The Gulf Killifish Fundulus grandis is a popular baitfish for recreational fisheries and is a candidate for commercial‐scale culture; however, its relatively low fecundity compared to other baitfish presents a production challenge. Egg output in relation to broodstock densities and spawning mat surface areas was evaluated in recirculating aquaculture systems (RASs). In experiment 1, broodstock were stocked at two densities (45 and 90 fish/m³), with eggs collected three times per week for 8 weeks. Broodstock (15–17 g) that were held at 45 fish/m³ had greater reproductive output (3.93 eggs·female−1·d−1) than broodstock that were held at 90 fish/m³ (2.22 eggs·female−1·d−1). However, there was no difference between the number of eggs per collection (1,005 and 1,107 eggs per 3,600‐L tank) or total egg production (23,202 and 25,802 eggs per 3,600‐L tank), indicating that reproductive output was limited by another factor, such as available spawning substrate. Therefore, in experiment 2, broodstock (12–16 g) were stocked at a density of 100 fish/m³ in all tanks and were held for 8 weeks with one of three spawning mat (Spawntex) surface areas: 4,500 cm2 (one tier), 9,000 cm2 (two tiers), or 13,500 cm2 (three tiers). Reproductive output was 2.12, 2.27, and 2.21 eggs·female−1·d−1 for the one‐tier, two‐tier, and three‐tier treatments, respectively, with no differences among treatments. There was also no among‐treatment difference in the number of eggs per collection (1,724, 1,858, and 1,794 eggs/tank) or in the total number of eggs collected (27,587, 29,728, and 28,704 eggs/tank), suggesting that increasing the surface area of spawning material does not increase egg production. Therefore, a stocking density of 45 fish/m³ and a spawning mat surface area of 4,500 cm2 in commercial‐scale RAS broodstock tanks result in similar egg production compared to a higher stocking density and greater spawning mat surface area.
