Exploratory stock identification through morphometric trait analysis of the estuarine round herring, Gilchristella aestuaria (Pisces: Clupeidae).

As a first step in stock identification of estuarine round herring Gilchristella aestuaria, a morphometric analysis of multiple standardised traits and body condition was conducted on samples from sites across the distribution range of the species, including freshwater and estuarine populations. Multivariate analyses, including a principal component analysis (PCA), revealed that sites on the same river system and sites in close geographic proximity tend to share morphometric traits. Most of the variation in PCA was due to caudal fin length (CFL). Single traits were analysed with generalised additive models with river system location as the smooth term. CFL was strongly related to river systems, and to a lesser extent salinity and turbidity. Unlike previous local-scale studies, this broad-scale study did not support the notion of ecotypes based on eye diameter. Condition indices like body depth and relative weight were related to climatic conditions, salinity and turbidity, more than to river system. The findings agree with previous studies on phylogenetic history and limited gene flow in G. aestuaria, and suggest variation in environmental productivity that affects body condition in different populations. Subsequent research should examine temporal changes in traits like CFL and body condition based on long-term seasonal sampling. Safeguarding this potential resource may be enabled by adopting appropriate management methods based on delineation of stocks with different levels of productivity and connectivity, before widespread fishing of this species is promoted.

Probiotics in marine larviculture.

Owing to the problem of antibiotic resistance and subsequent reluctance of using antibiotics, the use of probiotics in larviculture is becoming increasingly popular. During the early stages of development, manipulation of the larval digestive system seems possible through the addition of probiotics either through the culture water or via the livefood. Well-studied probiotics used in human medicine and terrestrial agriculture have proved to be successful in aquaculture and therefore reduce the need for extensive biosafety trials. The selection of probiotics requires various in vitro screening experiments, which assay for the production of antagonist compounds, their growth in and attachment to fish intestinal mucus, and the production of other beneficial compounds such as vitamins, fatty acids and digestive enzymes. Further information regarding probiont suitability can be obtained from its identification, interaction with livefood and host pathogenicity. Finally, pilot-scale in vivo tests need be performed, after which a production cost-benefit analysis to determine its economic viability needs to be undertaken.

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.