The application of bioflocs technology to protect brine shrimp (Artemia franciscana) from pathogenic Vibrio harveyi

Biofloc Technology (BFT) in Aquaculture: What Goes Right, What Goes Wrong? A Scientific-Based Snapshot

Aquaculture is a crucial industry that can help meet the increasing demand for aquatic protein products and provide employment opportunities in coastal areas and beyond. If incorrectly manage, traditional aquaculture methods can have negative impacts on the environment and natural resources, including water pollution and overuse of wild fish stocks as aquafeed ingredients. Biofloc technology (BFT) may offer a promising solution to some of these challenges by promoting a cleaner and sustainable production system. BFT converts waste into bioflocs, which serve as a natural food source for fish and shrimp within the culture system, reducing the need for external inputs, such as feed and chemicals. Moreover, BFT has the potential to improve yields and economic performance while promoting efficient resource utilization, such as water and energy. Despite its numerous advantages, BFT presents several challenges, such as high energy demand, high initial/running costs, waste (effluent, suspended solids, and sludge) management, opportunistic pathogens (vibrio) spread, and a lack of understanding of operational/aquatic/microbial dynamics. However, with further training, research, and innovation, these challenges can be overcome, and BFT can become a more widely understood and adopted technique, acting as an effective method for sustainable aquaculture. In summary, BFT offers a cleaner production option that promotes circularity practices while enhancing performance and economic benefits. This technique has the potential to address several challenges faced by the aquaculture industry while ensuring its continued growth and protecting the environment. A more broad BFT adoption can contribute to meeting the increasing demand for aquaculture products while reducing the industry's negative impact on the environment and natural resources. In this context, this review provides an overview of the advantages and challenges of BFT and highlights key technical, biological, and economic aspects to optimize its application, promote further adoption, and overcome the current challenges.

Utilization of Sugarcane Bagasse (Saccharum officinarum Linn.) as a Carbon Source in Biofloc System of Vaname Shrimp Litopenaeus vannamei

<b>Background and Objective:</b> Vaname shrimp (<i>Litopenaeus vannamei</i>) is one of the main economic commodities in aquaculture in the world. Biofloc is a cultivation technology that effectively improves the growth and health status of vaname shrimp. This research aimed to analyze the use of bagasse as a carbon source in the biofloc system for white shrimp cultivation. <b>Materials and Methods:</b> The shrimp used were 18 g/individual shrimp obtained from the Bone Marine and Fisheries Polytechnic Pond. Sugarcane bagasse processed from sugar factory waste was dried in an oven at 60°C and ground using a flouring machine. The research treatments included biofloc application where sugarcane bagasse played a role as a carbon source (L), biofloc application where wheat flour's role was as a carbon source (T) and control or no biofloc application (K). <b>Results:</b> This research showed that sugarcane bagasse could be used as a carbon source for white shrimp biofloc cultivation where the growth value tended to be the same as wheat flour. Total hemolytic count (THC) and shrimp survival in sugarcane bagasse biofloc were as good as wheat flour biofloc. Sugarcane bagasse biofloc had the same ability as wheat flour biofloc in reducing ammonia levels in the rearing media. Sugarcane bagasse biofloc had the same ability as wheat flour biofloc in reducing ammonia levels in the rearing media. The application of bagasse had no effect on temperature, pH, dissolved oxygen and salinity of the rearing media because this treatment was in the optimal range for the growth of vaname shrimp. <b>Conclusion:</b> Sugarcane bagasse has the potential to be a carbon source in biofloc systems because it could improve growth, health status, survival and water quality.

Microbial inoculums improve growth and health of Heteropneustes fossilis via biofloc-driven aquaculture

Biofloc technology aims to maximize fish farming productivity by effectively breaking down ammonia and nitrite, promoting healthy flocculation, and enhancing the growth and immunity of cultured animals. However, a major limitation in this field is the suitable starter microbial culture and narrow number of fish species that have been tested with the biofloc system. Here, we investigated various microbial inoculum containing beneficial microbes with probiotics, immunostimulatory and flocs development and bioremediation properties would lead to the development of ideal biofloc development. Three treatment groups with different microbial combinations, viz., group 1 [Bacillus subtilis (AN1) + Pseudomonas putida (PB3) + Saccharomyces cerevisiae (ATCC-2601)], group 2 [B. subtilis (AN2) + P. fluorescens (PC3) + S. cerevisiae (ATCC-2601)] and group 3 [B. subtilis (AN3) + P. aeruginosa (PA2) + S. cerevisiae (ATCC-2601)] were used and compared with the positive control (pond water without microbial inoculums) and negative control (clear water: without microbial inoculums and carbon sources) on biofloc development and its characteristic features to improve the water quality and growth of fish. We demonstrated that microbial inoculums, especially group 2, significantly improve the water quality and microbiota of flocs and gut of the test animal, Heteropneustes fossilis. The study further demonstrates that biofloc system supplemented with microbial inoculums positively regulates gut histomorphology and growth performance, as evidenced by improved villous morphology, amylase, protease and lipase activity, weight gain, FCR, T3, T4 and IGF1 levels. The inoculums induced an antioxidative response marked by significantly higher values of catalase (CAT) and superoxide dismutase (SOD) activity. Furthermore, the supplementation of microbial inoculums enhances both specific and non-specific immune responses and significantly elevated levels of immune genes (transferrin, interleukin-1β and C3), and IgM was recorded. This study provides a proof-of-concept approach for assessing microbial inoculums on fish species that can be further utilized to develop biofloc technology for use in sustainable aquaculture.

Taurine metabolism is modulated in Vibrio-infected Penaeus vannamei to shape shrimp antibacterial response and survival

BACKGROUND

Numerous microorganisms are found in aquaculture ponds, including several pathogenic bacteria. Infection of cultured animals by these pathogens results in diseases and metabolic dysregulation. However, changes in the metabolic profiles that occur at different infection stages in the same ponds and how these metabolic changes can be modulated by exogenous metabolites in Penaeus vannamei remain unknown.

RESULTS

Here, we collected gastrointestinal tract (GIT) samples from healthy, diseased, and moribund P. vannamei in the same aquaculture pond for histological, metabolic, and transcriptome profiling. We found that diseased and moribund shrimp with empty GITs and atrophied hepatopancreas were mainly infected with Vibrio parahaemolyticus and Vibrio harveyi. Although significant dysregulation of crucial metabolites and their enzymes were observed in diseased and moribund shrimps, diseased shrimp expressed high levels of taurine and taurine metabolism-related enzymes, while moribund shrimp expressed high levels of hypoxanthine and related metabolism enzymes. Moreover, a strong negative correlation was observed between taurine levels and the relative abundance of V. parahaemolyticus and V. harveyi. Besides, exogenous taurine enhanced shrimp survival against V. parahaemolyticus challenge by increasing the expression of key taurine metabolism enzymes, mainly, cysteine dioxygenase (CDO) and cysteine sulfinic acid decarboxylase (CSD).

CONCLUSIONS

Our study revealed that taurine metabolism could be modulated by exogenous supplementation to improve crustacean immune response against pathogenic microbes. Video Abstract.

Heat Shock Proteins (Hsps) in Cellular Homeostasis: A Promising Tool for Health Management in Crustacean Aquaculture

Heat shock proteins (Hsps) are a family of ubiquitously expressed stress proteins and extrinsic chaperones that are required for viability and cell growth in all living organisms. These proteins are highly conserved and produced in all cellular organisms when exposed to stress. Hsps play a significant role in protein synthesis and homeostasis, as well as in the maintenance of overall health in crustaceans against various internal and external environmental stresses. Recent reports have suggested that enhancing in vivo Hsp levels via non-lethal heat shock, exogenous Hsps, or plant-based compounds, could be a promising strategy used to develop protective immunity in crustaceans against both abiotic and biotic stresses. Hence, Hsps as the agent of being an immune booster and increasing disease resistance will present a significant advancement in reducing stressful conditions in the aquaculture system.

Supplementation of ex situ produced bioflocs improves immune response against AHPND in Pacific whiteleg shrimp (Litopenaeus vannamei) postlarvae

The emergence of Vibrio diseases, including acute hepatopancreatic necrosis disease (AHPND) caused by Vibrio spp., had resulted in heavy losses in global shrimp production. Biofloc technology is a closed aquaculture system developed as one of the sustainable solutions to increase system resilience in the shrimp industry. In this study, biofloc was formed externally (ex situ biofloc) with probiotics Bacillus sp. strain BME and Bacillus sp. strain BCE, diatom microalgae Chaetoceros calcitrans, and a consortium of nitrifying bacteria, in the ratio of 1:1:6:6 as a starter. The study showed that the ex situ biofloc supplementation in Pacific whiteleg shrimp (L. vannamei) postlarvae culture can increase the shrimp culture performance (shrimp survival and growth), reduce Vibrio counts in the water and shrimp body, and provide stimulation of the shrimp immune response through humoral immune responses, such as pattern recognition protein (C-type lectin) and melanization process (proPO). Overall, the results indicate that the supplementation of ex situ biofloc provided protection to shrimp under Vibrio infection, regardless of the timing of addition (before, simultaneously, or after addition of Vibrio sp. strain VPA). This suggests that the ex situ biofloc can be effective as a preventive and a supportive treatment against potential AHPND infection in L. vannamei postlarvae culture. Taken together, the ability of the ex situ biofloc to modulate immune-related gene expression and resistance of L. vannamei against potentially AHPND-causing Vibrio sp. strain makes it an effective aquaculture technology for infectious disease control in shrimp production with high-density and minimal water exchange culture. KEY POINTS: • Supplementation of ex situ produced biofloc in shrimp postlarvae culture. • Ex situ biofloc reduces Vibrio counts in the water and shrimp body. • Ex situ biofloc stimulates shrimp humoral immune responses and survival.

A Solution for Sustainable Utilization of Aquaculture Waste: A Comprehensive Review of Biofloc Technology and Aquamimicry

Biofloc technology (BFT) is gaining traction as a strategic aquaculture tool for boosting feed conversions, biosecurity, and wastewater recycling. The significant aspect of BFT is aquaculture with highest stocking density and minimal water exchange. It not only improves the water quality of a system by removing inorganic nitrogen from wastewater but also serves as a suitable feed supplement and probiotic source for cultured species. This technology is commonly used for shrimp and tilapia culture and can be used for both semi-intensive and intensive culture systems. Biofloc, when combined with formulated diets, forms a balanced food chain that improves growth performance. Nutrients in this system are continuously recycled and reused and form an efficient alternative system in aquaculture. In addition to the reduction in water exchange, it is also considered as a bio-security measure, since it prevents entry of disease from outside sources. Aquamimicry is an innovative concept that simulates natural estuarine conditions by developing copepods that act as supplementary nutrition especially for shrimp culture. The review highlights the process, significance, and development of BFT, its microbial interactions, nutritional value, transition from biofloc to copefloc, and concept of aquamimicry to sustainably improve aquaculture production.

Acute Hepatopancreatic Necrosis Disease (AHPND): Virulence, Pathogenesis and Mitigation Strategies in Shrimp Aquaculture

Shrimp, as a high-protein animal food commodity, are one of the fastest growing food producing sectors in the world. It has emerged as a highly traded seafood product, currently exceeding 8 MT of high value. However, disease outbreaks, which are considered as the primary cause of production loss in shrimp farming, have moved to the forefront in recent years and brought socio-economic and environmental unsustainability to the shrimp aquaculture industry. Acute hepatopancreatic necrosis disease (AHPND), caused by Vibrio spp., is a relatively new farmed penaeid shrimp bacterial disease. The shrimp production in AHPND affected regions has dropped to ~60%, and the disease has caused a global loss of USD 43 billion to the shrimp farming industry. The conventional approaches, such as antibiotics and disinfectants, often applied for the mitigation or cure of AHPND, have had limited success. Additionally, their usage has been associated with alteration of host gut microbiota and immunity and development of antibiotic resistance in bacterial pathogens. For example, the Mexico AHPND-causing V. parahaemolyticus strain (13-306D/4 and 13-511/A1) were reported to carry tetB gene coding for tetracycline resistance gene, and V. campbellii from China was found to carry multiple antibiotic resistance genes. As a consequence, there is an urgent need to thoroughly understand the virulence mechanism of AHPND-causing Vibrio spp. and develop novel management strategies to control AHPND in shrimp aquaculture, that will be crucially important to ensure food security in the future and offer economic stability to farmers. In this review, the most important findings of AHPND are highlighted, discussed and put in perspective, and some directions for future research are presented.

Preliminary evaluation of the use of bacteria isolated from the digestive tract of shrimp Litopenaeus vannamei as a source to accelerate the process of formation and development of bioflocs

This study aims at investigating to follow the formation and development of biofloc aggregates in a system with the introduction of an in vitro selected bacterial consortium (Bacillus thuringiensis, Bacillus sp., Staphylococcus cohnii) in order to induce fast formation of biofloc and to compare it to the development of spontaneous formation biofloc. Two experimental groups were evaluated for biofloc formation, SFT and IFT. The first refers to spontaneous (conventional) formation of the flocs and the second to induced formation (IFT), achieved through the consortium of potentially inducing bacteria. Both treatments presented a constant increase of bioflocs, however, in the IFT treatment, the microbial aggregates were larger and more uniform. By the end of the experiment, we verified that the aggregates formed in the IFT showed higher volume and lower sedimentation rate in comparison to the spontaneously formed ones. The results show that domestication in microbial communities is efficient as related to bioflocs, reducing instability during its formation and development.

Antagonistic Activity Against Pathogenic Vibrio Isolates of Bioflocculant-Producing Bacteria Isolated from Shrimp Ponds

&lt;b&gt;Background and Objectives:&lt;/b&gt; Biofloc culture system has been used in aquaculture as an effective technology for water treatment due to many advantages of being biodegradable and environmentally friendly. This study aims to isolate bioflocculant-producing bacteria antagonistic to pathogenic &lt;i&gt;Vibrio&lt;/i&gt; species from Pacific white shrimp ponds in Thua Thien Hue, Vietnam. &lt;b&gt;Materials and Methods:&lt;/b&gt; &lt;i&gt;Vibrio&lt;/i&gt; isolates were isolated by screening on medium with and without antibiotics. The resistance of &lt;i&gt;Vibrio&lt;/i&gt; to antimicrobial agents was assessed by Minimum Inhibitory Concentration (MIC). Bioflocs formed in shrimp cultures were used to screen bioflocculant-producing bacteria. The identification of bacteria was performed by 16S rRNA sequencing. The flocculating activity was measured by a test with kaolin clay suspension. To evaluate the antagonistic activity against &lt;i&gt;Vibrio&lt;/i&gt; isolates, an agar well diffusion assay was used. &lt;b&gt;Results:&lt;/b&gt; The screening results have found that &lt;i&gt;Vibrio&lt;/i&gt; isolates such as &lt;i&gt;V. parahaemolyticus&lt;/i&gt; KS02 and &lt;i&gt;V. alginolyticus&lt;/i&gt; KS08 from shrimp ponds can be resistant to many antibiotics with the highest resistance rate up to 66.49%. Four bioflocculant-producing isolates were obtained and identified as &lt;i&gt;Bacillus&lt;/i&gt; species. Among them, &lt;i&gt;Bacillus velezensis &lt;/i&gt;B9 when grown in YPG medium supplemented with 3% sucrose and 0.7% peptone had the highest bioflocculation with an activity of 49.2%. Two isolates of &lt;i&gt;B. subtilis&lt;/i&gt; B2 and &lt;i&gt;Bacillus&lt;/i&gt; sp. B6 had quite strong antagonistic activities against vibriosis shown in the zones of inhibition on the assay plates with diameters of about 20 mm. &lt;b&gt;Conclusion:&lt;/b&gt; The present study has found some &lt;i&gt;Bacillus&lt;/i&gt; isolates had bioflocculant-producing efficiency and inhibited pathogenic &lt;i&gt;Vibrio&lt;/i&gt; bacteria. These &lt;i&gt;Bacillus&lt;/i&gt; isolates will potentially be used as inoculum for bioflocculation to improve shrimp production.

Biological and Genomic Characterization of a Novel Jumbo Bacteriophage, vB_VhaM_pir03 with Broad Host Lytic Activity against Vibrio harveyi

Vibrio harveyi is a Gram-negative marine bacterium that causes major disease outbreaks and economic losses in aquaculture. Phage therapy has been considered as a potential alternative to antibiotics however, candidate bacteriophages require comprehensive characterization for a safe and practical phage therapy. In this work, a lytic novel jumbo bacteriophage, vB_VhaM_pir03 belonging to the Myoviridae family was isolated and characterized against V. harveyi type strain DSM19623. It had broad host lytic activity against 31 antibiotic-resistant strains of V. harveyi, V. alginolyticus, V. campbellii and V. owensii. Adsorption time of vB_VhaM_pir03 was determined at 6 min while the latent-phase was at 40 min and burst-size at 75 pfu/mL. vB_VhaM_pir03 was able to lyse several host strains at multiplicity-of-infections (MOI) 0.1 to 10. The genome of vB_VhaM_pir03 consists of 286,284 base pairs with 334 predicted open reading frames (ORFs). No virulence, antibiotic resistance, integrase encoding genes and transducing potential were detected. Phylogenetic and phylogenomic analysis showed that vB_VhaM_pir03 is a novel bacteriophage displaying the highest similarity to another jumbo phage, vB_BONAISHI infecting Vibrio coralliilyticus. Experimental phage therapy trial using brine shrimp, Artemia salina infected with V. harveyi demonstrated that vB_VhaM_pir03 was able to significantly reduce mortality 24 h post infection when administered at MOI 0.1 which suggests that it can be an excellent candidate for phage therapy.

Biofloc-Based Enhanced Survival of Litopenaeus vannamei Upon AHPND-Causing Vibrio parahaemolyticus Challenge Is Partially Mediated by Reduced Expression of Its Virulence Genes

The biofloc system is a relatively new aquaculture technology that offers practical solution to maintain culture water quality by recycling nutrients and improves the health status and resistance of shrimps against microbial infection, yet the mode of action involved remains unclear. This study aimed to unravel the underlying mechanism behind the protective effect of a biofloc system using Litopenaeus vannamei and acute hepatopancreatic necrosis disease (AHPND)-causing Vibrio parahaemolyticus M0904 strain as a host-pathogen model. The results showed that a biofloc system maintained at a C/N ratio of 15, improves the water quality and contributes to the nutrition of cultured animals as bioflocs might serve as an additional protein source. Furthermore, the study demonstrated that the biofloc system enhances the survival of L. vannamei upon challenge with a V. parahaemolyticus AHPND strain. Remarkably, the results highlight that in the biofloc system, AHPND-causing V. parahaemolyticus possibly switch from free-living virulent planktonic phenotype to a non-virulent biofilm phenotype, as demonstrated by a decreased transcription of flagella-related motility genes (flaA, CheR, and fliS), Pir toxin (PirB^VP), and AHPND plasmid genes (ORF14) and increased expression of the phenotype switching marker AlkPhoX gene in both in vitro and in vivo conditions. Taken together, results suggest that biofloc steer phenotype switching, contributing to the decreased virulence of V. parahaemolyticus AHPND strain toward shrimp postlarvae. This information reinforces our understanding about AHPND in a biofloc setting and opens the possibility to combat AHPND not only by trying to eliminate the AHPND-causing V. parahaemolyticus from the system but rather to steer the phenotypic switch.

Exploring the Impact of the Biofloc Rearing System and an Oral WSSV Challenge on the Intestinal Bacteriome of Litopenaeus vannamei

We provide a global overview of the intestinal bacteriome of Litopenaeus vannamei in two rearing systems and after an oral challenge by the White spot syndrome virus (WSSV). By using a high-throughput 16S rRNA gene sequencing technology, we identified and compared the composition and abundance of bacterial communities from the midgut of shrimp reared in the super-intensive biofloc technology (BFT) and clear seawater system (CWS). The predominant bacterial group belonged to the phylum Proteobacteria, followed by the phyla Bacteroidetes, Actinobacteria, and Firmicutes. Within Proteobacteria, the family Vibrionaceae, which includes opportunistic shrimp pathogens, was more abundant in CWS than in BFT-reared shrimp. Whereas the families Rhodobacteraceae and Enterobacteriaceae accounted for almost 20% of the bacterial communities of shrimp cultured in BFT, they corresponded to less than 3% in CWS-reared animals. Interestingly, the WSSV challenge dramatically changed the bacterial communities in terms of composition and abundance in comparison to its related unchallenged group. Proteobacteria remained the dominant phylum. Vibrionaceae was the most affected in BFT-reared shrimp (from 11.35 to 20.80%). By contrast, in CWS-reared animals the abundance of this family decreased from 68.23 to 23.38%. Our results provide new evidence on the influence of both abiotic and biotic factors on the gut bacteriome of aquatic species of commercial interest.

Resurrection ecology in Artemia

Resurrection ecology (RE) is a very powerful approach to address a wide range of question in ecology and evolution. This approach rests on using appropriate model systems, and only few are known to be available. In this study, we show that Artemia has multiple attractive features (short generation time, cyst bank and collections, well‐documented phylogeography, and ecology) for a good RE model. We show in detail with a case study how cysts can be recovered from sediments to document the history and dynamics of a biological invasion. We finally discuss with precise examples the many RE possibilities with this model system: adaptation to climate change, to pollution, to parasites, to invaders and evolution of reproductive systems.

Biofloc technology application in aquaculture to support sustainable development goals

Biofloc technology (BFT) application offers benefits in improving aquaculture production that could contribute to the achievement of sustainable development goals. This technology could result in higher productivity with less impact to the environment. Furthermore, biofloc systems may be developed and performed in integration with other food production, thus promoting productive integrated systems, aiming at producing more food and feed from the same area of land with fewer input. The biofloc technology is still in its infant stage. A lot more research is needed to optimise the system (in relation to operational parameters) e.g. in relation to nutrient recycling, MAMP production, immunological effects. In addition research findings will need to be communicated to farmers as the implementation of biofloc technology will require upgrading their skills.

Bacterial community characterization of water and intestine of the shrimp Litopenaeus stylirostris in a biofloc system

BACKGROUND

Biofloc technology (BFT), a rearing method with little or no water exchange, is gaining popularity in aquaculture. In the water column, such systems develop conglomerates of microbes, algae and protozoa, together with detritus and dead organic particles. The intensive microbial community presents in these systems can be used as a pond water quality treatment system, and the microbial protein can serve as a feed additive. The current problem with BFT is the difficulty of controlling its bacterial community composition for both optimal water quality and optimal shrimp health. The main objective of the present study was to investigate microbial diversity of samples obtained from different culture environments (Biofloc technology and clear seawater) as well as from the intestines of shrimp reared in both environments through high-throughput sequencing technology.

RESULTS

Analyses of the bacterial community identified in water from BFT and "clear seawater" (CW) systems (control) containing the shrimp Litopenaeus stylirostris revealed large differences in the frequency distribution of operational taxonomic units (OTUs). Four out of the five most dominant bacterial communities were different in both culture methods. Bacteria found in great abundance in BFT have two principal characteristics: the need for an organic substrate or nitrogen sources to grow and the capacity to attach to surfaces and co-aggregate. A correlation was found between bacteria groups and physicochemical and biological parameters measured in rearing tanks. Moreover, rearing-water bacterial communities influenced the microbiota of shrimp. Indeed, the biofloc environment modified the shrimp intestine microbiota, as the low level (27 %) of similarity between intestinal bacterial communities from the two treatments.

CONCLUSION

This study provides the first information describing the complex biofloc microbial community, which can help to understand the environment-microbiota-host relationship in this rearing system.

Evaluation of Pacific white shrimp (Litopenaeus vannamei) health during a superintensive aquaculture growout using NMR-based metabolomics

Success of the shrimp aquaculture industry requires technological advances that increase production and environmental sustainability. Indoor, superintensive, aquaculture systems are being developed that permit year-round production of farmed shrimp at high densities. These systems are intended to overcome problems of disease susceptibility and of water quality issues from waste products, by operating as essentially closed systems that promote beneficial microbial communities (biofloc). The resulting biofloc can assimilate and detoxify wastes, may provide nutrition for the farmed organisms resulting in improved growth, and may aid in reducing disease initiated from external sources. Nuclear magnetic resonance (NMR)-based metabolomic techniques were used to assess shrimp health during a full growout cycle from the nursery phase through harvest in a minimal-exchange, superintensive, biofloc system. Aberrant shrimp metabolomes were detected from a spike in total ammonia nitrogen in the nursery, from a reduced feeding period that was a consequence of surface scum build-up in the raceway, and from the stocking transition from the nursery to the growout raceway. The biochemical changes in the shrimp that were induced by the stressors were essential for survival and included nitrogen detoxification and energy conservation mechanisms. Inosine and trehalose may be general biomarkers of stress in Litopenaeus vannamei. This study demonstrates one aspect of the practicality of using NMR-based metabolomics to enhance the aquaculture industry by providing physiological insight into common environmental stresses that may limit growth or better explain reduced survival and production.