Introduction
Marine aquaculture side-streams such as seaweeds are gaining increased attention as alternative aquafeed ingredients for fish relevent amino acid profile, presence of long-chain polyunsaturated fatty acids (PUFAs) and micronutrients. However, the complex carbohydrate structures in plant-based ingredients make them challenging to use directly in the diets of carnivorous species. Larvae of insects, as the black soldier fly larvae (BSFL) on the other hand, have emerged as promising candidates for convertion and valorization of different side streams and wastes into valuable feed ingredients . Particularly interesting are marine side-streams that have the potential to increase the PUFA and other nutritional values of the feed. Although some studies have explored the valorization of seaweeds through BSFL, salmon feeding trials incorporating algea derived BSFL are still scarse, particularly during sensitive developmental stages like the smoltification.
Atlantic salmon smoltification represent a critical life stage, when the fish undergo profound physiological changes to prepare for seawater (SW) . A key adaptation during this transition is the developemnt of intestine and the fluid transport capacity which enables effective water absorption , a crucial osmoregulat ory step in marine environments. Proper intestinal development during smoltification is therefore essential for successful seawater acclimation. To date, no studies have specifically investigated the impact of marine side-stream derived insect-based diets on intestinal fluid transport development and growth performance in Atlantic salmon during smoltification . Therefore, the main aim of the present study was to investigate how side-streams from seaweed farming, valorized through BSFL influence intestinal development in Atlantic salmon smolts with a particular focus on intestinal fluid transport capacit y.
Materials and methods
Atlantic salmon smolts were produced at the Lerang Research Station, Norway (Skretting Aquaculture Innovation), using a winter light regime (12 hours light:12 hours darkness, 12:12) for 6 weeks, followed by continuous light (24:0) at 12 °C. During the winter light period in freshwater (FW), triplicate groups of parr ( ̴ 35 g) were assigned either a control diet or one of two insect meal diets: BSFL fed on Ulva (Ulva fenestrata ) or BSFL fed on Kelp (Saccharina latissima). Insect meal diets (5% inclusion) were formulated to partially replace fishmeal in the control diet. The feeding trial was conducted for 12 weeks. Fish were then transferred to full-strength SW at 504 degree-days (dd). During the SW period, all fish were fed the same commercial diet for six weeks. Sampling was conducted in FW at 0 dd, 336 dd, and 504 dd as well as at the final sampling in SW at 1008 dd. At each sampling point, fish body weight (g) and intestinal mucosal weight (mg) were measured. Plasma was analyzed for ion concentrations, osmolality, and hormone levels (including IGF-1 and cortisol). Intestinal fluid transport rate was measured using the non-everted gut sac method, and Na⁺/K⁺-ATPase (NKA) enzyme activity using a kinetic enzyme assay. The intestinal nutrient transport capacity was also determined using Ussing chamber techniques.
Results
Mucosal weight increased over time, with significantly higher mucosal mass observed in fish sampled after 6 weeks in SW at 1008 dd. No significant differences in mucosal mass were found between diets (Fig 1-A) . Intestinal fluid transport capacity also increased over time with significantly higher values in SW fish. A general dietary effect was observed, where fish fed the control diet exhibited significantly higher intestinal fluid transport capacity compared to those fed BSFL meal based on kelp (Fig 1-B) . Growth r esults showed that body weight increased by nearly 20% in the smolts at 504 dd compared to the parr at 0 dd. After six weeks in seawater (1008 dd) body weight increased by approximately 50% compared to the parr stage (Table 1) . However, body weight was not significantly affected by different diets (Table 1). These findings suggest that BSFL-based diets supported similar growth performance to the control diet, with Ulva-based diet being the most promising candidate as fish fed the kelp-based BSFL diet showed lower intestinal fluid transport capacity. In-depth physiological results, including intestinal nutrient transport, barrier function, and NKA activity will be presented at the conference. Overall, these findings highlight both the potential and the challenges of using BSFL-valorized seaweeds diets to support intestinal development and seawater adaptation in Atlantic salmon smolts.