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The disease crisis facing shrimp aquaculture may be propelled, in part, by an interaction between management practices that cause inbreeding, and the amplification by inbreeding of susceptibility to disease and environmental stresses. The study describes and numerically simulates gene flow from Penaeus (Litopenaeus) vannamei hatcheries that employ a ‘Breeder Lock’ to discourage use of their PL as breeders, through ‘copy hatcheries’ that breed the locked PL, to inbred shrimp in farm ponds. Re-analysis of published data shows that inbreeding depression under stress is exceptionally strong in shrimp. Inbreeding is currently overlooked as a problem because: (1) procedures recommended for well-managed hatcheries do not consider their implications for the copy hatcheries that supply most farmed shrimp (estimated 70%), (2) inbreeding in hatcheries is often reported as zero even though zero is the mathematical expectation of the usual estimator (Fis, fixation index) whatever the true genealogy of the broodstock. Simulation shows, however, that inbreeding can be estimated with Wang's trioML estimator, that Fis can differentiate Breeder Locked from copy PL and that simple tests can verify the lock status of PL. The importance of inbreeding should be re-evaluated in the context of disease and environmental stress. Unrecognized inbreeding may increase the incidence, prevalence and lethality of WSSV, IHHNV, EMS (AHPND) and other diseases.

Mortality between “seeding” and harvest can cause serious loss of production in crustacean aquaculture. When a species is brought under cultivation for its complete life cycle, genetic changes inevitably result from natural selection in the controlled environment (domestication selection). Such changes can, in a properly designed program, lead to a reduction in the variability that is the source of much of the economic loss. Domestication selection resulted in a two-fold increase in productivity in a model aquaculture program which involved cultivating the estuarine amphipod Gammarus lawrencianus in the laboratory. Approximately 75% of this gain was due to increased survivorship, the remainder to increased growth rate. Domestication selection will be beneficial in any crustacean with similar genetic relationships among the quantitative traits which determine yield. In aquaculture programs designed with genetic stock improvement in mind, it may be advantageous to reduce mortality by domestication selection before beginning conventional selection or inbreeding for individual traits such as growth rate. If this is not done, conventional programs may be counter-productive because of their concentrating effect on “unfit” genes which are present in the population. Domestication can be accomplished by systematic feedback of progeny from the outgrowth ponds into the broodstock, and/or selection for lifespan, and/or selection for survivorship on a family basis.

Growth of three strains of Oreochromis niloticus L. fry exposed to salinity stress in the presence of an internal reference fish were compared. The Central Luzon State University (CLSU) strain was obtained from the Freshwater Aquaculture Center, CLSU, Philippines. The ISRAEL strain was acquired from the Philippine government's Bureau of Fisheries and Aquatic Resources National Freshwater Fisheries Technology Center (BFAR-NFFTC), Munoz, Nueva Ecija. The National Inland Fisheries Institute (NIFI) strain was obtained from the NIFI, Bangkok, Thailand. Eight to nine full-sib families (replicates) per strain were split into two groups. One group was grown in freshwater for 2 weeks, acclimated to 32 ppt and reared for 2 weeks and finally grown in freshwater for another 2 weeks. Another group was contemporaneously grown in freshwater polyethylene tanks for 6 weeks. Each replicate family included a size-matched internal reference population of red tilapia strain. Two-way analysis of variance (anova) revealed no significant strain differences (P=0.081; r2=0.106). However, analysis of covariance with the internal reference strain used as a covariate showed significant (P=0.049; r2=0.638) strain effects on specific growth (based on standard length measurements). The ISRAEL strain showed consistently better growth rate in both saline and freshwater environments than the NIFI and CLSU strains. We estimated the statistical power of the two-way anova (ϕ=√(k′−1)(factor MS−s2)/(k′s2); Zar 1984) to be ∼0.30. There was a 70% probability of a Type II error and no true difference in the growth of the three strains was detected. The use of internal reference strain as a covariate improved the r2 from 0.106 to 0.638 and increased the efficiency of the test in detecting a true difference. Other strain comparison studies in our laboratory at the Southeast Asian Fisheries Development Center Aquaculture Department showed that the ISRAEL strain shows better growth than the NIFI and CLSU strains in a crowding stress tolerance experiment, when fed only with rice bran and under restrictive feeding regimes.

This study was funded by the Aquaculture Department of the Southeast Asian Fisheries Development Centre (SEAFDEC). The authors would like to thank Mr. Ronnie Gabasa for the use of his hatchery and nursery ponds and for providing some manpower support; and Messrs. Bonifacio Antazo and Romy Manibog for their technical assistance.

We report on evidence of long term stability in the geographic pattern of genetic differentiation among cod (Gadus morhua) collected from 5 spawning banks off Newfoundland and Labrador over a period spanning three decades (1964–1994) and 2 orders of magnitude of population size variation. Six microsatellite DNA loci amplified from archived otoliths (1964 and 1978) and contemporary (1990s) tissue samples revealed fidelity to natal spawning banks over this period. A two level (spawning bank and decade) hierarchical and multilocus AMOVA indicated that 1.55% of the total variation in allele frequencies could be attributed (P = 0.036) to spatial structure while no variance component could be attributed to temporal changes. A finer scale analysis among cod from just 3 of these spawning banks reveals, however, evidence consistent with some post-collapse mixing between cod from two banks. In the context of fisheries management and conservation, the survival of the spatial pattern of genetic differentiation during the population collapse suggests that if recovery eventually occurs it will likely be through population re-growth in situ rather than by migratory influx.

Summary Six sets of microsatellite primers were developed from Oreochromis shiranus nuclear DNA. These cross-primed in species of the genera Sarotherodon and Tilapia, and were polymorphic in most of the species. The total number of alleles ranged from 18 to 30 per locus; the mean heterozygosity per population ranged from 0.51 ± 0.12 to 0.82 ± 0.03, which is higher than reported in allozyme studies. Five sets of primers were used to characterize five putative populations of O. shiranus, an indigenous mouth brooding tilapia which has been widely distributed in fish farms and reservoirs in Malawi. Two postulated subspecies of O. shiranus (subspecies shiranus and subspecies chilwae) have been difficult to distinguish morphologically. Lake Chilwa, Lake Chiuta and Bunda Reservoir populations cluster together as O. sh. chilwae, while the Lake Malombe and Bishop Reservoir populations form a second cluster as O. sh. shiranus. The assignment of the Chiuta and Chilwa populations of O. sh. chilwae to a single subspecies is consistent with the fact that the two lakes were a single open lake until about 8000–9000 bp. There is no connection between them and the Lake Malawi-Malombe drainage system where the O. sh. shiranus subspecies is found.

A study was carried out to investigate the genetic diversity during domestication of Oreochromis shiranus (Trewavas) and to see if it could be associated with events in the known history of aquaculture development in Malawi. Five polymorphic microsatellite loci were scored in 14 populations of O. shiranus and one population of O. mossambicus (Peters). The mean number of alleles per locus ranged from 4.4 ± 1.03 to 13.2 ± 3.31 and was higher in the wild populations than in the domesticated populations. Other measures of genetic diversity were also lower in the domesticated compared with the wild populations, and the decline in diversity was correlated with the time elapsed since the founding of the farm stocks. Ordination analysis grouped domesticated populations into three: (1) those that trace their genealogy from Lakes Chiuta and Chilwa populations and are now spread all over the country; (2) those that come from Lakes Malawi and Malombe; and (3) hybrids between O. shiranus and O. mossambicus. Genetic differentiation among farms was strongly influenced by the pattern of known exchanges among the farmers and introgressive hybridization that had occurred between O. shiranus and O. mossambicus in the farmers’ ponds. Thus, the process of genetic changes in the species subsequent to domestication are best explained and predicted by socio-economic factors that influence the behaviour of farmers, rather than by the time-and-distance models of standard population genetics.

One generation of mass selection based on the collimation procedure (early culling of large fry) was applied on Nile tilapia, Oreochromis niloticus L., in net cages set in Laguna de Bay, Philippines. The objective was to test the effectiveness of a low-cost, small-scale broodstock improvement procedure in this culture environment. Directional selection was performed in two steps after initial removal of large fry at 21 days. Selection of parents and testing of the offspring were also conducted in hapa net cages set up in Laguna de Bay. The selection resulted in a significant positive response of 3% relative to the control, which represents a projected 34% gain over 5 years in Laguna cage culture. The realized heritability is ≈16%.

The present experiment was designed to assess the impact of the confounding of environmental tank effect with genetic family effect during early rearing periods. In November 1989, two sets of mixed family tanks were created in the Salmon Genetic Research Program (SGRP) hatchery by pooling equal numbers of eggs from four and eight families, respectively. In July 1990, tissue samples and length and weight measurements were collected on fish from both individual family tanks and mixed family tanks. Four polymorphic microsatellites were used to determine unambiguously the family of origin of 790 mixed group fish. There was a strong correlation of family survival rates measured in the mixed tanks and in the single family tanks. Differences among families for length and growth were observed in both the mixed tanks and the single family tanks. However, family growth performance in the single tank environment was poorly correlated with family growth performance in the mixed tank environment and appeared to reflect environmental differences among tanks rather than genetic differences among families. Neither family growth performance in the mixed tanks nor in the single family tanks appeared strongly correlated with subsequent family growth performance up to smolt stage.

Experiments were conducted to compare the growth and growth compensation of three strains of juvenile Oreochromis niloticus. Ten full sib families (10 replicates) per strain were split and grown under crowded and uncrowded conditions for 3 weeks (the treatment). Both treatments were then grown an additional 12 weeks under less crowded conditions (the compensation). Standard length measurements were made at the end of crowding and the end of compensation. Each replicate included a size-matched control population of a fourth (red) reference strain. ANCOVA with the reference strain used as a cofactor revealed significant strain effects on specific growth throughout the experiment. The reference strain removed most of the random among-replicate error variance as shown by an increase of r 2 from 0.06 to 0.91 when it was included in the statistical models. If the reference fish had not been used, approximately 450 replicate families would have been needed to achieve the sensitivity of the present experiment (a difference of 7% among strains significant at P=0.05). We conclude that the CLSU strain grows significantly more slowly than the Israel and NIFI strains under the experimental conditions, that the crowding effect was essentially eliminated after 12 weeks of compensation, and that the reference strain greatly improved the resolution of the strain-testing experiment.

This study was initiated to assess the feasibility of establishing pedigrees in mixed aquaculture populations and of selection programs for commercial aquaculture operations based on genetic profiling data from microsatellite markers. Complete factorial crosses between ten sires and ten dams were performed in a small rainbow trout farm. The largest and smallest progenies were sampled after 1 year of communal rearing, and their parentage was established with four or five microsatellite markers. About 91% of the fish could be traced to one or two parental couples out of the 100 possible couples. There were significant differences among sires and dams for the growth and survival of their progeny. There were also indications that progeny of inbred crosses have depressed performances. Based on these results, a breed improvement program has been implemented in this small hatchery.

Modern intensive fish farming encourages the use of superior strains of fish developed in centralized research institutions. In many artisanal fish farms these strains may not be available or suitable: environmental conditions may differ from those optimal for the superior strains, or the species of fish desired in particular farming systems may not have been the subject of a selection programme. In such cases, the option would be for farmers to improve their own strains using procedures that are within their financial and technical capabilities. One generation of mass selection for size-specific growth rate of tilapia was performed on a fish farm in West Java, Indonesia. Selection was applied in two steps over the 6 month grow out period following the initial stocking of size-graded fingerlings acquired from the farmer. Size grading, rather than age-matching, was a key step in simplifying the procedure for on-farm use. Selection and testing of the offspring were conducted in hapa nets set up in farm ponds. The selection resulted in a significant positive response of 2.3% as measured by the percent differences of the lengths of the selected offspring from the control offspring. A rough estimate of realized heritability amounted to 0.12.

Microsatellites, in particular (dG-dT)nand (dG-dA)ndinucleotide repeats, are abundant and display a high degree of length polymorphism and heterozygosity in eukaryotic genomes. Here, we report the cloning and characterization of 64 microsatellite sequences from Atlantic cod, Gadus morhua. The microsatellites were classified as perfect, imperfect, and compound repeats. The length and integrity of these repeats were compared with microsatellites characterized from two other teleosts, rainbow trout (Oncorhynchus mykiss) and Atlantic salmon (Salmo salar), and from three mammalian genomes, human, porcine, and canine. Differences were found in the proportions of the repeat classes; however, the most significant difference between microsatellites from teleost fishes and mammals was the propensity of the former to be of greater length: some cod and rainbow trout microsatellites were more than twice the size of the longest microsatellite repeats reported for any mammalian genome. Primers for PCR amplification were constructed for seven of the cod microsatellites. Allele frequencies, degree of polymorphism, and heterozygosity were estimated for a sample population. Amplification with these cod primers was also carried out on a number of related gadids. These polymorphic microsatellite loci have enormous potential utility as genetic markers for use in population, breeding, and evolutionary studies.

Experiments were conducted to determine the effects of poor nutrition on the growth of three Oreochromis niloticus strains fed protein-deficient diets. Four-week-old fry from the three “test” strains were paired with a fourth “reference” strain of tilapia (red) of the same size and stocked in 60-1 aquaria. The treatment lasted 6 weeks, with fish being fed commercial fish feed crumbles for the first and last 2-week periods and rice bran during weeks 3 and 4. Control fish were fed commercial diet throughout. Both control and treatment fish were fed at 20% of fish biomass per day. Lengths and weights were measured every 2 weeks. Significant strain effects were noted when the growth of test fish over the whole experimental period was analysed by analysis of covariance using the reference fish growth as a concomitant variable. The relative growth of the three test strains differed at each feeding phase. The nifi strain grew best during the commercial feed phases, the Israel strain performed best during the rice bran phase while the clsu strain, regardless of the type of nutritional environment, usually ranked last. Different performance rankings at each feeding phase represent strong genotype X environment interaction among these commercially important lines. This was statistically confirmed by analysis of covariance of the growth of the Israel and nifi strains during the different feeding phases using the reference strain as a covariate.

Estimates of growth over a short period are often difficult to obtain from fish unless individuals are tagged or experimental conditions are highly controlled. We demonstrate that the outer edge of scales (the osseoid layer) in tilapia (Oreochromis sp.) can be used to recognize nongrowing fish. We measured the width of the osseoid tissue at various positions of the anterior edge of the scale as well as inward to the first circulus on the edge of the scale in fed and starved fish. Our results indicated that response of the osseoid layer to growth rates occurred in 1 wk or less. Discriminant analysis based solely on osseoid variables classified over 84% of ail fish correctly on the basis of the feeding treatment after 8 d of growth. This technique was field-tested in cage-cultured tilapia in Java, Indonesia. We were able to classify correctly 86% of the growing and nongrowing fish in the population after a 21-d period. These measurements can supplement the information collected from the spacing of circuli on the calcified portion of the scale to completely specify the recent growth history of individual fish.

The genetic diversity of aquaculture stocks can be maintained, and their genetic impact on wild stocks minimized, by breeding programmes that deliberately generate genetic diversity. Current animal breeding practices are likely to reduce the diversity of domestic stocks if they are extended to aquaculture. It is proposed that national breeding programmes for aquaculture should, instead, try to develop numerous breeds specially adapted to local environments and aquaculture systems. An economic model is presented of decision-making by individual farmers who, in choosing which breed to produce, determine the "fitness" of the breeds in a meta-population that includes all breeds. As long as strong genotype-environment interaction for production traits is maintained by artificial selection, the economic self-interest of farmers should ensure the stability of genetic polymorphisms among breeds. Genetic variation would be conserved (in the among-breed component of genetic diversity) but not the primordial distribution of gene and genotype frequencies. Economic benefits to farmers, plus a high return on investment at the national or supra-national level, makes breed diversification an attractive conservation strategy even though it is admittedly a compromise from a purely genetic viewpoint.

Growth and the stabilities of growth and development were examined in inbred, outbred, and interstrain hybrid lines of tilapia (Oreochromus niloticus) under two diet regimes. Developmental stability was quantified as the inverse of the number of deformed scale circuli (calcified ridges). Growth stability was measured as the inverse of the individual variability of circulus spacing (circulus spacing is an indirect measure of growth rate). Each line was fed an alternating diet of low-protein rice bran and high-protein commercial pellet or a constant diet of commercial pellet. Diet regime had a significant effect on growth, developmental stability, and growth stability. Genetic composition of line had a significant effect only on developmental stability. Inbred lines had lower developmental stabilities than outbred lines. Hybrid progeny had higher developmental stability than the average of the parent strain values. Faster growing individuals had higher developmental and growth stabilities and lower growth stabilities (only within the commercial pellet regime). Composite indices of "merit" can be constructed by combining the variables, according to the correlations among variables and breeding objectives.

Induced spawning of Indian major carps by hypophysation has been practiced extensively since the mid-nineteen-fifties. At present, all hatcheries function as isolated genetically closed units raising their own stock of breeders and producing fish seed for distribution to grow-out areas. The individuals for replenishment of broodstock are those which were retained after the hatchery has reached its seed production target for a given year. There is a growing concern that hatchery stocks may be severely inbred because their overall performance has been rapidly deteriorating. Exact estimates of the rate of inbreeding accumulation, however, is difficult because the effective population size is not known and systematic records of various broodstock management operations are not available. In this paper, effective population size ( N e ) and the rate of inbreeding ( ΔF ) have been estimated in terms of the numbers of new individuals entering the broodstock population each year and the variance of their reproductive success. This study is based on data collected during the 1987–1988 breeding season from 18 fish hatcheries in Karnataka State, southern India. N e ranges from as low as 3 to a maximum of 30, and ΔF from 2% to 17% per year. Rapid inbreeding of stocks is evident. Possible ways of reducing further accumulation of inbreeding in broodstock populations are discussed.

We have estimated the phenotypic correlation between growth rate and interactive behavior in a laboratory population of hybrid tilapia (O. mossambicus × hornorum). Observations were made on growth and several measures of interactive behavior (in both high and low competitive feeding situations) in group-reared juveniles over 52 days. Growth was not food-limited. Four feeding behavior variables: BITES, BOUTS, LATENCY, and FFI plus one agonistic behavior variable, DONOR, were significantly correlated with size-specific growth rate. The total proportion of growth rate variance “explained” by the observed behavior (R2) was high, ranging from 0.69 to 0.82. The correlations were quantitatively, but not qualitatively influenced by the size of the individual relative to its competitors. Growth was positively correlated with measures of the intensity of feeding activity, and negatively correlated with agonistic behavior. Sex accounted for significant growth rate variation but for almost no behavioral variation. On the basis of these phenotypic correlations we suggest that agonistic interaction (aggression) in this tilapia is unlikely to be indirectly selected for during simple growth rate selection of fish reared under adequate food resource conditions.

Several strains of Oreochromis niloticus, O. mossambicus and their hybrids occur in the Philippines, and others are likely to be introduced or developed locally in the near future. Our objective was to study biological interaction. The paper reports on an experimental design in which “reference” fish are included in each replicate to provide internal statistical control. Growth of 10 full-sib families from each of two domestic strains was compared. A third strain of red tilapia was mass spawned to provide reference fish. Thirty equal-sized fry from each family were matched with 30 red tilapia fry and reared for 8 weeks in laboratory aquaria. The fish were deliberately crowded to provide a “worst-case scenario” for the application of the reference-strain technique. The objective was to see whether behavioural interaction causes statistical or genotype × environment interactions that create problems in the analysis. The test strains interacted biologically with the reference strain in different ways: the growth (change in length) of strain 1 only was negatively correlated with reference growth. Statistical interaction did occur in this extreme situation. We speculate that the reference-fish technique will be more useful in experiments in ponds or cages, where variable environmental factors induce positive, rather than negative, correlations between reference and test strains.

The multivoltine, estuarine amphipodGammarus lawrencianus has four generations per year in an environment where temperatures range seasonally from −1° to 25°C. Temperature-response curves for rates of brood production and development were determined by laboratory experiments and field observation. The life history and population dynamics were observed over a full annual cycle (1981) for a field population located at Rocky Run, Porter's Lake, Nova Scotia, Canada. On a natural (i.e., sidereal) time scale, the generations appear to have very different life histories: the two summer generations have short lives, rapid development and mature at small size (classicr-selection), whereas the overwintering generations have relatively low rates of mortality, slow development and mature at large size (classicK-selection). This pattern (larger size at maturity at lower temperatures) is widespread in aquatic poikilotherms. Similar life-history differences are evident among cohorts of the summer generations that mature at different temperatures. When time is expressed on a physiological scale that removes the effect of temperature on embryonic development and reproductive rate, the variation within and among generations is greatly reduced. In particular, an apparent alternation betweenr- andK-selection largely disappears. Because the generations are temporally isolated, it might be surmised that natural selection acting on the summer generations might antagonize the effects of natural selection acting on the fall and winter generations. However, the scaling of the rates of development, maturation, growth, reproduction and mortality on the physiological time scale derived from the temperature dependence of development and reproductive rate gives a very different and more homogeneous pattern.

Artificial selection for genetic improvement is not carried on in Indian fish hatcheries. However, the process of selection of potential breeders and artificial propagation involves indirect selection, the nature of which will be determined by the type of breeders selected for breeding. The present study is focused on females belonging to two carp species, Labeo rohita and Cirrhinus mrigala , from two fish farms in Karnataka State, south India. Quantitative estimates of pre-reproductive growth rate, age at first spawning and size at first spawning were obtained by a confirmatory factor analysis of measurements made on fish scales. Phenotypic correlations and the resulting correlated selection intensities during a single episode of selection have been estimated. The intensity of indirect selection was negative for pre-reproductive growth rate and positive for age at first spawning. This indicates an undesirable tendency to breed slower growing and later maturing individuals in both farms.

Three shell-shape parameters of Littorina saxatilis were measured and found to vary in a regular pattern with distance up an estuary. The translation rate of the shell increased, the rate of whorl expansion decreased and the circularity of the aperture decreased proceeding from the exposed shore to the protected shore. The genetic variance of these traits was estimated from the full-sib covariance and the motheroffspring covariance. The genetic variance of the translation rate and the circularity of the aperture was low in all populations, but the genetic variance of the rate of whorl expansion was high on the exposed shore and low on the protected shore. It is argued that the change in mean phenotype of these traits is the result of natural selection produced by varying degrees of wave action and desiccation. The observed genetic variance is consistent with the theory that a trait under selection will show little additive genetic variance.

A simple, hyperbolic model describes ;he relationship between the rate at which Corophiunz. volutator (Crustacea: Amphipoda) ingests sediment and the particle-size spectrum in the sediment. Mixtures of natural sedimentary floc and glass spheres were used in the feeding experiments. Ingestion rate depends not only on the concentration of edible material but also on its composition as measured by the ratio of sphere volume:floc volume. The parameters of the model are therefore functions of the nutritional value of the ingested material, as required by the hypothesis that feeding rate% adjusted to maximize the energy return.

There is widespread concern that artificial selection for rapid growth will indirectly select for aggressive fish, thus nullifying genetic stock improvement programs based on selection. We show that this unfortunate result is highly unlikely. The argument is based on a game-theoretic analysis of two general features of fish competition: (1) the growth of some individuals is often suppressed even when food is abundant and (2) the behaviour of a fish towards other members of a population depends on its relative size. The commonsense view that growth rate selection favours aggressive fish is shown to be correct only when aggressive behaviour is independent of relative size and resources are severely limited. In experimental and aquaculture environments, growth rate selection is shown to favour a decrease in the value of both aggressive and submissive behaviour, i.e. fish which ignore each other. Artificial selection for rapid growth will therefore indirectly select for tameness, not aggression.

“Indirect selection” is exerted on a trait indirectly, by means other than artificial selection for the trait itself. Expressions are derived for calculating the intensity of indirect selection in aquaculture environments. The calculations are illustrated with growth-rate data on M. rosenbergii from farm ponds in Thailand, and with computer-generated data which simulate measurements made during a multiple mark-recapture experiment. Indirect selection for growth is probably negligible in Thai prawn farms and hatcheries at present, but small changes in management practice could exert strong indirect selection on growth rate. One very effective change appears to be collecting broodstock as early in the production cycle as possible. Control of indirect selection may be useful for the genetic improvement of aquaculture stock, especially in developing countries and other situations where an intensive artificial selection program is not economically or biologically desirable. Like all selection programs, the probability of success is critically dependent on the genetics of the traits being selected.

Mass selection has a higher predicted response than within-family selection unless the intrafamily correlation coefficient, t , exceeds 0.75 or selection intensities are higher within families. At the National Inland Fisheries Institute (NIFI) of Thailand, feasible selection intensities are higher using within-family selection. The within- and between-replicate variance of a large number of tilapia grow-out experiments in ponds and cages was analysed to aid the design of future selection experiments. Within-family selection is preferable at NIFI and probably other small-to-medium selection projects in Asia. Inbreeding and other practical considerations reinforce this view.

In G. lawrencianus , females of a given fecundity and proximity to copulation day are assessed by males to be more valuable as mates, the longer the males have amplexed with the females (i.e. the greater their past investment in the act of amplexus). This behaviour is consistent with the hypotheses that (1) males are uncertain of the true reproductive state of a female on first contacting her; (2) the prior distribution of unamplexed females likely to be encountered by the males affects their initial estimates of a female's state; and (3) the time spent amplexing with a female allows information to be gathered which reduces a male's uncertainty of the female's state.

A method for estimating natural mortality and evolutionary constraint on fish life histories is presented based on the assumption that observed life histories are evolutionarily stable. Inverse optimization techniques are used to determine the values of natural mortality, reproduction–mortality trade-offs, and energy conversion efficiencies that would make observed life histories evolutionarily stable. The life history method yields natural mortality estimates comparable with those based on population age–frequency data. Sensitivity analysis is used to determine the robustness of the predictions to errors in parameter estimation and density-dependent factors.

In the Gulf Stream in October of 1968, division of at least nine species of dinoflagellates occurred between 03.00 h and 06.00h indicating a high level of developmental synchronization. Whereas light-dark and temperature cycles act independently on each species, the synchronizing agent in the Gulf Stream apparently couples the development of many species of the dinoflagellate community. It is proposed that diurnal fluctuation in nutrients, acting through interspecific competition, is the coupling and synchronizing agent in this environment. A mathematical model of interspecific competition shows that regular, short-term changes in nutrient levels are extremely sffective in synchronizing the development of several species. Calculations also suggest that division eynchrony is an indicator of the occurrence of competition for nutrients and that the degree of synchronization increases with the severity of nutrient limitation on the phytoplankton.

Samples of Littorina saxatilis (Olivi) and L. obtusata (L.) were taken from populations in an estuary where selection for certain shell morphologies had previously been established. These samples were examined for allelic variation at a polymorphic esterase locus. The allelic variation in L. saxatilis was correlated with the distance from the mouth of the estuary. In addition two pairs of alleles in L. saxatilis each showed a significant correlation of frequency. The same pattern of variation in L. saxatilis was observed in another estuary. The allelic variation in L. obtusata was similar to that of L. saxatilis, but the correlation of allele frequency was not significant. It is tentatively concluded that the variation at the esterase locus of these two species is maintained by selection.

A trade-off is found between growth and the length of time male amphipods (Gammarus lawrencianus) spend in amplexus. Males spending the majority of time in amplexus showed 45% less growth than unamplexed males. The inability of males to use their gnathopods to feed while in amplexus appears to cause this reduced growth. Growth rates of females appear unaffected by amplexus.Since male size is correlated with male reproductive success in G. lawrencianus, a 45% decrease in size increment at the next molt would represent a similar loss in the incremental male reproductive success. Male mating decisions are therefore based not only on immediate past male investment in amplexus, male size, and population characteristics, but also on the trade-off between present reproduction and future size.

Effective population size and inbreeding rates are derived in terms of the variability of broodstock replacement; the parameters are more useful for extensive aquaculture than the usual formulation. It is shown that response to mass selection may improve if variability in birth date and age is reduced by weight-specific rather than age-specific selection for growth rate. Current studies on domestication selection in Asia suggest that traditional broodstock management may be causing stocks to deteriorate.

The settling behaviour of larvae of Spirorbis borealis from several ecologically contrasting environments was analysed in terms of means and genetic variances (the latter expressed as heritability) of two components—the probability of settling when presented with a particular substrate, and the gregarious behaviour of the larvae. Larval behaviour differed markedly according to the population from which they came: animals from a sheltered tidepool prefer Ascophyllum, and are distinctly gregarious, while those from a semi-exposed embayment settle much more readily on Fucus and are not as gregarious. Animals collected from an environmental cline ranging in conditions from tidepool-like to embayment-like exhibit a corresponding range in behaviour. This interpopulation variation in character means is interpretable in terms of adaptation to differential selection pressures known to be acting on the populations; consideration of intra-population genetic variances gives an indication of the mechanism of the adaptation.

A Leslie matrix model, fit by multivariate time series analysis methods, of the southwest Nova Scotia herring (Clupea harengus) stock indicates that there is feedback from mature stock abundance to recruitment. Median smoothing suggests dome-shaped nonlinearity in this stock–recruitment relationship. Evidence for compensatory density dependence appears most strongly in the relationships of abundances of individual mature age groups and total recruitment. We suggest that this arises from the separation in space and time of spawners based on their size or age. This results in the production of distinct larval cohorts. These cohorts and the spawners which produce them are the units relevant for the examination of density-dependent processes, rather than the total numbers of spawners and recruits.

A theory of habitat selection is developed which, although fairly general in form, is intended to apply with some realism to the settlement behavior of planktonic larvae of marine organisms. It is assumed that different substrates represent different habitats and that the environment can be heterogenous on two scales of distance: (1) a "fine grained" heterogeneity in which larvae are exposed to a homogeneous mixture of substrate types during the settlement period, and (2) a "course grained" heterogeneity in which the composition of the mixture varies from place to place. The objective is to predict whether settlement will occur on both substrates in the fine grained mixtures or only on one--in other words, whether larvae will exhibit habitat preference. Settlement behavior is treated as an absorbing Markov chain in which the transition probabilities are the probability on encountering a substrate, the probability of metamorphosing on it, the probability of adult survival and the probability of death in th...

Biological and statistical properties of spacing of the marginal circuli of scales (CIRC) were examined in an experimental population of tilapia (Oreochromis mossambicus/hornorum) hybrids. Biological, or "causal," interrelationships of body length, growth, and CIRC were analysed by maximum likelihood solution of a simultaneous equation model involving the three variables. Spacing varies directly with growth rate and inversely with fish length, and growth varies inversely with length. Consequently, spacing is relatively constant along the radial axis of the scale except where variation in ration level or other factors varies the rate of growth as a function of length. The correlation of CIRC with current growth rate in populations of mixed parentage, sizes, and ages is sufficiently high (r = 0.65–0.71) to render it a useful growth estimator in aquaculture and natural situations where ages are unknown and sequential size measurements are difficult to obtain. Statistical and procedural conditions for maximizing the utility of the growth estimator in several applications are examined: (a) comparison of individual growth rates within a population, (b) comparison of mean growth of populations in different environments, (c) indirect selection for growth in aquaculture genetic programs, and (d) response of growth to exogenous variables such as parasite load, behaviour, and microhabitat.

Tilapia of various species, hybrids, and strains are grown for food in farm ponds throughout West Java, Indonesia, but their relative efficiency has not been addressed. We compared the growth rate of two strains of tilapia (“red” and “grey”) in traditional farming systems in Indonesia. A new procedure that allows growth to be estimated in undisturbed aquaculture environments, the spacing of circuli on scales (CIRC), was used to estimate size-specific growth rates from a single sample. Overall, grey morphs grew faster than reds, but a significant interaction term between colour morphs and farm made within-farm comparisons necessary. Absolute and relative growth (by colour) differed significantly among farms. The body length/scale radius allometry also varied significantly between farms and colour morphs, indicating genetic variability between farm and strain. We conclude that grey tilapia grow faster than reds, but that comparative strain evaluation is complicated by genotype/environment interaction and/or by undetected strain variation within colour morphs.

The heritability of oyster (Crassostrea virginica) larval growth rate was estimated to be in the range of 0.25 to 0.50 and a significant part of this genetic variation is of the additive type. Larval growth rate and spat growth rate were found to be highly correlated. These results suggest that a selection program for faster growing larvae and spat would be successful.

Two crustaceans, Gammarus lawrencianus and Idotea baltica , grazed selectively on Enteromorpha spp. and Ectocarpus spp. which often grow epiphytically on Chondrus crispus in culture. Idotea baltica readily consumed C. crispus when no other food was available while G. lawrencianus did not. Results indicate that these animals could also be used to control epiphytes on Gracilaria tikvahiae and Gymnogongrus sp. but not on Palmaria palmata as the latter was consumed together with the epiphytes.

The amphipod, Gammarus lawrencianus Bousfield, was maintained for ≈ 26 generations in a laboratory environment that permitted constantly increasing population size. The intrinsic rate of population growth, rm, increased 1.7-fold as a result of heritable changes in survivorship and fertility. Yield of biomass of large animals (“harvestable” biomass) increased 2.6-fold for each 45-day harvest cycle. Yield is correlated with fitness in this environment and therefore increased, although not selected for directly. Changes in life history traits are interpreted in terms of selection for Darwinian fitness in a controlled environment. The potential for genetic domestication of Crustacea is proportional to the variance of the major components of fitness, which can be estimated experimentally.

Duration of spontaneous feeding was observed at three times each day in a laboratory population of nine juvenile tilapia (Oreochromis mossambicus). Growth of the fish was measured as change in length and weight, and also as uptake of 14C-labelled glycine by isolated scales. Duration of the first morning feeding was highly correlated with all measures of growth and was independent of initial size. Later feedings were not correlated with growth. Selection on feeding duration could be used to select indirectly for growth rate; this might be a valuable procedure where individuals in the population are not exactly the same age (i.e., where size-at-age is an inaccurate measure of growth).

The present study on the carp species Labeo rohita in Karnataka State, south India, shows that it is possible to obtain quantitative estimates of pre-reproductive growth rate, age at first spawning and present age by appropriate interpretation of scale features such as the number of growth checks, circuli counts and spacing between circuli. The unobserved life history variables and their causal relationships with the observed scale features are specified a priori and estimated in a confirmatory factor analysis model. The estimates of pre-reproductive growth rate, age at first spawning and present age can be used to guide selection programmes and to compare populations in different aquaculture regimes. As a preliminary indication of the usefulness of this technique in studying domestication, it is noted that the fish chosen as breeders are not the fastest growing fish in the particular fish farms studied.