Horst Kaiser

In vitro growth characteristics of five candidate aquaculture probiotics and two fish pathogens grown in fish intestinal mucus

The selection of probiotics for aquaculture is usually based on their antagonism towards pathogens. However, other criteria such as growth, attachment to intestinal mucus and production of beneficial compounds should also be considered. We suggest a protocol for the isolation and selection of potential probiotic bacteria based on their in vitro growth characteristics and propose a ranking index (RI) to screen potential aquaculture probionts. We suggest that the lag period and doubling time are the most important criteria for the comparison of growth curves, hence the RI is based on the doubling time (t(d)) and lag period (lambda) obtained from the growth profile of each bacterium. Bacteria were isolated from the gut of the common clownfish, Amphiprion percula, and screened for antagonistic activity towards seven aquatic pathogens. All five candidate probiotics showed antagonism to various aquatic pathogens. When grown in intestinal fish mucus no probiotic had a RI higher than the two tested pathogens (Aeromonas hydrophila and Vibrio alginolyticus). However, candidate probiont AP1 had a faster specific growth rate (micro) (0.05) than the pathogens (0.049 and 0.047 respectively), while AP5 grown in marine broth had a shorter lag period than the pathogens. Strategies to increase probiotic concentration include the inoculation of high concentrations and the preconditioning of these bacteria to reduce the lag period. It should be tested whether or not such strategies will allow the probiotic bacteria to dominate initially and thereby gain a competitive advantage. This could become an important aspect under in vivo conditions where both attachment and nutrient supply differ from that found in in vitro studies.

The effect of stocking density on growth, survival and agonistic behaviour of African catfish

A high rate of sibling cannibalism is one of the principal obstacles in the rearing of larvae and juveniles of the African catfish Clarias gariepinus. This paper examines the underlying behavioural components of agonistic behaviour under culture conditions. Ten day old catfish larvae were stocked at 30 larvae l−1 in three tank designs with different surface areas and equal volumes. Stocking densities were 1.2, 0.6 and 0.3 fish cm−2 bottom surface. Growth did not differ between treatments. The highest mortality over 30 days was recorded at the medium density (0.6 larvae l−1). Fish stocked at this density showed the highest rate of aggression, while there was no difference in aggression between the highest and the lowest stocking densities. The results indicate that stocking density should be at least as high as 1.2 larvae cm−2 bottom surface area to obtain high production at best survival rates.

Studies on the captive breeding of the Knysna seahorse, Hippocampus capensis

Seahorse populations throughout the world are vulnerable toexploitation for use in traditional Chinese medicines. Variouscaptive breeding ventures have been established in attempts tomeet the demand and to reduce the pressure on the naturalpopulations. Many of these programmes have relied on the captureof wild pregnant males which then give birth under captiveconditions. This would however limit production during the non-breeding season. In this study Knysna seahorses, Hippocampuscapensis, were bred in captivity during the non-breeding season(winter) using photothermal manipulation. Three constantphotoperiods (20L:4D, 16L:8D & 12L:12D) were tested incombination with three temperatures (22 °C, 25 °C& 28 °C). All combinations tested were successful inextending the breeding season of H. capensis. The frequency ofpregnancies as well as the number of young produced/pregnancy wasnot affected by the different photoperiod/temperaturecombinations. Each male gave birth to 39.38 ± 20.81juveniles every 34.03 ± 17.25 days during a four monthperiod. The mean juvenile weight and length was influenced by thephotoperiod and/or temperature regimes. Juvenile length decreasedwith increasing temperature (p ≤ 0.0001), while an increasein the light phase of the photoperiod regime at 20L:4D resultedin a significant increase (p ≤ 0.0006) in length whencompared to the 16L:8D and 12L:12D photoperiods. Juvenile weightdecreased with increasing temperature, while the combination ofphotoperiod 16L:8D and the lowest water temperature (22 °C)yielded the highest mean weights. Further studies are necessaryto determine whether mean juvenile weight or length influencesjuvenile survival

The effect of 2-phenoxyethanol and transport packing density on the post-transport survival rate and metabolic activity in the goldfish, Carassius auratus

To test the hypothesis that the anaesthetic 2-phenoxyethanol would reduce the metabolic rate and allow for higher transport packing densities, goldfish (3.93 +/- 5 g) were transported for 48 h at 25, 50 and 75 fish per 500 ml combined with anaesthetic concentrations of 0, 0.25 and 0.35 ml l-1. The anaesthetic did not affect the survival rate or the oxygen and ammonia concentrations. Thus, its use could not be recommended for the transport of goldfish. It is suggested that optimum packing densities be based on a minimum post-transport oxygen value of 4 mg l-1 for goldfish

Habitat simplification increases the impact of a freshwater invasive fish

Biodiversity continues to decline at a range of spatial scales and there is an urgent requirement to understand how multiple drivers interact in causing such declines. Further, we require methodologies that can facilitate predictions of the effects of such drivers in the future. Habitat degradation and biological invasions are two of the most important threats to biodiversity and here we investigate their combined effects, both in terms of understanding and predicting impacts on native species. The predatory largemouth bass Micropterus salmoides is one of the World’s Worst Invaders, causing declines in native prey species, and its introduction often coincides with habitat simplification. We investigated the predatory functional response, as a measure of ecological impact, of juvenile largemouth bass in artificial vegetation over a range of habitat complexities (high, intermediate, low and zero). Prey, the female guppy Poecilia reticulata, were representative of native fish. As habitats became less complex, significantly more prey were consumed, since, even although attack rates declined, reduced handling times resulted in higher maximum feeding rates by bass. At all levels of habitat complexity, bass exhibited potentially population de-stabilising Type II functional responses, with no emergence of more stabilising Type III functional responses as often occurs in predator-prey relationships in complex habitats. Thus, habitat degradation and simplification potentially exacerbate the impact of this invasive species, but even highly complex habitats may ultimately not protect native species. The utilisation of functional responses under varying environmental contexts provides a method for the understanding and prediction of invasive species impacts.

Does the endangered Knysna seahorse, Hippocampus capensis, have a preference for aquatic vegetation type, cover or height?

ABSTRACT The Knysna seahorse, Hippocampus capensis, is an endangered teleost confined to three South African estuaries. Its abundance within these systems is low and distributions are patchy. Consequently, monitoring population sizes is labour-intensive. The aim of this study was to establish if Knynsa seahorses are associated with specific regions within the estuaries, on which conservation efforts could concentrate. The relationship of Knysna seahorses with aquatic vegetation was analysed in the Knysna Estuary (the largest of the three estuarine systems inhabited by H. capensis) to determine whether this species shows a preference for a particular plant species, vegetation density or vegetation height. Seahorses were associated with five dominant aquatic plants: Zostera capensis, Caulerpa filiformis, Codium extricatum, Halophila ovalis and Ruppia cirrhosa. Together, these comprised 96% of the submerged objects with which seahorses were associated. The relative abundance of plant species changed along the estuary, but seahorses were present throughout the system, except at the estuary mouth, which was characterized by low plant densities and strong currents. No significant difference was found between the proportion of plant species present in a particular region of the estuary and the proportion of plants that seahorses used as holdfasts. However, when Z. capensis and C. filiformis were present at the same sites, adult seahorses preferred C. filiformis as holdfast. Adult seahorse density (individuals/m2) was significantly correlated with percentage vegetation cover and with holdfast length, but juvenile seahorse density was not. Nonetheless, significantly more adult and juvenile seahorses were found at sites characterized by high vegetation cover (>75%) than at sites with lower cover. Our results indicate that although there is some evidence that Knynsa seahorses prefer certain plant species over others, they are likely to be encountered anywhere in the estuary where aquatic plants are present. Conservation efforts in the Knysna Estuary should thus concentrate on such vegetated areas, which comprise approximately 11% of the total submerged surface area.

Infestation of the abalone, Haliotis midae, by the sabellid, Terebrasabella heterouncinata, under intensive culture conditions, and the influence of infestation on abalone growth

Abstract Heavy infestation by the shell-infesting sabellid, Terebrasabella heterouncinata, has been shown to cause a reduction in the growth rate and marketability of cultured abalone. This worm is endemic to South Africa and the locally cultured abalone, Haliotis midae, are under constant threat of infestation. An understanding of factors that influence abalone growth and sabellid infestation levels under intensive culture conditions is therefore imperative to controlling infestation levels. Abalone were held on an abalone farm at stocking densities of 18%, 23% and 28% of the surface area available to the abalone. They were fed kelp or a pelleted diet, Abfeed™. Their growth and infestation levels were monitored for 14 months. Within the range of abalone sizes used for this study kelp-fed abalone grew significantly better than Abfeed™-fed abalone (F1,197=7.99; P≤0.005), while stocking density had no effect (F2,196=1.41; P≤0.246) on growth. Abalone that became infested grew at the same rate as those that did not (0.09≤P≤1). No growth-limiting effect of infestation was detected. There was no effect on total intensity (i.e., the total number of worms per sample of 60 abalone; 0.41≤P≤1 and 0.17≤P≤1), or prevalence (i.e., the percentage of infected abalone within a sample; 0.50≤P≤1 and 0.19≤P≤1) of infestation by diet and stocking density, respectively. There was a significant positive coefficient of correlation of 86.2% between the level at which an increase in total intensity starts, and the average increase in total intensity per day (F1,10=62.2; P≤0.0001). A regression model is presented to estimate total intensity based on prevalence. The greatest increase in total intensity levels coincided with the onset of spring, and a possible seasonal component to the reproduction of this sabellid is discussed.

The function of aggression in the swordtail, Xiphophorus helleri: resource defence

Competition for mating opportunities may involve exclusion of intrasexual competitors (direct) or defending resources necessary to attract mates (indirect). Male swordtails (Xiphophorus helleri) engage in direct competition. Moreover in natural populations they defend a home range. This study aimed to test whether this home range defence is a form of food resource defence, which may therefore have a female attraction function. Male swordtails did defend a food resource and showed increased aggression in the presence of both food and females. However, food resource defence decreased when females were present, suggesting that both food and females are treated as defendable resources.

Effect of Feed Type and Age-at-weaning on Growth and Survival of Clownfish Amphiprion percula (Pomacentridae)

To simplify the weaning of clownfish Amphiprion percula from live food to artificial food, and to reduce the costs of feeding live food, three trials were undertaken to determine the best age for weaning. Twenty-nine-day-old post-hatch A. percula were weaned onto a fishmeal/caesin-based dry food which was an acceptable substitute for either Artemia or a moist food comprising Penaeus indicus and Donax serra. Larvae weaned 4 days after hatch (DAH) had lower survival than fish weaned 7 or 10 DAH. In juveniles weaned from 10 to 30 DAH good survival was obtained between 15 and 20 DAH suggesting that this is a suitable window for the weaning of this species.

Using functional responses to quantify interaction effects among predators

Summary Predator diversity alterations have been observed in most ecosystems as a result of the loss and/ or addition of species. This has implications for predator–prey dynamics as non-trophic interactions among predators, so-called multiple predator effects (MPE), are known to influence predation success. In addition, there is often a density-dependent relationship between prey availability and prey consumption (functional response). While MPE investigations are common in the literature, functional responses have rarely been incorporated into this field of predation ecology. Here, we outline an experimental procedure that incorporates functional responses into multiple predator effect studies. Using three fish species with different functional traits as model predators (bluegill Lepomis macrochirus, southern mouthbrooder Pseudocrenilabrus philander and banded tilapia Tilapia sparrmanii), we assess intra- and interspecific predator interaction outcomes on predator–prey dynamics. This was done by contrasting observed functional responses of heterospecific and conspecific combinations of predators with expected responses based on those of individual predators. Multipredator combinations produced variable results. Bluegill were the only species in which observed conspecific multipredator functional responses matched those of expected based on individual performance (prey risk neutral effects). In contrast, prey risk reduction was observed for both mouthbrooder and tilapia conspecific multipredator trials. Heterospecific combinations revealed strong prey risk reduction effects for mouthbrooder–tilapia and bluegill–tilapia trials, while mouthbrooder–bluegill multipredator functional responses combined additively. These results are discussed within the context of behavioural traits of the species and the development of a trait-based predictive framework. Using a functional response approach allowed for the assessment of multiple predator effects across a range of prey densities. We propose that the incorporation of within-guild predator combinations into classic functional response investigations will enhance predictive capacity development in competition and predation ecology. A lay summary is available for this article.