Population structure, long-term connectivity, and effective size of mutton snapper (Lutjanus analis) in the Caribbean Sea and Florida Keys

Genetic structure and average long-term connectivity and effective size of mutton snapper (Lutjanus analis) sampled from offshore localities in the U.S. Caribbean and the Florida Keys were assessed by using nuclear-encoded microsatellites and a fragment of mitochondrial DNA. No significant differences in allele, genotype (microsatellites), or haplotype (mtDNA) distributions were detected; tests of selective neutrality (mtDNA) were nonsignificant after Bonferroni correction. Heuristic estimates of average long-term rate of migration (proportion of migrant individuals/generation) between geographically adjacent localities varied from 0.0033 to 0.0054, indicating that local subpopulations could respond independently of environmental perturbations. Estimates of average longterm effective population sizes varied from 341 to 1066 and differed significantly among several of the localities. These results indicate that over time larval drift and interregional adult movement may not be sufficient to maintain population sustainability across the region and that there may be different demographic stocks at some of the localities studied. The estimate of long-term effective population size at the locality offshore of St. Croix was below the minimum threshold size considered necessary to maintain the equilibrium between the loss of adaptive genetic variance from genetic drift and its replacement by mutation. Genetic variability in mutton snapper likely is maintained at the intraregional level by aggregate spawning and random mating of local populations. This feature is perhaps ironic in that aggregate spawning also renders mutton snapper especially vulnerable to overexploitation.

Observations on the mating behavior, spawn mass and larval development of Hydatina physis Linne, 1758 (Cephalaspidea: Hydatinidae) from Karachi

Reproduction of Hydatina physis was studied in a population from Karachi, Pakistan, including mating and egg laying behavior, spawn characteristics and development.Individuals first appear in the field in October and remain until March. The spawning occurs from mid-November till mid-February with a peak in December. During this period the individuals were also observed pairing. In captivity, mating lasts for 30 minutes, second mating occurs two days later. Oviposition occurs in a very interesting and unusual manner. The mother turns "up-side-down" with its food fully expanded and the shell completely hidden underneath, the expanded foot serves as protective cover to the eggs. Eggs are deposited in a complexly folded mass with a short stem and an adhesive disc. Capsules, arranged in a single layer, contain 4-6 eggs each of wich is 70 um in diameter. Development is planktotrophic and veligers hatch after 14 days at a temperature of 26-28 degrees Celsius.

Population Characteristics of Largemouth Bass Associated with Changes in Abundance of Submersed Aquatic Vegetation in Lake Seminole, Georgia

Population characteristics of largemouth bass ( Micropterus salmoides L.) including growth, body condition (relative weight), survival, and egg production were examined in relation to abundance of submersed aquatic vegetation (SAV) coverage (primarily hydrilla [ Hydrilla verticillata L.f. Royle]) in three embayments of Lake Seminole, GA, and compared to a previous study conducted in 1998. (PDF has 8 pages.)

Population Dynamics of Cyrtobagous salviniae on Common Salvinia in South Florida

Common salvinia (Salvinia minima Baker) is an exotic floating fern that has been in the U.S. from at least 1928(Small 1931). Its pest status in Florida is less clear perhaps due to the presence of the specialized herbivore Cyrtobagous salviniae (Coleoptera: Curculionidae). Our objective was to sample populations of adult C. salviniae in south Florida in order to assess temporal abundance and estimate density on common salvinia. (PDF has 4 pages.)

The Use of Herbicides to Control Hydrilla and the Effects on Young Largemouth Bass Population Characteristics and Aquatic Vegetation in Lake Seminole, Georgia

From 1997 to 2003, we examined the impacts of two aquatic herbicides, fluridone (Sonar; 1-methyl-3-phenyl-5-[3-(trifluromethl) phenyl]-4(1H)-pyridinone), and dipotassium salt of endothall (Aquathol K; 7-oxabicyclo[2.2.1]heptane-2,3-dicarboxylic acid), used to control dense hydrilla (Hydrilla verticillata L. f. Royle), on population characteristics of juvenile largemouth bass (Micropterus salmoides Lacepede) in small coves (<10 ha) in Lake Seminole, Georgia. In addition, we estimated areal coverage and species composition of submersed aquatic vegetation (SAV) communities in each cove. Fish and plants were sampled in both control (hydrilla infested)and herbicide treated coves in November and March- April each year. Electrofishing catch-per-effort for both number and weight of age-0 and age-1 fish for the 1997 to 2002 year classes was either the same or higher (p < 0.05) in herbicide treated than in control coves. Age-0 fish were larger (p <0.05) in treated, than in control coves in November, but at age-1 in the following spring, fish were slightly longer (p <0.05) in the control coves. Higher age-0 catches were associated with greater percent reductions in numeric catch between age-0 and age-1 and reduced lengths of fish in November indicating density-dependent effects. Age-0 fish lengths were also negatively correlated to percent cover of both total and native SAV. Total or native SAV coverages were not associated with catch-per-unit effort for number and weight, but nearly all control and herbicide treated coves had total SAV coverage greater than 40%. Applications of both Sonar and Aquathol K reduced total SAV coverage and hydrilla, permitted the establishment of native SAVs, and had either neutral or positive impacts on young largemouth bass in small coves in Lake Seminole. (PDF contains 7 pages.)

Do tissue carbon and nitrogen limit population growth of weevils introduced to control waterhyacinth at a site in the Sacramento-San Joaquin Delta, California?

Waterhyacinth (Eichhornia crassipes(Mart.) Solms), is a serious problem in the Sacramento Delta. Two weevil species (Neochetina bruchi Hustache and N. eichhorniae Warner) have been introduced as biological control agents. The purpose of this study was to test the hypothesis that nitrogen (N) in the tissue of waterhyacinth was not sufficient to support weevil growth and reproduction. Because it grows better on plants with high N content and because it has a greater impact on the growth of high N plants, N. bruchi may be a more effective biological control agent in the Sacramento Delta.

The influence of disparate levels of submersed aquatic vegetation on largemouth bass population characteristics in a Georgia reservoir

Population characteristics of largemouth bass (Micropterous salmoides L.) including growth, body condition (relative weight), size structure, survival, and fecundity were examined in relation to abundance of submersed aquatic vegetation (SAV) coverage (primarily hydrilla Hydrilla verticillata L.f. Royle) in three major embayments of Lake Seminole, Georgia. Relative weight, fecundity, and growth of large-mouth bass in the Spring Creek embayment (76% areal SAV coverage) was considerably less than measured in the Chattahoochee and Flint river arms that contained lower SAV coverages (26% and 32%). It took fish 1.8 years longer to reach 406 mm in Spring Creek compared to the Chattahoochee-Flint arms. Consequently, fish were smaller in Spring Creek than in the Chattahoochee-Flint arms. In addition, due to slower growth rates and lower fecundity-to-body weight relation, we predicted a 47% reduction in total potential ova production in Spring Creek compared to the other two reservoir embayments. The annual survival rate of 3 to 10 year old largemouth bass was higher in Spring Creek (84%) than in the Chattahoochee-Flint arms (72%) and suggested either lower harvest and/or lower accessibility of particularly larger fish to angling in dense vegetation. Contrary to our expectaions, the fit between number-at-age and age in a catch-curve regression was weaker for fish collected in Spring Creek and suggested greater recruitment variability has occurred over time in this highly vegetated embayment. In Lake Seminole, spatial differences in largemouth bass population characterstics were associated with disparate levels of SAV. Our data suggest that a reduction in hydrilla, but maintenance of an intermediate level of SAV in Spring Creek, should improve largermouth bass population in this arm of the reservoir.

Population response of triploid grass carp to declining levels of hydrilla in the Santee Cooper Reservoirs, South Carolina

Approximately 768,500 triploid grass carp ( Ctenopharyngodon idella Valenciennes) were stocked into the Santee Cooper reservoirs, South Carolina between 1989 and 1996 to control hydrilla ( Hydrilla verticillata (L.f.) Royle). Hydrilla coverage was reduced from a high of 17,272 ha during 1994 to a few ha by 1998. During 1997, 1998 and 1999, at least 98 triploid grass carp were collected yearly for population monitoring. Estimates of age, growth, and mortality, as well as population models, were used in the study to monitor triploid grass carp and predict population trends. Condition declined from that measured during a previous study in 1994. The annual mortality rate was estimated at 28% in 1997, 32% in 1998 and 39% in 1999; however, only the 1999 mortality rate was significantly different. Few (2 out of 98) of the triploid grass carp collected during 1999 were older than age 9. We expect increased mortality due to an aging population and sparse hydrilla coverage. During 1999, we estimated about 63,000 triploid grass carp system wide and project less than 3,000 fish by 2004, assuming no future stocking. management, population size Ctenopharyngodon idella, Hydrilla

Toxic pollutants, health indices and population dynamics of Harbor Seals in San Francisco Bay, 1989-1992

(PDF contains 300 pages)

Population assessment of two stocks of white grunt, Haemulon plumieri, from the southeastern coast of the United States

Changes in the age structure and population size of white grunt, Haemulon plumieri, from North Carolina through the Florida Keys were examined using records of landings and size frequencies of fish from commercial, re~reational, and headboat fisheries from 1986-1998. Data were stratified into two geographical areas: North Carolina and South Carolina; and southeast Florida. Population size in numbers at age was estimated for each year and geographical area by applying an uncalibrated separable virtual population analysis (SVPA) to the landings in numbers at age. A calibrated virtual population analysis, FADAPT, was also run for data from North Carolina and South Carolina. SVPA and FADAPT were used to estimate annual, age-specific fishing mortality (F) for four levels of natural mortality (M = 0.20, 0.25, 0.30, and 0.35). The best estimate of M for white grunt is 0.30. Landings of white grunt in the Carolinas for the three fisheries have generally decreased in recent years, but have held fairly steady for the species in southeast Florida. Age at entry and age at full recruitment were age-1 and age-4 for the Carolinas, and age-l and age-3 for southeast Florida. With M = 0.30, levels of fishing mortality (F) on the fully-recruited ages were 0.23 for the Carolinas and 0.33 for southeast Florida. Spawning potential ratio (SPR) at M = 0.30 was 57% for the Carolinas and 61% for southeast Florida, which indicates that the species, by definition, has not been over-exploited by fishing. The results of this assessment of the white grunt population off the Carolinas agree with the recent F/FMSY analysis of white grunt (Anonymous, 1999). (PDF contaons 72 pages)

Impact of the commercial fishery on the population of bait shrimp (Penaeus spp.) in Biscayne Bay, 1986

Monthly population size of bait shrimp in the Bay was estimated from December 1984 to July 1985. Growth rates for male and female P. duorarum showed that pink shrimp exhibit a mean residence time in the nursery area (Biscayne Bay) of approximately 21 weeks. Monthly mortality rates were determined for each sex of pink shrimp. It was estimated that 23% and 26% of the male and female monthly population size, respectively, was absorbed by both the fishery and ecosystem monthly. Monthly proportion of the standing stock expected to die exclusively through fishing was 6.5% and 6.0% for males and females respectively. Estimates of emigration rates showed that approximately 4.0% of the population was lost from the Bay system each month. This surplus production was about 50% of the average monthly catch by the fleet. Fishing mortality represents only 8 - 9% of the losses to the shrimp population. The biggest source of loss is emigration, suggesting that most shrimp beyond the size at recruitment (to the fishery) are not utilized for food while in the Bay. Thus, it appears that the direct impact of the fishery on the bait shrimp population is relatively small. (PDF contains 46 pages)

Population assessment of the vermilion snapper, Rhomboplites aurorubens, from the Southeastern United States

Changes in the age structure and population size of vermilion snapper, Rhornboplites aurorubens, from North Carolina through the Florida Keys were examined using records of landings and size frequencies of fish from commercial, recreational, and headboat fisheries from 1986-1996. Population size in numbers at age was estimated for each year by applying separable virtual population analysis (SVPA) to the landings in numbers at age. SVPA was used to estimate annual, age-specific fishing mortality (F) for four levels of natural mortality (M = 0.20, 0.25, 0.30, and 0.35). Although landings of vermilion snapper for the three fisheries have declined, minimum fish size regulations have resulted in an increase in the mean size of fish landed. Age at entry and age at full recruitment were age-1 andage-3 fDr 1986-1991, compared with age-1 and age-4, respectively, for 1992-1996. Levels of mortality from fishing (F) ranged from 0.38 - 0.61 for the entire period. Current spawning potential ratio (SPR) is 21% or 27% depending on the natural mortality estimate. SPR could be raised to 30% or 40% with a reduction in F, or by increasing the age at entry to the fisheries. The latter could be enhanced now if fishermen, particularly recreational, comply with minimum size regulations. However, released fish mortality, modeled in the assessment at 27%, will continue to make the achievement of 30% and 40% SPR more difficult. (PDF contains 63 pages)

A study of measures of population density and of concentration of fishing effort in the fishery for yellowfin tuna, Neothunnus macropterus, in the Eastern Tropical Pacific Ocean, from 1951 to 1956

ENGLISH: This study shows how the catch and effort statistics, from 1951 to 1956, of the fishery for yellowfin tuna, Neothunnus macropterus, in the Eastern Tropical Pacific Ocean, have been used to compute: (i) two indices of average population density; (ii) an index of concentration of effort on areas of greatest density of available yellowfin. These three indices were then used to determine: (i) quarterly and annual variation in each of them; (ii) the relationship between the two indices of density; (iii) the relationship of each of the indices to the number of exploited one-degree rectangles. To remove extreme sampling variation at low levels of effort, the data from all one-degree rectangles subjected to less than five logged days' fishing in a quarter were eliminated, and the computations were repeated for comparison with those of the original data. SPANISH: Este estudio da a conocer cómo las estadísticas sobre la pesca y el esfuerzo de pesca de la pesquería del atún aleta amarilla, Neothunnus macropterus, en el Océano Pacífico Oriental Tropical, durante 1951 a 1956, han servido para computar: (i) dos índices del promedio de la densidad de la población; (ií) un índice de la concentración del esfuerzo en las áreas de mayor densidad de atún aleta amarilla disponible. Estos tres índices han sido luego usados para determinar: (i) la variación trimestral y anual en cada uno de ellos; (ií) la relación entre los dos índices de densidad; (iii) la relación de cada uno de los índices con el número de rectángulos de un grado explotados. Para evitar la extrema variación del muestreo a bajos niveles de esfuerzo, se eliminaron los datos de todos los rectángulos de un grado sujetos a menos de cinco días de actividad pesquera durante un trimestre según los registros de los cuadernos de bitácora, y las computaciones se repitieron para compararlas con las de los datos originales.

Population characteristics of gulf menhaden, Brevoortia patronus

The status of the Gulf menhaden, Brevoortia patronus, fishery was assessed with purse-seine landings data from 1946 to 1997 and port sampling data from 1964 to 1997. These data were analyzed to determine growth rates, biological reference points for fi shing mortality from yield per recruit and maximum spawning potential analyses, spawner-recruit relationships, and maximum sustainable yield (MSY). The separable virtual population approach was used for the period 1976–97 (augmented by earlier analyses for 1964–75) to obtain point estimates of stock size, recruits to age 1, spawning stock size, and fishing mortality rates. Exploitation rates for age-1 fi sh ranged between 11% and 45%, for age-2 fi sh between 32% and 72%, and for age-3 fi sh between 32% and 76%. Biological reference points from yield per recruit (F0.1: 1.5–2.5/yr) and spawning potential ratio (F20: 1.3–1.9/yr and F30: 0.8–1.2/yr) were obtained for comparison with recent estimates of F (0.6–0.8/yr). Recent spawning stock estimates (as biomass or eggs) are above the long-term average, while recent recruits to age 1 are comparable to the long-term average. Parameters from Ricker-type spawner-recruit relations were estimated, although considerable unexplained variability remained. Recent survival to age-1 recruitment has generally been below that expected based on the Ricker spawner-recruit relation. Estimates of long-term MSY from PRODFIT and ASPIC estimation of production model ranged between 717,000 t and 753,000 t, respectively. Declines in landings between 1988 and 1992 raised concerns about the status of the Gulf menhaden stock. Landings have fl uctuated without trend since 1992, averaging about 571,000 t. However, Gulf menhaden are short lived and highly fecund. Thus, variation in recruitment to age 1, largely mediated by environmental conditions, infl uences fi shing success over the next two years (as age-1 and age-2 fi sh). Comparisons of recent estimates of fi shing mortality to biological reference points do not suggest overfishing. (PDF file contains 22 pages.)

Measures of population density and concentration of fishing effort for yellowfin and skipjack tuna in the eastern Tropical Pacific Ocean 1951-1959

ENGLISH: In a previous Bulletin of this Commission, Griffiths (1960) discussed two indices of population density and an index of concentration of fishing effort of bait boats for yellowfin tuna in the Eastern Tropical Pacific for the 1951-1956 period. Yellowfin and skipjack tuna occur in the same general fishing areas and many of the commercial catches are composed of a mixture of the two species. It is desirable, therefore, to extend the investigation to skipjack and to the two species combined. SPANISH:En un Boletín anterior de esta Comisión, Griffiths (1960) se refiere a dos índices de la densidad de la población y a un índice de la concentración del esfuerzo de pesca de los barcos de carnada sobre el atún aleta amarilla en el Pacífico Oriental Tropical, correspondientes al período 1951-1956. Los atunes aleta amarilla y barrilete se encuentran en las mismas áreas generales de pesca y muchas de las pescas comerciales están compuestas de una mezcla de las dos especies. Es deseable, por lo tanto, ampliar la investigación en lo que se refiere al barrilete y a las dos especies combinadas.