Gastrointestinal microbiota have an important impact on fish health and disease, stimulating interest in a better understanding of how these gastrointestinal microbial communities are composed and consequently affect host fitness. In this respect, probiotic microorganisms have been extensively used in recent aquaculture production. To study the use of probiotics in the treatment of infectious diseases, the establishment of a method of experimental infection to obtain consistent results for mortality and infection in challenge tests is important. In pathogen-screening tests, 4 candidate pathogenic bacteria strains (Edwardsiella ictaluri gly09, E. ictaluri gly10, E. tarda LMG2793 and Streptococcus agalactiae LMG15977) were individually tested on xenic Nile tilapia larvae. Only Edwardsiella strains delivered via Artemia nauplii, with or without additional pathogen delivery via the culture water, led to increased mortality in fish larvae. A gnotobiotic Nile tilapia larvae model system was developed to provide a research tool to investigate the effects and modes-of-action of probiotics under controlled conditions. A double disinfection procedure using hydrogen peroxide and sodium hypochlorite solution was applied to the fish eggs, which were subsequently incubated in a cocktail of antibiotic and antifungal agents. In the gnotobiotic challenge test, E. ictaluri gly09R was added to the model system via Artemia nauplii and culture water, resulting in a significant mortality of the gnotobiotic fish larvae. The developed gnotobiotic Nile tilapia model can be used as a tool to extend understanding of the mechanisms involved in host-microbe interactions and to evaluate new methods of disease control.

The commercial farming of juvenile lumpfish requires monitoring of gonadal development to achieve synchronized production. Conventional methods such as gonadosomatic index (GSI), sex hormone analyses, gonadal histology, endoscopy, and gene expression analyses are costly, invasive, and often involve sacrificing the fish. We assessed the efficiency of ultrasound as a non‐invasive method for monitoring gonadal development in lumpfish. Based on ultrasound observations, we categorized the fish into six stages; F0 to F5 for females and M0 to M5 for males, that represented maturity levels from immature to spent. Importantly, the ultrasound gonadal stages aligned with histological gonadal stages. Additionally, ultrasound stages aligned with profiles of GSI, testosterone (T), 11‐ketotestosterone, and 17β‐estradiol throughout gonadal development including the spawning period. Moreover, these parameters exhibited significant positive correlations with each other reflecting their parallel trends during gonadal development. To minimize the frequency of ultrasound usage and fish handling, we established F3 and M3/M4 as arbitrary thresholds for identifying ripe females and males, respectively. By using these thresholds, the need for regular ultrasound monitoring could be reduced during most of the rearing period. Ultrasound proves to be useful and reliable for monitoring gonadal development in lumpfish, enabling synchronized production of juvenile fish.

A successful control of sexual maturation is crucial for year-round production of lumpfish juveniles destined as cleaner fish in Atlantic salmon aquaculture. This study investigated the combined effects of photoperiod and temperature manipulations on sexual maturation and spawning in lumpfish females. Lumpfish juveniles were exposed to simulated natural and nine-month compressed annual photoperiods, with subsequent temperature elevation. Body weight (BW), condition factor (K), gonadosomatic index (GSI), ovarian development, plasma levels of 17β-estradiol (E2), testosterone (T) and 11-ketotestosterone (11-KT), and spawning were assessed. Compressing the natural photoperiod caused a clear increase and decrease in GSI, T, 11-KT and E2 towards and during the spawning period. Before the temperature elevation, GSI, T, 11-KT, E2 and ovarian development were advanced in the compressed photoperiod. After the temperature elevation, GSI, T, 11-KT and E2 fluctuated more in the compressed photoperiod, while in the natural photoperiod, E2 declined, and GSI, T and 11-KT increased. Spawning was advanced by 1 month in the compressed photoperiod compared to the natural photoperiod. Temperature elevation led to higher levels, earlier peaks and declines of T, 11-KT or E2 in both photoperiods, and advanced spawning by 1.5 months in the compressed photoperiod compared to the natural photoperiod. Temperature elevation also led to increased ovulation recruitment and increased cumulative weight of spawned eggs in the natural photoperiod. Compressing the natural photoperiod and elevating temperature can thus advance sexual maturation and spawning in lumpfish females. Due to the lower amounts of spawned egg weights in the high temperature compressed photoperiod, further studies on effects of photoperiod and timing of temperature manipulations on spawning, fecundity and egg quality could optimize the photothermal manipulations on lumpfish broodstock.

In an Artemia survey conducted along the coast of Tanzania between March and July 2017, 32 salt pans in Tanga, Dar es salaam, Pwani, Lindi, and Mtwara were assessed. Of all visited salt pans, 16 (50%) had either Artemia biomass or cysts or both. Body length ranged from 4.0 mm to 9.5 mm, while the mean and modal lengths were 6.2 mm and 6.0 mm, respectively. Temperature ranged from 27.2°C to 48.7°C, salinity from 20 g L⁻¹ to >140 g L⁻¹, pH from 5.8 to 7.8, dissolved oxygen (DO) from 3.1 mg L⁻¹ to 4.9 mg L⁻¹, water depth from 10 cm to 75 cm and conductivity from 42.0 ms cm⁻¹ to 176.6 ms cm⁻¹. A bisexual population of Artemia franciscana is suggested. Observed cyst states included concave, biconcave, spherical and cracking cysts. This is the first to report on the occurrence of Artemia in Tanzania.

The demand for lumpfish (Cyclopterus lumpus) as a cleaner fish for the Atlantic salmon industry to battle sea lice infestations results in an increased interest of juvenile production in aquaculture. For successful control of the lumpfish reproduction, it is crucial to adequately understand its reproductive biology. Female and male lumpfish were sampled four times during a 24-week experiment where gonadal development in relation to endocrine factors was studied. Histologically, we identified and described ten oogenic and four spermatogenic stages during gamete development. The female gamete stages were grouped into ovarian development stages to simplify categorization of ovarian development. The ovarian development and the associated endocrine factors were typical for fish with group-synchronous ovarian organization, and from our histological observations it is possible that lumpfish females release up to four batches of eggs in one spawning season. During early stages of testis development, spermatogenic cells were observed in cysts. The cystic spermatogenesis and the distribution of spermatogonial cysts through the germinal compartment indicated an unrestricted spermatogonial distribution type. Despite its variance, the expression of genes gnrh2 and gnrh3 in the brain of lumpfish decreased in females and increased in males during gonadal development. Gonadal development was also associated with increasing trends of gene expression in gonadotropin receptors, vitellogenin receptors, steroidogenic enzymes, and blood plasma concentrations of sex steroids. The genes fshr, lhr, vtgr, cyp19a1 and the sex steroid E2 increased up to oocyte maturation and decreased at ovulation, while GSI, T and 11-KT increased up to ovulation. While fshr and lhr peaked at spermatocyte stage and decreased later, cyp17a1, T and 11-KT increased up to the spermatid stage and decreased at the spermatozoa stage in males. Our findings improve the current understanding on lumpfish reproductive biology and suggest future research directions.