited.