The effects of controlled propagation on an endangered species: genetic differentiation and divergence in body size among native and captive populations of the Socorro Isopod (Crustacea: Flabellifera)

The endangered Socorro Isopod, Thermosphaeroma thermophilum, is endemic to a single thermal spring in Socorro, NM. This species is cannibalistic, with males more cannibalistic than females, and with females and juveniles more vulnerable than males as prey. In 1990, the New Mexico Department of Game and Fish, created the Socorro Isopod Propagation Facility (SIPF) near the natural habitat, Sedillo Spring (SS), to increase total population size and to examine the effects of habitat heterogeneity on population growth. We report the genetic and morphological effects of this experiment, using the natural population as a control. Captive subpopulations experienced bottlenecks of known intensity and duration, as well as different intensities of cannibalism. Using 57 AFLP markers, we show that in 6 years (1990–1996), captive subpopulations diverged significantly from the natural population. Also during this 6-year period, body lengths of captive isopods diverged nearly 2-fold from the natural population, evidently because cannibalism and thus selection favoring large size was more intense in captive subpopulations than in nature. This hypothesis is supported by the fact that cannibalism and the apparent response to selection on body size became variable among captive subpopulations when physical structure was added to three of the four SIPF pools in April 1997. As expected if cannibalism was the source of selection for large body size, by August 1998 (15 months = 7–8 generations), the rate at which body size increased became inversely proportional to the amount of physical structure within pools. Although we are unable to separate the specific effects of population subdivision and cannibalism, our results show that these conditions in combination caused rapid changes in genetic variation and the external morphology of these captive subpopulations. Our results have important implications for future attempts to manage and propagate endangered species.     

Ontogenetic constraints and diet shifts in Perch (Perca fluviatilis): mechanisms and consequences for intra‐cohort cannibalism

1. In many populations, sufficient size variation to allow for cannibalism may develop not only among age cohorts but also within them. Here, we used data on resource dynamics, consumer body size distribution and gape size limitation to unravel mechanisms promoting cannibalism within cohorts of young‐of‐the‐year (YOY) perch (Perca fluviatilis). 2. Perch are strongly gape limited when feeding on large zooplankton during early ontogeny. As a consequence, only initially large fish were able to shift to feeding on abundant large invertebrates, necessary to sustain fast growth. 3. We suggest that a combination of high initial size variation and exclusive access to resources for individuals with an initial size advantage is a prerequisite for the development of a size distribution sufficient for intra‐cohort cannibalism to occur. 4. During the time when cannibalism was observed, growth of the largest individuals in YOY perch cohorts was faster than that of smaller individuals. However, the energy gain from cannibalism did not increase growth rate enough to reach a size necessary to feed on more abundant size classes of victims, and therefore, the effect of cannibalism on overall cohort density was minor. 5. In addition to a high energy gain from cannibalism allowing for fast growth, strong resource limitation and slow growth rates of small individuals (i.e. potential victims) are a prerequisite not only for the development of intra‐cohort cannibalism but also for its persistence.     

Influence of predator density,diet and living substrate on developmental fitness of Orius laevigatus

Mass rearing of Orius laevigatus on non‐insect foods could substantially increase the cost‐effectiveness of the production of this biological control agent which is largely based on the use of expensive eggs of the Mediterranean flour moth Ephestia kuehniella. In this study, the effect of substrate quality and predator density on nymphal development of O. laevigatus fed on E. kuehniella eggs, honeybee pollen or an egg yolk based artificial diet was assessed using several types of substrates as shelter materials in the rearing containers (wax paper, bean pod or no extra substrate). In general, E. kuehniella eggs proved to be a nutritionally superior food compared to pollen and artificial diet. Pollen supported nymphal development of O. laevigatus better than the artificial diet. Overall, increasing nymphal density resulted in higher mortality, which may be due in part to cannibalism. The addition of a bean pod compensated for the nutritionally suboptimal artificial, but had a negative effect when O. laevigatus was fed on pollen. The non‐insect foods tested could not adequately replace lepidopteran eggs as a food source for O. laevigatus but they may be useful as an alternative food or in a part of the rearing process.     

Causes of Growth Depensation in Red Drum, Sciaenops Ocellatus, Larvae

Individual growth rates, and thus sizes at a given age, vary within cohorts of fishes reared in the laboratory, field enclosures, and the wild, but the causes of this variability are poorly understood. We examined effects of three variables (food availability, a diet shift, and fish density) on growth depensation in red drum, Sciaenops ocellatus, larvae in laboratory experiments. Absolute variance in length of larvae was greater with higher food availability, and relative variance (CV) for a cohort at 14d posthatching was positively correlated with its CV at 7d posthatching, over which time CV doubled. Supplementation of a rotifer diet with Artemia nauplii (diet shift) increased growth rate and length variance, but not CV. Individually-reared and group-reared larvae had similar growth rates, but group-reared larvae exhibited higher CVs than individually-reared larvae. The observation that food availability and diet shifts affected absolute size variance, while fish density affected relative size variance of red drum larvae, indicates that both absolute and relative size variance are important to understand how environmental factors affect growth depensation.     

Effects of resources and mortality on the growth and reproduction of Nile perch in Lake Victoria

1. A collapse of Nile perch stocks of Lake Victoria could affect up to 30 million people. Furthermore, changes in Nile perch population size‐structure and stocks make the threat of collapse imminent. However, whether eutrophication or fishing will be the bane of Nile perch is still debated. 2. Here, we attempt to unravel how changes in food resources, a side effect of eutrophication, and fishing mortality determine fish population growth and size structures. We parameterised a physiologically structured model to Nile perch, analysed the influence of ontogenetic diet shifts and relative resource abundances on existence boundaries of Nile perch and described the populations on either side of these boundaries. 3. Our results showed that ignoring ontogenetic diet shifts can lead to over‐estimating the maximum sustainable mortality of a fish population. Size distributions can be indicators of processes driving population dynamics. However, the vulnerability of stocks to fishing mortality is dependent on its environment and is not always reflected in size distributions. 4. We suggest that the ecosystem, instead of populations, should be used to monitor long‐term effects of human impact.     

Diet of spiny dogfish, Squalus acanthias Linnaeus, on the east coast, South Island, New Zealand

Stomach content data from 7283 spiny dogfish, Squalus ucunthius Linnaeus, from east coast South Island of New Zealand are presented. The major dietary components based on occurrence were 60% crustaceans and 15% fish. The principal food items were the post-larval phase of the squat lobster, Munida greguria Fabricius (30%), and the euphausiid, Nyctiphanes australis Sars (20%). Commercial fish species contributed to less than 5% of the diet. Diet of dogfish varied with dogfish size and sex. Compared to northern hemisphere studies, spiny dogfish in this study area had (1) a high proportion of crustaceans in their diet, (2) a relatively low percentage of empty stomachs, (3) a relatively high incidence of cannibalism.     

The response of burbot (Lota lota) to change in lake trout (Salvelinus namaycush) abundance in Lake Opeongo,Ontario

The purpose of this study was to test the hypothesis that the burbot and lake trout compete with or prey on each other, and that a change in the abundance of lake trout has triggered a response in the burbot population. Burbot growth, length-weight relationship and population size did not change, and it appears that at these population levels, the changes in lake trout abundance had no detectable effect on burbot. It is suggested that the burbot population in Lake Opeongo is limited at the larval stage by competition with, or predation by, a planktivore, the lake herring.     

Non-interactive effects of habitat complexity and adult crayfish on survival and growth of juvenile crayfish (Pacifastacus leniusculus)

1.  In a 1-month outdoor stream channel experiment, we investigated the relative importance of habitat complexity (i.e. cobbled area) and the presence of adult signal crayfish ( Pacifastacus leniusculus ) males on the survival and growth of juveniles.
2.  In treatments with high habitat complexity, more juveniles survived, more were newly moulted and they had a higher specific growth rate (SGR) at the end of the experiment than juveniles in treatments with low habitat complexity. The presence of adult males did not affect survival, moulting stage or growth of the juveniles.
3.  The presence of adult males decreased juvenile activity during night. Juveniles in treatments with low habitat complexity were more active than juveniles in high habitat complexity during both day and night.
4.  There was no difference in total invertebrate biomass between treatments. However, some invertebrate taxa, such as Chironomidae larvae, were affected by habitat complexity or the presence of adult crayfish. Juvenile crayfish in all channels had consumed detritus, algae and Chironomidae larvae and there were no differences in gut contents or stable isotope signals (carbon and nitrogen) between treatments, indicating a similar diet among the juveniles across treatments. However, the biomass of chironomids was significantly higher in channels with adult crayfish present, indicating a decreased consumption of chironomids by juveniles in the presence of adults.
5.  Our results suggest that the recruitment of juvenile crayfish is mostly affected by habitat complexity. The competition for food and shelter and aggressive interactions between the juveniles were most pronounced in low habitat complexity, indicating that habitats with a good access to shelter will enhance recruitment of juvenile crayfish in streams.